Metabolic responses induced by isoproterenol in ractopamine-fed pigs

Ractopamine at 0 or 20 mg/kg and protein at 130 or 170 g/kg diet were used in a 2 x 2 factorial experiment arranged in six randomized complete blocks of 64-kg pigs. On d 21, pigs were intravenously administered 1 microgram isoproterenol/kg body wt, and blood was sampled at -60, -30, 0, 5, 10, 15, 20, 30, 60, 90 and 120 min relative to injection. Subcutaneous adipose tissue samples were used to measure in vitro lipolytic and lipogenic responses to isoproterenol. Regardless of dietary treatment, isoproterenol challenge induced a rapid increase in plasma glucose, nonesterified fatty acids and lactate concentrations. Pigs fed ractopamine in a 170 g protein/kg diet had a greater plasma nonesterified fatty acid response to isoproterenol challenge than pigs fed the same dietary protein but without ractopamine. There was an attenuation of plasma glucose and lactate responses to isoproterenol by chronic ractopamine feeding at both protein concentrations. Ractopamine feeding had no effect on basal lipolysis and lipogenesis in vitro. Lipolysis was elevated and lipogenesis was depressed by isoproterenol. The results suggest that ractopamine, when it improves performance and carcass composition, elicits a shift in metabolism involving increased use of fat for oxidation and potentiates the responsiveness of the adipose tissue to lipolytic cues.