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Detubulation of rat ventricular myocytes has been used to investigate the role of the t-tubules in Ca2+ cycling during excitation-contraction coupling in rat ventricular myocytes. Ca2+ was monitored using fluo-3 and confocal microscopy. In control myocytes, electrical stimulation caused a spatially uniform increase in intracellular Ca2+ across the cell width. After detubulation, Ca2+ 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 Ca2+; 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 Ca2+ as Ca2+ was extruded from the cell; this rise was significantly smaller in detubulated cells, and the remaining increase was blocked by the sarcolemmal Ca2+ ATPase inhibitor carboxyeosin. The treatment used to produce detubulation had no significant effect on Ca2+ efflux in atrial cells, which lack t-tubules. Detubulation of ventricular myocytes also resulted in loss of Na+-Ca2+ exchange current, although the density of the fast Na+ current was unaltered. It is concluded that Na+-Ca2+ exchange function, and hence Ca2+ efflux by this mechanism, is concentrated in the t-tubules, and that the concentration of Ca2+ flux pathways in the t-tubules is important in producing a uniform increase in intracellular Ca2+ on stimulation.
Yang et al. (Fri,) studied this question.