Thyrotropin-releasing hormone-induced growth hormone secretion in dogs with primary hypothyroidism

Primary hypothyroidism in dogs is associated with increased release of growth hormone (GH). In search for an explanation we investigated the effect of intravenous administration of thyrotropin-releasing hormone (TRH, 10 microg/kg body weight) on GH release in 10 dogs with primary hypothyroidism and 6 healthy control dogs. The hypothyroid dogs had a medical history and physical changes compatible with hypothyroidism and were included in the study on the basis of the following criteria: plasma thyroxine concentration < 2 nmol/l and plasma thyrotropin (TSH) concentration > 1 microg/l. In addition, (99m)TcO(4)(-) uptake during thyroid scintigraphy was low or absent. TRH administration caused plasma TSH concentrations to rise significantly in the control dogs, but not in the hypothyroid dogs. In the dogs with primary hypothyroidism, the mean basal plasma GH concentration was relatively high (2.3+/-0.5 microg/l) and increased significantly (P=0.001) 10 and 20 min after injection of TRH (to 11.9+/-3.5 and 9.8+/-2.7 microg/l, respectively). In the control dogs, the mean basal plasma GH concentration was 1.3+/-0.1 microg/l and did not increase significantly after TRH administration. We conclude that, in contrast to healthy control dogs, primary hypothyroid dogs respond to TRH administration with a significant increase in the plasma GH concentration, possibly as a result of transdifferentiation of somatotropic pituitary cells to thyrosomatotropes.     

The effect of floor type or relocation on calves’ pulsatile growth hormone and cortisol secretion

Abstract

To examine if hard floor or relocation to a new room affects the pulsatile secretion of growth hormone (GH) and cortisol in calves, we housed individually 12 calves on concrete floors and 12 on rubber mats for 67–74 days. Half of the calves in each pen were relocated to an identical but strange room. Serial blood samples were taken 24 h prior to and 24 h following relocation at 20-min intervals and were assayed for GH and cortisol. Both of the hormones had a clear ultradian pulsatile variation. We detected (mean±/se) 4.6±/0.6 GH pulses/24 h. GH concentrations peaked four times during the day and pulses were higher in daytime. Cortisol peaked 5.8±0.6 times/24 h. Cortisol concentrations were highest at feeding. No overall effects of flooring or relocation were found but calves on concrete floors had greater cortisol concentrations than on rubber mats, especially during the night.     

Secretion pattern of thyroid-stimulating hormone in dogs during euthyroidism and hypothyroidism

In as many as one third of dogs with primary hypothyroidism a plasma thyrotropin (TSH) concentration within the reference range for euthyroid dogs is found. To determine whether this is due to fluctuations in the release of TSH, the plasma profiles of TSH were analyzed in 7 beagle bitches by collecting blood samples every 10 min for 6 hr, both before and after induction of primary hypothyroidism. After induction of primary hypothyroidism, a 37-fold increase in mean basal plasma TSH concentration and a 34-fold increase in mean area under the curve for TSH were found. Analysis by the Pulsar program demonstrated pulsatile secretion of TSH in the hypothyroid state, characterized by relatively low amplitude pulses (mean [+/-SEM]) amplitude 41 +/- 3% of basal plasma TSH level) and a mean pulse frequency of 2.0 +/- 0.5 pulses/6 hr. In the euthyroid state, significant TSH pulses were identified in only 2 dogs. The mean basal plasma TSH level correlated positively (r = 0.84) with the mean amplitude of the TSH pulses, and correlated negatively (r = -0.88) with the TSH pulse frequency. The results of this study demonstrate pulsatile secretion of TSH in dogs during hypothyroidism and only small fluctuations in plasma TSH concentrations during euthyroidism. The findings also suggest that the low TSH values occasionally found in dogs with spontaneous primary hypothyroidism may in some cases in part be the result of ultradian fluctuations.     

