ACh exerted a biphasic effect in the in vitro working heart of Rana esculenta. High concentrations (10⁷ M) of ACh depressed stroke volume (SV) and stroke work (SW) by ~30% with a shorter systolic phase and reduced peak pressure. Doses from 10¹⁰ M induced a positive response peaking at 10⁸ M (SV: +8.6%; SW: +6.5%) and a prolonged systolic phase without affecting peak pressure. Atropine and pirenzepine blocked both the positive and the negative effects of ACh. Pretreatment with Triton X-100 (0.1 ml, 0.05%) or with nitric oxide (NO)-cGMP pathway antagonists (N^G-nitro-L-arginine, N^G-nitro-L-arginine methyl ester, N^G-monomethyl-L-arginine, and 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one) abolished the positive and negative cholinergic effects. Infusion of 8-bromoguanosine 3',5'-cyclic monophosphate reverted the positive effect of ACh to a negative effect. Milrinone blocked the positive inotropism but did not change the negative cholinergic response. The NO donor 3-morpholinosydnonimine generated a biphasic dose-response curve with a maximum positive effect at 10⁸ M (SV: +8%; SW: +5.6%; systolic phase: +28 ms) and a negative effect at 5 × 10⁸ M (SV and SW: about 12%; systolic phase: 70 ms; peak pressure: 1.50 mm). We conclude that in the avascular frog heart the endocardial endothelium mediates the inotropic effect of luminal cholinergic stimuli via a NO-cGMP pathway.

acetylcholine; nitric oxide; signal transduction

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