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

摘要

class="enumerated" style="list-style-type:decimal">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.