TRH and Dopamine Interactions Affecting Pituitary Hormone Secretion

Abstract

Dopamine infusion consistently elevated baseline serum growth hormone values in normal males in contrast to a suppression of baseline serum prolactin and perhaps TSH values. The mean peak serum TSH concentration in response to 100 fjig TRH was 14 ± 2 μU/ml in normal males and fell to 7 ± 2 μU/ml when a dopamine infusion was begun 5 min before the TRH bolus and continued for 120 minutes. Under similar conditions, the mean peak serum prolactin response (36 ± 3 ng/ml) was obliterated and was markedly attenuated when the dopamine infusion was begun 5 min after the TRH bolus (17 ± 2 ng/ml) (P < .01). Administration of 100 μg of T₃ 4 h previously resulted in a mean peak serum prolactin concentration of 46 ± 6 ng/ml in response to TRH ad ministered 5 min before a 60 min dopamine infusion, compared to 17 ± 2 ng/ml without triiodothyronine (P < .01). Serum growth hormone rose to a mean peak value of 13 ± 2 ng/ml at 60 min in response to dopamine from a mean baseline value of 1 ±0.1 ng/ml (P < .001). When 100 μg of TRH was administered 5 min before the dopamine infusion, the mean growth hormone only rose to 6 ± 2 ng/ml. Pretreatment with triiodothyronine blocked the TRH inhibition of the growth hormone response to dopamine. When TRH was administered 5 min after dopamine, the growth hormone concentration was elevated to 26 ± 6 ng/ml. The data suggest that dopamine and TRH are antagonistic in their effects in pituitary hormone secretion in man.