SELECTIVE REGULATION OF MYOFIBER DIFFERENTIATION BY ENERGY STATUS DURING POSTNATAL-DEVELOPMENT

The role of energy status in postnatal regulation of porcine skeletal muscle development has been determined in littermate animals kept for 3-4 wk on a high (H) or low (L) energy intake (H = 2L), at a thermally neutral [26 degrees C (26H and 26L, respectively)] or low [10 degrees C (10H and 10L, respectively)] environmental temperature. A variety o f skeletal muscles was assessed at 7 wk. of age for changes in myofibe r hypertrophy and differentiation. In contrast with findings in adult humans and rats, there was no selective preservation of type I slow-ox idative fiber size during energy restriction. However, differentiation between mature skeletal myosin heavy-chain isoforms was markedly affe cted by energy status, and in rhomboideus there were particularly stri king effects of both nutrition and temperature: proportions of type I fibers from the four groups 26H, 26L, 10H, and 10L were 34 +/- 2, 50 /- 4, 73 +/- 2, and 72 +/- 3 (P < 0.005 for diet at 26 degrees C; P < 0.001 for temperature). These changes may have been induced by alterat ions in both thyroid status and contractile activity. They support the hypothesis of a key role for rhomboideus muscle in thermoregulation a nd demonstrate the plasticity of skeletal muscle differentiation to en vironmental change during postnatal life.