Submicroscopic calcium signals as fundamental events of excitation--contraction coupling in guinea-pig cardiac myocytes.
1. Subcellularly localized Ca2+ signals have been proposed to represent elementary events of cardiac Ca2+ signalling (Ca2+ sparks), whereby an individual sarcolemmal L-type Ca2+ channel locally controls opening of a single (or a few) Ca2+ release channels in the sarcoplasmic reticulum (SR). 2. To investigate directly the elementary nature of this Ca(2+)-induced Ca2+ release mechanism we used flash photolysis of caged Ca2+ while simultaneously measuring the intracellular Ca2+ concentration ([Ca2+]i) with a laser-scanning confocal microscope. 3. Power spectral analysis of the confocal images performed in the spatial domain revealed that only Ca2+ signalling events involving the L-type Ca2+ channel pathway gave rise to Ca2+ sparks. In contrast, SR Ca2+ release triggered by photolytic [Ca2+]i jumps resulted in Ca2+ transients that were always spatially homogeneous. 4. From these findings we conclude that the fundamental event of Ca2+ signalling in cardiac muscle may be smaller in size or amplitude than a Ca2+ spark. 5. We term this event a 'Ca2+ quark' possibly resulting from gating of a single SR Ca2+ release channel. It is proposed that concerted activation of several 'Ca2+ quarks' may be required for a Ca2+ spark. The 'Ca2+ quark' could also be the fundamental event in other cell types implementing a hierarchical Ca2+ signalling concept.
Lipp
P
P
Department of Physiology, University of Bern, Switzerland.
Niggli
E
E
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology; D006168:Guinea Pigs; D018613:Microscopy, Confocal; D009200:Myocardial Contraction / Q000502:physiology*; D018408:Patch-Clamp Techniques; D012508:Sarcolemma / Q000502:physiology; D012519:Sarcoplasmic Reticulum / Q000502:physiology; D016276:Ventricular Function
D015220:Calcium Channels; D002118:Calcium