Optical Bioimaging: From Living Tissue to a Single Molecule: Atrio-Ventricular Difference in Myocardial Excitation-Contraction Coupling — Sequential Versus Simultaneous Activation of SR Ca2+ Release Units —

Abstract

ABSTRACT: Rapid-scanning cofocal microscopy has been applied to the analysis of early phase Ca2+ transients in ventricular and atrial cardiomyocytes. On electrical stimulation of ventricular myocytes, Ca2+ concentration begins to rise earliest at the Z-line level and becames uniform throughout the cytoplasm within about 10 ms after the onset of the action potential; transsarcolemmal Ca2+ influx triggers Ca2+ release from release sites on the junctional sarcoplasmic reticulum (SR) coupled to T-tubules at the Z-line throughout the cytoplasm. In atrial myocytes lacking the T-tubular network, transsarcolemmal Ca2+ influx during an action potential triggers SR Ca2+ release only at subsarcolemmal region. SR Ca2+ release then spreads towards the central region of the cell thourgh a propagated Ca2+-induced-Ca2+ release mechanism. The atrio-ventricular difference in excitation-contraction coupling mechanisms underlies some of the atrio-ventricular difference in response to physiological and pharmacological stimuli.