Abstract
Plasma Growth hormone (GH) profiles, secretion rates (SR), metabolic clearance rates (MCR) of intravenously infused GH, and response to 0.0165, 0.067, and 0.267 μg of growth hormone-releasing factor (GHRF)/kg body weight (bwt) were studied in intact and castrated male and female cattle (4 of each) at 5, 8, 12, and 15 mo of age. As cattle aged, overall GH levels in the plasma decreased (P<.01), GH baseline concentrations declined (P<.05), SR decreased (P<.01), half-life of GH decreased (P<.01), and the GH response to intravenous injections of GHRF declined (P<.05). Bulls had elevated overall and baseline GH concentrations (P<.05), greater amplitude and number of GH secretory periods (P<.01), and greater SR (P<.05) than steers, heifers, or ovariectomized heifers. Males had more frequent GH spikes and secretory periods of higher amplitudes, greater SR, and greater GH responses to 0.0165 μg GHRF/kg bwt (P<.05) than females. Castration had no impact on the GH status of heifers. It is suggested that testosterone or its metabolites enhanced GH secretion in bulls.
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