Effects of dexamethasone,glucose infusion,adrenocorticotropin, and propylthiouracil on plasma leptin concentrations in horses

In experiment 1, nine light horse geldings (three 3 x 3 Latin squares) received dexamethasone (DEX; 125 microg/kg BW, i.m.), glucose (0.2 g/kg BW, i.v.), or nothing (control) once per day for 4 days. DEX increased (P < 0.001) glucose, insulin, and leptin concentrations and resulted in a delayed increase (P < 0.001) in IGF-I concentrations. In experiment 2, mares were similarly treated with DEX (n = 6) or vehicle (n = 6). DEX again increased (P < 0.01) glucose, insulin, and leptin concentrations; the delayed elevation in IGF-I concentrations occurred on day 10, 12, and 19, relative to the first day of treatment. In experiment 3, six light horse geldings received either 200 IU of adrenocorticotropin (ACTH) i.m. or vehicle twice daily for 4 days. ACTH increased (P < 0.001) cortisol concentrations. Further, ACTH resulted in increases (P < 0.01) glucose, insulin, and leptin concentrations. In experiment 4, plasma samples from four light horse stallions that were fed 6-n-propyl-2-thiouracil (PTU) at 6 mg/kg BW for 60 days to induce hypothyroidism were compared to samples from control stallions. On day 52, stallions receiving PTU had lower concentrations of thyroxine (P < 0.05) and triiodothyronine (P < 0.01) and higher (P < 0.01) concentrations of TSH. Leptin concentrations were higher (P < 0.01) in PTU-fed stallions from day 10 through 52. In conclusion, circulating concentrations of leptin in horses was increased by administering DEX. Treatment with ACTH increased cortisol and resulted in lesser increases in leptin, glucose, and insulin. In addition, PTU feeding results in lesser increases in leptin concentrations.     

Effect of GnRH and hCG administration on plasma LH and testosterone concentrations in normal stallions, aged stallions and stallions with lack of libido

Gonadotrophin-releasing hormone (GnRH) (a single intravenous injection with 0.042 mg busereline acetate) was administered to control stallions (n=5), aged stallions (n=5) and stallions with lack of libido (n=5). Jugular blood samples were taken at -10, 0, 10, 20, 40 and 80 minutes after treatment and measured for luteinizing hormone (LH) and testosterone concentrations. A single intravenous injection of hCG (3000 IE) was given 1 day later. Venous blood samples were taken at -60, 0, 15, 30, 60, 120, and 240 minutes after treatment and measured for the testosterone concentration. The experiment was performed in the breeding season. There was a wide variation between stallions in basal concentrations of LH and testosterone. The treatment groups all showed a significant increase in LH and testosterone concentrations after treatment with GnRH. There was a significant difference (P<0.05) between the control, the lack of libido stallions and the aged stallions in the production of LH before and after stimulation with GnRH. The aged stallions had higher basal LH concentrations. GnRH induced a rise in plasma LH in all groups, but the greatest response was observed in aged stallions. No response to GnRH was seen with respect to plasma testosterone. There was an increase in plasma testosterone following hCG; however, this increase was very small in aged stallions. After stimulation with hCG the control and lack of libido stallions had a significant increase (P<0.05) in testosterone production. In conclusion, stimulation with either GnRH or hCG can be a valuable method to test whether the function of the stallion's reproductive endocrine system is optimal.     

Increased serum leptin and insulin concentrations in canine hypothyroidism

Serum concentrations of leptin and insulin were compared between gender-matched hypothyroid (n=25) and healthy (n=25) client-owned dogs within comparable age and body condition score (BCS) ranges. Fasted blood samples were collected from each dog and analysed for glucose, cholesterol, triglyceride, leptin and insulin concentrations. Leptin and insulin concentrations were significantly higher in the hypothyroid compared to normal dogs (P=0.006 and P=0.001, respectively) following adjustment for potential confounders. A nearly significant (P=0.051) interaction with BCS was found in the association between hypothyroidism and leptin. Leptin concentrations were significantly higher in hypothyroid dogs compared to normal dogs, in separate analyses for BCS 6 (P=0.036) and 7 (P=0.049). There was no significant difference in glucose concentration between the hypothyroid and normal groups (P=0.84) following adjustment for BCS. This study showed that canine hypothyroidism is associated with increased serum leptin and insulin concentrations, neither of which may be attributed to obesity alone.     

