Rapid inhibition of the Na+‐K+ pump affects Na+‐Ca2+ exchanger‐mediated relaxation in rabbit ventricular myocytes

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Abstract
The direct influence of Na+-K+ pump activity on the ability of the Na+-Ca2+ exchanger to remove Ca2+ was investigated in isolated adult rabbit ventricular myocytes. Cell shortening was measured using an edge-detection system. Cytoplasmic [Ca2+] was monitored using the fluorescent indicator indo-1. Electrophysiological parameters were recorded using high-resistance microelectrodes. The Na+-K+ pump was rapidly inhibited by removal of extracellular K+ and measurements were taken almost immediately to minimise effects on other cellular compartments. Activity of the Na+-Ca2+ exchanger was monitored during release of Ca2+ from the sarcoplasmic reticulum (SR) elicited by rapid application of 15 mM caffeine. When Na+-K+ pump activity was affected by K+ removal, cell relaxation and indo-1 fluorescence decline were slowed by approximately 40 %. The charge calculated by integrating the caffeine-induced transient inward current was unchanged, suggesting that there was no difference in the SR Ca2+ content in the two conditions. However Ca2+ flux via the Na+-Ca2+ exchanger was slower when the Na+-K+ pump was inhibited. Similar experiments were performed by inhibiting the Na+-K+ pump using 0.5 mM strophanthidin. In this condition similar results to the ones observed by K+ removal were obtained, suggesting a specific role of the Na+-K+ pump in the phenomenon observed. This study suggests that the activity of the Na+-K+ pump influences Na+-Ca2+ exchanger function in the absence of changes in SR Ca2+ content. This can be explained by a slower removal of Na+ from the subsarcolemmal space. The source of the increase in subsarcolemmal [Na+] requires further investigation. However, calculations derived from modelling suggest that the Na+-Ca2+ exchanger itself could be involved.