Endocrinology, Vol 115, 1203-1209, Copyright © 1984 by Endocrine Society

ARTICLES

Presence of an adenylate cyclase dually regulated by somatostatin and human pancreatic growth hormone (GH)-releasing factor in GH-secreting cells

A Spada, L Vallar and G Giannattasio

We have investigated the effects of SRIF and human pancreatic GH- releasing factor-44 (hpGRF-44) on adenylate cyclase (AC) activity of male rat anterior pituitaries (in which somatotrophs are present in large proportion) and of human GH-secreting pituitary adenomas (which are almost homogeneously constituted by somatotrophs). The adenoma's responsiveness to both agents in terms of secretion was previously demonstrated in in vitro experiments. SRIF inhibited in a dose- dependent fashion the GH release from monolayer cultures of the tumors. The inhibition ranged from 32-66% at the maximal effective concentration (10(-6) M). hpGRF-44 stimulated GH release in a dose- dependent fashion. The stimulation was 78-172% at 10(-7) M. SRIF and hpGRF-44 markedly affected AC activity in both systems. SRIF elicited a pronounced inhibition of the enzyme activity in a dose-dependent manner. The inhibition was about 40% in the rat and ranged from 16-49% in adenomas at the maximal effective concentration (10(-5) M SRIF). The inhibitory effect was GTP-dependent. hpGRF-44 markedly stimulated AC activity. The stimulation was dose dependent and GTP dependent. The stimulation was about 650% in the rat and 26-350% in adenomas at the maximal effective concentration (10(-6) M). These results suggest the presence of a dually regulated (by SRIF and hpGRF-44) AC in GH- secreting cells; an involvement of cAMP in the intracellular mechanisms transducing the signals of SRIF and hpGRF-44 in somatotrophs.

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