Orexin-B modulates luteinizing hormone and growth hormone secretion from porcine pituitary cells in culture

To test the hypothesis that orexin-B acts directly on the anterior pituitary to regulate LH and growth hormone (GH) secretion, anterior pituitary cells from prepuberal gilts were studied in primary culture. On day 4 of culture, 10(5) cells/well were challenged with 0.1, 10 or 1000 nM GnRH; 10, 100 or 1000 nM [Ala15]-hGRF-(1-29)NH2 or 0.1, 1, 10 or 100 nM, orexin-B individually or in combinations with 0.1 and 1000 nM GnRH or 10 and 1000 nM GRF. Secreted LH and GH were measured at 4 h after treatment. Basal LH and GH secretion (control; n = 6 pigs) was 183 +/- 18 and 108 +/- 4.8 ng/well, respectively. Relative to control at 4 h, all doses of GnRH and GRF increased (P < 0.0001) LH and GH secretion, respectively. All doses of orexin-B increased (P < 0.01) LH secretion, except for the 0.1 nM dose. Basal GH secretion was unaffected by orexin-B. Addition of 1, 10 or 100 nM orexin-B in combinations with 0.1 nM GnRH increased (P < 0.001) LH secretion compared to GnRH alone. Only 0.1 nM (P = 0.06) and 100 nM (P < 0.001) orexin-B in combinations with 1000 nM GnRH increased LH secretion compared to GnRH alone. All doses of orexin-B in combination with 1000 nM GRF suppressed (P < 0.0001) GH secretion compare to GRF alone, while only 0.1 nM orexin-B in combination with 10 nM GRF suppressed (P < 0.01) GH secretion compared to GRF. These results indicate that orexin may directly modulate LH and GH secretion at the level of the pituitary gland.     

Vitamin D status before and after hypophysectomy in dogs with pituitary-dependent hypercortisolism

Both spontaneous hypercortisolism and chronic glucocorticoid treatment are associated with osteoporosis and low circulating concentrations of 1,25-dihydroxy-vitamin D in humans. Pituitary-dependent hypercortisolism (PDH) is a common disorder in dogs, but little is known about the vitamin D status of affected dogs. Pituitary-dependent hypercortisolism in dogs can be treated effectively by hypophysectomy and subsequent hormone supplementation. Because hormone supplementation does not include GH, dogs that have undergone hypophysectomy have low circulating concentrations of GH and IGF-1, which may result in low plasma 1,25-dihydroxy-vitamin D concentrations and consequently increased parathyroid hormone secretion. The aim of this study was to determine whether dogs with PDH need vitamin D supplementation before and/or after hypophysectomy. To this end, we measured plasma concentrations of GH, IGF-1, parathyroid hormone, calcium, phosphate, and vitamin D metabolites in 12 dogs with PDH before and 8 wk after hypophysectomy and in 12 control dogs. Although plasma GH concentrations were lower in dogs with PDH than in control dogs both before and after hypophysectomy, the vitamin D status was similar. In conclusion, there is no need for vitamin D supplementation in dogs with PDH, either before or after hypophysectomy.     

Pituitary adenylate cyclase-activating polypeptide induces secretion of growth hormone in cattle.

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a hypothalamic neuropeptide that stimulates release of growth hormone (GH) from cultured bovine anterior pituitary gland cells, but the role of PACAP on the regulation of in vivo secretion of GH in cattle is not known. To test the hypothesis that PACAP induces secretion of GH in cattle, meal-fed Holstein steers were injected with incremental doses of PACAP (0, 0.1, 0.3, 1, 3, and 10 microg/kg BW) before feeding and concentrations of GH in serum were quantified. Compared with saline, injection of 3 and 10 microg PACAP/kg BW increased peak concentrations of GH in serum from 11.2 ng/ml to 23.7 and 21.8 ng/ml, respectively (P < 0.01). Peak concentrations of GH in serum were similar in steers injected with 3 or 10 microg PACAP/kg BW. Meal-fed Holstein steers were then injected with 3 microg/PACAP/kg BW either 1 hr before feeding or 1 hr after feeding to determine if PACAP-induced secretion of GH was suppressed after feeding. Feeding suppressed basal concentrations of GH in serum. Injection of PACAP before feeding induced greater peak concentrations of GH in serum (19.2 +/- 2.6 vs. 11.7 +/- 2.6 ng/ml) and area under the response curve (391 +/- 47 vs. 255 +/- 52 ng. ml(-1) min) than injection of PACAP after feeding, suggesting somatotropes become refractory to PACAP after feeding similar to that observed by us and others with growth hormone-releasing hormone (GHRH). We concluded that PACAP induces secretion of GH and could play a role in regulating endogenous secretion of GH in cattle, perhaps in concert with GHRH.     

