Pharmacological Characteristics Of The Responses In The Muscle Strips Isolated From Different Regions Of Rat Stomach

Objective: To investigate the effects of stretch on M receptor- and 5-HT receptor-mediated contractile responses in the isolated circular muscle strips taken from the rat stamoch, and the mechanisms of phenylephrine-induced relaxant responses in the isolated longitudinal muscle strips taken from the rat gastric fundus and body.Methods: The isolated circular muscle strips were prepared from the rat gastric fundus, body, antrum, cardia and pylorus, and their contractile responses to carbachol?CCh? or 5-hydroxytryptamine?5-HT? were recorded under different stretch. The isolated longitudinal muscle strips were prepared from the rat gastric fundus and body, and phenylephrine-induced relaxant responses were recorded in the preparations precontracted with CCh.Results:1 Effects of stretch on M receptor- and 5-HT receptor-mediated contractile responses in the isolated circular muscle strips taken from the rat stamochThe-Log EC50 values for CCh in the isolated circular muscle strips of the gastric fundus, body and pylorus under 1 g preload were the largest, and the values decreased slightly with higher preloads significantly?P < 0.05 and 0.01?. Similar result was obtained in the isolated circular muscle strip of the gastric body when 5-HT was used as agonist. The Emax values of contractile responses to CCh and 5-HT in the circular muscle strips of the gastric cardia under 3 g preload were increased by 117.4% and 75.7% when compared to that under 1 g preload. The Emax value of contractile response to 5-HT in the circular muscle strips of the gastric body under 3 g preload was also increased by 115.9% when compared to 1 g preload. The Emax?g? values of contractile responses to CCh in four types of muscle strips were 10.453 ± 2.956?cardia under 3 g preload?, 13.878 ± 2.618?fundus under 2.5 g preload?, 10.244 ± 1.843?body under 2.5 g preload? and 2.585 ± 1.098?pylorus under 2.5 g preload?, respectively. The Emax?g? values of contractile responses to 5-HT in three types of muscle strips were 4.363 ± 1.705?cardia under 3 g preload?, 3.931 ± 0.615?fundus under 3 g preload? and 3.161 ± 0.680?body under 3 g preload?, respectively.2 Relaxant responses induced by ?₁-adrenoceptor agonst in the isolated longitudinal muscle strip of the rat gastric fundus and body5-HT(0.0001³0 ?mol ? L^-1) or carbachol(0.001¹0 ?mol ? L^-1) produced contractile responses in a concentration-dependent manner in the isolated longitudinal muscle strips of the rat gastric body. The concentration-response curves for 5-HT or carbachol were constructed for 4 times, and solvent did not affect the concentration-response curves for 5-HT or carbachol significantly?P>0.05?. The results of two way ANOVA analysis indicated that doxazosin(30 ?mol?L^-1) decreased the contractile responses to 5-HT in the isolated longitudinal muscle strips of the rat gastric body significantly?P?27?0.01?. Phenylephrine(30 and 300 ?mol?L^-1) also decreased the contractile responses to carbachol in the isolated longitudinal muscle strips of the rat gastric body significantly?P?27?0.01?.Phenylephrine(0.01³00 ?mol?L^-1) administered cumulatively or non-cumulatively produced relaxant responses in the isolated longitudinal muscle strips of the rat gastric body in a concentration-dependent manner, and phenylephrine(300 ?mol?L^-1) produced the relaxant responses by 16.36±4.62%?cumulative administration? and 38.05±6.43%?non-cumulative administration?.Propranolol(0.001¹ ?mol?L^-1) concentration-dependently and significantly inhibited the relaxant responses induced by phenylephrine in the isolated longitudinal muscle strips of the rat gastric fundus. Propranolol at 0.001, 0.01, 0.1 and 1 ?mol ? L^-1 inhibited the phenylephrine-induced relaxant response by 22.02±14.09%, 25.86±11.51%, 34.35±9.81% and 37.16±7.78% significantly?P?27?0.01?. Doxozosin at 0.001⁰.1 ?mol?L^-1 did not significantly affect phenylephrine-induced relaxant responses in the isolated longitudinal muscle strips of the rat gastric fundus?P>0.05?, but 1 ?mol?L^-1 doxozosin significantly inhibited the relaxant responses by 61.81±26.53%?P?27?0.01?. Yohimbine(0.03¹ ?mol?L^-1) did not affect the relaxant responses induced by phenylephrine in the isolated longitudinal muscle strips of the rat gastric fundus?P>0.05?. TTX(0.001⁰.1 ?mol?L^-1) also did not show the significant effects on the relaxant responses induced by phenylephrine in the isolated longitudinal muscle strips of the rat gastric fundus?P>0.05?. Though prasosin at 0.001⁰.1 ?mol?L^-1 did not affect the relaxant responses induced by phenylephrine in the isolated longitudinal muscle strips of the rat gastric fundus, it at 1 ?mol?L^-1 inhibited the responses by 52.41±18.61% significantly?P?27?0.01?.Moreover, propranolol, doxazosin, prazosin, yohimbine and TTX produced the similler effects on the relaxant responses induced by phenylephrine in the isolated longitudinal muscle strips of the rat gastric body like that of the rat gastric fundus.Conclusion:1 Results of the present study indicate that 0.5 g or 1.0 g preload is inadequate to accurately measure the contractile responses mediated by M receptors or 5-HT receptors in the isolated circular muscle strips taken from the rat gastric cardia, fundus, body and pylorus.2 The relaxant responses to phenylephrine are partially mediated by ?1- or ?2-adrenoceptors in the isolated longitudinal muscle strips of the rat gastric fundus and body precontracted by carbachol, and a large portion of the relaxant response induced by phenylephrine beongs prazosin-resistant response.