Intracellular proteolysis in rat cardiac and skeletal muscle cells in culture

Abstract

Treatment of adult rats with dexamethasone resulted in an increase in cardiac muscle weight but a decrease in skeletal muscle weight. The different response of skeletal and cardiac muscles to the glucocorticoid was also reflected by a dexamethasone‐induced enhancement of myofibrillar protease activity in the gastrocnemius muscle and an inhibition of a similar proteolytic activity in the heart. Newborn rats also exhibit the same, tissue‐specific response to the glucocorticoid hormone. Consequently, the difference between cardiac and skeletal muscle responsiveness to conditions of wasting was investigated in culture. Average rates of degradation of intracellular proteins were determined in cultured cells derived from rat skeletal and cardiac muscle by following the release of radioactivity from cells prelabelled with ¹⁴C‐phenylalanine. The release of label into the TCA soluble medium as measured during 12 hours of incubation, conformed to a first‐order reaction and both cell types were found to degrade intracellular proteins at a similar rate. After 12 hours of incubation in a complete Ham F‐10 medium supplemented with serum approximately 18% of total cellular protein was degraded. Incubation in a minimal medium or serum‐deprivation enhanced the average rate of proteolysis to a value of 29% degradation at 12 hours indicating that intracellular proteolysis in these cells is responding to nutritional deprivation by increased activity. However, addition of glucose (22.2 nM) or dexamethasone (10⁻⁶M) to the incubation medium failed to affect the rate of net protein degradation. Under no experimental condition could a difference be found between the proteolytic response of skeletal muscle cells to that of cardiac muscle cells and both cell types displayed similar changes in rates of protein degradation under various nutritional and hormonal conditions in culture. Thus, protein sparing in the heart of intact animals under catabolic conditions which enhance protein loss in skeletal muscle can probably not be ascribed to intrinsic differences in the direct response of cellular proteases to the tested hormones and nutrients. Rather, an extracellular factor(s) is apparently required for induction of the differential response of these tissues in the intact animal to protein wasting conditions. Alternatively, cells in culture might have lost the property of differential degradative response which operates in vivo.