AJP - Cell Physiology, Vol 251, Issue 6 920-C927, Copyright © 1986 by American Physiological Society
Effects of Ca2+/Mg2+ removal on aiNa, aiK, and tension in cardiac Purkinje fibers
R. A. Chapman, H. A. Fozzard, I. R. Friedlander and C. T. January
Sheep cardiac Purkinje fibers were exposed to solutions free of divalent
cations for hour-long periods, while intracellular Na+ and K+ activities
were measured using ion-sensitive microelectrodes. Intracellular Na+
activity (aiNa) increased to 50.1 +/- 8.1 mM, and intracellular K+ activity
(aiK) decreased to 76.7 +/- 3.5 mM. These ionic changes could be blocked by
the presence of Mg2+ or the Ca2+ channel blocking agents D 600 and
nifedipine. The rise in aiNa and the fall in aiK was accentuated by the
inhibition of the Na+-K+ pump with acetylstrophanthidin or by removal of
extracellular K+. These results demonstrate that in cardiac Purkinje fibers
removal of divalent cations produces intracellular loading of Na+ by Na+
entry through the Ca2+ channel. On reexposure to Ca2+-containing solutions,
the cells become loaded with Ca2+, and the fibers exhibit large
contractures. These observations implicate Na+-Ca2+ exchange in the entry
of Ca2+ into these cells during Ca2+ repletion and in the etiology of the
calcium paradox.