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.     

Chronic administration of recombinant ovine leptin in growing beef heifers: effects on secretion of LH, metabolic hormones, and timing of puberty

Serum concentrations of leptin increase linearly from approximately 16 wk before until the week of pubertal ovulation in beef heifers. To test the hypothesis that exogenous leptin can hasten the onset of puberty in heifers, we examined the effects of chronic administration of recombinant ovine leptin (oleptin) on timing of puberty, pulsatile and GnRH-mediated release of LH, and plasma concentrations of GH, IGF-I, and insulin. Fourteen fall-born, prepubertal heifers (Brahman x Hereford, 12 to 13 mo; 304.7+/-4.12 kg) were used. Heifers were stratified by age and BW and assigned randomly to one of two groups (seven animals per group): 1) Control; heifers received s.c. injections of saline twice daily (0700 and 1900) for 40 d; and 2) Leptin; heifers received s.c. injections of oleptin (19.2 microg/kg) twice daily at 0700 and 1900 for 40 d. Blood samples were collected at 10-min intervals for 5 h on. d 0, 5, 10, 20, 30, and 40, and twice daily, just before each treatment injection, throughout the study. On d 41, heifers received i.v. injections of GnRH at 0 (0.0011 microg/kg) and 90 min (0.22 microg/kg), with additional sampling for 5.5 h to examine releasable pools of LH. Diets promoted a gain of 0.32+/-0.09 kg/d, which did not differ between groups. Plasma concentrations of leptin increased markedly in leptin-treated heifers and were greater (P < 0.001) than controls throughout (27.8+/-0.8 vs. 4.9+/-0.12 ng/mL). None of the heifers reached puberty during the experiment, but did so within 45 d of its termination. Mean concentrations of plasma LH, GH, IGF-I, and insulin were not affected by treatment, nor was there an overall effect on the frequency of LH pulses. However, a treatment x day interaction (P = 0.02) revealed that the frequency of LH pulses (pulses/ 5 h) was greater (P = 0.03) in controls (3.6+/-0.36) than in leptin-treated heifers (1.7+/- 0.28) on d 10. Characteristics of GnRH-induced release of LH were not affected by treatment. In summary, chronically administered leptin failed to induce puberty or alter endocrine characteristics in beef heifers nearing the time of expected puberty.     

Ghrelin-stimulation test in the diagnosis of canine pituitary dwarfism

This study investigated whether ghrelin, a potent releaser of growth hormone (GH) secretion, is a valuable tool in the diagnosis of canine pituitary dwarfism. The effect of intravenous administration of ghrelin on the release of GH and other adenohypophyseal hormones was investigated in German shepherd dogs with congenital combined pituitary hormone deficiency and in healthy Beagles. Analysis of the maximal increment (i.e. difference between pre- and maximal post-ghrelin plasma hormone concentration) indicated that the GH response was significantly lower in the dwarf dogs compared with the healthy dogs. In none of the pituitary dwarfs, the ghrelin-induced plasma GH concentration exceeded 5 microg/l at any time. However, this was also true for 3 healthy dogs. In all dogs, ghrelin administration did not affect the plasma concentrations of ACTH, cortisol, TSH, LH and PRL . Thus, while a ghrelin-induced plasma GH concentration above 5 microg/l excludes GH deficiency, false-negative results may occur.     

Comparison of luteinizing hormone, leptin and progesterone levels in the systemic circulation (Vena jugularis) and near the ovarian circulation (Vena cava caudalis) during the oestrous cycle in Mangalica and Landrace gilts

