Effect of different growth hormone-releasing factors on the concentrations of growth hormone, insulin and metabolites in the plasma of sheep maintained in positive and negative energy balance

Abstract

Three experiments were conducted to compare the ability of different preparations of growth hormone-releasing factor (GRF) to stimulate GH secretion in sheep maintained in positive and negative energy balance. In experiment 1 five sheep were injected (i.v.) with three preparations of human pancreatic GRF (hpGRF-44, hpGRF-40, hpGRF-29-NH₂) and one preparation of rat hypothalamic GRF (rhGRF-29-NH₂) all at 98·0 pmol/kg, or control vehicle, in a Latin square design when the animals either had free access to food or were fed half their maintenance requirements. Analysis of plasma samples, obtained before and for 150 min after injection, revealed that the reduced food intake resulted in the expected changes in body weight and circulating GH, insulin, glucose, urea and non-esterified fatty acids. The maximum post-injection concentrations of GH did not differ between either the two levels of feeding or the four GRF preparations but the mean post-injection concentration of GH was significantly higher for all GRF treatments on the restricted ration (P < 0·001). The mean post-injection response to rhGRF-29-NH₂ was less than that obtained with hpGRF-44 for sheep with food available ad libitum (P < 0·05) and was clearly more persistent for all GRF treatments in animals fed the reduced diet (P < 0·001). In experiment 2 the same five sheep were injected i.v. with rhGRF-29-NH₂ (98·0 pmol/kg) when they had free access to food and after food had been withdrawn for 3 days. The peak concentrations of plasma GH and the speed of the response did not differ between feeding and food deprivation but the average post-injection concentration of GH was higher (P < 0·05) when the animals were deprived because the response was more persistent. In experiment 3 the effects of i.v. injection of hpGRF-10-NH₂, hpGRF-10-OH and hpGRF-10-OCH₃ were examined at three different doses (8, 80 and 800 nmol/kg) each in three sheep. None of these treatments stimulated a significant increase in circulating GH. J. Endocr. (1985) 105, 113–119