Inside-Out IP(3)-Mediated G Protein-Coupled Receptor Activation Drives Intercellular Ca(2+) 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 IP(3)-evoked Ca(2+) 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 IP(3)-evoked Ca(2+) signals occurs are poorly understood. Here, in intact resistance arteries, precision photolysis of IP(3) combined with high-resolution mesoscale imaging, targeted drug application, and advanced analytical techniques was used to determine the mechanisms underlying regenerative propagation of IP(3)-evoked Ca(2+) signals in the endothelium. Elevated IP(3) in the initiating cell triggers a noncanonical inside-out signaling mechanism that leads to transcellular activation of a G(αq/11)-coupled receptor in a neighboring (receiving) cell. This, in turn, initiates canonical outside-in signaling via PLC, leading to the hydrolysis of PIP(2) and production of IP(3). This process creates a regenerative, IP(3)-dependent signaling cascade operating between adjacent cells. Notably, neither Ca(2+) nor IP(3) diffusion through gap junctions plays a significant role in intercellular communication. Our findings uncover a previously unrecognized mechanism of endothelial communication, in which noncanonical IP(3)-driven transcellular activation of G protein-coupled receptors sustains a regenerative signaling loop, highlighting a novel framework for intercellular coordination in the vascular endothelium.