REGULATION OF PORCINE INSULIN-LIKE GROWTH-FACTOR-I AND GROWTH-HORMONERECEPTOR MESSENGER-RNA EXPRESSION BY ENERGY STATUS

Regulation of insulin-like growth factor I (IGF-I) and growth hormone (GH) receptor mRNA in Liver and muscle by energy status was assessed i n 2-mo-old pigs by altering thermoregulatory demand and energy intake over a 5-wk period to produce a range of plasma IGF-I concentrations f rom 3.5 +/- 0.7 to 28.9 +/- 6.2 nmol/l. These values were related dire ctly to growth rates (0.06 +/- 0.02 to 0.44 +/- 0.01 kg/day) and total hepatic IGF-I mRNA levels. Increased growth rates were accompanied by an increase in hepatic class 1 and class 2 IGF-I mRNA levels and an i ncrease in the ratio of class 2 to class 1 IGF-I mRNA in liver, sugges ting a distinct role for class 2 expression in the endocrine growth re sponse. High levels of class 1 transcripts and a virtual absence of cl ass 2 transcripts characterized all muscle tissues examined, and there was no correlation with plasma IGF-I levels. This suggests that growt h promotion in response to increased energy status is regulated via en docrine hepatic IGF-I rather than via a paracrine response. The levels of GH receptor mRNA were positively correlated with overall growth ra te (P < 0.005) in liver and negatively correlated (P < 0.05) in muscle , indicating distinct tissue-specific effects of energy status.