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 μM 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.