Effect of withholding feed on concentration and composition of plasma very low density lipoprotein and serum nonesterified fatty acids in horses

OBJECTIVE: To measure and compare the concentration and composition of very low-density lipoprotein (VLDL) in plasma and selected lipids in serum of horses fed mixed grass hay ad libitum or denied feed for 36 hours. ANIMALS: 4 healthy adult mares. PROCEDURE: Mares were either fed mixed grass hay ad libitum or denied feed for 36 hours beginning at 8:00 AM. Blood samples were collected every 2 hours during the study period and analyzed for nonesterified fatty acid (NEFA), triglyceride (TG), VLDL, and glucose concentrations and composition of VLDL. RESULTS: Withholding feed significantly increased mean serum concentrations of NEFA. By 36 hours, a 16-fold increase in mean serum NEFA concentration and 2-fold increase in mean plasma VLDL concentration, compared with baseline values, were detected. Mean plasma TG concentrations significantly increased with time in feed-deprived horses. Significantly lower overall mean plasma glucose concentrations were detected in feed-deprived horses. Mean percentage of protein in VLDL was significantly lower in feed-deprived horses. Plasma VLDL concentrations varied widely among horses in response to withholding feed. Plasma TG and VLDL concentrations remained unaltered in 2 horses. CONCLUSIONS AND CLINICAL RELEVANCE: Withholding feed significantly increases blood lipid concentrations in horses, but individual horses respond differently. Serum NEFA concentrations were increased in all 4 horses denied feed, indicating mobilization of tissue triglyceride stores. Variation in plasma VLDL concentration in response to withholding feed suggests that its metabolism is strongly influenced by other, as yet undetermined, factors in horses. Differences in the plasma VLDL concentrations among horses in response to withholding feed may be used as an indication of susceptibility to the hyperlipemic syndrome of Equidae.     

Effect of hypothyroidism on blood lipid concentrations in horses.

OBJECTIVE: To measure and compare concentrations of selected blood lipids before and after thyroidectomy in horses. ANIMALS: 5 healthy adult mares. PROCEDURE: Mares were confirmed to be euthyroid. Thyroidectomy was performed, and hypothyroidism was confirmed. Selected blood lipid variables were measured before hypothyroidism was induced and weekly for 4 weeks after induction. Plasma concentrations of very low-density lipoprotein (VLDL), low-density lipoprotein (LDL), serum triglyceride (TG), total cholesterol (TC), and nonesterified fatty acid (NEFA) were measured. The composition of VLDL and LDL also was examined. RESULTS: Mean plasma concentrations of VLDL and LDL increased significantly after thyroidectomy. By 4 weeks after thyroidectomy, a ninefold increase in mean plasma concentration of VLDL and a threefold increase in LDL, compared with baseline values, were detected. After thyroidectomy, mean percentage of TG in VLDL increased significantly, whereas free cholesterol and cholesterol ester content decreased. Mean percentage of TG in LDL also increased by 3 to 4 weeks after thyroidectomy. Serum concentrations of TG and TC increased, whereas serum NEFA concentration decreased. CONCLUSIONS: Hypothyroidism significantly alters blood lipid concentrations of horses. After thyroidectomy, markedly high VLDL concentration, appearance of TG-rich VLDL, increased serum concentrations of TG and TC, and decreased blood concentration of NEFA were evident. CLINICAL RELEVANCE: Examination of blood lipid concentrations of horses may be useful for detecting naturally acquired hypothyroidism.     

Effects of rice bran oil on plasma lipid concentrations, lipoprotein composition, and glucose dynamics in mares

