Species comparisons of tissue protein metabolism: effects of age and hormonal action.

Ontogenic responses in whole body and tissue protein metabolism are compared between ruminants and nonruminants. In sheep and cattle, the liver makes a smaller contribution to body protein mass and synthesis but is more responsive to age and nutrition than in the rodent. Protein synthesis increases within the gastrointestinal tract (to > 25% of total whole body protein synthesis) during the weaning phase in sheep and rats and in both species is sensitive to nutritional state. Protein synthesis in skin is also sensitive to intake but the sheep has a slower ontogenic decline compared with rats. Translational efficiency (g protein synthesis.d-1.g RNA-1) is unchanged for ovine muscle during the transition from the milk-fed to weaned state but is lower for the suckling rat. In contrast, in growing ruminants muscle translational efficiency decreases, whereas for the rat it is maintained. In young rodents, muscle protein synthesis is sensitive to nutrition and insulin, but the latter has little effect in sheep or indeed in nongrowing adult rats and humans. In ruminants the chronic response in muscle protein synthesis to intake may be exerted through the growth hormone and insulin-like growth factor 1 axis, although conclusive proof is lacking.