The objective of this study was to evaluate luteinizing hormone (LH) and luteal progesterone (P4) secretion in systemic blood and blood near the ovaries in Mangalica (M) and Landrace (L) gilts by implanting catheters into the Vena jugularis and the Vena cava caudalis via the Vena saphena, respectively. Furthermore, leptin was analyzed in jugular vein blood. Blood was collected twice daily from day 7 to day 19 of the oestrous cycle and frequently (10-min intervals for 6 h) on day 9, day 12 and day 15 in M (n=3) and L gilts (n=4). L gilts had congruent pulsatile LH secretion in both veins, but the LH concentrations in M were always below the assay sensitivity during the luteal phase. In both breeds, episodic P4 secretion was found in the jugular and caval veins, and both sampling site and breed had an influence on P4 secretion (P<0.05). The mean concentration of P4 was higher (P<0.01) in utero-ovarian blood (75.8+/-5.3 in M; 49.6+/-4.2 ng/ml in L) than in the periphery (31.3+/-2.0 in M; 21.2+/-1.8 ng/ml in L). M pigs had a lower number of corpora lutea (9.7+/-2.3 vs. 20.5+/-4.4), and analysis of the P4 secretion ratio per corpus luteum revealed an influence of breed (P<0.01). This ratio was significantly higher in M (3.8+/-0.3 and 8.7+/-0.7 ng/ml) compared with the L gilts (1.4+/-0.1 and 2.8+/-0.3 ng/ml) in the jugular and caval veins, respectively. Blood sampling from the Vena cava caudalis is potentially more precise than from the Vena jugularis for evaluation of ovarian P4 secretion. Both the higher P4 concentration and increased leptin secretion (11.3+/-0.6 vs. 3.0+/-0.1 ng/ml, P<0.05) and consequently the altered LH secretion pattern in the Mangalica may contribute to the lower fecundity of this breed.     

Differential effects of the endocrine-disrupting compounds bisphenol-A and octylphenol on gonadotropin secretion, in prepubertal ewe lambs

This study examined the effects of long-term exposure to the endocrine-disrupting compounds (EDCs) Bisphenol-A (BPA) and Octylphenol (OP) on gonadotrophin secretion in pre-pubertal female sheep. Four-week-old, female lambs were randomly allocated to four groups (n=6), and twice each week treated with i.m. injections of either corn oil (vehicle controls), diethylstilbestrol (DES; 0.175mg/kg), BPA (3.5mg/kg) or OP (3.5mg/kg). After 5 weeks of treatment, animals were ovariectomized (ovx) and ovary weights recorded. Two weeks later, blood samples were collected from lambs every 15min for 6h, for LH pulse analysis. Animals were then euthanased and adrenal and kidney weight recorded. An age-related increase in tonic LH secretion was noted in Control, BPA- and OP-treated lambs, but was absent in DES-treated lambs. Following ovx, LH secretion increased in all except DES-treated lambs; FSH concentrations increased in all groups. BPA and DES significantly suppressed LH pulse frequency (C: 6.7+/-0.3pulses/6h, DES: 1.5+/-0.8pulses/6h, BPA: 2.3+/-0.8pulses/6h) and amplitude (C: 7.1+/-1.0ng/ml, DES: 1.9+/-0.6ng/ml, BPA: 1.6+/-0.4ng/ml). OP had no effect on LH secretion (Frequency: 5.8+/-0.5pulses/6h, amplitude: 8.0+/-2.0ng/ml). Ovary weight was similar among all groups. Results show that chronic in vivo exposure of prepubertal female lambs to BPA, at levels lower than those reported previously, can have significant effects on LH secretion that are comparable to those seen following exposure to the known xenoestrogen, DES. Exposure to an equal dose of the EDC OP, over the equivalent period of time was without effect on gonadotropin secretion in the prepubertal ewe lamb. These results indicate that exposure of prepubertal female lambs to the EDC BPA can induce significant effects on gonadotropin secretion, the potential long-term effects of exposure and the effects of these changes on reproductive performance and efficacy, therefore, merit further study.     

Use of prostaglandins and bromocriptine mesylate for pregnancy termination in bitches

