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(+), with the highest maximum I (P) and lowest Na(+) 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 Ca(2+) channels less outward current through Ca(2+)-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(+), with I (ibCa) = 0, should create a gradient in Ca(2+). To test this hypothesis, we measured resting Ca(2+) using two methods, based on either transport or the Ca(2+)-sensitive dye Fura2. Both methods demonstrated a significant transmural gradient in resting Ca(2+), with Endo > Mid > Epi. This gradient was eliminated by exposing Epi to sufficient ouabain to partially inhibit Na/K pumps, thus increasing Na(+) to values similar to those in Endo. These data support the existence of a transmural gradient for Ca(2+) removal by NCX. This gradient is not due to differences in expression of NCX; rather, it is generated by a transmural gradient in Na(+), which is due to a transmural gradient in plasma membrane expression of the Na/K pump.