H+-induced membrane depolarization in canine cardiac Purkinje fibers

The H+-induced membrane depolarization in canine cardiac Purkinje cells in false tendons was studied. In some electrically paced Purkinje cells ("sensitive" cells), exposure to a pH 6.0 superfusate produced a large membrane depolarization [44.5 +/- 6.7 (SD) mV], whereas other Purkinje cells ("resistant" cells) developed only a small depolarization (9.8 +/- 5.6 mV) even after 60 min of exposure to the low-pH superfusate. Cs+, Ba2+, tetraethylammonium, 9-aminoacridine, verapamil, or exposure to Ca2+- or K+-deficient or hypertonic solutions were capable of converting resistant cells to sensitive cells. Increasing extracellular K+ concentration [( K+]) or rapid electrical pacing failed to convert resistant cells to sensitive cells. Membrane depolarizations of approximately equal magnitude produced in Purkinje cells by either increasing [K+] to 18.2 mM, decreasing [K+] to 0.5 mM, reducing extracellular pH to 4.1, or ouabain administration were associated with membrane resistances of approximately 45, 377, 386, or 45%, respectively, of the membrane resistances in the control solution. The results suggest that the H+-induced membrane depolarization in sensitive Purkinje cells is caused by a mechanism similar to that responsible for a low [K+]-induced depolarization.