Plasma lipid concentrations, lipoprotein composition, and glucose dynamics were measured and compared between mares fed diets containing added water, corn oil (CO), refined rice bran oil (RR), or crude rice bran oil (CR) to test the hypothesis that rice bran oil lowers plasma lipid concentrations, alters lipoprotein composition, and improves insulin sensitivity in mares. Eight healthy adult mares received a basal diet fed at 1.5 times the DE requirement for maintenance and each of the four treatments according to a repeated 4 x 4 Latin square design consisting of four 5-wk feeding periods. Blood samples were collected for lipid analysis after mares were deprived of feed overnight at 0 and 5 wk. Glucose dynamics were assessed at 0 and 4 wk in fed mares by combined intravenous glucose-insulin tolerance tests. Plasma glucose and insulin concentrations were measured, and estimated values of insulin sensitivity (SI), glucose effectiveness, and net insulin response were obtained using the minimal model. Mean BW increased (P = 0.014) by 29 kg (range = 10 to 50 kg) over 5 wk. Mean plasma concentrations of NEFA, triglyceride (TG), and very low-density lipoprotein (VLDL) decreased (P < 0.001) by 55, 30, and 39%, respectively, and plasma high-density lipoprotein and total cholesterol (TC) concentrations increased (P < 0.001) by 15 and 12%, respectively, over 5 wk. Changes in plasma NEFA (r = 0.58; P < 0.001) and TC (r = 0.44; P = 0.013) concentrations were positively correlated with weight gain over 5 wk. Lipid components of VLDL decreased (P < 0.001) in abundance over 5 wk, whereas the relative protein content of VLDL increased by 39% (P < 0.001). Addition of oil to the basal diet instead of water lowered plasma NEFA and TG concentrations further (P = 0.002 and 0.020, respectively) and increased plasma TC concentrations by a greater magnitude (P = 0.072). However, only plasma TG concentrations and VLDL free cholesterol content were affected (P = 0.024 and 0.009, respectively) by the type of oil added to the diet. Mean plasma TG concentration decreased by 14.2 mg/dL over 5 wk in the CR group, which was a larger (P < 0.05) decrease than the one (-5.3 mg/dL) detected in mares that received water. Consumption of experimental diets lowered S(I), but glucose dynamics were not affected by oil supplementation. Addition of oil to the diet altered blood lipid concentrations, and supplementation with CR instead of water specifically affected plasma TG concentrations and VLDL free cholesterol content.     

Effects of oral administration of levothyroxine sodium on concentrations of plasma lipids, concentration and composition of very-low-density lipoproteins, and glucose dynamics in healthy adult mares

OBJECTIVE: To evaluate glucose and lipid metabolism in healthy adult horses administered levothyroxine sodium (L-T4). ANIMALS: 12 healthy adult mares. PROCEDURE: 8 horses received an incrementally increasing dosage of L-T4 (24, 48, 72, or 96 mg of L-T4/d) for weeks 1 to 8. Each dose was provide between 7 AM and 8 AM in the morning grain meal for 2 weeks. Four additional horses remained untreated. Serum concentrations of nonesterified fatty acids, triglyceride (TG), total cholesterol (TC), and very-low-density lipoprotein (VLDL) were measured and composition of VLDL examined in samples obtained between 8 AM and 9 AM at weeks 0, 2, 4, 6, and 8. Glucose dynamics were assessed by use of a combined IV glucose-insulin tolerance test (IVGITT) conducted before and at the end of the 8-week treatment period. Data for each combined IVGITT were interpreted by use of the minimal model. RESULTS: Plasma TG, TC, and VLDL concentrations significantly decreased over time in treated horses. At the completion of the 8-week treatment period, mean plasma VLDL concentration was 46% of the mean value for week 0 in treated horses. Insulin sensitivity significantly increased (> 2-fold) in treated horses, but glucose effectiveness and net insulin response were not affected. Levothyroxine sodium significantly increased the rate of insulin disposal. CONCLUSIONS AND CLINICAL RELEVANCE: Administration of L-T4 decreases blood lipid concentrations, improves insulin sensitivity, and increases insulin disposal in horses. Levothyroxine sodium may have potential as a treatment for horses with reduced insulin sensitivity.     

Effect of hypothyroidism on the blood lipid response to higher dietary fat intake in mares

Blood lipid and lipoprotein concentrations were measured and compared between euthyroid and thyroidectomized mares on low-fat or high-fat diets to test the hypothesis that hypothyroidism alters the blood lipid response to higher dietary fat intake. Four healthy adult mares and four adult mares that had been thyroidectomized 3 to 6 mo earlier were placed on low-fat or high-fat diets according to a replicated 2 x 2 Latin square design consisting of two 5-wk feeding periods separated by a 2-wk washout interval. Plasma lipid concentrations were measured at 0, 3, 4, and 5 wk, and plasma lipase activities were measured at the end of each 5-wk feeding period. Compared with euthyroid mares (0.46 ng/mL [range 0.34 to 0.68 ng/mL T3], and 21.5 ng/mL [range 18.1 to 25.1 ng/mL T4], respectively), median serum concentrations of T3 and T4 were lower (P = 0.029 and P = 0.021, respectively) in thyroid-ectomized mares (0.26 ng/mL [range 0.23 to 0.26 ng/ mL T3], and undetectable T4). Serum T4 concentrations were below the limits of detection in thyroidectomized horses. Alterations in body weight over 5 wk did not differ between groups. Mean plasma very low density lipoprotein (VLDL) and triglyceride (TG) concentrations were higher (P = 0.045 and 0.034, respectively) in hypothyroid mares (55.42 +/- 35.05 mg/dL and 52.83 +/- 34.46 mg/dL, respectively) compared with euthyroid mares (28.28 +/- 13.76 mg/dL and 23.53 +/- 9.84 mg/dL, respectively). Mean plasma total cholesterol (TC) concentrations increased from 88.73 +/- 25.49 mg/dL at baseline to 103.93 +/- 24.42 mg/dL after 5 wk on the low-fat diet, but increased by a greater magnitude (P = 0.006 diet +/- time interaction) in mares that were on the high-fat diet (81.05 +/- 17.24 mg/dL and 123.84 +/- 32.27 mg/ dL, respectively). Mean plasma TC concentrations were higher (P = 0.099) in hypothyroid mares (116.16 +/- 32.89 mg/dL) than in euthyroid mares (89.56 +/- 14.45 mg/ dL). Higher post-heparin plasma lipoprotein lipase and hepatic lipase activities (P = 0.012 andP = 0.017, respectively) were detected in mares that were on the high-fat diet (2.66 +/- 0.91 micromol FA x mL(-1) x h(-1) and 2.95 +/- 0.49 micromol FA x mL(-1) x h(-1), respectively) vs. a low-fat diet (1.75 +/- 0.55 micromol FA x mL(-1) x h(-1) and 2.27 +/- 0.59 micromol FA x mL(-1) x h(-1), respectively). We conclude that plasma VLDL and TG concentrations are elevated in hypothyroid mares, but the blood lipid response to higher dietary fat intake is not influenced by hypothyroidism.     

