Pituitary growth hormone response in rats during a 24-hour infusion of growth hormone-releasing factor

The pituitary GH response during a 24-h iv infusion of GH-releasing factor (hpGRF-44; 15 micrograms/h) and to a subsequent bolus injection of hpGRF-44 (2 micrograms) was studied in conscious, freely moving male rats pretreated with antiserum against somatostatin. Within 2 h of the initiation of the hpGRF-44 infusion, plasma GH concentrations rose from 169 +/- 16 to 2465 +/- 307 (+/- SE) ng/ml. By 6 h, plasma GH concentrations began to fall. They decreased slowly and reached a nadir of 490 +/- 107 ng/ml by 12 h. Rats infused for 24 h with hpGRF-44 failed to respond to a 2-micrograms bolus injection (iv) of hpGRF-44, whereas rats infused for 24 h with saline responded with a normal increase in plasma GH. The pituitary GH content of rats treated with saline was significantly greater than that of rats treated with hpGRF-44. These results demonstrate that the capacity of the pituitary to respond to GRF can be exhausted after the chronic administration of hpGRF-44 and that this lack of response appears to be due, in part, to a depletion of pituitary stores of GH.