Effect of intravenous infusion of recombinant ovine leptin on feed intake and serum concentrations of GH, LH, insulin, IGF-1, cortisol, and thyroxine in growing prepubertal ewe lambs

In sheep, serum concentrations of leptin change congruently with increases or decreases in nutritional status, while intracerebroventricular infusions of leptin dramatically suppress feed intake in well-fed lambs, and may also increase growth hormone (GH), and/or luteinizing hormone (LH) in undernourished lambs. The objective of the present study was to determine the effects of peripherally delivered ovine leptin, via intravenous infusions, on feed intake and serum concentrations of GH, LH, insulin, IGF-1, cortisol, and thyroxine. Twelve ewe lambs weighing 29.4 +/- 0.7 kg were infused intravenously with a linearly increasing dose of leptin or saline (n = 6 per group) for 10 days, reaching a maximum dose delivered of 0.5mg/h on day 10. Feed intake was assessed twice daily, and blood samples were collected every 10 min for 6 h on days 0, 2, 5, 8, and 10. Serum concentrations of leptin increased in leptin-treated lambs by day 2 (P = 0.05), and continued to increase to concentrations 9-fold greater than saline-infused lambs by day 10 (P < 0.001). Despite the substantial increase in serum leptin, feed intake did not differ between leptin and saline-infused lambs except on day 3.5 (P = 0.01). Furthermore, intravenous infusions of leptin did not significantly influence serum concentrations of insulin, cortisol, IGF-1, thyroxine, LH, or GH. Collectively, these observations contrast with the potent hypophagic effects of leptin when delivered intracerebroventricularly into well-fed lambs. The reasons for the disparate response of lambs treated intravenously with leptin, versus that reported for lambs treated intracerebroventricularly with leptin are not known, but may provide insight into the mechanism(s) of leptin resistance.     

Effect of short-term feed restriction and refeeding on serum concentrations of leptin, luteinizing hormone and insulin in ovariectomized gilts

Ovariectomized gilts were either placed on full feed (FF) or restricted to one-third of the full feed amount (RST) for 7 days. Blood samples were taken through jugular catheters every 15 min for 4 h at the end of the 7-day period. Then dietary treatments were reversed and 7 days later samples were taken as before. Serum concentrations of leptin, insulin and luteinizing hormone (LH) were determined by radioimmunoassay. LH pulse frequency and mean serum leptin and insulin concentrations were lower (P < 0.01) in RST than FF gilts. Reversal of treatment reversed the patterns of hormone secretion. These results confirm previous observations that feed restriction can inhibit pulsatile LH secretion and also decrease leptin and insulin secretion.     

Effect of ovariectomy and ad libitum feeding on body composition, thyroid status, ghrelin and leptin plasma concentrations in female dogs

The objective of this study was to evaluate the effects of ovariectomy (i) and ad libitum feeding (ii) on energy intake, body weight (BW), body composition, thyroid status, leptin and ghrelin plasma concentrations. Four young adult female Beagle dogs were fed a maintenance diet for 6 weeks prior to ovariectomy, then 6 months after. Food allowance was adjusted in order to maintain optimal BW. Then, a diet slightly higher in energy concentration was fed ad libitum for 4 months. The maintenance diet was then fed ad libitum for one additional month. The maintenance of optimal BW after ovariectomy required a significant decrease in energy allowance. No increase in fat mass was observed. Ghrelin concentration remained unchanged. During the first month of ad libitum feeding, plasma ghrelin concentration and energy intake increased, then they decreased. Mean BW, plasma leptin, thyrotropin (TSH), total triiodothyronine (TT3) and total thyroxine (TT4) concentrations significantly increased over the study. The BW increase was exclusively due to an increase in body fat. In conclusion, energy allowance should be strictly controlled in spayed female dogs. The results suggest that in dogs, thyroid hormones, leptin and ghrelin concentrations change in response to a positive energy balance in an attempt to limit weight gain. However, the significant weight gain shows that this goal was not achieved.     