Effect of hypothyroidism on kinetics of metabolism of very-low-density lipoprotein in mares

OBJECTIVE: To compare kinetics of the metabolism of very-low-density lipoprotein (VLDL) apolipoprotein B (apoB) before and after thyroidectomy in mares. ANIMALS: 5 healthy adult mares. PROCEDURE: Thyroidectomy was performed in euthyroid mares. Kinetics of VLDL apoB metabolism were measured before and after thyroidectomy by use of a bolus IV injection of 5,5,5-2H3 (98%) leucine (5 mg/kg) and subsequent isolation of labeled amino acid from plasma and VLDL. Labeled leucine was quantified by use of gas chromatography-mass spectrometry. Production rate (PR), delay time, and fractional catabolic rate (FCR) were calculated for the 2 forms of equine VLDL, apoB-48 VLDL, and apoB-100 VLDL. Plasma lipid concentrations were measured, and VLDL composition was determined. RESULTS: Physical appearance of horses was not altered by thyroidectomy. Significantly lower mean blood concentrations of thyroid hormones and non-esterified fatty acids were detected following thyroidectomy. Mean percentage of free cholesterol in VLDL was significantly higher after thyroidectomy. Mean plasma VLDL concentration or kinetics of apoB-48 or apoB-100 were not significantly altered by thyroidectomy. Mean +/- SEM PR was significantly lower (8.70 +/- 1.61 mg/kg/d) and mean delay time significantly longer (1.58 +/- 0.12 hours) for apoB-48 VLDL in euthyroid mares, compared with values for thyroidectomized mares (16.15 +/- 2.24 mg/kg/d and 0.93 +/- 0.10 hours, respectively). CONCLUSIONS AND CLINICAL RELEVANCE: Hypothyroidism did not significantly alter plasma VLDL concentrations or kinetics of VLDL apoB metabolism. Metabolism of apoB-48 VLDL differed significantly from that of apoB-100 VLDL in euthyroid mares.     

An association between the level of oxidative stress and the concentrations of NEFA and BHBA in the plasma of ketotic dairy cows

The aim of this study was to evaluate the oxidative status in ketotic cows. We observed changes in the oxidative status and correlations between the oxidative and metabolic status in non‐ketotic (n = 10), subclinical ketotic (n = 10) and ketotic cows (n = 10). Plasma samples were analysed by standard biochemical techniques and ELISA to determine traditional metabolic parameters: triglyceride (TG), phosphonium (P), calcium (Ca), aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), total protein (TP), albumin (ALB), immune globulin (Ig), total cholesterol (TC), high‐density lipoprotein (HDL), very low‐density lipoprotein (VLDL) and lactate dehydrogenase (LDH); energy metabolism indices: glucose, β‐hydroxybutyrate (BHBA) and non‐esterified fatty acids (NEFA); and indices of oxidative status: malondialdehyde (MDA), hydrogen peroxide (H₂O₂), vitamin C, vitamin E, superoxide dismutase (SOD), glutathione peroxidase (GSH‐Px), catalase (CAT), xanthine oxidase (XOD) and total antioxidant capacity (TAOC). The results of this study showed that plasma glucose levels were lower in ketotic and subclinical ketotic cows than in non‐ketotic cows; however, the plasma NEFA and BHBA concentrations were higher. In addition, significant decreases in TC, HDL and VLDL and significant increases in AST, ALT and LDH were observed in the plasma of the ketotic cows. The ketotic cows showed decreased plasma SOD, CAT, vitamin C and vitamin E, inhibited hydroxyl radical capacity and increased plasma H₂O₂ and MDA. There were positive correlations between the plasma NEFA and ALT, AST, LDH and MDA and negative correlations between the plasma NEFA and TC, HDL, VLDL, SOD, vitamin C, vitamin E, 1542280 uric acid and inhibited hydroxyl radical capacity. In addition, there were positive correlations between BHBA concentrations and ALT, AST and LDH and negative correlations between plasma BHBA concentrations and TC, HDL, VLDL, vitamin E and inhibited hydroxyl radical capacity. Overall, ketotic dairy cows experience oxidative stress, which is presumably associated with hyperketonemia and higher NEFA.     

