Effect of somatotropin on adipose tissue metabolism: ontogeny of the enhanced response to adrenergic challenge in the lactating cow.

Seven multiparous Holstein cows (> 150 d postpartum) were used to evaluate the time course of the chronic adaptation in lipolytic response to adrenergic challenge with bovine somatotropin (bST) treatment. Cows received daily bST (sometribove; 40 mg/d) or excipient injections for 7 d (single reversal design) with a 7-d interim between periods. Epinephrine challenges (1.4 microgram/kg body weight intravenously) were administered on Days -2, -1, 1, 2, 3, 5, and 7 of treatment at 10:00 a.m. (15 hr after bST or excipient injection). Frequent blood samples were collected, and concentrations of plasma glycerol (GLY) and nonesterified fatty acids (NEFA) were determined. Treatment with bST increased milk yield 23% (P < 0.05) and milk fat content 33% (P < 0.001) compared with controls. Somatotropin-treated cows entered negative energy balance by Day 3 and had higher basal plasma concentrations of GLY and NEFA than did controls by Day 2 and Day 3, respectively. Response to epinephrine, expressed as area under the response curve corrected for basal, was enhanced by bST treatment, regardless of energy balance. GLY response was greater than control by Day 1 of bST treatment (P < 0.01), and had plateaued by Day 2 (P < 0.001). The NEFA response area was higher than control and had plateaued by Day 1 of bST treatment (P < 0.001). Day 1 represented 15 hr after the first bST injection. Results illustrate that bST treatment results in enhanced in vivo lipolytic response to catecholamine challenge, and the metabolic adaptation is in place by 15 hr after the first bST injection.