MODULATION OF CA2- INSIGHT FROM SUBCELLULAR RELEASE PATTERNS REVEALEDBY CONFOCAL MICROSCOPY( RELEASE IN CULTURED NEONATAL RAT CARDIAC MYOCYTES )

It is well established that in heart muscle the influx of Ca2+ through Ca2+ channels during the action potential is the main trigger for Ca2 + release from the sarcoplasmic reticulum (SR), but intact cardiac tis sue and single myocytes are also known to exhibit spontaneous Ca2+ rel ease from the SR under a variety of circumstances. Although conditions favoring spontaneous activity have been examined extensively, mechani sms modulating or regulating spontaneous as well as triggered Ca2+ rel ease are still largely unknown. Using the high spatial and temporal re solution of laser-scanning confocal microscopy, we investigated subcel lular aspects of spontaneous and triggered Ca2+ release in isolated ra t neonatal myocytes loaded with the Ca2+-sensitive fluorescent dye flu o 3. Three distinct patterns of spontaneous Ca2+ release were identifi ed: (1) a homogeneous Ca2+ release, presumably corresponding to Ca2+ r elease during a spontaneous action potential, (2) a focal or spatially restricted Ca2+ release with no or only limited subcellular propagati on, and (3) a Ca2+ release propagating as a wave throughout the entire cell, Pharmacologic tools that interfere with the SR revealed that al l release types were critically dependent on the Ca2+ release and upta ke function of the SR. From our results we conclude that the probabili ty, extent, and pattern of Ca2+ release are modulated on the subcellul ar level. The observed spectrum of release patterns can be explained b y a space- and time-dependent variability in the positive feedback of the Ca2+-induced Ca2+-release mechanism within an individual myocyte. Presumably, this variability depends on the existence of subcellular f unctional elements of the SR, The actual degree of positive feedback m ay be modulated locally by the Ca2+-loading state of each SR element.