Analysis of origin and spreading of Ca^<2+> transients by digital imaging system in isolated cardiac cells.

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To examine the origin and spreading of the Ca^<2+> transients following electrical stimulation of isolated myocyte, a system capable of recording intracellular Ca^<2+> distribution with sufficient temporal and spatial resolution was constructed. The system consists of an inverted epifuorescent microscope with a computer-controlled CCD camera and a digital image analyzer. With ths system, it is possible to collect a fluorescent picture every 4 msec. In this study, single myocytes obtained from guinea pig or rat hearts were used. Ca^<2+> transient was monitored using fura-2 as the Ca^<2+> indicator. This was a two year study conducted from 1992 throught 1993. Experimental results are summarized as follows.Ca^<2+> transients triggered by electrical stimulation originated from one or a few sites, or almost entire part of a myocyte. When the Ca^<2+> transient initiated at one site, it took approximately 30 msec to propagate to the other end of the myocyte. A peak of the Ca^<2+> transient was observed about 80 msec after electrical stimulation. The mumber of the initiating sites was increased by isoproterenol which stimulates Ca^<2+> channels, and decreased by verapamil which inhibits Ca^<2+> channels. Ouabain, which increases intracellular Ca^<2+> concentration but has n direct effect on Ca^<2+> channels, failed to alter the number of initiating sites. Doxorubicin, shown to inhibit the Ca^<2+> release mechanism of sarcoplasmic reticulum, did not alter the number of initiating sites athough this agent delayd time to peak Ca^<2+> transients remarkably.These results ndicate that each cell has preferential site(s) at which Ca^<2+> transients originate responding to membrane depolarization. Cz^<2+> transients may be initiated by Ca^<2+> influx via Ca^<2+> channels, and propagate within a cell as the wave of Ca^<2+> induced Ca^<2+> release from sarcoplasmic reticulum.