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Abstract

Objective

To investigate the signal transduction pathways by which endotoxin stimulates in vitro pituitary cell growth hormone (GH) release.

Animals

Pituitary cell cultures derived from 6 sheep.

Procedure

Signal transduction pathways involved in endotoxin-mediated GH release from sheep pituitary cell cultures were evaluated by the use of specific blockers of arachidonic acid and its metabolites, extracellular calcium, protein kinase C, and protein kinase A. Cell cultures were exposed to the specific blockers in the presence or absence of endotoxin (Escherichia coli O55:B5, 10 μg/ml) for 24 hours. In addition, effects of endotoxin on GH cell content and GH mRNA values were determined.

Results

Nordihydroquairetic acid (lipoxygenase blocker, 10 μM, 30 μM) and eicosatetraynoic acid (arachidonic acid competitor, 10 μM) decreased endotoxin-stimulated GH release. The calcium channel blocker verapamil (25 μM) decreased baseline and endotoxin-stimulated GH release. Phorbol myristate acetate-induced down-regulation of protein kinase C, indomethacin, or the protein kinase A blocker H89 did not alter endotoxin-stimulated GH release. Endotoxin increased GH mRNA values by 50.1 ± 6.0%, but the cell content of GH was not affected.

Conclusions

A direct effect of endotoxin on the pituitary gland to stimulate GH secretion was evident, an effect mediated predominantly by arachidonic acid and its metabolites through the lipoxygenase pathway. Endotoxin-stimulated GH release requires extracellular calcium and is associated with increased cell GH mRNA content.

Clinical Relevance

A better understanding of the signal transduction pathways involved in endotoxin-mediated effects will allow more appropriate therapeutic intervention in clinical cases of endotoxemia. (Am J Vet Res 1996;57:1662–1667)

Free access
in American Journal of Veterinary Research

SUMMARY

Unbound or free cortisol constitutes a small fraction of total plasma cortisol, but is believed to represent the biologically active portion of this circulating glucocorticoid. We tested the hypothesis that the percentage free cortisol was altered in plasma from dogs with hyperadrenocorticism, which could account for a greater target tissue response to this circulating hormone. The percentage free cortisol in plasma samples from human beings, healthy dogs, and dogs with hyperadrenocorticism was estimated, using centrifugal ultrafiltration-dialysis. Total cortisol concentrations were determined by use of radioimmunoassay. Total cortisol concentrations appeared greater in plasma from human beings than in plasma from either group of dogs. However, the percentage free cortisol was lower in plasma from human beings, resulting in a calculated concentration of free cortisol that was quite similar between plasma from human beings and healthy dogs. Total plasma cortisol concentrations were greater (P < 0.01) in samples from dogs with hyperadrenocorticism (190 ±113 nmol/L; mean ± SD) than in healthy dogs (102 ± 85 nmol/L), but the percentage free cortisol was not different between these 2 groups (dogs with hyperadrenocorticism, 16 ± 9%; healthy dogs, 13 ± 6%). However, plasma free cortisol concentrations (product of total and the percentage of free cortisol) were greater (P < 0.01) in samples from dogs with hyperadrenocorticism (36 ± 41 nmol/L) than in those from healthy dogs (16 ± 9 nmol/ L). Significant (P < 0.001) positive linear relationships were found between total cortisol concentrations and percentage free cortisol in plasma samples from healthy dogs and dogs with hyperadrenocorticism. Furthermore, the slope of these lines was not different between the 2 groups, providing no evidence for alterations in cortisol binding associated with hyperadrenocorticism. The higher total cortisol concentrations in dogs affected with this disease do, however, result in greater concentrations of free cortisol in circulation, contributing to the development of clinical signs observed in this disease.

Free access
in American Journal of Veterinary Research

Abstract

Objective

To determine whether cytokines of homologous species might mediate the stimulatory effects of endotoxin on release of luteinizing hormone (LH) from pituitary cells.

Sample Population

Cells from pituitary glands collected from 8- to 14-month-old wethers.

