Inhibition of ovine prolactin and thyrotropin responses to thyrotropin-releasing hormone by triiodothyronine

Hypothalamic control of endocrine thermogenesis

Hypothalamic and ruminal cooling raised serum thyrotropin (TSH), adrenocorticotropin (ACTH), norepinephrine (NE), and glucose in conscious goats in 20 degree C ambient temperature. Cooling of the preoptic anterior hypothalamus (POAH) for 2 h initially evoked shivering and vasoconstriction, leading to 1.5 degree C rise in rectal temperature (Tr). Pituitary-thyroid activation by POAH cooling was shown by peak rises in TSH of 60% at 40 min, in triiodothyronine (T3) of 54% at 80 min, and in thyroxine (T4) of 40% at 140 min. At 60 min, ACTH and NE peaked at 57 and 65%, respectively. TSH, ACTH, and NE declined during the 2nd h of POAH cooling as Tr plateaued; when POAH cooling was stopped, these hormones fell below basal level as vasodilation and panting restored Tr to normal. In contrast to the core hyperthermia evoked by POAH cooling, ruminal intubation with O degree C water (1 liter/10 kg) led to general hypothermia, Tpoah and Tr falling 1.6 degree C at 40 min. Pituitary-thyroid responses were less but ACTH and NE more, compared with POAH cooling. TSH peaked at 37% at 20 min, T3 at 55% at 60 min, and T4 at 18% at 200 min. ACTH peaked at 250% at 30 min and NE at 120% at 20 min. Thermosensitive neurons in the POAH seem to mediate more sensitive and complete control over TSH than over ACTH, or NE release, whereas extrahypothalamic core thermosensitivity (e.g., brain stem, spinal cord, abdomen) may influence ACTH and NE more than TSH release.

Influence of prostaglandins and thyrotropin releasing hormone (TRH) on hormone secretion and growth in wether lambs

A series of experiments were conducted in ewes and whether (castrate male) lambs to evaluate the influence of prostaglandins on secretion of anabolic hormones and to determine if repeated injections of prostaglandin (PG) F2alpha would chronically influence the secretion of these hormones and perhaps growth rate as well. A single intravenous injection of PGA1 and PGB1 (100 microgram/kg) exerted no significant (P greater than .10) influence on plasma concentrations of prolactin (PRL), growth hormone (GH) or thyrotropin (TSH) in ewes. PGA1, but not PGB1, stimulated an increase in the plasma concentration of insulin. Infusion of PGF2alpha for 5.5 hr into ewes resulted in increased (P less than .05) plasma concentrations of both GH and ARL while TSH and insulin were not significantly influenced. Prostaglandin F2alpha, when injected subcutaneously into wether lambs (10 mg twice weekly) stimulated (P less than .05) plasma GH concentrations after the first injection, but not after 3 weeks of treatment. Changes in plasma PRL or TSH were not observed consistently in the lambs treated chronically with PGF2alpha or TRH. Prostaglandin F2alpha, in the present studies, and PGE1 in previously reported studies (1-3), has been demonstrated to be stimulatory to the secretion of PRL and GH. In contrast, PGA1 and PGB1, which lack an 11-hydroxyl group, failed to influence the secretion of either PRL or GH. It would, therefore, appear that the 11-hydroxyl group is a structural requirement for prostaglandins to influence the secretion of these two hormones in sheep. Treatment with thyrotropin releasing hormone (TRH), alone or in combination with PGF 2alpha, significantly (P less than .05) increased growth rate (average daily gains) while PGF2alpha did not, despite the fact that both compounds exerted similar effects on plasma GH.