Role of caveolae in signal-transducing function of cardiac Na+/K+-ATPase

Ouabain binding to Na+/K+-ATPase activates Src/epidermal growth factor receptor (EGFR) to initiate multiple signal pathways that regulate growth. In cardiac myocytes and the intact heart, the early ouabain-induced pathways that cause rapid activations of ERK1/2 also regulate intracellular Ca2+ concentration ([Ca2+](i)) and contractility. The goal of this study was to explore the role of caveolae in these early signaling events. Subunits of Na+/K+-ATPase were detected by immunoblot analysis in caveolae isolated from cardiac myocytes, cardiac ventricles, kidney cell lines, and kidney outer medulla by established detergent-free procedures. Isolated rat cardiac caveolae contained Src, EGFR, ERK1/2, and 20-30% of cellular contents of alpha(1)- and alpha(2)-isoforms of Na+/K+-ATPase, along with nearly all of cellular caveolin-3. Immunofluorescence microscopy of adult cardiac myocytes showed the presence of caveolin-3 and alpha-isoforms in peripheral sarcolemma and T tubules and suggested their partial colocalization. Exposure of contracting isolated rat hearts to a positive inotropic dose of ouabain and analysis of isolated cardiac caveolae showed that ouabain caused 1) no change in total caveolar ERK1/2, but a two-to threefold increase in caveolar phosphorylated/activated ERK1/2; 2) no change in caveolar alpha(1)-isoform and caveolin-3; and 3) 50-60% increases in caveolar Src and alpha(2)-isoform. These findings, in conjunction with previous observations, show that components of the pathways that link Na+/K+-ATPase to ERK1/2 and [Ca2+](i) are organized within cardiac caveolae microdomains. They also suggest that ouabain-induced recruitments of Src and alpha(2)-isoform to caveolae are involved in the manifestation of the positive inotropic effect of ouabain.