Effects of long-term infusion of growth hormone (GH)-releasing factor on pulsatile GH secretion in the male rat.

The effects of long-term infusion of GRF on pulsatile GH secretion were investigated in conscious, freely-moving male rats. GH-releasing factor (GRF) was intravenously administered for 14 days using an osmotic minipump. Six-hour iv infusion of 0.5 microgram/h GRF significantly increased mean GH concentration in the conscious rats. In contrast, 14-day iv infusion of 0.5 microgram/h GRF significantly inhibited pulsatile GH secretion. The pituitary GH content was significantly increased by the 14-day treatment. The GH response to bolus injection of 1 microgram GRF under urethane anesthesia was enhanced by the 14-day infusion of GRF. Pretreatment with somatostatin (SS) antiserum did not restore the inhibited GH secretion in rats treated with 14-day infusion of GRF. These results suggest that neither pituitary GH depletion nor high somatostatin tone is involved in the GRF-induced GH inhibition.