Desynchronising effect of the endothelium on intracellular Ca2+ concentration dynamics in vascular smooth muscle cells of rat mesenteric arteries

The kinetics of the intracellular Ca2+ concentration (Ca2+]i) of vascular smooth muscle cells (VSMCs) in rat small mesenteric arteries was investigated by confocal laser scanning microscopy using the fluorescent Ca2+ indicator fluo-3 AM. One micromole noradrenaline (NA) induced randomly distributed transient elevations of Ca2+]i in several single VSMCs which were weakly temporally coupled. Higher NA concentrations of 3 or 10 microM, however, induced strongly synchronised Ca2+]i oscillations in VSMCs. In preparations with intact endothelium, the synchronisation of Ca2+]i signals was attenuated by acetylcholine (ACh) but augmented by the NO synthase antagonist L-NAME, pointing to a desynchronising effect of the endothelium even under basal conditions. In preparations with or without intact endothelium sodium nitroprusside (SNP) as well as the gap-junction uncoupler heptanol reversibly desynchronised the Ca2+]i transients. The effect of ACh but not that of SNP was influenced by L-NAME. Propagated intracellular Ca2+]i waves had a velocity of 25 microm/s. The phase shift of Ca2+]i oscillations between single VSMCs were maximally 2s and independent of the distance of up to 90 microm between individual cells. Therefore, we consider intercellular Ca2+]i waves to be too slow to account for the synchronisation of Ca2+]i oscillations.We conclude that the coupling of Ca2+]i signals in vascular smooth muscle cells is not constant but highly regulated by NA and by endothelium derived NO. Oscillations of vessel contraction at high sympathetic tone may be induced by synchronisation of Ca2+]i transients of distinct VSMCs whereas endothelium derived NO inhibits vasomotion by desynchronising Ca2+]i transients of single VSMCs.