Na+-Ca2+ exchange in regulation of contractility in canine cardiac Purkinje fibers.
To study Na+-Ca2+ exchange, intracellular Na+ activity (aiNa), twitch tension, and transmembrane potential were simultaneously measured in canine cardiac Purkinje fibers driven at a constant rate (1 Hz) in the absence and presence of strophanthidin (5 X 10(-7) M) at normal, low, and high extracellular [Na+] ([Na+]o) or [Ca2+] ([Ca2+]o). Intracellular Ca2+ activity (aiCa) of the fibers was also measured in a normal Tyrode solution. Reductions of [Na+]o by 20, 40, and 60% decreased the ratio of extracellular Na+ activity (aoNa) and aiNa in the steady state but steeply increased twitch tension. This finding is consistent with the view that a decrease in aoNa/aiNa increases intracellular Ca2+ through Na+-Ca2+ exchange. In further agreement with this view, a Na+-free solution virtually depleted intracellular Na+ and increased the resting tension of the fibers. The slope of the relation of the logs of twitch tension and aiNa that was determined at normal [Na+]o and [Ca2+]o may reflect the properties of the Na+-Ca2+ exchange. Slope of log tension-aiNa relationship decreased when reducing [Na+]o or increasing [Ca2+]o had decreased the level of aiNa. On the other hand, the slope increased when a rise in [Na+]o or a reduction in [Ca2+]o had increased the level of aiNa. These results indicate that as the aiNa level increased, slope of tension-aiNa relation increased, which suggests that Na+-Ca2+ exchange may depend on level of aiNa.(ABSTRACT TRUNCATED AT 250 WORDS)
Department of Physiology and Biophysics, Cornell University Medical College, New York, NY 10021.
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Lee
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D004285:Dogs; D005260:Female; D006329:Heart Conduction System / Q000502:physiology*; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects; D011690:Purkinje Fibers / Q000187:drug effects / Q000502:physiology*; D012964:Sodium / Q000378:metabolism / Q000494:pharmacology; D019831:Sodium-Calcium Exchanger; D013327:Strophanthidin / Q000494:pharmacology
D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; D013327:Strophanthidin; D012964:Sodium; D002118:Calcium