Enhanced spontaneous Ca2+ events in endothelial cells reflect signalling through myoendothelial gap junctions in pressurized mesenteric arteries

SummaryIncreases in global Ca2+ in the endothelium are a crucial step in releasing relaxing factors to modulate arterial tone. In the present study we investigated spontaneous Ca2+ events in endothelial cells, and the contribution of smooth muscle cells to these Ca2+ events, in pressurized rat mesenteric resistance arteries. Spontaneous Ca2+ events were observed under resting conditions in 34% of cells. These Ca2+ events were absent in arteries preincubated with either cyclopiazonic acid or U-73122, but were unaffected by ryanodine or nicotinamide. Stimulation of smooth muscle cell depolarization and contraction with either phenylephrine or high concentrations of KCl significantly increased the frequency of endothelial cell Ca2+ events. The putative gap junction uncouplers carbenoxolone and 18α-glycyrrhetinic acid each inhibited spontaneous and evoked Ca2+ events, and the movement of calcein from endothelial to smooth muscle cells. In addition, spontaneous Ca2+ events were diminished by nifedipine, lowering extracellular Ca2+ levels, or by blockers of non-selective Ca2+ influx pathways. These findings suggest that in pressurized rat mesenteric arteries, spontaneous Ca2+ events in the endothelial cells appear to originate from endoplasmic reticulum IP3 receptors, and are subject to regulation by surrounding smooth muscle cells via myoendothelial gap junctions, even under basal conditions.