Effect of feeding allowance level on embryonic survival, IGF-1, insulin, GH, leptin and progesterone secretion in early pregnancy gilts

The present experiment was conducted to evaluate the effects of feeding allowance level on embryonic survival, uterine development and reproductive hormone secretion in early gestation gilts. A total of 54 F1 crosses of Landrace x Large white gilts were randomly allocated to three treatment groups of high (H, 2 x maintenance), medium (M, 1.2 x maintenance) and low (L, 0.6 x maintenance) feeding level after mating. Blood samples and uterine flushings were collected on day 12, 25 and 35 of pregnancy, and embryonic survival rate was estimated. Concentrations of insulin-like growth factor I (IGF)-1, insulin, growth harmone (GH), leptin and progesterone in serum and uterine flushings were determined by enzyme-linked immunosorbent assay. Embryonic survival was affected by dietary treatment; total number of viable embryos and embryo survival of group M were higher than other groups in the early pregnancy (p < 0.05). Greater foetal weight in M gilts was achieved when gestation advanced to day 35 (p < 0.05), though there was no difference on day 25 of pregnancy among treatments. No appreciable differences were observed in the crown-rump length on day 25 and 35 of pregnancy among groups. Greater weight of uterus and products of conception were identified in M gilts compared with group H and L (p = 0.024 and p = 0.005, respectively) on day 25 of pregnancy. The hormone level was greatly affected by feeding allowance level. In serum, concentrations of IGF-1, leptin and insulin tended to be greater in H than in M and L during the early gestation, while concentrations of GH were greater in M and progesterone were the lowest in H. At the same time, feed allowance level affected the concentration of IGF-1, insulin, GH, leptin and progesterone in uterine flushings. These data demonstrated that feed allowance level after mating has important consequence on embryo survival, embryo development and uterine development, possibly mediated by nutrition level inducing changes in concentrations of reproductive hormones and/or intermediary metabolites in early pregnancy.     

Effects of administration of glucocorticoids and feeding status on plasma leptin concentrations in dogs

OBJECTIVE: To investigate effects of short- and long- term administration of glucocorticoids, feeding status, and serum concentrations of insulin and cortisol on plasma leptin concentrations in dogs. ANIMALS: 20 nonobese dogs. PROCEDURE: For experiment 1, plasma leptin concentrations and serum concentrations of insulin and cortisol were monitored for 24 hours in 4 dogs administered dexamethasone (0.1 mg/kg, IV) or saline (0.9% NaCl) solution for fed and nonfed conditions. For experiment 2, 11 dogs were administered prednisolone (1 mg/kg, PO, q 24 h for 56 days [7 dogs] and 2 mg/kg, PO, q 24 h for 28 days [4 dogs]) and 5 dogs served as control dogs. Plasma leptin and serum insulin concentrations were monitored weekly. RESULTS: For experiment 1, dexamethasone injection with the fed condition drastically increased plasma leptin concentrations. Furthermore, injection of saline solution with the fed condition increased plasma leptin concentrations. These increases in plasma leptin concentrations correlated with increases in serum insulin concentrations. Dexamethasone injection with the nonfed condition increased plasma leptin concentrations slightly but continuously. Injection of saline solution with the nonfed condition did not alter plasma leptin concentrations. For experiment 2, prednisolone administration at either dosage and duration did not alter plasma leptin concentrations in any dogs. CONCLUSIONS AND CLINICAL RELEVANCE: Dexamethasone injection and feeding increased plasma leptin concentrations in dogs. In addition, dexamethasone administration enhanced the effect of feeding on increases in plasma leptin concentrations. Daily oral administration of prednisolone (1 or 2 mg/kg) did not affect plasma leptin concentrations in dogs.     

