MYOSIN HEAVY-CHAIN TURNOVER IN CULTURED NEONATAL RAT-HEART CELLS - EFFECTS OF [CA2+](I) AND CONTRACTILE ACTIVITY

Blockade of L-type Ca2+ channels in spontaneously contracting cultured neonatal rat ventricular myocytes causes contractile arrest, myofibri llar disassembly, and accelerated myofibrillar protein turnover. To de termine whether myofibrillar atrophy results indirectly from loss of m echanical signals or directly from alterations in intracellular Ca2+ c oncentration ([Ca2+](i)), contractile activity was inhibited with vera pamil (10 mu M) or 2,3-butanedione monoxime (BDM), and their effects o n cell shortening, [Ca2+](i), and myosin heavy chain (MHC) turnover me re assessed. Control cells demonstrated spontaneous [Ca2+](i) transien ts (peak amplitude 232 +/- 15 nM, 1-2 Hz) and vigorous contractile act ivity. Verapamil inhibited shortening by eliminating spon taneous [Ca2 +](i) transients. Low concentrations of BDM (5.0-7.5 mM) had no effect on basal or peak [Ca2+](i) transient amplitude but reduced cell short ening, whereas 10 mM BDM reduced both [Ca2+](i) transient amplitude an d shortening. Both agents inhibited MHC synthesis, but only verapamil accelerated MHC degradation. Thus MHC half-life does not change in par allel with contractile activity but rather more closely follows change s in [Ca2+](i). [Ca2+](i) transients appear critical in maintaining my ofibrillar assembly and preventing accelerated MHC proteolysis.