Abstract 10762: Inositol Triphosphate Receptor Activation Provides Inotropic Support to Human Ventricular Myocytes
Ca2+ release from the endoplasmic reticulum via inositol 1,4,5-triphosphate receptors (IP3Rs) regulate the growth and fate of human cardiac stem cells, but whether IP3Rs are operative in the derived myocyte progeny, and modulate Ca2+ homeostasis and electrical activity in this cell compartment is unknown. In this study, IP3R function was determined in left ventricular (LV) myocytes from normal human hearts declined for transplantation and integrated with assays in mouse cells. Transcripts for the three IP3R subtypes were identified in human LV myocytes by qRT-PCR and the expression of IP3R-2 was detected by Western blotting. In field-stimulated cells, IP3R activation via Gq-protein-coupled receptor agonists (endothelin-1, ATP) increased Ca2+ transient amplitude and contractility. Moreover, extra-systolic Ca2+ release and after-contractions were induced. These effects were prevented by IP3R blockade with xestopspongin-C. Similarly, myocytes obtained from mice infected in vivo with small hairpin RNA targeting IP3R-2, failed to respond to Gq-protein receptor agonists. By patch-clamp, IP3R stimulation in human LV myocytes resulted in a 1.5-fold increase in the time to 50% repolarization of the action potential and promoted the occurrence of early and delayed after-depolarizations. In voltage-clamp, IP3R activation in human and mouse cells elicited transient inward currents preceded by small elevations in intracellular Ca2+. To test the possibility that IP3R-Ca2+ mobilization promoted the electrogenic extrusion of this cation causing electrical instability, the activity of the Na/Ca exchanger (NCX) was measured with nickel. Current-voltage relationships documented that IP3R agonists induced larger inward currents in the range of potentials corresponding to forward mode NCX. Alterations in the electromechanical characteristics of human cardiomyocytes following IP3R activation were coupled with a ~30% increased isometric twitch of LV trabeculae, and with a prolongation of epicardial monophasic action potentials and occurrence of arrhythmic events in perfused human LV myocardium. Thus, IP3R signaling provides inotropic reserve, which is hampered by the initiation of electrical disorders and contractile abnormalities.
- © 2012 by American Heart Association, Inc.