Extracellular Calcium Ion Depletion in Frog Cardiac Ventricular Muscle

The extracellular free [Ca++] in frog ventricular muscle strips was monitored using single-barrel calcium ion-selective microelectrodes. During trains of repetitive stimulation, a heart rate-dependent, sustained fall (depletion) of the extracellular free [Ca++] occurs, which is most likely a consequence of net Ca++ influx into ventricular cells. The magnitude of the [Ca++]0 depletion increases for higher Ringer's solution [Ca++], and is reversibly blocked by manganese ion. Prolonged repetitive field stimulation (20-30 min) activates additional cellular Ca++ efflux, which can balance the additional Ca++ influx caused by stimulation, resulting in abolition of extratrabecular [Ca++]0 depletion in 20-30 min, and hence zero net transmembrane Ca++ flux at steady state. In the poststimulation period of quiescence, cellular Ca++ efflux persists and causes an elevation (accumulation) of the extracellular free [Ca++]. From these [Ca++]0 depletions, quantitative estimates for the net transmembrane Ca++ flux were derived using an analytical solution to the diffusion equation. In the highest Ringer's solution [Ca++] used (1 mM) the calculated net increase of the total intracellular calcium per beat was 6.5 +/- 1.4 mumol/l of intracellular space. This corresponds to an average net transmembrane Ca++ influx of 0.81 +/- 0.17 pmol/cm2/s during the 800-ms action potential. In lower bath [Ca++] the net transmembrane [Ca++] flux was proportionately reduced.