Growth Hormone Receptor Gene Expression in Porcine Skeletal and Cardiac Muscles Is Selectively Regulated by Postnatal Undernutrition

During mild postnatal undernutrition, growth hormone receptor (GHR) mRNA abundance decreases in liver but increases in longissimus dorsi muscle. We tested the following hypotheses: 1) GHR gene expression is related to the metabolic and contractile characteristics of different muscles, and 2) the GHR response to nutrition depends on muscle type. Eight pairs of littermate pigs were weaned at 3 wk and given an optimal [60 g/(kg·d)] or low [(20 g/(kg·d)] food intake for the next 3 wk. All pigs grew, but at a slower rate in the low food intake group (P < 0.001). Functionally distinct muscles were assessed for GHR mRNA (RNase protection analysis), oxidative myofibers (succinate dehydrogenase histochemistry) and type I slow myofibers (myosin immunocytochemistry). There were striking muscle-specific differences in GHR gene expression (P < 0.001) and in its regulation by nutritional status. Relative expression of GHR mRNA in the optimal food intake group occurred in ascending order as follows: longissimus < diaphragm ≈ rhomboideus < cardiac < soleus. There was a positive correlation with the proportion of oxidative myofibers (P < 0.001) but not with type I myofibers (P > 0.10). Compared with the high intake pigs, hepatic GHR mRNA was downregulated in the low intake pigs by 59% (P < 0.01), whereas in the four muscles examined it was upregulated as follows: longissimus, 124% (P < 0.05); rhomboideus, 19% (P > 0.4); soleus, 65% (P < 0.05); cardiac, 51% (P < 0.05). Moreover, the proportion of skeletal muscle fibers with high oxidative capacity was also greater in the low intake group (P < 0.05). We conclude that postnatal GHR gene expression and its regulation by mild undernutrition are related to the metabolic, contractile and specific functional properties of different muscles.