Continuous infusion of growth hormone-releasing factor: effects on pulsatile growth hormone secretion in normal rats

The effects of a continuous infusion of growth hormone-releasing factor (GRF) on pulsatile growth hormone (GH) secretion were studied in conscious, freely moving rats. Male Sprague-Dawley rats were prepared with 2 indwelling venous catheters and were then housed individually for 6-16 days in isolation chambers. After this period, normal saline, 1.5 microgram/h GRF or 15 micrograms/h GRF were continuously infused for 8 or 31 h via one of the catheters. Blood samples were drawn at approximately 20-min intervals via the other catheter during the last 6-7 h of the infusion. Plasma GH concentrations were determined by RIA. Saline-treated animals exhibited the typical pattern of spontaneous GH pulses. The frequency of GH pulses in animals infused with either dose of GRF was not different from that of the saline-infused rats. Likewise, trough GH concentrations were similar in all three treatment groups. However, peak GH concentrations in the rats receiving the 15-micrograms/h GRF infusion were remarkably higher than the concentrations observed in the other two groups, regardless of the duration of the infusion. This resulted in mean GH concentrations being significantly higher (p less than 0.01) in the 15-micrograms/h GRF-infused rats. These results demonstrate that pituitary episodic GH secretion continues even in the presence of a continuous GRF stimulation.