Characterization of Porcine Growth Hormone (pGH) Binding to Porcine Liver Microsomes: Chronic Administration of pGH Induces pGH Binding*

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Abstract
The effect that GH has on regulating GH binding to its receptors has not been resolved. This report describes the characterization of porcine (p) GH binding to pig liver membranes and clarifies the issue of regulation of GH binding by measuring pGH binding to liver membranes prepared from pigs treated daily for 35 days with different doses of pGH (0, 10, 30 or 70 .mu.g/kg BW). Specific binding of [125I]pGH was dependent on time, pH, and membrane protein concentration. At 23 C, pGH binding reached a steady state after 24 h. Maximal pGH binding was observed at pH 7. Binding increased linearly as membrane protein concentration was increased from 150 to 450 .mu.g/tube. Specificity studies indicated that the hepatic GH receptor was somatogenic, since porcine PRL poorly inhibited [125I] pGH binding (cross-reactivity, 0.1%). Treatment of microsomes from control pigs with 4 M MgCl2 to remove endogenously bound pGH did not affect pGH binding, whereas binding was significantly increased to microsomes from pGH-treated pigs. Binding of pGH increased in a linear manner with the dose of pGH given for 35 days (r = 0.79), thus establishing the inductive effect of chronic pGH administration on pGH binding in pig liver. GH binding was highly correlated with weight gain in pigs treated with pGH (r = 0.76). In addition, the serum insulin-like growth factor I (IGF-I) concentration was increased linearly (r = 0.87) by pGH. This increase in serum IGF-I was also highly correlated with the increase in pGH binding (r = 0.71). These results suggest that hepatic GH binding plays an important role in regulating pig growth, which may be mediated, in part, by an increase in hepatic IGF-I synthesis and secretion. The present report is also the first to establish that exogenous pGH induces pGH binding to pig hepatic GH receptors and to relate this increase in binding to an enhancement in pig growth.