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1 Cardiology Department,
We have previously demonstrated that the sarcolemmal
Na+-K+
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. Swelling-induced pump
stimulation altered the voltage dependence of
Ip. Pump
stimulation persisted in the absence of extracellular
Na+ and under conditions designed
to minimize changes in intracellular Ca2+, 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.
osmolarity; tyrosine kinase; ion transport; phosphatases
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