Abstract 160: Contribution of IP3 Receptors in the Electromechanical Response of LV Myocytes to Gq-protein Coupled Receptor Agonists
Gq-protein coupled receptor (GPCR) stimulation promotes PLC function, generating diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3). The latter may promote Ca2+ translocation from intracellular stores altering Ca2+ homeostasis in cardiomyocytes. The aim of this study was to establish whether GPCR agonists enhance IP3 receptor (IP3R) activity, affecting the electromechanical properties of LV myocytes. For this purpose, the functional responses of myocytes to GPCR agonists ATP or ET-1 were established. In field-stimulated cells, GPCR activators increased diastolic Ca2+, transient amplitude, and contractility; extra-systolic Ca2+ releases and aftercontractions were promoted. These responses were prevented by inhibition of PLC, or blockade of IP3Rs. Since DAG promotes PKC activity, the effects of GPCR stimulation were tested in the presence of the PKC inhibitor chelerythrine. This compound failed to abrogate the effects of ATP and ET-1, indicating that PKC-independent pathways play a critical role in mediating the observed cellular responses to GPCR stimulation. Additionally, an AAV9 vector carrying EGFP and sh-RNA targeting IP3R type-2 was employed in vivo to downregulate IP3Rs. GPCRs activation failed to increase Ca2+ transients and to induce extra-systolic Ca2+ elevations in EGFP-positive myocytes. Conversely, these responses were preserved in EGFP-negative cells. In patch-clamped myocytes, changes in Ca2+ transient properties following GPCR activation were accompanied by a decrease in resting potential, action potential (AP) prolongation, and emergence of arrhythmic events. Similar electrical disturbances were detected by direct activation of IP3R with IP3 dialysis, or by enhancing the affinity of the receptors to its ligand, with thimerosal. To establish whether Ca2+ mobilized from the sarcoplasmic reticulum (SR) to the cytoplasm via IP3Rs was responsible for the electrical alterations caused by GPCR agonists, experiments were performed in which SR Ca2+ was depleted, or cytosolic Ca2+ was buffered. Under these conditions, ATP and ET-1 failed to prolong the AP and to induce arrhythmias. In conclusion, the GPCR/IP3R axis regulates Ca2+ homeostasis, contractile performance and the electrical stability of LV myocytes.
- © 2013 by American Heart Association, Inc.