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Antibodies raised against dog cardiac Na(+)-Ca2+ exchanger were employed to determine the presence and distribution of the exchanger in arterial smooth muscle (ASM) cells. The antiserum cross-reacted with protein bands of approximately 70, 120, and 150-160 kDa from the membranes of ASM cells, as well as heart sarcolemma. A cardiac Na(+)-Ca2+ exchanger cDNA probe hybridized to 7-kilobase (kb) mRNA from myocytes of the mesenteric artery. Thus ASM cells possess a "cardiac type" Na(+)-Ca2+ exchanger. The relative amounts of 7-kb mRNA and antigen detected on Northern and Western blots, respectively, however, indicate that vascular myocytes contain much less of this transporter than do cardiac myocytes. Immunofluorescence studies on cultured arterial myocytes suggest that the exchanger molecules are organized in reticular patterns over the cell surfaces. A similar pattern is observed when cells are stained for sarcoplasmic reticulum (SR) Ca(2+)-ATPase. This raises the possibility that the exchanger in the plasmalemma of arterial myocytes may be associated, perhaps functionally as well as structurally, with the underlying SR. The antiserum also cross-reacted with endothelial cell membranes, but labeling was lighter and more diffuse than in the myocytes.

We investigated the localization of the Na-Ca exchanger in fixed, isolated heart cells from rat and guinea pig using immunocytochemical methods with epifluorescence and confocal microscopy. We found that the Na-Ca exchanger is distributed throughout all membranes in contact with the extracellular space, including the sarcolemma, the transverse tubules (T-tubules), and the intercalated disks. Microscopic nonuniformities in the fluorescent labeling appear to reflect varying views of the membranes containing Na-Ca exchanger protein. Confocal thin-section imaging reveals a regular grid of discrete foci of fluorescence, which represent Na-Ca exchanger in T-tubules viewed en face. These foci are 1.80 +/- 0.01 microns apart from sarcomere to sarcomere and are aligned with the Z-line. Along each Z-line, these foci are spaced at 1.22 +/- 0.11-microns intervals. Longitudinal sections of the sarcolemma-T-tubule junction show a comblike appearance, with T-tubules extending inward from the heavily labeled sarcolemma. Our finding that the Na-Ca exchanger is widely distributed over the cell surface may provide further insight into the role of Na-Ca exchange in the heart.

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