THE DEPENDENCE OF THE RELAXATION OF TENSION OF FROG ATRIAL TRABECULAE ON THE SODIUM‐CALCIUM EXCHANGE: A VOLTAGE‐CLAMP STUDY

The dependence of the relaxation of tension of isolated frog atrial trabeculae, upon membrane potential, [Na] o and [Ca] o , has been studied under voltage‐clamp conditions. The change in tension following the repolarization of the membrane potential can be resolved into two phases: an initial phase which opposes relaxation and has an exponential time constant of about 80 ms and is unaffected by changes in either membrane potential, [Na] o or [Ca] o ; and a subsequent exponential fall in tension, the rate of which is slowed by depolarization, raised [Ca] o or lowered [Na] o . The dependence of the second phase of relaxation upon the membrane potential is consistent with Ca 2+ being removed from the sarcoplasm by an Na‐Ca exchange in the cell membrane which has a coupling ratio close to 3 Na + for each Ca 2+ . To explain the full effects of changes in [Na] o and [Ca] o upon relaxation with the same Na‐Ca exchange stoicheiometry it is necessary to assume that these changes in the bathing fluid affect [Na] i and that relaxation is dependent upon a single unbinding step, involving Ca 2+ and the regulatory proteins.