Abstract
Summary
To determine if thyrotropin-releasing hormone (TRH) stimulates growth hormone (GH) secretion by acting on the central nervous system (CNS), we measured plasma GH concentrations in unanesthe-tized calves at various times after cerebro-ventricular and jugular injection of TRH. We found that even after the highest dose of TRH (200 μg), lateral ventricle injection of TRH had no detectable effect on plasma GH levels. With jugular injection, on the other hand, 50-, 100-, and 200-μg doses of TRH were effective in significantly elevating plasma GH levels. Maximum increases occurred within 10 min of injection and usually involved a threefold increase in GH from preinjection levels. In all cases, plasma GH concentrations returned to basal levels within 60 min postinjection. These results indicate that stimulation of GH secretion by exogenous TRH is probably due only to direct stimulation of the pituitary gland and does not involve action of TRH on the CNS.
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