Nutritional regulation of growth hormone receptor gene expression.

The role of energy intake in regulating growth hormone receptor (GHR) gene expression has been assessed in young growing pigs living at thermal neutrality (26 degrees C) for a 4-wk period. To determine the importance of altering metabolic demand while maintaining food intake constant, littermates were also studied in a cold environment (10 degrees C). Results were tissue-specific: the level of GHR mRNA per unit total RNA in liver was greater on high than low (high = 2 x low) food intake (P < 0.001), whereas in muscle it was elevated on the low compared with the high intake diet (P < 0.02) and also at 10 degrees C compared with 26 degrees C (P < 0.04). When results for GHR mRNA were expressed per unit weight of tissue, only the effects of diet on liver and temperature on muscle remained significant. The changes in hepatic GHR mRNA may have been driven in part by nutritionally induced changes in thyroid status, because both plasma 3,5,3'-triiodothyronine concentration and liver 5'-deiodinase activity were greater on the high than the low intake diet (P < 0.001). Levels of liver GHR mRNA probably had a direct influence on growth of the animals, as they were positively correlated with plasma IGF-I and growth rate (P < 0.001), whereas muscle GHR mRNA may have had a metabolic role when energy supplies were limited.