The role of the sodium-calcium exchanger for calcium extrusion in coronary arteries

Calcium ionophores, such as the A23187, cause endothelium-dependent relaxation of arterial strips with intact endothelium, whereas the effect of the ionophore should result from the combination of a relaxation caused by the endothelium-dependent factors and of a contraction of the smooth muscles. In addition, the application of a calcium ionophore to a strip of pig coronary arteries without endothelium does not change cytosolic free calcium concentration and force developed by the smooth muscle cells. To explain these paradoxes, the hypothesis that active calcium extrusion would match the entry of extracellular calcium caused by the ionophore was tested. We see that the sodium-calcium exchanger extrudes calcium that enters the smooth muscle cells in the absence of the ionophore. This exchanger is efficient enough to expel the increased influx of calcium created by the additional calcium carriers formed by the ionophore. This explains the inefficiency of calcium ionophores to increase cytosolic free calcium of smooth muscle cells and consequently, the fact that the ionophore does not cause a contraction of a strip without endothelium. This makes evident that a calcium ionophore fully relaxes, in an endothelium-dependent manner, an intact strip of porcine coronary artery

Role of smooth muscle cells on endothelial cell cytosolic free calcium in porcine coronary arteries

We tested the hypothesis that the cytosolic free calcium concentration in endothelial cells is under the influence of the smooth muscle cells in the coronary circulation. In the left descending branch of porcine coronary arteries, cytosolic free calcium concentration ([Ca(2+)](i)) was estimated by determining the fluorescence ratio of two calcium probes, fluo 4 and fura red, in smooth muscle and endothelial cells using confocal microscopy. Acetylcholine and potassium, which act directly on smooth muscle cells to increase [Ca(2+)](i), were found to indirectly elevate [Ca(2+)](i) in endothelial cells; in primary cultures of endothelial cells, neither stimulus affected [Ca(2+)](i), yet substance P increased the fluorescence ratio twofold. In response to acetylcholine and potassium, isometric tension developed by arterial strips with intact endothelium was attenuated by up to 22% (P < 0.05) compared with strips without endothelium. These findings suggest that stimuli that increase smooth muscle [Ca(2+)](i) can indirectly influence endothelial cell function in porcine coronary arteries. Such a pathway for negative feedback can moderate vasoconstriction and diminish the potential for vasospasm in the coronary circulation

Cytosolic-free calcium in smooth-muscle and endothelial cells in an intact arterial wall from rat mesenteric artery in vitro

The regulation of cytosolic-free calcium concentration of smooth-muscle and endothelial cells was mainly studied on cultured cells where the cross talk between these two coupled cell types is lost. In the present study, the cytosolic-free calcium concentration in the endothelial and the smooth-muscle cells was examined in an intact arterial wall in vitro. Strips of the main branch of rat mesenteric artery were used. Cytosolic-free calcium concentration [Ca2+]i was estimated by determining the fluorescence ratio of the two calcium probes, Fluo-4 and Fura red. The emitted fluorescence of both probes was measured with a confocal microscope. We showed that potassium and phenylephrine, which increase the cytosolic -free calcium concentration of the smooth-muscle cells, also indirectly influence the calcium concentration in the endothelial cells. By simultaneously determining [Ca2+]i in the endothelial and the smooth-muscle cells of an arterial strip, we observed that when calcium increases in the endothelial cells in response to acetylcholine, it slightly decreases in the smooth-muscle cells. We conclude that the regulation of [Ca2+]i in the arterial endothelial cell, depends according to the stimuli either upon the endothelial cells themselves, or upon the coupled smooth-muscle cells