Na$^+$-Ca$^2+$ exchange activity is localized in the T-tubules of rat ventricular myocytes.
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Circ. Res. 91 (4): 315-22 (August 2002)

Detubulation of rat ventricular myocytes has been used to investigate the role of the t-tubules in Ca$^2+$ cycling during excitation-contraction coupling in rat ventricular myocytes. Ca$^2+$ was monitored using fluo-3 and confocal microscopy. In control myocytes, electrical stimulation caused a spatially uniform increase in intracellular Ca$^2+$ across the cell width. After detubulation, Ca$^2+$ rose initially at the cell periphery and then propagated into the center of the cell. Application of caffeine to control myocytes resulted in a rapid and uniform increase of intracellular Ca$^2+$; the distribution and amplitude of this increase was the same in detubulated myocytes, although its decline was slower. On application of caffeine to control cells, there was a large, rapid, and transient rise in extracellular Ca$^2+$ as Ca$^2+$ was extruded from the cell; this rise was significantly smaller in detubulated cells, and the remaining increase was blocked by the sarcolemmal Ca$^2+$ ATPase inhibitor carboxyeosin. The treatment used to produce detubulation had no significant effect on Ca$^2+$ efflux in atrial cells, which lack t-tubules. Detubulation of ventricular myocytes also resulted in loss of Na$^+$-Ca$^2+$ exchange current, although the density of the fast Na$^+$ current was unaltered. It is concluded that Na$^+$-Ca$^2+$ exchange function, and hence Ca$^2+$ efflux by this mechanism, is concentrated in the t-tubules, and that the concentration of Ca$^2+$ flux pathways in the t-tubules is important in producing a uniform increase in intracellular Ca$^2+$ on stimulation.
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