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 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(+)]( i ), with I (ibCa) = 0, should create a gradient in [Ca(2+)]( i ). To test this hypothesis, we measured resting [Ca(2+)]( i ) 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+)]( 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 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(+)]( i ), which is due to a transmural gradient in plasma membrane expression of the Na/K pump.
Wang
Wei
W
Department of Physiology and Biophysics, SUNY at Stony Brook, NY 11794-8661, USA.
Gao
Junyuan
J
Entcheva
Emilia
E
Cohen
Ira S
IS
Gordon
Chris
C
Mathias
Richard T
RT
D000818:Animals; D015153:Blotting, Western; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D004285:Dogs; D006352:Heart Ventricles / Q000378:metabolism; D008954:Models, Biological; D032342:Muscle Cells / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium; D002118:Calcium