The effect of Exchanger Inhibitory Peptide (XIP) on sodium-calcium exchange current in guinea pig ventricular cells

Abstract

We investigated the effect of exchanger inhibitory peptide (XIP) on Na-Ca exchange current (INa-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 INa-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 μM XIP, outward INa-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 INa-Ca was blocked secondary to the blockade of outward INa-Ca. L-type Ca2+ current (ICa) was not measurably affected by XIP. It appears that XIP might be successfully used to separate the contribution of INa-Ca and ICa to excitation-contraction coupling and the regulation of contraction in isolated heart cells.