ACELL May 45/5

Bewick, N. L., C. Fernandes, A. D. Pitt, H. H. Rasmussen, and D. W. Whalley. Mechanisms of Na1-K1 pump regulation in cardiac myocytes during hyposmolar swelling. Am. J. Physiol. 276 (Cell Physiol. 45): C1091–C1099, 1999.—We have previously demonstrated that the sarcolemmal Na1-K1 pump current (Ip) 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 Ip stimulation during exposure of rabbit ventricular myocytes to a hyposmolar solution. Ip was measured by the whole cell patch-clamp technique. Swellinginduced pump stimulation altered the voltage dependence of Ip. Pump stimulation persisted in the absence of extracellular Na1 and under conditions designed to minimize changes in intracellular Ca21, excluding an indirect influence on Ip 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 Ip 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.