EFFECTS OF LONG-TERM INFUSION OF GROWTH-HORMONE (GH)-RELEASING FACTORON PULSATILE GH SECRETION IN THE MALE-RAT

The effects of long-term infusion of GRF on pulsatile GH secretion wer e investigated in conscious, freely-moving male rats. GH-releasing fac tor (GRF) was intravenously administered for 14 days using an osmotic minipump. Six-hour iv infusion of 0.5 mug/h GRF significantly increase d mean GH concentration in the conscious rats. In contrast 14-day iv i nfusion of 0.5 mug/h GRF significantly inhibited pulsatile GH secretio n. The pituitary GH content was significantly increased by the 14-day treatment. The GH response to bolus injection of 1 mug GRF under ureth ane anesthesia was enhanced by the 14-day infusion of GRF. Pretreatmen t with somatostatin (SS) antiserum did not restore the inhibited GH se cretion in rats treated with 14-day infusion of GRF. These results sug gest that neither pituitary GH depletion nor high somatostatin tone is involved in the GRF-induced GH inhibition.