Mechanisms of Na+-K+ pump regulation in cardiac myocytes during hyposmolar swelling

We have previously demonstrated that the sarcolemmal Na+-K+ pump current (I-p) in cardiac myocytes is stimulated by cell swelling induced by exposure to hyposmolar solutions. However, the underlying mechanism has not been examined. Because cell swelling activates stretch-sensitive ion channels and intracellular messenger pathways, we examined their role in mediating I-p stimulation during exposure of rabbit ventricular myocytes to a hyposmolar solution. I-p was measured by the whole cell patch-clamp technique. Swelling-induced pump stimulation altered the voltage dependence of I-p. Pump stimulation persisted in the absence of extracellular Na+ and under conditions designed to minimize changes in intracellular Ca2+, excluding an indirect influence on I-p mediated via fluxes through stretch-activated channels. Pump stimulation was protein kinase C independent. The tyrosine kinase inhibitor tyrphostin A25, the phosphatidylinositol 3-kinase inhibitor LY-294002, and the protein phosphatase-1 and -2A inhibitor okadaic acid abolished I-p stimulation. Our findings suggest that swelling-induced pump stimulation involves the activation of tyrosine kinase, phosphatidylinositol 3-kinase, and a serine/threonine protein phosphatase. Activation of this messenger cascade may cause activation by the dephosphorylation of pump units.