Inside-Out IP3-Mediated G Protein-Coupled Receptor Activation Drives Intercellular Ca2+ Signaling in the Vascular Endothelium.

The endothelium's control of nearly all vascular function relies on rapid intercellular communication to coordinate cellular activity across scale. A key form of intercellular communication arises from the regenerative propagation of IP3-evoked Ca2+ signals from cell to cell, which regulate vessel tone, modulate vascular permeability, and determine immune responses. Despite their importance, the mechanisms by which regenerative propagation of IP3-evoked Ca2+ signals occurs are poorly understood. Here, in intact resistance arteries, precision photolysis of IP3 combined with high-resolution mesoscale imaging, targeted drug application, and advanced analytical techniques was used to determine the mechanisms underlying regenerative propagation of IP3-evoked Ca2+ signals in the endothelium. Elevated IP3 in the initiating cell triggers a noncanonical inside-out signaling mechanism that leads to transcellular activation of a Gαq/11-coupled receptor in a neighboring (receiving) cell. This, in turn, initiates canonical outside-in signaling via PLC, leading to the hydrolysis of PIP2 and production of IP3. This process creates a regenerative, IP3-dependent signaling cascade operating between adjacent cells. Notably, neither Ca2+ nor IP3 diffusion through gap junctions plays a significant role in intercellular communication. Our findings uncover a previously unrecognized mechanism of endothelial communication, in which noncanonical IP3-driven transcellular activation of G protein-coupled receptors sustains a regenerative signaling loop, highlighting a novel framework for intercellular coordination in the vascular endothelium.