RESUMEN
AIM: To find if tacrine exerts a sensitizing effect on the cholinergic receptors of gastric smooth muscles, and study some of the mechanisms inducing it and measure the relative intensity of tacrine's effects on contractile activity. MATERIAL AND METHODS: Isometric recording of the mechanical activity of gastric smooth muscle preparations; determination of acetyl-cholinesterase activity in smooth-muscle tissue homogenates using Ellman's method. RESULTS: We found that the threshold concentration for tacrine not reducing the acetylcholinesterase activity and not having an effect on the smooth muscle preparations was 1 x 10(-8) mol/l. This concentration, however, significantly increased the acetylcholine-induced contraction compared with the controls, after the smooth-muscle tissue was incubated for 60 or 100 min. Treating smooth-muscle preparations with tacrine in a concentration of 5 x 10(-6) mol/l triggered a contraction induced by the drug's anti-cholinesterase activity. A secondary contraction was induced after 38.6 +/- 5.6 min. There was no secondary contraction after the control acetylcholine-induced effect. Atropine (1 x 10(-6) mol/l) inhibits this effect. Preliminary treatment of smooth muscle preparations with hexamethonium (1 x 10(-6) mol/l) did not change significantly the intensity of the first phase of tacrine-induced contraction and shifted in time the appearance of the second contractile phase. CONCLUSION: Tacrine has a sensitizing effect on M-cholinergic receptors; it occurs after a long incubation of the gastric smooth muscles with the drug and is manifested as a secondary contraction which is shifted in time and is significantly inhibited by atropine.
Asunto(s)
Inhibidores de la Colinesterasa/farmacología , Músculo Liso/efectos de los fármacos , Receptores Muscarínicos/efectos de los fármacos , Estómago/efectos de los fármacos , Tacrina/farmacología , Acetilcolina/farmacología , Animales , Masculino , Contracción Muscular/efectos de los fármacos , Músculo Liso/fisiología , Ratas , Ratas Wistar , Estómago/fisiologíaRESUMEN
INTRODUCTION: Tacrine is an acetylcholinesterase inhibitor. It has an indirect cholinomimetic effect inducing contractions of the gastric smooth muscles. The contractions are related to the effect of the accumulated acetylcholine in tissues on the respective choline receptors. There is a well defined direct correlation between tacrine concentrations and the inhibition of cholinesterase activity. That suggest simultaneous increase of the strength of tacrine-induced contractions. Instead, at concentrations above 1 x 10(-5) mol/l, tacrine causes permanent relaxation with yet unknown causing mechanism. AIM: The aim of this study was to investigate if tacrine induces reduction of calcium ions through chelation and/or inhibits directly calmodulin's participation in the contractile processes, thus causing smooth muscle relaxation which is not characteristic of a typical acetylcholinesterase blocker. METHODS: The contractile activity of smooth muscle preparations was measured isometrically with a Microtechna (Czech Republic) amplifier and recorded by a Linseis (Germany) recorder. The absorption electron spectrum of tacrine (1 x 10(-4) mol/l) was determined with a Cary 1 (Varian, Australia) spectrophotometer. The concentration of ionized CaCa2+ was measured with the ISE-block of a clinical-chemical analyzer Konelab 60 (Finland). RESULTS: The presence of Ca2+ (10(-2) mol/l) does not alter tacrine characteristic absorption spectrum at pH values corresponding to the SM cell cytosolic pH. The presence of 1 x 10(-4) mol/l tacrine does not affect Ca2+ concentration in the Krebs solution (pH = 7.4). In the presence of trifluoperazine (a calmodulin blocker) 1 x 10(-4) mol/l tacrine causes relaxation which is commensurable with that in the controls. CONCLUSIONS: Tacrine-induced smooth muscle relaxation is not a result of the reduction of the effective Ca2+ concentrations as a result of chelation between tacrine and Ca2+ and it is not related to the tacrine effects on calmodulin.