Differential effects of somatostatin on the release of bioactive and immunoactive rat growth hormone in vitro.

Abstract

The effects of linear (L) or cyclic (C) somatostatin (SRIF) on the release of growth hormone (GH) as measured both by bioassay and by radioimmunoassay (RIA) were studied in a series of incubation experiments with rat adenohypophyses. Although both forms of SRIF inhibited release of RIA-GH in all experiments, only C-SRIF appeared to inhibit release of bioactive GH. L-SRIF even enhanced release of bioassayable GH in one study. Addition of C-SRIF to medium containing secreted GH strongly inhibited the tibial growth response of the bioassay animals, while it had no effect on the measurement of GH by the RIA. Thus, the apparent inhibitory effect of C-SRIF on release of bioactive GH may be an artifact. The conclusion was supported by experiments in which the SRIF was not removed from the medium prior to assay by dialysis or by neutralization with anti-SRIF antiserum. After these manipulations, the inhibition by C-SRIF of the secretion of bioactive GH was eliminated, whereas the inhibitory effect on RIA-GH release was not affected. These results support other evidence for the differential release of rat GH with varying immuno-and bioactivities by different stimuli.