Is potassium co-transported by the cardiac Na-Ca exchange?

It has been suggested that the stoichiometry of the electrogenic Na-Ca exchange is 3Na:1Ca. Recently, however, it was reported in rod outer segments that the stoichiometry of Na-Ca exchange is not 3Na:1Ca but 4Na:1Ca + 1K. In cardiac cells, the reversal potential has always been measured in the absence of K or at a very low K concentration. We have, therefore, re-examined the reversal potential of the Na-Ca exchange current by whole-cell voltage clamp in single guinea-pig ventricular cells in the presence of K on both sides of the membrane. The Na-Ca exchange current reversed at potentials close to the calculated values for 3Na:1Ca stoichiometry even in the presence of K. The magnitude of the Na-Ca exchange current did not change in 1 and 10 mM [K]o. We therefore conclude that K is not co-transported by cardiac Na-Ca exchange and its stoichiometry is 3Na:1Ca.