Procedure

Cells from the anterior pituitary gland were cultured in the presence of recombinant ovine or bovine cytokines (interleukin [IL]-1α, IL-1β, and IL-2), tumor necrosis factor-α (TNF), and interferon-γ (IFN-γ). Luteinizing hormone that was released into the medium was measured. Cells were also cultured with modulators of signal transduction pathways to evaluate the second messenger system used by IL-1α and IL-1β.

Results

Similar to effects of endotoxin, IL-1α and IL-1β stimulated release of LH. Interleukin 2, TNF, and IFN-γ did not have a detectable effect on release of LH. Stimulation of LH release by IL-1αand IL-1β required activation of voltage-dependent Ca2+ channels and appeared to involve protein kinase C.

Conclusions

IL-1αand IL-1β may mediate the direct stimulatory effect of endotoxin on release of LH in vitro. Interleukin 2, TNF, and IFN-γ do not have a direct effect on release of LH; therefore, they do not mediate this effect of endotoxin.

Clinical Relevance

Stressors, including infection, are often associated with reduced fertility. Infection resulting in endotoxin release, production of interleukins, or both, can lead to direct stimulation of LH release from the pituitary gland. Inopportune release of LH via cytokines may interfere with normal pulsatile release of LH, thereby suppressing gonadal function. (Am J Vet Res 1998;59:1488–1493)

Free access
in American Journal of Veterinary Research

Summary

Pituitary cells, collected from five healthy dogs, were cultured and treated with various doses of ovine corticotropin-releasing hormone (crh), arginine vasopressin (avp), oxytocin (ot), or angiotensin II (aii) to determine which of these hypothalamic peptides affected adrenocorticotropin (acth) secretion. Of the 4 peptides, only crh significantly increased acth secretion from cultured canine anterior pituitary cells. The lowest dose of crh tested, 0.01 nM, significantly stimulated acth release. Co-addition of avp, ot, or aii with crh did not increase acth secretion beyond that caused by addition of crh alone. Similarly, neither co-addition of avp with ot, avp with aii, or ot with aii significantly stimulated acth secretion. These results support a role for crh in the physiologic regulation of acth secretion from the canine anterior pituitary, but do not support regulatory roles for avp, ot, or aii.

Free access
in American Journal of Veterinary Research

Abstract

Objective

To determine whether an estradiol-progesterone (EP) growth implant would have an effect on febrile responses and on the catabolic component of Eimeria bovis infection.

Animals

27 Holstein bull calves.

Procedure

Calves were assigned to treatment groups as: control (n = 5), EP implant (EP, n = 5), E bovis-inoculated (coccidia: C, n = 7), pair fed (n = 4), or EP plus E bovis-inoculated coccidia (EP/C, n = 6) groups. Calves were provided subcutaneous EP implants at 8 weeks of age, and were inoculated with 2 × 105 oocysts of E bovis at 11 weeks of age. Body weight was measured on postinoculation day (PID) 0, 14, and 28. Rectal temperature and food intake were determined and fecal samples were collected daily from PID 15 to 28. Blood samples were collected on PID 24 for analysis of CD2+, CD4+, and CD8+ antigens and plasma insulin-like growth factor I concentration. Blood samples were collected at 15-minute intervals for measurement of pulsatile growth hormone release.

Results

Group-EP/C calves had fever for 2 days versus 5 days for group-C calves (P < 0.05). These calves had diarrhea for fewer days than did their group-C counterparts (P < 0.05). Fibrinogen and glucose values were high in group-C (P < 0.05) but not group-EP/C calves. The latter had positive weight gain from PID 14 to 28, whereas group-C calves had weight loss (P < 0.05). Plasma insulin-like growth factor I concentration was reduced by infection (P < 0.05). EP-treated noninfected calves had increased numbers of CD2+, CD4+, and CD8+ blood mononuclear cells (P < 0.05).

Conclusions

EP has a protective effect in calves infected with E bovis. This may relate to changes in immune function induced by EP.

