In isolated rings of guinea-pig aorta not responding to acetylcholine, the diuretic etozoline did not influence basal vascular tone but inhibited noradrenaline- and histamine-induced contractions. The inhibition was evident at concentrations of the diuretic (10 microM-1 mM) suitable to inhibit, in a competitive manner, the contractions evoked by a K+ channel blocker, tetraethylammonium, in the same preparation (Dorigo et al., 1989, 1990). In isolated rings of guinea-pig aorta, etozoline, at very low concentrations (1 nM-0.1 microM), inhibited also serotinin-induced contractions. The contractile effect of serotonin was abolished by nifedipine associated with 2-nitro-4-carboxyphenyl N,N-diphenyl-carbamate (an inhibitor of phospholipase C) or with etozoline, thus suggesting that the diuretic, besides inhibiting extracellular Ca++ uptake, also prevents intracellular Ca++ mobilization mediated by inositol triphosphate. In isolated rings of rat aorta responding to acetylcholine, etozoline did not influence basal vascular tone either in the absence or in the presence of superoxide-dismutase. In the same preparation, the diuretic inhibited vascular contractions induced by the three spasmogenic agents used, i.e. noradrenaline, histamine and serotonin. This inhibition occurred at concentrations of etozoline ranging from 10 microM to 1 mM and was uninfluenced by indomethacin (10 microMs). In isolated rings of rat aorta, the contractile effect of noradrenaline was not influenced by the addition of either 100 microM pyrogallol, or 10 microM methylene blue or 100 U/ml superoxide-dismutase, while the contractile responses to histamine and to serotonin were potentiated by pyrogallol and by methylene blue and reduced by superoxide-dismutase. This indicates that, in rat aorta, noradrenaline evokes only a direct contractile response, whereas both serotonin and histamine have a double effect: direct contraction of vascular smooth muscle and release of a relaxing factor from the endothelium. The inhibitory activity of etozoline towards serotonin- and histamine-induced contractions was reduced by pyrogallol and by methylene blue, whereas it was potentiated by superoxide-dismutase. The ability of etozoline to reverse the noradrenaline-induced contraction was unaffected by pyrogallol, methylene blue or superoxide-dismutase. These results emphasize the spasmolytic activity of etozoline, which seems to involve only the muscular component of rat and guinea-pig aorta.

Acetylcholine↗

PubMedDate not supplied

Mechanism of the vasodilating action of etozoline and its metabolites in guinea-pig aorta.

The mechanism of the vasodilating action of the diuretic etozoline and of its metabolites d- and l-ozolinone was studied in isolated aortic strips from reserpine-treated guinea-pigs. Etozoline (10 microM to 1 mM) induced a concentration-dependent inhibition of the contraction evoked by increasing K+ concentrations (15 to 45 mM) in the perfusion medium. The inhibition was of a noncompetitive type. Etozoline also inhibited contractions induced by increasing Ca++ concentrations in the perfusion medium of aortic strips depolarized by 40 mM K+. The inhibition was concentration-dependent and of a noncompetitive type. Tetraethylammonium, at concentrations reported to increase Ca++ influx into smooth muscle cells by closing K+ channels, induced contractions of aortic strips that were highly sensitive to inhibition by etozoline. Analysis of the concentration-response curves for tetraethylammonium showed that the antagonism by etozoline was of a competitive type. The pA2 value for etozoline was 5.01 +/- 0.16. Also arterial spasm, obtained by prolonging the exposure of vascular strips to tetraethylammonium (30 min), was completely suppressed by etozoline. l- and d-Ozolinone antagonized tetraethylammonium-induced contractions of aortic strips in an apparently competitive manner, but only at very high concentrations (1 and 3 mM, respectively). These results suggest that the vasodilating action of etozoline and of its metabolites is mediated by an inhibition of Ca++ influx into smooth muscle cells following a specific opening of K+ channels, but only etozoline may act through this mechanism at concentrations likely to be operative in vivo.

Animals↗

PubMedDate not supplied

Antispasmodic effect of amiodarone on gastrointestinal smooth muscle: possible involvement of calcium.

The influence of amiodarone on contractions induced by acetylcholine (ACh) was studied in isolated preparations of guinea-pig ileum, duodenum and stomach fundus as well as in rat stomach fundus. In the guinea-pig ileum a concentration-dependent antispasmodic effect of amiodarone (20-70 microM) was observed after 30 min exposure to the drug, but not after 15 min. The inhibition of ACh-induced contraction further increased with time after removal of amiodarone from the bathing fluid. Similar results were obtained in ileum preparations maintained in a low Ca2+ medium (0.35 mM CaCl2) and under these conditions the response to ACh was restored by washing the tissue with a normal Ca2+ medium (1.4 mM CaCl2). Both low Ca2+ and amiodarone depressed the tonic component of ACh-induced contraction more than the phasic one. Guinea-pig duodenum was more susceptible than the ileum to the antispasmodic action of amiodarone and again this effect was slow in onset and not reversible. The highest inhibition of ACh-induced contractions by amiodarone was obtained in guinea-pig stomach fundus. In these preparations treated with a low amiodarone concentration (20 microM) the response to ACh was restored after drug removal. In rat stomach fundus the effect of amiodarone was low and not reversible. Like amiodarone, Ca2+ lowering caused a decrease in the response to ACh with the following order of effectiveness: guinea-pig stomach greater than guinea-pig duodenum greater than guinea-pig ileum greater than rat stomach. The inhibitions caused by amiodarone and by low Ca2+ were always additive. These results indicate that amiodarone exerts an antispasmodic effect on the gastrointestinal tract and that regional and species differences exist for this action. The possible involvement of Ca2+ in this effect is discussed.

Acetylcholine↗