Abstract

The recognition that ryanodine receptor (RyR) dysfunction is associated to arrhythmogenic diseases raises the possibility that alternative Ca2+ release channels alter the electrical properties of ventricular myocytes. The aim of this study was to determine whether inositol triphosphate receptor (IP3R)-mediated Ca2+ mobilization modulates Ca2+ cycling and electrical behavior in LV myocytes. Cells were obtained from human and mouse hearts and the expression and function of IP3Rs were evaluated. IP3Rs were identified in isolated myocytes by immunocytochemistry and Western blotting. In field-stimulated cells, IP3R activation via Gq-protein receptor agonists (ET-1, ATP) or enhancer of ligand affinity (thimerosal) increased diastolic Ca2+ and transient amplitude by 11% and 44%, respectively. Additionally, extra-systolic Ca2+ release and sustained Ca2+ elevations were detected. These effects were prevented by inhibition of IP3 production or by IP3R blockade. Importantly, myocytes obtained from mice infected in vivo with small hairpin RNA (shRNA) targeting IP3R type 2, failed to respond to Gq-protein receptor agonists. In patch-clamped human and mouse cells, changes in Ca2+ transient properties following IP3R activation were accompanied by a decrease in resting potential, action potential (AP) prolongation, and emergence of arrhythmic events. In mouse cardiomyocytes, assessment of excitation-contraction coupling gain and blockade of RyR channels under IP3R stimulation, excluded the contribution of RyRs to the effects induced by IP3R activation. In voltage-clamped cells, IP3R agonists promoted transient inward currents at the membrane potential of -70 mV and increased a nickel-sensitive current in the range of potentials corresponding to forward mode operation of the Na-Ca exchanger. To establish whether enhanced Ca2+ load was responsible for the altered electrical properties under IP3R activation, conditions buffering cytosolic Ca2+ levels were employed. Under these circumstances, IPR stimulation failed to prolong the AP and to induce arrhythmias. In conclusions, our observations demonstrate that Ca2+ mobilization via IP3Rs directly alters the electrical properties of human and rodent myocytes.