We recently reported that insulin inhibits basal and cortisone- and T3-stimulated GH secretion by GH3 rat pituitary tumor cells. The effects of purified semisynthetic human insulin were therefore tested on long-term GH secretion in normal pituitary cells. Primary monolayer cultures derived from male rat pituitaries were grown in serum-free defined medium. Insulin (7 nM) maximally inhibited basal GH secretion by almost 60% after 72 h, with 50% of maximal GH suppression occuring with 1.75 nM insulin. Insulin receptor antiserum blocked the suppression of GH by 7 nM insulin, but had no effect on the suppression of GH by IGF-I (3.25 nM). GRF (100pM) stimulated GH two- to threefold during 48 h. Insulin (7 nM) prevented the stimulation of GH induced by up to 1 nM GRF (P < 0.01) and this suppression was also selectively blocked by insulin receptor antiserum. The inhibition of GRF-stimulated GH required a lag period of 48 h and the dose of insulin required for 50% inhibition of GRF stimulation was 3.5nM. Insulin did not alter the degradation rate of 125I-GH in these cultures and medium glucose concentrations were not different in control or insulin-treated wells for up to 72 h of incubation. The insulin-induced suppression of GH was also observed when cells were grown in glucose-free medium. Insulin did not nonselectively suppress cell secretion, as PRL secretion was mildly stimulated (P < 0.01) by insulin in the same cultures. Fibroblast growth factor, epidermal growth factor, and insulin A-chain at similar doses did not alter basal or GRFstimulated GH secretion. Northern gel analysis of cell extracts showed that insulin-treated cells exhibited a decrease of the GH mRNA band. Using nitrocellulose dot-blot hybridization of cellular RNA with specific 32P-cDNA for rat GH, insulin suppressed pituitary GH mRNA levels by 56 ± 5% (P < 0.001) after 72 h of incubation. The results show that selective suppression of basal and GRF-stimulated GH secretion by insulin is time and dose dependent. Insulin modulates GH secretion at the level of the somatotroph.

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