Heart: excitation-contraction coupling

The study of E-C coupling in heart muscle has been facilitated by the recent availability of reasonably reliable voltage clamp techniques and a method of "skinning" cardiac cells. We have also had the introduction of several new ideas, including a Na:Ca exchange pump, metabolically controlled Ca storage capacity of the SR, and length dependence of Ca release. Consideration of the mechanism of E-C coupling in striated muscle as a general model has enabled transfer of insights gained studying fast skeletal muscle to heart muscle. On the other hand, many of the complexities of regulation of heart muscle contraction are manifested in fast skeletal muscle, as investigators explore the details of E-C coupling. On the whole, it is interesting to be an investigator in this field, as the E-C coupling mechanisms under investigation are being located in many nonmuscle cells, for such varied functions as control of cell shape during growth and excitation-secretion coupling. The last few years have seen the establishment of the existence and importance of a channel in the membrane that admits Ca as a function of electric field. We remain uncertain, however, of the details of relation of this current to the size of contraction. We have begun to explore the characteristics and role of the Na:Ca exchange mechanism in regulating the magnitude of intracellular Ca stores. Most investigators feel that this finally represents the necessary link in understanding digitalis action. A powerful but technically demanding tool is available in the "skinned" cardiac cell, permitting direct studies of Ca release from the SR in more-or-less intact cells. One dramatic finding with that technique is the demonstration of length-dependence of Ca release. On the horizon are methods of monitoring any possible transient potentials across subcellular organelle membranes and directly determining transient changes in free Ca in the sarcoplasm. This reviewer cannot help but feel that the next three or four years will be exciting ones in this field, and that the next review of E-C coupling will make interesting reading.