The effect of intake level on whole body kinetics and hepatic removal of somatotropin in growing beef steers

The effect of level of intake of a high concentrate diet (0.6, 1.0 and 1.6 X maintenance requirements, M) on whole body somatotropin (St) kinetics was evaluated in six growing, multicatheterized beef steers (398 +/- 27 kg), u sing a double 3 X 3 Latin Square design with 21 d-periods. Simultaneously t o St kinetics, net hepatic removal of St was measured in 4 of the 6 steers. On the last day of each period, concentrations and net fluxes of St were d etermined, first in basal conditions for 5 hr, and then, during a primed (0 .5 mg of St) infusion of bovine St (1.5 mg/hr) administered for 3 hr. The f ollowing results are LSM +/- SEM for 0.6, 1.0, and 1.6 X M, respectively. I ncreasing feed intake linearly decreased (P < 0.01) basal St concentrations (5.6, 4.6, 3.1 +/- 0.62 ng/ml), mainly through a linear increment (P < 0.0 1) in the metabolic clearance rate (32.7, 37.1, 43.4 +/- 2.60 l/hr), althou gh secretion rate also tended to decrease (P = 0.09; 189, 185, 135 +/- 27.2 mu g/hr). During the infusion period, net liver removal of immunoreactive ST averaged 60% of the total inflow of St. This confirms the liver is capab le of removing large amounts of St, suggesting it has an important role in metabolic clearance of the hormone. Net liver removal of St, however, was n ot affected by intake. There was a strong correlation between the metabolic clearance rate of St with either whole body protein synthesis (r = 0.75, P < 0.01) or protein retention (r = 0.68, P < 0.01). Together these results indicate the importance of postsecretory metabolism of St in determining bo th arterial plasma concentrations of St and whole body protein anabolism. ( C) 2000 Elsevier Science Inc. All rights reserved.