OBJECTIVE: To assess the efficacy and safety of 2 protocols using bromocriptine mesylate and prostaglandins to terminate unwanted pregnancy in bitches. DESIGN: Prospective randomized single-blind controlled study. ANIMALS: 34 crossbred and purebred bitches referred for possible pregnancy termination. Seven additional pregnant bitches were used as controls. PROCEDURE: Pregnancy was assessed by ultrasonographic examination from day 25 after mating in all bitches. Of the 34 bitches, 25 were pregnant and were randomly allocated to a treatment group. Group-1 dogs (n = 12) received a combination of increasing amounts of bromocriptine mesylate (15 to 30 microg/kg [6.8 to 13.6 microg/lb], p.o., q 12 h) and dinoprost tromethamine (0.1 to 0.2 mg/kg [0.045 to 0.09 mg/lb], s.c., q 24 h). Group-2 dogs (n =13) received a combination of increasing amounts of bromocriptine mesylate (the same schedule as group-1 dogs) and cloprostenol sodium (1 microg/kg [0.45 microg/lb], s.c., q 48 h). Both groups were treated until pregnancy termination. Results-Treatment success was 100% in both groups. Days of treatment required for pregnancy termination did not significantly differ between groups (5.0 +/- 0.6 vs 3.7 +/- 0.6 days, group-1 and group-2 dogs, respectively) although adverse effects only developed in group-1 dogs. At the end of the protocols, pseudopregnancy was observed in 3 of 12 and 6 of 13 group-1 and group-2 dogs, respectively. Pregnancy termination was followed by a mucoid sanguineous vulvar discharge for 3 to 10 days. CONCLUSIONS AND CLINICAL RELEVANCE: Results of this study indicate that protocols that combine the use of bromocriptine mesylate and prostaglandins for the termination of unwanted pregnancy in bitches are efficient and safe. The use of bromocriptine mesylate and cloprostenol had the best results and could be easily used on an outpatient basis.     

Evaluation of the concentration of marbofloxacin in alveolar macrophages and pulmonary epithelial lining fluid after administration in dogs

OBJECTIVE: To determine concentrations of marbofloxacin in alveolar macrophages (AMs) and epithelial lining fluid (ELF) and compare those concentrations with plasma concentrations in healthy dogs. ANIMALS: 12 adult mixed-breed and purebred hounds. PROCEDURE: 10 dogs received orally administered marbofloxacin at a dosage of 2.75 mg/kg every 24 hours for 5 days. Two dogs served as nontreated controls. Fiberoptic bronchoscopy and bronchoalveolar lavage procedures were performed while dogs were anesthetized with propofol, approximately 6 hours after the fifth dose. The concentrations of marbofloxacin in plasma and bronchoalveolar fluid (cell and supernatant fractions) were determined by use of high-performance liquid chromatography with detection of fluorescence. RESULTS: Mean +/- SD plasma marbofloxacin concentrations 2 and 6 hours after the fifth dose were 2.36 +/- 0.52 microg/mL and 1.81 +/- 0.21 microg/mL, respectively. Mean +/- SD marbofloxacin concentration 6 hours after the fifth dose in AMs (37.43 +/- 24.61 microg/mL) was significantly greater than that in plasma (1.81 +/- 0.21 microg/mL) and ELF (0.82 +/- 0.34 microg/mL), resulting in a mean AM concentration-to-plasma concentration ratio of 20.4, a mean AM:ELF ratio of 60.8, and a mean ELF-to-plasma ratio of 0.46. Marbofloxacin was not detected in any samples from control dogs. CONCLUSIONS AND CLINICAL RELEVANCE: Marbofloxacin concentrations in AMs were greater than the mean inhibitory concentrations of major bacterial pathogens in dogs. Results indicated that marbofloxacin accumulates in AMs at concentrations exceeding those reached in plasma and ELF The accumulation of marbofloxacin in AMs may facilitate treatment for susceptible intracellular pathogens or infections associated with pulmonary macrophage infiltration.     

Pulsatile secretion pattern of vasopressin under basal conditions, after water deprivation, and during osmotic stimulation in dogs

Measurement of plasma osmolality (Posm) and plasma vasopressin (VP) concentration in response to hypertonicity is regarded as the gold standard for the assessment of VP release in polyuric conditions. Yet the interpretation of the VP curve as a function of Posm may be hampered by the occurrence of VP pulses. To determine whether VP is secreted in a pulsatile fashion in the dog and whether stimulation of VP release changes the secretion pattern of VP, we measured VP at 2-min intervals for 2 h under basal conditions, after 12 h of water deprivation, and during osmotic stimulation with hypertonic saline (20%) in eight healthy dogs. Vasopressin was secreted in a pulsatile fashion with a wide variation in number of VP pulses, VP pulse duration, and VP pulse amplitude and height. After water deprivation, total and basal VP secretion, the number of significant VP pulses, as well as the pulse characteristics did not differ from the basal situation. During osmotic stimulation, there was a large increase in both basal and pulsatile VP secretion, and the number of VP pulses and VP pulse height and amplitude were significantly increased. The VP pulse amplitude correlated significantly with the basal plasma VP concentration during osmotic stimulation. It is concluded that VP is secreted in a pulsatile manner in healthy dogs. The basal and pulsatile VP secretion increases during osmoreceptor-mediated stimulation. The VP pulses may occur to the magnitude that they may be interpreted as erratic bursts, when occurring in the hypertonic saline infusion test.     

