title: Effects of Ca2+/Mg2+ removal on aiNa, aiK, and tension in cardiac Purkinje fibers.
abstract: 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.
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D006329:Heart Conduction System / Q000502:physiology*; D008274:Magnesium / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects; D009200:Myocardial Contraction / Q000187:drug effects*; D011188:Potassium / Q000494:pharmacology; D011690:Purkinje Fibers / Q000187:drug effects / Q000502:physiology*; D012756:Sheep; D012964:Sodium / Q000378:metabolism
chemical_list: D012964:Sodium; D008274:Magnesium; D011188:Potassium; D002118:Calcium