SUBMICROSCOPIC CALCIUM SIGNALS AS FUNDAMENTAL EVENTS OF EXCITATION-CONTRACTION COUPLING IN GUINEA-PIG CARDIAC MYOCYTES

1. Subcellularly localized Ca2+ signals have been proposed to represen t 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 reti culum (SR). 2. To investigate directly the elementary nature of this C a2+-induced Ca2+ release mechanism we used flash photolysis of caged C a2+ while simultaneously measuring the intracellular Ca2+ concentratio n ([Ca2+](i)) with a laser-scanning confocal microscope. 3. Power spec tral analysis of the confocal images performed in the spatial domain r evealed that only Ca2+ signalling events involving the L-type Ca2+ cha nnel pathway gave rise to Ca2+ sparks. In contrast, SR Ca2+ release tr iggered 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 b e smaller in size or amplitude than a Ca2+ spark. 5. We term this even t a 'Ca2+ quark' possibly resulting from gating of a single SR Ca2+ re lease channel. It is proposed that concerted activation of several 'Ca 2+ quarks' may be required for a Ca2+ spark. The 'Ca2+ quark' could al so be the fundamental event in other cell types implementing a hierarc hical Ca2+ signalling concept.