THE EFFECT OF EXCHANGER INHIBITORY PEPTIDE (XIP) ON SODIUM CALCIUM EXCHANGE CURRENT IN GUINEA-PIG VENTRICULAR CELLS

We investigated the effect of exchanger inhibitory peptide (XIP) on Na-Ca exchange current (I(Na-Ca)) in guinea pig ventricular cells. Cells were voltage-clamped with microelectrodes containing 20 mM Na+ and 14.0 mM EGTA ([Ca]i=100 nM). An outward putative exchange current was stimulated when extracellular Na+ was reduced from 144 mM to zero (Li+ replaced Na+). This outward current showed a significant dependence on extracellular Ca2+. When Na+ removal was delayed for up to 40 minutes (in the absence of extracellular K+ or the presence of 3.0 mM ouabain to block the Na+ pump), outward I(Na-Ca) increased presumably because [Na]i increased. Time-dependent increases of outward current in the absence of K+ could be abolished by reapplication of K+, which presumably reactivates the Na+ pump and reduces intracellular Na+. This effect is blocked in the presence of 3.0 mM ouabain. The dependence of this current on extracellular Ca2+, its dependence on intracellular Na+, and activation by extracellular Na+ reduction, together with its resistance to ouabain all suggest that it is a Na-Ca exchange current. After dialyzing the cell with 10 muM XIP, outward I(Na-Ca) was largely abolished. This indicates that XIP, which is a rather large molecule, can enter the heart cell via the microelectrode in sufficient quantities to inhibit exchange. Inward I(Na-Ca) was blocked secondary to the blockade of outward I(Na-Ca). L-type Ca2+ current (I(Ca)) was not measurably affected by XIP. It appears that XIP might be successfully used to separate the contribution of I(Na-Ca) and I(Ca) to excitation-contraction coupling and the regulation of contraction in isolated heart cells.