Pituitary Growth Hormone Response in Rats during a 24-Hour Infusion of Growth Hormone-Releasing Factor*

Abstract

The pituitary GH [growth hormone] response during a 24-h i.v. infusion of [human pancreatic] GH-releasing factor (hpGRF-44; 15 .mu.g/h) and to a subsequent bolus injection of hpGRF-44 (2 .mu.g) 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-.mu.g bolus injection (i.v.) of hpGRF-44; 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. The capacity of the pituitary to respond to GRF can be exhausted after the chronic administration of hpGRF-44 and this lack of response appears to be due, in part, to a depletion of pituitary stores of GH.