Determination of canine beta2-microgloblin in plasma and urine by enzyme-linked immunosorbent assay

An enzyme-linked immunosorbent assay was developed for the determination of canine beta2-microglobulin (beta2-m) in plasma and urine. The detectable sensitivity for pure canine beta2-m was 0.05 microg/l and the analytical range was 0.1 to 50 microg/l. The mean analytical recovery when pure canine beta2-m was added to normal plasma was 101.9%. The mean analytical recovery in the urine was 102.1%. The intra-day variation coefficient was 3.1% in plasma, 4.3% in serum and 1.9% in urine. No difference was found between the concentration of beta2-m in plasma and serum (n=17). The concentration of beta2-m in the plasma of normal dogs was 1.82 +/- 0.57 mg/l (n=31). The mean excretion in 24 hr urine collected from normal dogs was 17.6 +/- 9.2 microg/l, 0.22 +/- 0.12 microg/kg of body weight or 14.2 +/- 9.4 microg/g of urine creatinine. The beta2-m creatinine index of random urine samples was 23.5 +/- 16.6 microg/g (n=26). There was a close correlation between the beta2-m creatinine index of 24 hr urine samples and that of random urine samples (r=0.872).     

Plasma aldosterone concentrations and plasma renin activity in healthy dogs and dogs with hyperadrenocorticism

The mean (se) basal plasma aldosterone concentrations were significantly lower in 31 dogs with pituitary-dependent hyperadrenocorticism (PDH) (75 [9] pmol/litre) than in 12 healthy dogs (118 [14] pmol/litre), whereas in five dogs with hyperadrenocorticism due to an adrenocortical tumour they were significantly higher (205 [109] pmol/litre). The mean basal renin activity was not significantly different between the dogs with PDH (303 [48] fmol/litre/second), the dogs with an adrenocortical tumour (141 [63] fmol/litre/second), and the control dogs (201 [25] fmol/litre/second). At three and four hours after the intravenous administration of 0.1 mg/kg dexamethasone, the concentrations of aldosterone decreased significantly to about 60 per cent of their initial values in the control dogs but did not change in the dogs with PDH or an adrenocortical tumour. In the dogs with PDH the renin activity increased significantly after the administration of dexamethasone.     

Growth hormone in mares and stallions: pulsatile secretion, response to growth hormone-releasing hormone, and effects of exercise, sexual stimulation, and pharmacological agents.

Short-term patterns of growth hormone (GH) secretion and factors affecting it were studied in mares and stallions. In Exp. 1, hourly blood samples were collected from three mares and three stallions in summer and winter. Although GH concentrations varied in a pulsatile manner in all horses, there was no effect of sex or season (P greater than .1) on plasma GH concentrations and no indication of a diurnal pattern of GH secretion. In Exp. 2, 10-min blood samples were drawn for 8 h from 12 mares; after 6 h, porcine GH-releasing hormone (GHRH) was administered i.v. at 0, 45, 90, or 180 micrograms/mare (three mares per dose). Pulsatile secretion of GH occurred in all mares and averaged 2.4 +/- .3 peaks/6 h; amplitudes were variable and ranged from 2.6 to 74.4 ng/mL. Eight of nine mares responded within 20 min to GHRH injection, but there was no difference (P greater than .1) among the three doses tested. In Exp. 3, plasma GH concentrations in stallions increased (P less than .05) 8- to 10-fold after 5 min of acute physical exercise or exposure to an estrual mare. Restraint via a twitch (5 min) and epinephrine administration (3 mg i.v.) also increased (P less than .05) plasma GH concentrations by approximately fourfold. In Exp. 4 and 5, administration of either .4, 2, or 10 mg of thyrotropin-releasing hormone (TRH) or 100 or 500 mg of sulpiride (a dopamine receptor antagonist) increased (P less than .01) plasma prolactin concentrations but had no effect (P greater than .1) on GH concentrations during the same period of time.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of castration on plasma luteinizing hormone concentrations in prepubertal boars

