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, myofibrillar disassembly, and accelerated myofibrillar protein turnover. To determine whether myofibrillar atrophy results indirectly from loss of mechanical signals or directly from alterations in intracellular Ca2+ concentration ([Ca2+](i)), contractile activity was inhibited with verapamil (10 mu M) or 2,3-butanedione monoxime (BDM), and their effects on cell shortening, [Ca2+](i), and myosin heavy chain (MHC) turnover mere assessed. Control cells demonstrated spontaneous [Ca2+](i) transients (peak amplitude 232 +/- 15 nM, 1-2 Hz) and vigorous contractile activity. 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 shortening, whereas 10 mM BDM reduced both [Ca2+](i) transient amplitude and shortening. Both agents inhibited MHC synthesis, but only verapamil accelerated MHC degradation. Thus MHC half-life does not change in parallel with contractile activity but rather more closely follows changes in [Ca2+](i). [Ca2+](i) transients appear critical in maintaining myofibrillar assembly and preventing accelerated MHC proteolysis.