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

1. The direct influence of Na+-K+ pump activity on the ability of the Na+-C a2+ exchanger to remove Ca2+ was investigated in isolated adult rabbit vent ricular myocytes. 2. Cell shortening was measured using an edge-detection system. Cytoplasmic [Ca2+] was monitored using the fluorescent indicator indo-1. Electrophysio logical parameters were recorded using high-resistance microelectrodes. The Na+-K+ pump was rapidly inhibited by removal of extracellular K+ and measu rements were taken almost immediately to minimise effects on other cellular compartments. Activity of the Na+-Ca2+ exchanger was monitored during rele ase of Ca2+ from the sarcoplasmic reticulum (SR) elicited by rapid applicat ion of 15 mM caffeine. 3. When Na+-K+ pump activity was affected by K+ removal, cell relaxation an d indo-1 fluorescence decline were slowed by approximately 40 %. The charge calculated by integrating the caffeine-induced tl transient inward current was unchanged, suggesting that there was no difference in the SR Ca2+ cont ent in the two conditions. However Ca2+ flux via the Na+-Ca2+ exchanger was slower when the Na+-K+ pump was inhibited. 4. Similar experiments were performed by inhibiting the Na+-K+ pump using 0 .5 mM strophanthidin. In this condition similar results to the ones observe d by K+ removal were obtained, suggesting a specific role of the: Na+-K+ pu mp in the phenomenon observed. 5. 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 investi gation. However, calculations derived from modelling suggest that the Na+-C a2+ exchanger itself could be involved.