Effect of somatostatin withdrawal and growth hormone (GH)-releasing factor on GH release in vitro: amount available for release after disinhibition.

The secretion of GH is strikingly episodic. We have suggested that the timing of the episodic bursts of GH secretion is set by somatostatin (SRIF) withdrawal, whereas the magnitude of the bursts is determined by the amount of GH-releasing factor (GRF) impinging on the somatotrophs before and during SRIF withdrawal. We have now used an in vitro model of perifused rat pars distalis cells to examine the interaction of SRIF and GRF on GH release and, in particular, to examine the effect of GRF on the magnitude of the burst of GH release that follows SRIF withdrawal. After 30 min of perifusion with SRIF (10(-9) M), there follows an immediate but small burst of GH release. The burst of GH release following concurrent perifusion with SRIF plus GRF (10(-10) M) is increased, with a 7.5- to 9.5-fold increase in the peak secretion rate. When GRF is maintained after the withdrawal of SRIF, the peak secretion rate is not different from that seen after simple withdrawal of both SRIF and GRF, but the duration of the burst is increased. These data demonstrate that the presence of GRF during SRIF perifusion, while not altering basal release, does strikingly increase the post-SRIF release of GH. We propose that a similar relation applies in vivo, where SRIF withdrawal sets the timing of the episodic bursts of GH release, whereas GRF determines the magnitude.