Effect of feeding glucose, fructose, and inulin on blood glucose and insulin concentrations in normal ponies and those predisposed to laminitis

Identification of ponies (Equus caballus) at increased risk of pasture-associated laminitis would aid in the prevention of the disease. Insulin resistance has been associated with laminitis and could be used to identify susceptible individuals. Insulin resistance may be diagnosed by feeding supplementary water-soluble carbohydrate (WSC) and measuring blood glucose and insulin concentrations. The aim of this study was to assess the glycemic and insulinemic responses of 7 normal (NP) and 5 previously laminitic (PLP), mixed breed, native UK ponies fed glucose, fructose, and inulin [1 g/(kg(.)d) for 3 d] or no supplementary WSC (control) in spring and fall after a 7-d adaptation to a pasture or hay diet. Blood samples were taken for 12 h after feeding on each day, and baseline and peak concentrations and area under the curve (AUC) for glucose and insulin were recorded. Linear mixed models were used for statistical analysis. Differences between PLP and NP groups were most marked after glucose feeding with differences in peak glucose (P = 0.02) and peak insulin (P = 0.016) concentrations. Season and diet adaptation also affected results. Peak concentrations of glucose and insulin occurred 2 to 4 h after WSC feeding. Peak insulin concentration was greater and more variable in fall, particularly in PLP adapted to fall pasture. Baseline glucose and insulin concentrations varied between individuals and with season and diet adaptation but were not greater in PLP than NP. Insulin AUC was greater in PLP than NP after feeding both glucose and fructose (P = 0.017), but there were no differences between PLP and NP in glucose AUC. Glycemic and insulinemic changes were less (P ≤ 0.05) after feeding fructose than glucose, although differences between PLP and NP were still evident. Minimal changes in glucose and insulin concentrations occurred after inulin feeding. Measurement of peak insulin 2 h after feeding of a single dose of glucose (1 g/kg) may be a simple and practical way to aid identification of laminitis-prone ponies before the onset of clinical disease, particularly when ponies are adapted to eating fall pasture.     

Effects of fasting and refeeding on plasma concentrations of leptin, ghrelin, insulin, growth hormone and metabolites in swine

The effects of nutritional status, such as fasting and refeeding, on leptin and ghrelin secretion in swine were examined. The swine (n = 4) were fasted for 54 h and plasma hormone levels were measured before, during and after fasting. Plasma leptin and insulin concentrations began to decrease 12 and 6 h into the fasting period, respectively (P < 0.05), and maintained a low level for the remaining period of fasting. Plasma leptin and insulin returned to the pre‐fasting value 6 and 12 h after refeeding, respectively. Plasma ghrelin concentrations showed a nocturnal periodicity during the fasting period; it increased nocturnally at 36 and 42 h into the fasting period (P < 0.05). Plasma growth hormone levels did not show any remarkable changes during the fasting. Plasma glucose levels showed a modest fall during fasting and significantly decreased (P < 0.05) at 24 h into the fasting period, returning to pre‐fasting levels after refeeding. Plasma nonesterified fatty acid levels increased (P < 0.05) at 12 h into the fasting period and returned to the pre‐fasting level 6 h after refeeding. These results indicate that plasma leptin, insulin and ghrelin play an important role in maintaining energy homeostasis in swine. The plasma ghrelin did not continuously increase, but showed nocturnal periodicity during fasting. This may suggest that ghrelin is also involved in physiological processes other than energy homeostasis.     

Exercise-induced alterations in plasma concentrations of ghrelin, adiponectin, leptin, glucose, insulin, and cortisol in horses

Six Standardbred (STB) mares (11+/-2 years, 521+/-77 kg; means+/-SD) performed an exercise trial (EX) where they underwent an incremental exercise test (GXT) as well as a parallel control trial (CON) to test the hypothesis that short-term, high intensity exercise would alter plasma concentrations of glucose, leptin, adiponectin, ghrelin, insulin and cortisol. Plasma samples were taken before (0 min), during (last 10s at 6, 8m/s, and the velocity eliciting VO(2max)), and after exercise (2, 10, 30, 60 min; 12 and 24h post-GXT). A second set of blood samples was collected before and after an afternoon meal given at 1515 h (at 1500, 1514, 1530, and 1545 h). Data were analyzed using ANOVA for repeated measures and Tukey's test. During the GXT, there were no changes (P>0.05) in the plasma concentrations of glucose, leptin, adiponectin or ghrelin. However, there was a 29% increase (P<0.05) in mean plasma cortisol concentration and a 35% decrease (P<0.05) in mean plasma insulin concentration. Substantial increases (P<0.05) in the mean plasma concentrations of glucose and cortisol of 36% and 102%, respectively, were seen in the EX trial during the first 60 min post-GXT. Plasma leptin concentration, measured at the 24h post-GXT time point, was 20% lower (P<0.05) during the EX trial compared with the parallel time point in the standing control (CON) trial. Plasma ghrelin concentration was 37% lower (P<0.05) in the EX trial compared with CON before and after the afternoon meal, but was 43% higher (P<0.05) 12h post-GXT. There were no differences between EX and CON for plasma concentrations of insulin or adiponectin during recovery. It was concluded that short-term high intensity exercise alters plasma leptin and ghrelin concentrations in STB mares post-exercise, which may signal the exercised animals to alter energy intake.     