Effects of oral administration of levothyroxine sodium on serum concentrations of thyroid gland hormones and responses to injections of thyrotropin-releasing hormone in healthy adult mares

OBJECTIVE: To determine the effects of levothyroxine sodium (L-T4) on serum concentrations of thyroid gland hormones and responses to injections of thyrotropin-releasing hormone (TRH) in euthyroid horses. ANIMALS: 12 healthy adult mares. PROCEDURE: 8 horses received an incrementally increasing dosage of L-T4 (24, 48, 72, or 96 mg of L-T4/d) for weeks 1 to 8. Each dose was provided for 2 weeks. Four additional horses remained untreated. Serum concentrations of total triiodothyronine (tT3), total thyroxine (tT4), free T3 (fT3), free T4 (fT4), and thyroid-stimulating hormone (TSH) were measured in samples obtained at weeks 0, 2, 4, 6, and 8; 1.2 mg of TRH was then administered i.v., and serum concentrations of thyroid gland hormones were measured 2 and 4 hours after injection. Serum reverseT3 (rT3) concentration was also measured in the samples collected at weeks 0 and 8. RESULTS: Treated horses lost a significant amount of weight (median, 19 kg). Significant treatment-by-time effects were detected for serum tT3, tT4, fT3, fT4, and TSH concentrations, and serum tT4 concentrations were positively correlated (r, 0.95) with time (and therefore dosage) in treated horses. Mean +/- SD serum rT3 concentration significantly increased in treated horses (3.06 +/- 0.51 nmol/L for week 8 vs 0.74 +/- 0.22 nmol/L for week 0). Serum tT3, tT4, fT3, and TSH concentrations in response to TRH injections differed significantly between treated and untreated horses. CONCLUSIONS AND CLINICAL RELEVANCE: Administration of levothyroxine sodium increased serum tT4 concentrations and blunted responses toTRH injection in healthy euthyroid horses.     

Effects of plasma sample storage on blood ammonia, bilirubin, and urea nitrogen concentrations: cats and horses

Ten horses, a pony, and 13 cats were used to evaluate base-line blood ammonia, bilirubin, and urea nitrogen concentrations and to determine The effects of prolonged cold storage (-20 degrees C) before assay. Base-line plasma ammonia concentrations in cats (0.992 +/- 0.083 [SE] micrograms/ml) did not change significantly after 48 hours of storage (0.871 +/- 0.073 micrograms/ml); however, they were increased 4.2- and 13-fold after 168 and 216 hours of storage, respectively. In contrast to base-line plasma-ammonia values in cats, those of horses were significantly (0.265 +/- 0.044 micrograms/ml) lower, and significantly increased from base-line values after 48 hours of storage (0.861 +/- 0.094 micrograms/ml) and continued to increase 25.6-fold at 168 hours and 18.4-fold at 216 hours. Plasma urea nitrogen concentrations in cats (25.8 +/- 1.06 mg/dl) and horses (11.2 +/- 0.749 mg/dl) did not change significantly during 168 hours of storage. Total plasma bilirubin values from both cats (0.19 +/- 0.049 mg/dl) and horses (0.75 +/- 0.064 mg/dl) also did not change significantly during storage. These results indicate that feline plasma samples for ammonia determinations may be stored at -20 degrees C for up to 48 hours, whereas equine plasma ammonia values tend to increase during that time. The reason for the increase remains unexplained. Both feline and equine plasma urea nitrogen and total bilirubin are stable for at least 168 hours of storage at -20 degrees C.     

