REGULATION OF RAT VENTRICULAR MYOSIN HEAVY-CHAIN EXPRESSION BY SERUM AND CONTRACTILE ACTIVITY

To quantitatively analyze the effects of serum stimulation and contrac tile activity and their interaction on cellular growth and cardiac myo sin heavy chain (MHC) gene expression, spontaneously contracting neona tal rat ventricular myocytes in primary culture were maintained in ser um-free growth medium or growth medium supplemented with fetal bovine serum. Contractile activity in paired cultures was inhibited by additi on of the calcium channel blocker verapamil (10 mu M) to the culture m edium. Both serum stimulation and contractile activity produced myocyt e hypertrophy as assessed by increases in total protein, total RNA, pr otein-to-DNA ratios, and total MHC protein content. MHC isoenzyme anal ysis indicated that both MHC-alpha and MHC-beta proteins accumulated i n response to serum stimulation and/or contractile activity. The incre ases in MHC-beta protein resulting from serum stimulation and contract ile activity occurred in parallel with increases in MHC-beta mRNA. In contrast, MHC-alpha mRNA levels were relatively unaffected by serum st imulation but appeared to decrease in response to contractile activity . The protein kinase inhibitor staurosporine (5 nM) reduced MHC-beta e xpression in serum-free, contracting cultures and also prevented the s erum-induced increase in MHC-beta mRNA observed in both contracting an d arrested myocytes. Staurosporine also increased MHC-alpha mRNA level s in serum-free, contracting, and verapamil-arrested myocytes. These d ata suggest that both humoral and mechanical factors regulate MHC isoe nzyme expression and cellular growth in neonatal ventricular myocytes.