Effect of a protein-free diet on muscle protein turnover and nitrogen conservation in euthyroid and hyperthyroid rats

Abstract

Although protein turnover in skeletal muscle is increased in hyperthyroidism and decreased in hypothyroidism, a deficient protein intake tends to increase serum T3 (tri-iodothyronine) while decreasing muscle protein turnover. To determine whether this diet-induced decrease in protein turnover can occur independent of thyroid status, we have examined muscle protein turnover and nitrogen conservation in hyperthyroid rats fed on a protein-free diet. After inducing hyperthyroidism by giving 20 micrograms of T3/100g body wt. daily for 7 days, groups of euthyroid and hyperthyroid animals were divided into subgroups fed on basal and protein-free diets. Muscle protein turnover was measured by N tau-methylhistidine excretion and [14C]tyrosine infusion. Urinary nitrogen output of euthyroid and hyperthyroid animals fed on the protein-free diet was also measured. Although hyperthyroidism increased the baseline rates of muscle protein synthesis and degradation, it did not prevent a decrease in these values in response to protein depletion. Furthermore, hyperthyroid rats showed greatly decreased nitrogen excretion in response to the protein-free diet, although not to values for euthyroid rats. These findings suggest that protein depletion made the experimental animals less responsive to the protein-catabolic effects of T3.

Effects of Dietary Protein Restriction on the Fractional Rates of Protein Synthesis in Perfused Rat Hindlimb.

“…The more rapid fall in the synthesis would be probably due to younger age of rats used in vivo, suggesting that the age of the animal may influence the response of skeletal muscle to protein deficiency. Similar suppressions of muscle protein synthesis have been observed in vivo in protein-deficient rats (4,7,21). It is clear that the overall changes in rates of protein synthesis obtained in the perfused hindlimb are consistent with those measurements in vivo.…”

“…If, therefore, dietary protein is withheld, the effects of hormones and muscular activity would be questioned. O n the other hand, the rates of protein synthesis in skeletal muscle have been obtained in the whole animal by constant infusion (7,8) or a large dose of labeled amino acid in vivo (3,9) and in isolated muscles by incubation in medium with labeled amino acid in vitro (3,10). In contrast to the number of the reports in vivo and in vitro, there have been few in the perfused rat hindlimb preparation (11, 12).…”

Nutrient Regulation of Skeletal Muscle Protein Metabolism in Animals. The Involvement of Hormones and Substrates

“…Thyroid hormones increase protein synthesis in skeletal and heart muscle (Brown & Millward, 1983;Carter et al 1984;AngerPs & Hasselgren, 1987). The expression of specific genes encoding muscle proteins essential for muscle contraction and glucose uptake (GLUT 4), enzymes of glycolysis, pentose phosphate shunt and energy metabolism are affected.…”

Membrane Receptor for Thyroid Hormone: Physiologic and Pharmacologic Implications

“…This is not a useful descriptive term because such actions are usually demonstrated in the artificially defined, thyroid hormone-deprived environment to which hormone is then added; in the intact organism, however, thyroid hormone levels in tissues and blood are relatively constant. Rather than being rapid in onset, many nongenomic actions of thyroid hormone appear to contribute to basal levels of activity of a variety of proteins, including ion pumps [Ca 2+ -ATPase (14), Na,KATPase (13), Na + /H + antiporter (15)], and contribute to intracellular protein trafficking (16) and protein turnover (17). In the realms of both genomic and nongenomic actions, thyroid hormone contributes to rates of specific gene expression.…”