Studies on equine lipid metabolism. 2. Lipolytic activities of plasma and tissue lipases in large horses and ponies

The enzymatic fundamentals of lipid metabolism of equine have not been thoroughly investigated at this point in time. It is still unclear why ponies in contrast to horses may become hyperlipaemic when coming negative energy balance. In this study, the activities of the triglyceride-cleaving key enzymes of ponies are large bred horses were investigated in order to obtain insight into the aetiology of the syndrome. The objective of the study was to measure the activities of hormone-sensitive lipase (HSL), lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) in ponies and horses in ex vivo in vitro assays. Norepinephrine (NE) stimulated pony adipocytes to release FFA in a linear fashion (4.57 +/- 2.09 nmol FFA.10(5) cells-1.min-1). This was not observed in horses. Lipolysis was significantly higher in fat cells of ponies than in horses when adenosine deaminase (ADA) and NE were added (12.71 +/- 3.12 vs. 1.96 +/- 1.22 nmol FFA.10(5) cells-1.min-1). Relative inhibition of lipolysis by the action of insulin was comparable in adipocytes of horses and ponies. However, absolute FFA release in pony fat cells was as high as the maximal NE and ADA stimulated lipolysis in horse adipocytes. Postheparin plasma lipase activities in ponies and horses did not differ between the sub-species. This finding was supported by the results obtained from measurement of LPL activity in adipose and muscle tissue showing only a tendency of increased activities in pony explants when compared to horse tissue incubations. This study further supports the hypothesis that differences in regulation of TG release from fat stores rather than clearance of TG from plasma is causative for the development of hyperlipaemia in ponies. Abbreviations used: ADA, adenosine deaminase; BW, body weight; FFA, free fatty acid; HSL, hormone-sensitive lipase; HTGL, hepatic triglyceride lipase; LPL, lipoprotein lipase; NE, norepinephrine; SDS, sodium dodecyl sulfate; TG, triglyceride; VLDL, very low density lipoprotein.     

Effects of exploratory laparotomy on plasma and peritoneal coagulation/fibrinolysis in horses

Plasma and peritoneal fluid samples were collected before and after surgery from 6 horses undergoing a ventral midline exploratory laparotomy and from 6 anesthetized control horses. Coagulation/fibrinolytic components measured in the plasma and peritoneal fluid of these horses included the functional activity of antithrombin III, alpha-2 antiplasmin, plasminogen, and protein C, and the concentrations of fibrinogen and fibrin degradation products. Peritoneal fluid antithrombin III, fibrin degradation products, and plasminogen values were significantly increased after surgery (over time) in principal horses. Compared with control horses, postoperative peritoneal fluid from horses undergoing laparotomy had significantly increased antithrombin-III activity at 12 and 72 hours, alpha-2 antiplasmin activity at 24 hours, fibrin degradation product concentrations at 6, 12, 24, 72, 96, and 144 hours, plasminogen activity at 6, 12, 24, 48, 72 and 96 hours, and protein-C activity at 12, 24, 72, and 96 hours. There were no significant changes in the peritoneal fibrinogen concentration in principal horses. Plasma plasminogen activity was significantly decreased at 24 hours after surgery in principal horses, compared with controls. Changes were minimal in the remaining plasma coagulation/fibrinolytic components of horses undergoing laparotomy. Plasma and peritoneal fluid values of anesthetized control horses did not change.     

Quantitative assessment of hepatic function by means of 99mTc-mebrofenin in healthy horses

99mTc-mebrofenin is used in humans and small animals to assess hepatic function. This study was undertaken to measure hepatic clearance of 99mTc-mebrofenin in healthy horses and to determine whether feed deprivation and increased serum total bilirubin (TBIL) concentration alter 99mTc-mebrofenin clearance. Plasma clearance of 99mTc-mebrofenin was determirned in 7 healthy horses at 0, 48, and 96 hours of feed withholding. Serum TBIL and nonesterified fatty acid (NEFA) concentrations were measured every 24 hours. 99mTc-mebrofenin (4.16 +/- 0.62 mCi, mean +/- SD) was injected into a jugular vein, and blood samples were retrieved from the contralateral jugular vein. A plasma time-activity curve of 99mTc-mebrofenin was generated, from which the area under the curve (AUC) and the T1/2 of the fast-phase (T1/2) and slow-phase (T1/2f) were calculated. Mean +/- SD AUC was 17,700 +/- 4,257, 18,616 +/- 8,078, and 16,168 +/- 6,031 counts per minute (cpm) at 0, 48, and 96 hours, respectively; mean +/- SD T1/2f was 2.80 +/- 0.38 minutes, 3.52 +/- 1.46 minutes, and 3.82 +/- 1.29 minutes at 0, 48, and 96 hours, respectively; median T1/2s was 63.9, 49.2, and 45.8 minutes at 0, 48, and 96 hours, respectively. No difference was detected between the values of AUC, T1/2f, and T1/2s at 0, 48, and 96 hours. There was a significant increase in TBIL with fasting, with a mean +/- SD of 6.3 +/- 1.3 mg/dL at 26 hours. NEFAs increased, reaching a plateau at 48 hours (650 +/- 152 micromol/L). Plasma TBIL concentrations did not correlate with AUC or T1/2s but correlated weakly with T1/2f (r = 0.50). Plasma NEFA concentrations did not correlate with AUC, T1/2s, or T1/2f values. This study suggests that 99mTc-mebrofenin plasma clearance is not affected by feed withholding and that hyperbilirubinemia associated with feed withholding does not affect the hepatic extraction efficiency of this radiopharmaceutical.     

