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

Hiroyasu Fukuta, Hikaru Hashitani, Yoshimichi Yamamoto and Hikaru Suzuki

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.

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).

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.

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⁺.

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.

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.

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.

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.

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