PLASMA GROWTH-HORMONE, INSULIN-LIKE GROWTH-FACTOR-I, AND INSULIN-LIKEGROWTH-FACTOR BINDING-PROTEINS IN PIGS WITH DIVERGENT GENETIC MERIT FOR POSTWEANING AVERAGE DAILY GAIN

Characteristics of growth hormone (GH), IGF-I, and IGF-binding protein s (IGFBP) were studied in gilts sampled from lines of pigs selected fo r either fast (line F, n = 14) or slow (line S, n = 14) postweaning AD G. Repeated blood samples were obtained from gilts (approximately 55 k g BW) during a period of feed deprivation and again during refeeding. Averaged across time, the difference in mean plasma GH concentrations of F and S gilts was not significant (7.7 vs 6.4 ng/mL; P > .20) durin g feed deprivation, and frequency, height, and amplitude of GH pulses did not differ (P > .25) for F and S pigs. Overall, F gilts had greate r concentrations of plasma IGF-I than S gilts during feed deprivation (217.3 vs 145.1 ng/mL; P < .03). Across line, plasma IGF-I decreased ( P < .01) during feed deprivation. Average GH did not differ (P > .40) for F and S gilts during the refeeding period. Average plasma IGF-I te nded (P = .05) to be greater in F gilts than in S gilts during refeedi ng. Consistent with changes over time during feed deprivation, plasma IGF-I averaged across Line increased (P < .01) in response to refeedin g. Averaged across time (0 and 48 h refeeding), activity of IGFBP-2 (s inglet band at 34 kDa) did not differ significantly (P = .17) in F and S gilts. However, there was a tendency (P = .13) for a line x time in teraction. At time 0, the lines did not differ for IGFBP-2 activity, b ut S gilts had greater (P < .01) activity than F gilts at 48 h after r efeeding. Similarly, the lines did not differ for activity of IGFBP-3 (doublet band at 40 and 44 kDa) when averaged across time, but there w as a tendency (P = .12) for a line x time interaction. At 0 h, the lin es were not different, but at 48 h after refeeding, the S gilts expres sed greater (P = .05) IGFBP-3 activity than F gilts. Response to diver gent selection for postweaning ADG has occurred, to some extent, throu gh changes in the growth hormone/IGF axis. Throughout periods of feed deprivation and refeeding, pigs selected for fast gain expressed great er concentrations of circulating IGF-I than their contemporaries selec ted for slow gain.