The functional relationship between thyroid, adrenal and gonadal hormones was investigated using adult male rats. Hypothyroidism was produced by the administration of 4-methyl-2-thiouracil (thiouracil) in the drinking water for 2 weeks. Plasma concentrations of TSH dramatically increased, whereas plasma concentrations of tri-iodothyronine and thyroxine decreased in thiouraciltreated rats as compared with euthyroid rats. Hypothyroidism increased basal levels of plasma ACTH and pituitary content of ACTH. The pituitary responsiveness to CRH for ACTH release markedly increased, whereas the adrenal responsiveness to ACTH for corticosterone release decreased. These results indicated that hypothyroidism causes adrenal dysfunction in adult male rats. Pituitary contents of LH and prolactin decreased in hypothyroid rats as compared with euthyroid rats. In addition, hypothyroidism lowered pituitary LH responsiveness to LHRH. Testicular responsiveness to human chorionic gonadotrophin for testosterone release, however, was not different between euthyroid and hypothyroid animals. These results indicated that hypothyroidism causes adrenal dysfunction and results in hypersecretion of ACTH from the pituitary gland. Adrenal dysfunction may contribute to the inhibition of LHRH secretion from the hypothalamus, possibly mediated by excess CRH.

ABSTRACT. Effects of ether stress on the hypothalamo-hypophysial-gonadal axis in adult male rats were examined. To clarify the role of adrenal glucocorticoids in gonadal function, the effects of adrenalectomy and Dexamethasone treatment were also investigated. Ether stress increased the plasma concentrations of ACTH and corticosterone, but decreased the plasma concentrations of LH, FSH, inhibin and testosterone. The pituitary responsiveness to LH-RH for LH release and testicular responsiveness to the endogenous LH for testosterone release were maintained in stressed rats. Adrenalectomy caused an increase in the plasma concentrations of ACTH, but decreased the plasma concentrations of LH, FSH and testosterone. Dexamethasone treatment in adrenalectomized rats recovered the levels of plasma gonadotropins to control levels. The concentration of plasma inhibin did not change in adrenalectomized rats, but it was decreased compared to control rats by Dexamethasone treatment. Treatments of Dexamethasone in intact male rats resulted in a decline in plasma levels of testosterone and inhibin without a decrease in the levels of LH and FSH, indicating the direct effect of Dexamethasone on the testes. These results indicate that increased ACTH secretion in stressed rats is probably due to hypersecretion of CRH from the hypothalamus, which suppresses gonadotropin secretion via the inhibition of LH-RH. The decreased levels of testosterone may be caused by a stress-induced decrease in plasma LH concentrations and increased secretion of corticosterone in the ether stressed rats. The low levels of plasma inhibin in stressed rats was also probably due to the direct effect of corticosterone on the Sertoli cells. -KEY WORDS: corticoid, CRH, gonad, inhibin, stress.

To determine dose-dependent cardiovascular effects of dobutamine and phenylephrine during anesthesia in horses, increasing doses of dobutamine and phenylephrine were infused to 6 healthy Thoroughbred horses. Anesthesia was induced with xylazine, guaifenesin and thiopental and maintained with sevoflurane at 2.8% of end-tidal concentration in all horses. The horses were positioned in right lateral recumbency and infused 3 increasing doses of dobutamine (0.5, 1.0 and 2.0 µg/kg/min) for 15 min each dose. Following to 30 min of reversal period, 3 increasing doses of phenylephrine (0.25, 0.5 and 1.0 µg/kg/min) were infused. Cardiovascular parameters were measured before and at the end of each 15-min infusion period for each drug. Blood samples were collected every 5 min during phenylephrine infusion period. There were no significant changes in heart rate throughout the infusion period. Both dobutamine and phenylephrine reversed sevoflurane-induced hypotension. Dobutamine increased both mean arterial blood pressure (MAP) and cardiac output (CO) as the result of the increase in stroke volume, whereas phenylephrine increased MAP but decreased CO as the result of the increase in systemic vascular resistance. Plasma phenylephrine concentration increased dose-dependently, and these values at 15, 30 and 45 min were 6.2 ± 1.2, 17.0 ± 4.8 and 37.9 ± 7.3 ng/ml, respectively.

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