Transient tension responses of voltage-clamped frog atrial muscle related to sudden changes in external Ca or Na

Frog atrial muscle strips were placed in a double sucrose gap chamber and perfusion of the central node was arranged as to allow rapid changes of external Ca or Na concentration during long-lasting (15 s) depolarizing clamps. When the superfusing fluid was suddenly switched, the intercellular space inside the fibre bundle equilibrated with a time constant in the order of 1 s. A fast change of perfusate during clamp was followed by a delayed change of tonic (sustained) contraction. When [Ca]0 was increased from 0.25 to 4 mM, tension rose in a sigmoid manner and the level reached at the end of the clamp was almost identical with the steady control in 4 mM-Ca-Ringer. A similar tension increase occurred upon a reduction of [Na]0 from 100 to 25% of normal. At a given depolarization time course and height of the tension responses followed the ratio of [Ca]0/[Na]20. Transient tension responses are interpreted in terms of a sudden perturbation of a transmembrane Ca-Na exchange system leading to a depression of Ca pumping activity.