Effect of Experimental Hypothyroidism on Glomerular Filtration Rate and Plasma Creatinine Concentration in Dogs

Background: Hypothyroidism affects renal function in a manner opposite the effects of hyperthyroidism.
Objective: To evaluate the effects of experimentally induced hypothyroidism on glomerular filtration rate (GFR) and basal plasma creatinine concentration in dogs.
Animals: Sixteen anestrous, female dogs.
Methods: Hypothyroidism was induced by administration of ¹³¹I in 8 dogs, and 8 healthy euthyroid dogs acted as controls. Exogenous plasma creatinine clearance (an estimate of GFR) was measured in all dogs before (control period) and 43–50 weeks after induction of hypothyroidism (posttreatment period). Other pharmacokinetic parameters of creatinine were also determined.
Results: No significant difference was observed for basal plasma creatinine concentration and creatinine clearance between control and hypothyroid dogs in the control period. In the posttreatment period, mean ± SD creatinine clearance in the hypothyroid group (2.13 ± 0.48 mL/min/kg) was lower ( P < .001) than that of the control group (3.20 ± 0.42 mL/kg/min). Nevertheless, basal plasma creatinine concentrations were not significantly different between the hypothyroid and control groups (0.74 ± 0.18 versus 0.70 ± 0.08 mg/dL, respectively) because endogenous production of creatinine was decreased in hypothyroid dogs (22 ± 3 versus 32 ± 5 mg/kg/d, P =.001).
Conclusion and Clinical Importance: Hypothyroidism causes a substantial decrease in GFR without altering plasma creatinine concentrations, indicating that GFR evaluation is needed to identify renal dysfunction in such patients.     

Studies on the functional relationship between thyroid, adrenal and gonadal hormones

In order to clarify the functional relationship between thyroid, adrenal and gonadal hormones, hypothyroidism was induced by administration of thiuoracil in adult male and female rats, and the effects of hypothyroidism on the adrenal and the gonadal axes were investigated in the present study. 1. The functional relationship between thyroid and adrenal hormones: Adrenal weights and corticosterone were lowered, whereas the secretion of ACTH, corticotrophin-releasing hormone (CRH) and arginine vasopressin (AVP) increased in hypothyroid rats compared to euthyroid rats. These results indicate that hypothyroidism causes adrenal dysfunction directly and results in hypersecretion of CRH and AVP from the hypothalamus. 2. The functional relationship between thyroid and gonadal hormones: The pituitary response to LHRH was lowered, whereas the testicular response to hCG was not changed in hypothyroid rats. Hypothyroidism suppressed copulatory behavior in male rats. These results suggest that hypothyroidism probably causes dysfunction in gonadal axis at the hypothalamic-pituitary level in male rats. In adult female rats, hypothyroidism inhibited the follicular development accompanied estradiol secretion, whereas plasma concentrations of progesterone and prolactin (PRL) increased in hypothyroid female rats. Hypothyroidism significantly increased the pituitary content of vasoactive intestinal peptide (VIP) though it did not affect dopamine synthesis. These results suggest that hypothyroidism increases pituitary content of VIP and this increased level of VIP likely affects PRL secretion in a paracrine or autocrine manner. In female rats, inhibition of gonadal function in hypothyroid rats mediated by hyperprolactinemia in addition to hypersecretion of endogenous CRH.     

Methimazole and thyroid hormone replacement in broilers

Seven-day-old chickens were fed diets containing 18% crude protein + 0 or 1g methimazole/kg to produce either euthyroid or hypothyroid groups of birds at 28 days of age. These two groups were then offered diets containing either 0 or 1mg triiodothyronine (T(3))/kg diet. Birds were sampled at 0, 2, 5, and 8 days following the onset of the T(3) treatment. Measurements taken at these intervals included in vitro hepatic lipogenesis (IVL), growth and feed consumption, hepatic enzyme activities (malic enzyme, ME; isocitrate dehydrogenase, ICD; and aspartate amino transferase, AAT), plasma hormones (T(3); thyroxine, T(4); insulin like growth factors I, IGF-I; and insulin like growth factors II, IGF-II) and metabolites (glucose; fatty acids, NEFA; triglyerides; uric acid). Hypothyroidism decreased IVL and ME at 28 days of age; however, T(3) supplementation for 2 days restored both IVL and ME. Paradoxically, continuing T(3) replenishment for an additional 3-6 days decreased IVL without affecting ME activity. In contrast, supplemental T(3) decreased IVL in euthyroid birds, regardless of the dosing interval, but had no effect on ME activity. Methimazole decreased plasma T(3), T(4), uric acid, and IGF-I, but did not affect IGF-II at 28 days. Giving T(3) to birds previously on methimazole increased plasma IGF-I as did feeding a control diet. Supplemental T(3) increased NEFA in both euthyroid and hypothyroid birds, but only for a short period following the initiation of supplementation (2 days post-supplementation). These data may help to explain some of the apparent reported dichotomies in lipid metabolism elicited by changes in the thyroid state of animals. In addition, most metabolic changes in response to feeding T(3) occurred within 2-5 days, suggesting that changes in intermediary metabolism preceded morphological changes. In conclusion, the thyroid state of the animal will determine responses to exogenous T(3).     

