Calcium ion-selective microelectrodes made with Simon's neutral carrier were used to measure simultaneously sarcoplasmic Ca2+ activity (aiCa) and resting tension (Tr) of rabbit ventricular muscle during reduction and restoration of external sodium ion concentration, [Na]0. Under the same experimental conditions the change in contractile tension (Ta) also measured. In resting muscle the aiCa was 38 +/- 17 nanomolar (mean +/- standard deviation; N = 10). The reduction of [Na]O from 153 to 20 millimolar led to about a threefold increase in aiCa with parallel increases in Tr and Ta. The time course of the change in aiCa was similar to that of the changes in Tr and Ta. The results are consistent with an important role of the sodium-calcium exchange system for regulating sarcoplasmic Ca2+ activity.

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Intracellular sodium was estimated in ventricular myocytes using the new Na-sensitive fluorescent indicator SBFI. Membrane potential and contraction were also measured simultaneously. Using an in situ calibration method, we found that intracellular sodium activity (aiNa) was 2.9 mM in quiescent rabbit cells. When the digitalis analogue strophanthidin inhibited the Na-K pump of myocytes with action potentials (APs), changes of contraction and aiNa were dissociated in time. There was also marked hysteresis between contraction and aiNa. When strophanthidin was applied to the same myocytes under voltage-clamp conditions, temporal dissociation between contraction and aiNa was dramatically reduced. This suggests that much of the dissociation and hysteresis was due the change in AP shape with strophanthidin. A small amount of residual hysteresis still existed even with voltage-clamp, and this persisted when the pump was blocked by removal of external potassium as an alternative method. We suggest that a gradient of sodium concentration from the subsarcolemmal space to the bulk cytoplasm might be responsible for hysteresis. Whereas SBFI probably signals the average Na level of the cytoplasm, subsarcolemmal Na may control Ca influx and contraction via Na-Ca exchange.

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