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

¹ Department of Physiology, University of Leicester, Leicester LE1 7RH

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 Ca²⁺ 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 Ca²⁺. 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 Ca²⁺ and the regulatory proteins.

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