Effects of alpha2-adrenergic receptor agonists on urine production in horses deprived of food and water

OBJECTIVE: To quantitate the dose- and time-related effects of IV administration of xylazine and detomidine on urine characteristics in horses deprived of feed and water. ANIMALS: 6 horses. PROCEDURE: Feed and water were withheld for 24 hours followed by i.v. administration of saline (0.9% NaCI) solution, xylazine (0.5 or 1.0 mg/kg), or detomidine (0.03 mg/kg). Horses were treated 4 times, each time with a different protocol. Following treatment, urine and blood samples were obtained at 15, 30, 60, 120, and 180 minutes. Blood samples were analyzed for PCV and serum concentrations of total plasma solids, sodium, and potassium. Urine samples were analyzed for pH and concentrations of glucose, proteins, sodium, and potassium. RESULTS: Baseline (before treatment) urine flow was 0.30 +/- 0.03 mL/kg/h and did not significantly change after treatment with saline solution and low-dose xylazine but transiently increased by 1 hour after treatment with high-dose xylazine or detomidine. Total urine output at 2 hours following treatment was 312 +/- 101 mL versus 4,845 +/- 272 mL for saline solution and detomidine, respectively. Absolute values of urine concentrations of sodium and potassium also variably increased following xylazine and detomidine administration. CONCLUSIONS AND CLINICAL RELEVANCE: Xylazine and detomidine administration in horses deprived of feed and water causes transient increases in urine volume and loss of sodium and potassium. Increase in urine flow is directly related to dose and type of alpha2-adrenergic receptor agonist. Dehydration in horses may be exacerbated by concurrent administration of alpha2-adrenergic receptor agonists.     

Cutaneous analgesia,hemodynamic and respiratory effects,and beta-endorphin concentration in spinal fluid and plasma of horses after acupuncture and electroacupuncture

OBJECTIVE: To determine cutaneous analgesia, hemodynamic and respiratory effects, and beta-endorphin concentration in spinal fluid and plasma of horses after acupuncture and electroacupuncture (EA). ANIMALS: 8 healthy 10- to 20-year-old mares that weighed between 470 and 600 kg. PROCEDURE: Each horse received 2 hours of acupuncture and 2 hours of PAES at acupoints Bladder 18, 23, 25, and 28 on both sides of the vertebral column as well as sham needle placement (control treatment). Each treatment was administered in a random order. At least 7 days elapsed between treatments. Nociceptive cutaneous pain threshold was measured by use of skin twitch reflex latency (STRL) and avoidance to radiant heat (< or = 50 degrees C) in the lumbar area. Skin temperature, cardiovascular and respiratory variables, and beta-endorphin concentration in spinal fluid (CSF-EN) and plasma (plasma-EN) were measured. RESULTS: Acupuncture and PAES significantly increased STRL and skin temperature. The CSF-EN was significantly increased from baseline values 30 to 120 minutes after onset of PAES, but it did not change after acupuncture and control treatments. Heart and respiratory rates, rectal temperature, arterial blood pressure, Hct, total solids and bicarbonate concentrations, base excess, plasma-EN, and results of blood gas analyses were not significantly different from baseline values after acupuncture, PAES, and control treatments. CONCLUSIONS AND CLINICAL RELEVANCE: Administration of PAES was more effective than acupuncture for activating the spinal cord to release beta-endorphins into the CSF of horses. Acupuncture and PAES provided cutaneous analgesia in horses without adverse cardiovascular and respiratory effects.     

