Role of inositol 1,4,5-trisphosphate in the regulation of ventricular Ca2+ signaling in intact mouse heart

Inositol 1,4,5-trisphosphate (InsP(3)R)-mediated Ca2+ signaling is a major pathway regulating multiple cellular functions in excitable and non-excitable cells. Although InsP(3)-mediated Ca2+ signaling has been extensively described, its influence on ventricular myocardium activity has not been addressed in contracting hearts at the whole-organ level. In this work, InsP(3)-sensitive intracellular Ca2+ signals were studied in intact hearts using laser scanning confocal microscopy and pulsed local-field fluorescence microscopy. Intracellular [InsP(3)] was rapidly increased by UV flash photolysis of membrane-permeant caged InsP(3). Our results indicate that the basal [Ca2+] increased after the flash photolysis of caged InsP(3) without affecting the action potential (AP)-induced Ca2+ transients. The amplitude of the basal [Ca2+] elevation depended on the intracellular [InsP3] reached after the UV flash. Pretreatment with ryanodine failed to abolish the InsP(3)-induced Ca2+ release (IICR), indicating that this response was not mediated by ryanodine receptors (RyR). Thapsigargin prevented Ca2+ release from both RyR- and InsP(3)R-containing Ca2+ stores, suggesting that these pools have similar Ca2+ reuptake mechanisms. These results were reproduced in acutely isolated cells where photorelease of InsP(3) was able to induce changes in endothelial cells but not in AP-induced transients from cardiomyocytes. Taken together, these results suggest that IICR does not directly regulate cardiac excitation-contraction coupling. To our knowledge, this is the first demonstration of IICR in intact hearts. Consequently, our work provides a reference framework of the spatiotemporal attributes of the IICR under physiological conditions. (C) 2012 Elsevier Ltd. All rights reserved.