Mean concentrations and the occurrence of pulsatile release of luteinizing hormone (LH) were determined in 14-wk-old crossbred boars (50.5 +/- 1.5 kg) after bilateral or unilateral castration at 10 wk of age. Blood was collected at 10-min intervals for 5 h. Then gonadotropin releasing hormone (GnRH; 40 micrograms) was given and sampling was continued at 5-min intervals for 1 h. Compared with intact boars, bilateral castration increased (P less than .001) mean LH (982 +/- 56 vs 389 +/- 56 pg/ml), pulsatile releases of LH (7.0 +/- .6 vs 2.0 +/- .6 pulses/5 h) and LH pulse amplitude (617 +/- 29 vs 360 +/- 58 pg/ml). Unilaterally castrated boars did not differ from intact boars in any of the above measures of LH secretion. Testis weight increased more between 10 and 14 wk of age in the unilateral castrates than in the intact boars (432 +/- 42 vs 245 +/- 34%; P less than .05). Thus, compensatory hypertrophy occurred within 4 wk of castration. Plasma testosterone was lower for bilateral castrates than for intact animals (.1 +/- .8 vs 3.6 +/- .9 ng/ml; P less than .05) while unilateral castrates (3.8 +/- 1.0 ng/ml) and intact boars did not differ. Plasma estradiol concentrations in bilateral and unilateral castrates were not different from levels found in intact boars (1.8 +/- 1.8, 8.8 +/- 2.1 and 6.0 +/- 1.8 pg/ml, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)     

Pharmacokinetics of Amikacin in Lactating Sheep

The pharmacokinetics of amikacin (AMK) were investigated after intravenous (i.v.) and intramuscular (i.m.) administration of 7.5 mg/kg bw in 6 healthy lactating sheep. After i.v. AMK injection (as a bolus), the elimination half-life (t1/2beta), the volume of distribution (Vd,area), the total body clearance (ClB) and the area under the concentration-time curve (AUC) were 1.64 +/- 0.06 h, 0.19 +/- 0.02 L/kg, 1.36 +/- 0.1 ml/min per kg and 94.09 +/- 6.95 (microg.h)/ml, respectively. The maximum milk concentration of AMK (Cmax), the area under the milk concentration-time curve (AUCmilk) and the ratio AUCmilk/AUCserum were 1.18 +/- 0.22 microg/ml, 22.45 +/- 3.21 (micro.h)/ml and 0.24 +/- 0.02, respectively. After i.m. administration of AMK the t1/2beta, Cmax, time of Cmax (tmax) and absolute bioavailability (Fabs) were 1.29 +/- 0.1 h, 16.97 +/- 1.54 microg/ml, 1.0 +/- 0 h and 64.88% +/- 6.16%, respectively. The Cmax, AUCmilk and the ratio AUCmilk/AUCserum were 0.33 microg/ml, 1.67 (microg.h)/ml and 0.036, respectively.     

Effects of growth hormone and gonadotrophin releasing hormone antagonists on ovarian follicle growth in sheep

Our objective was to determine the effects of the administration of growth hormone (GH) alone or plus teverelix, a gonadotrophin releasing hormone antagonist (GnRHa), on follicle development in sheep. Ewes were treated daily for 6 days by the intramuscular route with 15 mg of GH alone (GH group; n = 6) or combined with two subcutaneous doses of GnRHa (1.5 mg) on days 0 and 3 of GH treatment (GH/GnRHa group; n = 6); the control group (n = 6) received similar treatment with saline solution. Plasma follicle stimulating hormone levels were significantly lower in the GH/GnRHa group than in the control (P < 0.001) and GH groups (P < 0.05). The number of follicles > or =2 mm increased to reach significant differences with control (18.7 +/- 0.6) on day 4 in GH/GnRHa group (22.7 +/- 0.5, P < 0.001) and on day 5 in GH group (20.3 +/- 0.4 vs. 17.0 +/- 0.6, P < 0.05). These results indicate that GH and GnRHa may be useful for increasing the number of gonadotrophin-responsive follicles in the ovary. However, follicle function could be affected as both GH and GH/GnRHa groups showed lower plasma inhibin A concentrations than control sheep (90-110 pg/mL vs. 170-185 pg/mL, P < 0.005).