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 of skeletal muscles was assessed at 7 wk of age for changes in myofiber hypertrophy and differentiation. In contrast with findings in adult humans and rats, there was no selective preservation of type I slow-oxidative fiber size during energy restriction. However, differentiation between mature skeletal myosin heavy-chain isoforms was markedly affected by energy status, and in rhomboideus there were particularly striking 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 alterations in both thyroid status and contractile activity. They support the hypothesis of a key role for rhomboideus muscle in thermoregulation and demonstrate the plasticity of skeletal muscle differentiation to environmental change during postnatal life.