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

1. Calcium responses induced by brief stimulation with acetylcholine (ACh) were assessed from the fluorescence changes in fura-2 loaded submucosal art erioles of the guinea-pig small intestine. 2. Initially, 1-1.5 h after loading with fura-2 (fresh tissues), ACh increa sed [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 tissue s. ACh further increased the Ba2+-elevated [Ca2+](i) in fresh tissues, but reduced it in old tissues. Responses were not affected by either indomethac in or nitroarginine. 4. In fresh mesenteric arteries, mechanical removal of endothelial cells ab olished 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 elev ated [Ca2+](i) in both fresh and old tissues. Subsequent addition of ACh fu rther increased [Ca2+](i) in fresh tissues without changing it in old tissu es. 6. Proadifen, an inhibitor of the enzyme cytochrome P450 mono-oxygenase, in hibited the ACh-induced changes in [Ca2+](i) in both fresh and Ba2+-stimula ted old tissues. It also inhibited the ACh-induced hyperpolarization. 7. In fresh tissues, the ACh-induced Ca2+ response was not changed by apami n, charybdotoxin (CTX), 4-aminopyridine (4-AP) or glibenclamide. In old tis sues in which [Ca2+](i) had previously been elevated with Ba2+, the ACh-ind uced Ca2+ response was inhibited by CTX but not by apamin, 4-AP or glibencl amide. 8. It is concluded that in submucosal arterioles, ACh elevates endothelial [Ca2+](i) and reduces muscular [Ca2+](i), probably through the hyperpolariz ation of endothelial or smooth muscle membrane by activating CTX-sensitive K+ channels.