Inhibitory actions of endothelial products on mechanical and calcium responses in aortic smooth muscle cells

Nitric oxide causes a cGMP-independent intracellular calcium rise in porcine endothelial cells-a paradox?

This study was undertaken to investigate the influence of exogenous NO on intracellular calcium levels of porcine aortic endothelial cell culture monolayers. Spontaneous NO liberating substances with different half-life periods (NOC-9 [10 micromol/L] approximately 1 min, SNAP [10 micromol/L] approximately 4 h), and an aqueous NO gas solution [130 nmol/L] were added onto the monolayers. All three solutions induced a rapid and similar calcium rise in the endothelial cells. NOC-9 as a rapidly NO releasing compound was selected to be investigated more thoroughly. The NOC-9 calcium rise is not dependent on the activation of the guanylate cyclase since preincubation with a specific guanylate cyclase inhibitor [ODQ, 10 micromol/L] did not alter the effect and a cGMP analogue [8-bromo-cGMP 10 micromol/L] did not significantly elevate calcium levels. The NOC-9 induced calcium rise could be completely blocked by removal of extracellular calcium and partly blocked by SKF 96365 [10 micromol/L], an unspecific inhibitor of the receptor operated calcium channels. Incubation with N-nitroarginine [100 micromol/L] slightly but significantly reduced basal calcium levels in the cell cultures. Therefore, we conclude that exogenous NO elevates [Ca(2+)](i) in cultured porcine aortic endothelial cells. This effect is not dependent on cGMP, and a calcium influx is involved. Moreover, constitutively formed endogenous NO seems to be necessary to maintain basal calcium levels.

Calcium responses induced by acetylcholine in submucosal arterioles of the guinea-pig small intestine

1. Calcium responses induced by brief stimulation with acetylcholine (ACh) were assessed from the fluorescence changes in fura-2 loaded submucosal arterioles of the guinea-pig small intestine. 2. Initially, 1-1.5 h after loading with fura-2 (fresh tissues), ACh increased [Ca2+]i in a concentration-dependent manner. This response diminished with time, and finally disappeared in 2-3 h (old tissues). 3. Ba2+ elevated [Ca2+]i to a similar extent in both fresh and old tissues. ACh further increased the Ba2+-elevated [Ca2+]i in fresh tissues, but reduced it in old tissues. Responses were not affected by either indomethacin or nitroarginine. 4. In fresh mesenteric arteries, mechanical removal of endothelial cells abolished the ACh-induced increase in [Ca2+]i, with no alteration of [Ca2+]i at rest and during elevation with Ba2+. 5. In the presence of indomethacin and nitroarginine, high-K+ solution elevated [Ca2+]i in both fresh and old tissues. Subsequent addition of ACh further increased [Ca2+]i in fresh tissues without changing it in old tissues. 6. Proadifen, an inhibitor of the enzyme cytochrome P450 mono-oxygenase, inhibited the ACh-induced changes in [Ca2+]i in both fresh and Ba2+-stimulated old tissues. It also inhibited the ACh-induced hyperpolarization. 7. In fresh tissues, the ACh-induced Ca2+ response was not changed by apamin, charybdotoxin (CTX), 4-aminopyridine (4-AP) or glibenclamide. In old tissues in which [Ca2+]i had previously been elevated with Ba2+, the ACh-induced Ca2+ response was inhibited by CTX but not by apamin, 4-AP or glibenclamide. 8. It is concluded that in submucosal arterioles, ACh elevates endothelial [Ca2+]i and reduces muscular [Ca2+]i, probably through the hyperpolarization of endothelial or smooth muscle membrane by activating CTX-sensitive K+ channels.