This preclinical study demonstrates that the relaxation of tension in frog atrial muscle is dependent on a sodium-calcium exchange mechanism with a stoichiometry of 3 Na+ per Ca2+.
Supports Na-Ca exchange in amphibian cardiac relaxation; leaves open its role in mammalian hearts and clinical calcium handling.
The dependence of the relaxation of tension of isolated frog atrial trabeculae, upon membrane potential, [Na]0 and [Ca]0, 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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Chapman et al. (1985) studied this question.
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