SPATIOTEMPORAL CHANGES OF CA2-EVOKED CONTRACTIONS IN ATRIAL AND VENTRICULAR CELLS( DURING ELECTRICALLY)

Spatial and temporal changes of intracellular calcium ion concentratio n ([Ca2+](i)) during stimulated contractions were observed by confocal microscopy in rat ventricular and guinea pig atrial myocytes. Fluores cence intensity profiles in flue 3-acetoxymethyl ester (fluo 3-AM)-loa ded cells were collected from the entire cell, selected regions of the cell, or along a single scanned line across the cell. In rat ventricu lar myocytes, the increase of [Ca2+](i) after a single stimulus from f ield electrodes occurred synchronously across the cell whether flue 3 fluorescence was monitored in a narrow region aligned with the long ax is of the cell or in line-scan images of a single z-line across the ce ll. However, during the onset of Ca2+ channel blockade by nifedipine ( 5 mu M), electrical stimulation produced spatially nonuniform, focal i ncreases of [Ca2+](i). In guinea pig atrial myocytes, stimulated incre ases of [Ca2+](i) first appeared in focal regions at the cell peripher y before spreading to the cell interior. Line-scan images showed the p eripheral 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-tubula r network ensures synchronous increases of [Ca2+](i) throughout the ce ll 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.