Effects of high and moderate non-structural carbohydrate hay on insulin, glucose, triglyceride, and leptin concentrations in overweight Arabian geldings

The objective of this study was to determine the effects of high and moderate non-structural carbohydrates (NSC) hay on insulin, glucose, triglyceride, and leptin concentrations in overweight Arabian geldings. Eight adult overweight (average BCS 7 [9-point scale]) Arabian geldings were fed each of two orchardgrass hays, high NSC (18% DM) and moderate NSC (12% DM), in a cross over design during two 28-day periods. Body weight and body condition score assessment along with blood sampling to measure insulin, glucose, leptin, and triglyceride concentrations were performed on days 0, 7, 14, 21 and 28 of each period. Effects of hay, period, day, and day*hay on plasma glucose and serum leptin were not detected. Serum insulin was influenced by hay (p = 0.001), day (p = 0.03), and day*hay (p = 0.04). Insulin concentrations were higher on day 7 in the high NSC group (15.6 μIU/ml) than the moderate NSC group (9.5 μIU/ml), but not by day 14 (p = 0.0007). Plasma triglyceride was influenced by period (p = 0.0003), day*period (p < 0.0001), and day*hay (p = 0.02). Hyperinsulinaemia was not observed in the overweight Arabian geldings fed either a moderate or high NSC hay.     

Effect of feeding on plasma secretin concentrations in sheep

Changes in plasma secretin concentration during the 24 hours after feeding were observed in five female sheep fed 1500 g of lucerne pellets and 300 g of orchard grass hay once daily. The pre-feeding concentration of plasma secretin was 50 +/- 9 pg ml-1. The plasma secretin concentration began to increase immediately after the beginning of feeding, reached values approximately double the pre-feeding values six hours after feeding, and remained significantly above the pre-feeding value for more than 10 hours after feeding (P less than 0.01 or less than 0.05).     

Effect of administration of adrenocorticotropic hormone on plasma concentrations of testosterone, luteinizing hormone, follicle stimulating hormone and cortisol in stallions

In boars and rabbits, administration of adrenocorticotropic hormone (ACTH) results in a testis-dependent, short-term increase in concentrations of testosterone in peripheral plasma. This experiment was designed to assess the short-term effects of a single ACTH injection on plasma concentrations of testosterone, luteinizing hormone (LH), follicle stimulating hormone (FSH) and cortisol in stallions. Eight light horse and two pony stallions were paired by age and weight and then one of each pair was randomly assigned to the treatment (ACTH, .2 IU/kg of body weight) or control (vehicle) group. Injection of ACTH increased (P<.01) plasma concentrations of cortisol by approximately twofold in the first 60 minutes; control stallions showed no change (P>.10) in concentrations of cortisol during the blood sampling period. Control stallions exhibited a midday increase (P>.05) in concentrations of testosterone similar to that reported previously; ACTH treatment prevented or delayed this increase such that concentrations of testosterone in treated stallions were lower (P<.05) than in controls 4 to 5 hours after injection of ACTH. Treatment with ACTH had no effect (P<.10) on plasma concentrations of LH or FSH up to 12 hours after injection.     

