Distribution of proteins implicated in excitation-contraction coupling in rat ventricular myocytes.
D. Scriven, P. Dan, and E. Moore.
Biophys. J. 79 (5): 2682-91 (November 2000)

We have examined the distribution of ryanodine receptors, L-type Ca$^2+$ channels, calsequestrin, Na$^+$/Ca$^2+$ exchangers, and voltage-gated Na$^+$ channels in adult rat ventricular myocytes. Enzymatically dissociated cells were fixed and dual-labeled with specific antibodies using standard immunocytochemistry protocols. Images were deconvolved to reverse the optical distortion produced by wide-field microscopes equipped with high numerical aperture objectives. Every image showed a well-ordered array of fluorescent spots, indicating that all of the proteins examined were distributed in discrete clusters throughout the cell. Mathematical analysis of the images revealed that dyads contained only ryanodine receptors, L-type Ca$^2+$ channels, and calsequestrin, and excluded Na$^+$/Ca$^2+$ exchangers and voltage-gated Na$^+$ channels. The Na$^+$/Ca$^2+$ exchanger and voltage-gated Na$^+$ channels were distributed largely within the t-tubules, on both transverse and axial elements, but were not co-localized. The t-tubule can therefore be subdivided into at least three structural domains; one of coupling (dyads), one containing the Na$^+$/Ca$^2+$ exchanger, and one containing voltage-gated Na$^+$ channels. We conclude that if either the slip mode conductance of the Na$^+$ channel or the reverse mode of the Na$^+$/Ca$^2+$ exchanger are to contribute to the contractile force, the fuzzy space must extend outside of the dyad.
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