Recent studies have demonstrated that the Na(+)/K(+)-ATPase is not only an ion pump, but also a membrane receptor that confers the ligand-like effects of cardiotonic steroids (CTS) such as ouabain on protein kinases and cell growth. Because CTS have been implicated in cardiac fibrosis, this study examined the role of caveolae in the regulation of Na(+)/K(+)-ATPase function and CTS signaling in cardiac fibroblasts. In cardiac fibroblasts prepared from wild-type and caveolin-1 knockout [Cav-1(-/-)] mice, we found that the absence of caveolin-1 did not affect total cellular amount or surface expression of Na(+)/K(+)-ATPase alpha1 subunit. However, it did increase ouabain-sensitive (86)Rb(+) uptake. While knockout of caveolin-1 increased basal activities of Src and ERK1/2, it abolished the activation of these kinases induced by ouabain but not angiotensin II. Finally, ouabain stimulated collagen synthesis and cell proliferation in wild type but not Cav-1(-/-) cardiac fibroblasts. Thus, we conclude that caveolae are important for regulating both pumping and signal transducing functions of Na(+)/K(+)-ATPase. While depletion of caveolae increases the pumping function of Na(+)/K(+)-ATPase, it suppresses CTS-induced signal transduction, growth, and collagen production in cardiac fibroblasts.
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