Although it is well known that exogenous bovine GH (bGH) increases milk yield in ruminants it has not been possible to determine whether an increase in endogenous GH secretion has the same effect. The recent isolation of human pancreatic GH-releasing factor (hpGRF-44) has enabled this comparison of the effects of bGH and hpGRF-44 on milk production in sheep. Three pairs of Dorset ewes underwent three 4-day treatments according to a Latin square design. Treatment 1 involved: 2-hourly i.v. injections (approximately 3.0 ml) of bGH (15 micrograms/kg; 1.8 units/mg); treatment 2: 2-hourly i.v. injections (approximately 3.0 ml) of hpGRF-44 (0.6 microgram/kg); treatment 3: 2-hourly i.v. injections (3.0 ml) of the vehicle. Treatment periods were separated by 10 days. Sheep were milked twice daily and the milk was analysed for fat, protein and lactose. Blood samples (5.0 ml) were taken before and at 15, 45, 75 and 100 min after every third injection throughout the 4 days. Plasma was analysed for insulin, glucose, urea and non-esterified fatty acids (NEFA). The changes in plasma GH stimulated by hpGRF-44 were consistent and repeatable throughout the 4 days of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

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 less than 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 less than 0.05) and was clearly more persistent for all GRF treatments in animals fed the reduced diet (P less than 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.(ABSTRACT TRUNCATED AT 250 WORDS)

It has recently been demonstrated that human pancreatic GH-releasing factor (hpGRF-44) and Tyr-D-Trp-Ala-Trp-D-Phe-NH2 (subsequently referred to as 'the peptide') release GH from rat pituitary glands maintained in vitro and, in the former case, increase circulating GH in rats and man. The commercial importance of discovering an agent capable of specifically enhancing GH secretion in ruminants stimulated the present study which examined: the intravenous administration of both peptides on plasma GH, prolactin, insulin, glucose, urea and non-esterified fatty acids in goats and the effect of the peptide on the release of GH from sheep pituitary glands maintained in vitro. The peptide was injected into the jugular vein of goats in three different forms and at several concentrations (dispersal by shaking, 0.07 microgram/kg; 0.7 microgram/kg; ball-milled, 7.0 micrograms/kg, 70 micrograms/kg; dimethyl sulphoxide (5%), 7.0 micrograms/kg, 70 micrograms/kg). None of the treatments stimulated a significant increase in circulating GH. Nevertheless the peptide (20 micrograms/ml medium) was found to stimulate a 50-60% increase in the production of GH from sheep pituitary glands maintained in vitro. The effect of intravenously injecting hpGRF-44 (1.0 microgram/kg) was investigated in the present and absence of passive immunization with sheep anti-somatostatin immunoglobulin G (IgG) (a bolus of 600 mg, 3 h before treatment with hpGRF-44). Plasma GH was increased (P less than 0.001) within 15 min of treatment and the magnitude of the response was the same for both the immunized and non-immunized goats. A second peak was measured after approximately 75 min which was only significant (P less than 0.05) in the immunized group.(ABSTRACT TRUNCATED AT 250 WORDS)

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