Long term intensive exercise training leads to a higher plasma malate/lactate dehydrogenase (M/L) ratio and increased level of lipid mobilization in horses

Continuous high intensity training may induce alterations to enzyme activities related to glucose and lipid metabolism in horses. In our study, five Thoroughbred race horses (3 male and 2 female, avg age=5 yrs old) were compared against five riding horses (1 male, 1 female, 3 gelding, avg age=13 yrs old) in terms of energy metabolism, by examining plasma malate (MDH) and lactate (LDH) dehydrogenase activities and M/L ratio. MDH is involved in NADH and ATP generation, whereas LDH can convert NADH back into NAD(+) for ATP generation. An increase in plasma M/L ratio can reflect heightened energy metabolism in the liver and skeletal muscle of horses adapted to continuous intensive exercise. Moreover, plasma lipid metabolism analytes (adiponectin, NEFA, total cholesterol (T-Cho), and triglycerides (TG)) can reflect changes to lipolysis rate, which can also indicate a change in energy metabolism. Overall, race horses demonstrated increased MDH and LDH activity in plasma (4x and 2x greater, respectively), in addition to a plasma M/L ratio twice as high as that of riding horses (2.0 vs 1.0). In addition, race horses also demonstrated significantly higher levels of plasma NEFA (50% greater), TG (2x greater), and T-Cho (20% greater) as compared to riding horses. Therefore, race horse muscles may have adapted to prolonged high intensity endurance exercise by gaining a higher oxidative capacity and an increased capacity for fat utilization as an energy source, resulting in heightened energy metabolism and increased rate of lipid mobilization.     

Effects of long-term oral administration of levothyroxine sodium on serum thyroid hormone concentrations, clinicopathologic variables, and echocardiographic measurements in healthy adult horses

OBJECTIVE: To determine the effects of long-term oral levothyroxine sodium (L-T(4)) administration on serum thyroid hormone concentrations, thyroid gland function, clinicopathologic variables, and echocardiographic examination measurements in adult euthyroid horses. ANIMALS: 6 healthy adult mares. PROCEDURES: Horses received L-T(4) (48 mg/d) orally for 48 weeks. Every 4 weeks, physical examinations were performed; blood samples were collected for CBC, plasma biochemical analyses, and assessments of serum total triiodothyronine (tT(3)) and thyroxine (tT(4)) concentrations. Plasma creatine kinase MB activity and cardiac troponin I concentration were also measured. Echocardiographic examinations were performed before and at 16, 32, and 48 weeks during the treatment period. RESULTS: During the treatment period, mean body weight decreased significantly; heart rate varied significantly, but the pattern of variation was not consistent. Significant time effects were detected for certain clinicopathologic variables, but mean values remained within reference ranges. Cardiac troponin I was only detectable in 8 of 24 plasma samples (concentration range, 0.01 to 0.03 ng/mL). Serum creatine kinase MB activity did not change significantly over time. Compared with the pretreatment value, 5.4-, 4.0-, and 3.7-fold increases in mean serum tT(4) concentrations were detected at 16, 32, and 48 weeks, respectively. Some cardiac measurements changed significantly over time, but mean values remained within published reference ranges. Mean fractional shortening was lower than the pretreatment mean value at 16 and 32 weeks. CONCLUSIONS AND CLINICAL RELEVANCE: In horses, long-term oral administration of 48 mg of L-T(4)/d significantly increased serum tT(4) concentrations and did not appear to adversely affect health.     

Influence of a high ambient temperature on lipid metabolism in the growing pig

Because pigs are fatter when they are heat-stressed, it was hypothesized that lipid metabolism is enhanced in heat-stressed pigs. To test this hypothesis, an experiment was conducted to determine the influence of a high ambient temperature on the level of plasma lipids, thyroid hormones, lipoprotein lipase activity, and on the composition of very low density lipoproteins (VLDL) and chylomicrons in the growing pig. Twelve Large White x Landrace castrated male pigs with an initial weight of 20 +/- 0.6 kg were allotted to one of the following treatments: 1) ambient temperature of 31 degrees C, with ad libitum access to feed or 2) ambient temperature of 20 degrees C and fed the amount consumed by those kept at 31 degrees C until 35 kg BW. Ambient temperature did not affect piglet performance. Compared to that in pigs kept at 20 degrees C, in pigs kept at 31 degrees C the lipid content of backfat was 26% higher and the proportion of flare fat was increased by more than twofold (P < 0.001). Lipoprotein lipase activity was increased more than twofold in backfat and nearly twofold in leaf fat at 31 vs 20 degrees C (P < 0.001). In warmth-exposed (31 degrees C), feed-restricted pigs, the plasma level of triiodothyronine was 30% lower than at 20 degrees C (P < 0.001), whereas VLDL-lipid concentration was more than fourfold higher, and plasma concentrations of NEFA and triglycerides were 2.6- and 3.6-fold higher, respectively (P < 0.001). In conclusion, the chronic exposure of growing pigs to a high ambient temperature enhances lipid metabolism in both the liver (VLDL production) and the adipose tissue (lipoprotein lipase activity). Consequently, plasma triglyceride uptake and storage are facilitated in the adipose tissue, which results in greater fatness.