A Transmural Gradient in the Cardiac Na/K Pump Generates a Transmural Gradient in Na/Ca Exchange

We previously demonstrated a transmural gradient in Na/K pump current (I _P) and Na⁺] _i , with the highest maximum I _P and lowest Na⁺] _i in epicardium. The present study examines the relationship between the transmural gradient in I _P and Na/Ca exchange (NCX). Myocytes were isolated from canine left ventricle. Whole-cell patch clamp was used to measure current generated by NCX (I _NCX) and inward background calcium current (I _ibCa), defined as inward current through Ca²⁺ channels less outward current through Ca²⁺-ATPase. When resting myocytes from endocardium (Endo), midmyocardium (Mid) or epicardium (Epi) were studied in the same conditions, I _NCX was the same and I _ibCa was zero. Moreover, Western blots were consistent with NCX protein being uniform across the wall. However, the gradient in Na⁺] _i , with I _ibCa = 0, should create a gradient in Ca²⁺] _i . To test this hypothesis, we measured resting Ca²⁺] _i using two methods, based on either transport or the Ca²⁺-sensitive dye Fura2. Both methods demonstrated a significant transmural gradient in resting Ca²⁺] _i , with Endo > Mid > Epi. This gradient was eliminated by exposing Epi to sufficient ouabain to partially inhibit Na/K pumps, thus increasing Na⁺] _i to values similar to those in Endo. These data support the existence of a transmural gradient for Ca²⁺ removal by NCX. This gradient is not due to differences in expression of NCX; rather, it is generated by a transmural gradient in Na⁺] _i , which is due to a transmural gradient in plasma membrane expression of the Na/K pump.