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

Inositol 1,4,5-trisphosphate (InsP3R)-mediated Ca2+ signaling is a major pathway regulating multiple cellular functions in excitable and non-excitable cells. Although InsP3-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, InsP3-sensitive intracellular Ca2+ signals were studied in intact hearts using laser scanning confocal microscopy and pulsed local-field fluorescence microscopy. Intracellular [InsP3] was rapidly increased by UV flash photolysis of membrane-permeant caged InsP3. Our results indicate that the basal [Ca2+] increased after the flash photolysis of caged InsP3 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 InsP3-induced Ca2+ release (IICR), indicating that this response was not mediated by ryanodine receptors (RyR). Thapsigargin prevented Ca2+ release from both RyR- and InsP3R-containing Ca2+ stores, suggesting that these pools have similar Ca2+ reuptake mechanisms. These results were reproduced in acutely isolated cells where photorelease of InsP3 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. ► Caged-InsP3 photolysis affects intracellular [Ca2+] in intact, beating hearts. ► InsP3 photolysis induced Ca2+ release in endothelial cells and epicardial myocytes. ► In ventricular myocytes, Ca2+ increase due to InsP3 photolysis did not impact ECC. ► InsP3 activates intracellular Ca2+ stores that share InsP3R and RyR.