The t-tubular network ensures synchronous increases of intracellular calcium throughout the cell during an action potential, whereas its absence or calcium channel blockade results in focal, nonuniform increases.
Spatial and temporal changes of intracellular calcium ion concentration (Ca2+i) during stimulated contractions were observed by confocal microscopy in rat ventricular and guinea pig atrial myocytes. Fluorescence intensity profiles in fluo 3-acetoxymethyl ester (fluo 3-AM)-loaded cells were collected from the entire cell, selected regions of the cell, or along a single scanned line across the cell. In rat ventricular myocytes, the increase of Ca2+i after a single stimulus from field electrodes occurred synchronously across the cell whether fluo 3 fluorescence was monitored in a narrow region aligned with the long axis of the cell or in line-scan images of a single z-line across the cell. However, during the onset of Ca2+ channel blockade by nifedipine (5 microM), electrical stimulation produced spatially nonuniform, focal increases of Ca2+i. In guinea pig atrial myocytes, stimulated increases of Ca2+i first appeared in focal regions at the cell periphery before spreading to the cell interior. Line-scan images showed the peripheral rise of Ca2+i led that at the center of the cell by 34 +/- 4 ms (mean +/- SE, n = 3). These data demonstrate that the t-tubular network ensures synchronous increases of Ca2+i throughout the cell during an action potential. In the absence of t tubules or when the number of sarcolemmal Ca2+ channels opened by membrane depolarization is greatly reduced, stimulated increases of Ca2+i can be observed to arise in focal regions of the cell.
Joshua R. Berlin (Fri,) studied this question.