Fasting-induced changes in ovulation rate, plasma leptin, gonadotropins, GH, IGF-I and insulin concentrations during oestrus in ewes

The aim of this experiment was to study the changes in the hormonal status and ovulation rate (OR) evoked by starvation during the follicular phase of the oestrous cycle in ewes. To achieve this goal, 12 female crossbreed sheep were synchronized and then half of them were fasted from the 12th to the 16th day of the oestrous cycle. On the 16th day, analysis of hormones and insulin-like growth factor-I (IGF-I) were performed in 10-min intervals. Then, on the 6th day of the following oestrous cycle, the OR in all ewes was determined by laparoscopy. Fasting reduced significantly (P < 0.05) the OR in ewes (1.25 +/- 0.50) in comparison with control (1.75 +/- 0.50). The drop in the OR was coincident with a significant (P < 0.001) decrease in the plasma concentration and pulse amplitude of leptin (0.29 +/- 0.08 ng/ml versus control 0.53 +/- 0.14 ng/ml), the plasma level of luteinizing hormone (LH) (0.19 +/- 0.06 IU/l versus 0.25 +/- 0.09 IU/l in control; P < 0.05) and the mean frequency of LH pulses (2.0/h versus 2.5/h in control). Fasting resulted also in a significant (P < 0.05) decrease in the plasma concentration and pulse amplitude of follicle stimulating hormone (FSH) in comparison with the control. Simultaneously, a significant (P < 0.001) drop in the IGF-I concentration in the fasted ewes (4.78 +/- 0.91 ng/ml) was found in comparison with control (7.63 +/- 1.85 ng/ml). Also the level of insulin were significantly (P < 0.001) lower in the fasted (178.99 +/- 39.08 pM/l respectively) than in the control sheep (302.66 +/- 49.01 pM/l respectively). Meanwhile, a double increase in the growth hormone (GH) pulses frequency and an augmentation in its plasma concentrations as a result of starvation was found. The obtained results shows that the acute fasting exerts an inhibitory effect on the ovulation rate in ewes coincident with suppression in leptin, FSH and LH secretion and changes in signalization mediated by GH.     

Plasma leptin concentrations are independently associated with insulin sensitivity in lean and overweight cats

This study investigated relationships between plasma leptin, insulin concentrations, insulin sensitivity and glucose tolerance in lean and overweight cats. Leptin concentrations were measured in 16 cats during glucose tolerance tests before and after gaining weight, and after feeding a test meal in overweight cats. An important finding of this study is that in both lean (r=-0.79) and overweight (r=-0.89) cats, the higher the leptin concentrations, the more insulin resistant the cat, independent of the degree of adiposity. Leptin concentrations at baseline and after consuming a meal tended to be higher in overweight cats with glucose intolerance, compared to overweight cats with normal glucose tolerance, although the difference was not significant. After feeding the test meal to overweight cats in the early morning, plasma leptin concentrations initially decreased before subsequently rising to peak 15 h later, which coincided with late evening. The leptin peak occurred 9 h after the insulin peak following ingestion of the test meal. Importantly, this study suggests that increased leptin concentrations may contribute to the diminished insulin sensitivity seen in overweight cats. Alternatively, the compensatory hyperinsulinaemia found with insulin resistance in overweight cats could stimulate leptin production.     

Effect of fat supplementation on leptin, insulin-like growth factor I, growth hormone, and insulin in cattle

We investigated the effect of fat supplementation on plasma levels of hormones related to metabolism, with special attention to leptin, in cows in early lactation and in feedlot steers. In experiment 1, 34 lactating cows received no fat or else 0.5 or 1.0 kg of partially hydrogenated oil per day in addition to their basal diet from day 20 before the expected calving date to day 70 postpartum. In experiment 2, part of the corn in the basal concentrate was replaced with 0.7 kg of the same oil such that the diets were isocaloric; 18 cows received the fat-substituted diet and 18 a control diet from day 20 before the expected calving date to day 75 postpartum. In experiment 3, calcium salts of fatty acids were added to the basal diet of 14 feedlot steers for 80 d; another 14 steers received a control diet. The basal plasma levels of leptin were higher in the cows than in the steers. Dietary fat supplementation did not affect the leptin levels in the lactating cows but lowered the levels in the feedlot steers despite greater energy intake and body fatness (body weight) in the steers receiving the supplement than in those receiving the control diet. The levels of insulin-like growth factor I and insulin were decreased with dietary fat supplementation in the lactating cows but were unaffected in the steers, suggesting that responses to fat ingestion depend on the physiological state of the animal, including age and sex. Finally, no effects of supplementary fat on the level of growth hormone were demonstrated in any of the models.     

