Primary role of sarcoplasmic reticulum in phasic contractile activation of cardiac myocytes with shunted myolemma

Homogeneous populations of single myocytes showing good preservation of ultrastructure were obtained by enzymatic digestion of rabbit and rat hearts, and maintained in a relaxed state in the presence of free Ca2+ concentrations less than 10(-7) M. Ultrastructural details such as a cytoskeleton of 100-A filaments connected to the sarcolemma at the Z lines were demonstrated especially well in these preparations. In spite of seemingly normal structure, electron probe analysis of cryosections reveals similar concentrations of electrolytes in the medium and in the cytoplasm, indicating the presence of electrochemical shunting across the external membrane. The dissociated myocytes display Ca uptake and phasic contractions that are apparently dependent on mitochondrial respiration, but are not affected by mitochondrial uncouplers when ATP and phosphocreatine are added. The uptake is augmented by oxalate and, based on identification of calcium oxalate crystals by electron microscopy and electron probe analysis, is localized to the sarcoplasmic reticulum (SR). An advantageous feature of the dissociated myocytes is that they are suitable for experiments using large numbers of cells in suspension. Thereby, velocities of calcium transport were measured directly by isotopic tracer and filtration methods. It was then found that the lowest CA2+ concentrations (5 x 10(-7) M for the rabbit and 1 x 10(-7) M for the rat) sustaining Ca transport also induce phasic contractile activity in all myocytes, even though the external membrane is electrochemically shunted. A stepwise rise in the Ca2+ concentration of up to one order of magnitude, increases transport velocities in parallel with the rates of phasic contractions. Both these parameters are affected by Mg2+, temperature, cyclic-AMP, and methylxanthines, even though the Ca2+ concentration is maintained constant in the medium. Therefore, Ca transport by SR is a requirement and a rate limiting factor for the occurrence of phasic contractile activation in dissociated cardiac cells retaining an electrochemically shunted external membrane. It is suggested that transient Ca release required for phasic contractile activation is due to equilibrium oscillations across the SR membrane. The sequential pattern of sarcomere activation is consistent with a self propagating mechanism of calcium release. SR in dissociated skeletal muscle cells sustains a greater Ca transport activity than in dissociated heart cells. However, the heart cells display a much higher phasic contractile activity, indicating that cardiac SR has a greater tendency to release accumulated calcium. If free Ca2+ in the medium is raised above 10(-6) M, both cardiac and skeletal myocytes undergo contractures and degenerative phenomena, accompanied by Ca, Mg, and phosphate accumulation in cardiac mitochondria.