Calcium signalling in cardiac muscle cells

Abstract

In heart cells, severa! distinct kinds of transient spatial patterns of cytoplasmic calcium ion concentration ([ Ca2 + )¡) can be observed: (1) [ Ca2 + )¡ waves, in which regions of spontaneously increased [ Ca2 + ] ; propagate at high velocity (100 ¡.im/s) through the cell; (2) Ca2 + 'sparks', which are spontaneous, non-propagating changes in [ Ca2 + ] ; that are localized in small ( == 2 ¡.im) subcellular regions; and (3) evoked [ Ca2 + )¡ transients that are elicited by electrical depolarization, in association with normal excitation-contraction (E­ C) coupling. In confocal [ Ca2 + ] ¡ images, evoked [ Ca2 + ] ; transients appear to be nearly spatially uniform throughout the cell, except during their rising phase or during small depolarizations. In contrast to [Ca2 + )¡ waves and spontaneous Ca2 + sparks, evoked [ Ca2 + ] ; transients are triggered by L-type Ca2 + channel current and they are 'controlled', in the sense that stopping the L-type Ca2 + current stops them. Despite their different characteristics, ali three types of Ca2 + transient involve Ca2 + -induced release of Ca2 + from the sarcoplasmic reticulum. Here, we address the question of how the autocatalytic process of Ca2 + -induced Ca2 + release, which can easily be understood to underlie spontaneous regenerative ('uncontrolled'), propagating [Ca2 + )¡ waves, might be 'harnessed', under other circumstances, to produce controlled changes in [ Ca2 + ]¡, as during normal excitation-contraction coupling, or changes in [ Ca2 + )¡ that do not propagate. We discuss our observations of Ca2 + waves, Ca2 + sparks and normal Ca2 + transients in heart cells and review our results on the 'gain' of Ca2 + -induced Ca2 + release. We discuss a model involving Ca2 + microdomains beneath L-type Ca2 + channels, and clusters of Ca2 + -activated Ca2 + release channels in the sarcoplasmic reticulum which may form the basis of the answer to this question