In vitro release of growth hormone-releasing factor (GRF) from the hypothalamus: somatostatin inhibits GRF release.

The manner of release of growth hormone-releasing factor (GRF) from the rat hypothalamus was studied in a perifusion system using a highly sensitive radioimmunoassay for rat GRF. The recovery of GRF in this system was 50-60%. The release of GRF from the rat hypothalamic blocks was almost stable for 20-240 min after the start of the perifusion and was stimulated by depolarization induced by high K+ concentration. The release of GRF was inhibited by somatostatin at concentrations of 10(-11) to 10(-8) M with maximum inhibition to 52.5% of the basal release at a concentration of 10(-9) M. These results suggest that this system is useful in studying the regulatory mechanism of GRF release and that, in addition to its action on the pituitary, somatostatin appears to act at the level of the hypothalamus in inhibiting GRF release in the regulation of GH secretion.