Evidence for signaling via gap junctions from smooth muscle to endothelial cells in rat mesenteric arteries: possible implication of a second messenger

We investigated heterocellular communication in rat mesenteric arterial strips at the cellular level using confocal microscopy. To visualize Ca2+ changes in different cell populations, smooth muscle cells (SMCs) were loaded with Fluo-4 and endothelial cells (ECs) with Fura red. SMC contraction was stimulated using high K+ solution and Phenylephrine. Depending on vasoconstrictor concentration, intracellular Ca2+ concentration ([Ca2+]i) increased in a subpopulation of ECs 5-11 s after a [Ca2+]i rise was observed in adjacent SMCs. This time interval suggests chemical coupling between SMCs and ECs via gap junctions. As potential chemical mediators we investigated Ca2+ or inositol 1,4,5-trisphosphate (IP3). First, phospholipase C inhibitor U-73122 was added to prevent IP3 production in response to the [Ca2+]i increase in SMCs. In high K+ solution, all SMCs presented global and synchronous [Ca2+]i increase, but no [Ca2+]i variations were detected in ECs. Second, 2-aminoethoxydiphenylborate, an inhibitor of IP3-induced Ca2+ release, reduced the number of flashing ECs by 75 ± 3% (n = 6). The number of flashing ECs was similarly reduced by adding the gap junction uncoupler palmitoleic acid. Thus, our results suggest a heterocellular communication through gap junctions from SMCs to ECs by diffusion, probably of IP3.