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

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 exa mined. Because cell swelling activates stretch-sensitive ion channels and i ntracellular messenger pathways, we examined their role in mediating I-p st imulation during exposure of rabbit ventricular myocytes to a hyposmolar so lution. I-p was measured by the whole cell patch-clamp technique. Swelling- induced pump stimulation altered the voltage dependence of I-p. Pump stimul ation persisted in the absence of extracellular Na+ and under conditions de signed to minimize changes in intracellular Ca2+, excluding an indirect inf luence 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 sti mulation. Our findings suggest that swelling-induced pump stimulation invol ves 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.