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]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.
D000818:Animals; D016275:Atrial Function; D002118:Calcium / Q000502:physiology*; D003593:Cytoplasm / Q000502:physiology; D007700:Kinetics; D008564:Membrane Potentials; D009126:Muscle Relaxation; D009200:Myocardial Contraction; D011898:Ranidae; D012964:Sodium / Q000502:physiology*
D012964:Sodium; D002118:Calcium