Contractile activity modulates actin synthesis and turnover in cultured neonatal rat heart cells

In this study, the role that active tension development plays in the formation and maintenance of cardiac myocyte myofibrillar structure and cellular shape was investigated. By use of the calcium channel blocker verapamil, spontaneous contractile activity of neonatal rat heart myocytes was inhibited for 24 to 96 hours. Confocal microscopy of rhodamine phalloidin-stained cells revealed that, within 24 hours of contractile arrest, actin filaments of myofibrils were no longer aligned with one another at their I bands and Z lines. Cellular shape was also affected, with the cells developing a less stellate appearance while remaining attached to the substrate as well as to one another. By 48 hours, actin fibrils were largely absent from these cells. The disappearance of actin was confirmed by measurements of actin synthesis and accumulation rates and by pulse-chase biosynthetic labeling experiments. It was revealed that, although actin synthesis was significantly reduced in arrested myocytes, the rapid disappearance of total cellular actin was largely due to increased rates of actin degradation. Contractile arrest produced by L-type calcium channel blockade with verapamil (or other calcium channel blockers) accelerated actin degradation to a greater extent than K+ depolarization. Chloroquine partially suppressed the accelerated rate of actin degradation, indicating that lysosomal proteolysis may be involved in actin degradative processing. Protein kinase C activation also partially inhibited the accelerated rate of actin degradation but did not restore actin filaments in arrested myocytes. The reformation of actin fibrils and their reassembly into striated myofibrils occurred when contractile activity was restored by removal of verapamil from the culture medium. The period of time required for myocytes to reassemble actin filaments and to regain their elongated morphology was proportional to the period of time that the cells were inhibited from contracting. Data are presented to indicate that active tension development by neonatal cardiac myocytes in culture is critical to the maintenance of filamentous actin structure via mechanisms involving actin assembly, disassembly, and degradation.