Effects of age, season and active immunization against estrogen on serum prolactin concentrations in stallions

Prolactin concentrations in stallion serum were measured by radioimmunoassay based on antiserum generated against equine prolactin and radioiodinated canine prolactin. Prolactin concentrations in serum collected from 152 stallions at a slaughterhouse were higher (P less than .01) in summer than in winter (5.7 +/- .15 vs 2.0 +/- .17 ng/ml). Moreover, there was an effect of age (P less than .02) in the analysis of variance; there was no interaction between age and season. In general, prolactin concentrations increased with age up to 3 to 5 years. Samples of serum collected from five control and five estrogen-immunized stallions from 6 to 26 months of age also were assessed for prolactin concentration. Active immunization against estrogen had no effect (P greater than .10) on prolactin concentrations. In contrast, whether analyzed by age or by month of the year, prolactin concentrations varied with time (P less than .01) in these stallions. Averaged over both groups, concentrations were high during summer and low during winter and were also gradually increasing with time as indicated by a linear trend (P less than .01) in the data. It is concluded that serum prolactin concentrations in stallions 1) are seasonal, as in many other seasonally breeding species, 2) in general are highest in older stallions and lowest in young stallions and 3) are not affected by active immunization against estrogens.     

Plasma concentrations of leptin, insulin-like growth factor-I, and insulin in relation to changes in body condition score in heifers

The objective of this study was to determine the relationships among plasma concentrations of leptin, insulin, and IGF-I with dynamic changes in body condition scores (BCS) in heifers. Nineteen Zebu-Brown Swiss crossbred heifers, 24 to 30 mo old, weighing 322 +/- 9 kg, and with an initial BCS of 2.6 +/- 0.11 (range = 1 to 9) were used. Heifers were fed 60% of their maintenance requirements until they reached a BCS of < or = 2. Heifers were then maintained at that level for 25 d, after which they were fed to gain 1 kg of body weight daily until a BCS of 6 was reached. Heifers were weighed weekly and BCS was measured every 2 wk. Plasma samples were collected twice weekly, and leptin and insulin were determined by RIA. An immunoradiometric assay was used to measure IGF-I from one sample every 2 wk. Plasma concentrations of leptin were positively correlated during nutritional restriction (NR) and weight gain (WG) periods with BCS (r = 0.47 for NR, and r = 0.83 for WG; P < 0.01) and body weight (r = 0.40 for NR, and r = 0.78 for WG; P < 0.01). Plasma concentrations of leptin decreased during nutritional restriction (P < 0.01) as BCS decreased. During weight gain, leptin concentration increased at BCS 3 and thereafter for each integer change in the BCS. Regression analysis showed that changes in body weight affect leptin concentrations within a given BCS. There was a decrease in IGF-I as BCS declined (P < 0.01). During weight gain, by contrast, IGF-I increased significantly (P < 0.01) with every unit change in body condition up to BCS of 4 and plateaued thereafter. Insulin concentrations did not change during nutritional restriction when BCS decreased from 3 to 1. However, once the diet was improved, there was a large increase in insulin concentrations in heifers with BCS 1 (P < 0.01). Among heifers of BCS 2 and 3, insulin did not differ and was lower than in heifers of BCS 1 (P < 0.01). Insulin increased (P < 0.01) among heifers at BCS 4 to 6. Leptin was positively correlated (P < 0.01) with both IGF-I (r = 0.34 for NR, and r = 0.36 for WG) and insulin (r = 0.18 for WG). Insulin was correlated with IGF-I (r = 0.60; P < 0.01). During nutritional restriction, insulin did not correlate with leptin (r = -0.05), BCS (r = -0.03), or IGF-I (r = 0.07). It was concluded that leptin serves as a dynamic indicator of body condition in heifers, as well as an indicator of nutritional status.