The regulation of protein turnover in newborn rat heart cell cultures.

The regulation of myocardial protein turnover was studied using newborn rat heart cells in monolayer cultures. Protein accumulation in this model system was regulated by the presence of serum in the culture medium. The influences of serum on the rates of protein synthesis and degradation were investigated. Serum deprivation or arginine deprivation did not modify protein synthesis. In contrast serum strongly inhibited the rates of [3H]leucine release from prelabeled long-lived myocardial proteins. The inhibiting serum factor was found heat-stable, displayed slight species specificity, was of plasmatic origin, and its activity was decreased in the plasma of hypophysectomized rats. Platelet growth factor, which promotes DNA synthesis in heart cell cultures, did not inhibit protein degradation. The influence of known regulators of myocardial protein balance was investigated. Leucine had no influence on protein synthesis and degradation. Supraphysiological concentrations of insulin and growth hormone inhibited protein degradation but only in the absence of serum. It is suggested that these compounds may not be the natural regulators of myocardial protein balance.