Sodium/calcium exchange in ventricular muscle

Ventricular cells possess two Ca extrusion mechanisms, a Na/Ca exchange system and a Ca pump. Reversing the exchanger by extracellular Na removal causes Na]i to decrease, and the cells take up mmolar quantities of calcium. Since Ca]i shows only a marginal increase the calcium load must be buffered. The capacity of the SR is limited so the mitochondria probably buffer a large part of this load. However, when Ca uptake into the mitochondria is blocked, the gain in Ca is still mmolar and the increase in Ca]i still marginal, suggesting an additional buffering site. Measurements of the Na/Ca stoichiometry on sarcolemmal vesicles gave a value of 3, but in ventricle values of around 2.5 or 3 are found. Reasons for this are discussed, as are the differences amongst the different methods of Ca measurement. The interaction of the sarcolemmal Ca pump and the exchanger are considered and it is suggested they could interact via Na]i. At rest both systems could remove Ca from the cell but on a large perturbation the Na/Ca exchange would be the more important of the two.