Clinical Relevance

Treatment of calves with EP could offer some protection against the often severe wasting and debilitation associated with E bovis infection. (Am J Vet Res 1997;58:891–896)

Free access
in American Journal of Veterinary Research

Summary

A study was initiated to determine whether development of a functional ruminant digestive system was associated with alterations in plasma growth hormone (gh) concentration. Holstein bull calves were fed milk or milk with grain until studied at the age of 1 month (n = 12). Calves placed on pasture with some grain supplementation were studied at the age of 3 months (n = 6) to determine plasma gh concentration in an animal with fully developed ruminant metabolism. Blood samples were taken at 10-minute intervals for 5 hours, followed by administration of bovine gh-releasing factor (0.075 μg/kg of body weight) and subsequent blood sample collection for 1 hour. On the following day, a blood sample was collected via jugular cannula, clonidine (10 μg/kg) was administered, and blood samples were subsequently obtained. Data indicated that milk-fed calves had higher mean plasma gh concentration than did either milk/grain-fed or older calves. The difference in mean plasma gh concentration was related to higher secretory pulse amplitude. Pituitary responses to bovine gh-releasing factor did not differ among the 3 groups, but response to clonidine were greater in milk-fed calves than in calves of the other groups. These data indicate that the change from a nonruminant to a ruminant-type gastrointestinal tract, perhaps attributable to subsequent changes in metabolism, may induce changes in hypothalmic function to decrease gh concentration.

Free access
in American Journal of Veterinary Research

SUMMARY

Excess production or long-term administration of glucocorticoids is detrimental to longitudinal growth in people and rats. A portion of this effect is attributed to cortisol inhibition of growth hormone (gh). Glucocorticoid effects are usually studied in subjects under long-term treatment with synthetic, more potent glucocorticoids, and, to the authors' knowledge, have not been examined in domestic animals. We sought to examine the effects of cortisol infusion on gh release in sheep. Cortisol infusion into castrated, male Suffolk sheep (1 to 1.5 years old) caused a significant (P < 0.0001) increase in cortisol concentretion. Basal gh release was not affected over the 4-hour period of infusion. Growth hormone-releasing hormone administration stimulated gh release in both groups (P < 0.001); however, the control group had a greater response to growth hormone-releasing hormone than did the cortisol infused group (P < 0.0001). These results were duplicated in cultured sheep pituitary cells. Cortisol inhibition of gh release may be mediated via enhanced somatostatin release, owing to a direct inhibition of somatotrope function, or a combination of both mechanisms. Because of effects of stress and disease in increasing cortisol concentration, additional study of the mechanisms for cortisol inhibition of gh release in sheep needs to be performed.

Free access
in American Journal of Veterinary Research

SUMMARY

Neospora caninum-induced abortion is a major production problem in the dairy cattle industry in the United States and worldwide. Abortions attributable to naturally acquired N caninum infection also have been observed in pygmy goats. We studied experimentally induced infections with N caninum in pregnant pygmy does to determine whether abortions attributable to N caninum infection would occur after inoculation. Seven pregnant pygmy does (1 control doe and 6 inoculated with N caninum) were studied. The control doe remained clinically normal throughout the study and delivered 2 healthy kids. Abortion, fetal death, and stillbirths were observed in some pregnant does inoculated with N caninum. Two pregnant pygmy does inoculated with N caninum early in gestation (day 51) had fetuses that died and were aborted, or died and were reabsorbed. Neospora caninum tachyzoites and lesions were observed in the brain, spinal cord, and heart of aborted fetuses; parasites also were isolated from the placenta. Four additional pregnant pygmy does (2 inoculated at mid-gestation [day 85], and 2 at late gestation [day 127]) did not abort after inoculation. However, 1 doe inoculated during midgestation delivered a stillborn fetus that had died about 1 week prior to parturition. This kid was congenitally infected with N caninum. Neospora caninum was isolated from the placentas of all inoculated does examined. Neonatal neosporosis was not observed in live-born kids, nor were stages of N caninum isolated from any live-born kid. Does did not undergo abortion or have congenitally infected kids when they were rebred and evaluated for neosporosis.

Free access
in American Journal of Veterinary Research