Selective Potentiation By Doxazosin Of 5-HT-induced Contractile Responses In The Isolated Longitudinal Muscle Of Rabbit Gastric Body

Objective: To investigate the potentiation of 5-HT-induced contraction by doxazosin enantiomers and its related mechanism in the isolated longitudinal muscle strips of the rabbit gastric body.Methods: Adult male New Zealand white rabbits (2.0³.0kg) were anesthetized by intravenous injection of sodium pentobarbital (30mg?kg-1) and then were killed by arterial exsanguination. After the abdomen was opened, the stomach was removed quickly and opened along the lesser curvature. The stomach was washed with ice-cold Krebs-Henseleit (K-H) solution. After removal of excess connective tissue and fat, the upper gastric body of anterior surface side was excised and pinned flat on a dish containing ice-cold K-H solution with the mucosa layer facing downwards. Four longitudinal, full-thickness muscle strips (8mm in length and 2mm in width) with the mucosal layer were obtained from the place situated at 1 cm-distance from the greater curvature in anterior surface side of the upper gastric body and then the mucosal layer was removed gently with fine tweezers. Silk ligatures were tied at each end of the muscle strips, one end was attached to a holder and the other to an isometric tension transducer coupled to a BL-420E+ biological and functional experimental system (Co Ltd TaiMeng Technology, Chengdu, China) to record a change in tension of the preparations. The longitudinal muscle strips were mounted in a 10 ml organ bath containing 37℃K-H solution aerated with 95% O2 and 5% CO2. An initial resting tension of 2g was applied to the preparations, which were then allowed to equilibrate for 1h. At the end of this period the tension in the strip was taken as the resting tension and no further mechanical adjustment was made during experimentation.Results:1. Effects of (±)DOX on the contractile responses to carbachol, histamine and 5-HT in the isolated longitudinal muscle strips of the rabbit gastric bodyCarbachol (0.01³0μmol·L^-1), histamine (0.03³00μmol·L^-1) and 5-HT (0.01³0μmol·L^-1) produced contractile responses concentration-dependently in the longitudinal muscle strips of the rabbit gastric body. The concentration-response curves for carbachol or 5-HT were constructed for 4 times, and those for histamine were constructed for 3 times in solvent groups. Solvent did not significantly affect the contractile responses to carbachol, histamine and 5-HT (P>0.05).The results of two way ANOVA analysis indicated that (±)DOX at 30μmol·L-1 decreased the contractile responses to carbachol in the isolated longitudinal muscle strips significantly (P<0.01), but a multiple comparison post test did not show a significant difference between two groups (P>0.05). (±)DOX (3³0μmol·L-1) did not significantly affect the contractile responses to histamine (P>0.05), however, the contractile responses to 5-HT were significantly potentiated by (±)DOX (3³0μmol·L-1) in a concentration-dependent manner in the longitudinal muscle strips of the rabbit gastric body (P<0.01).2. Effects of (+)DOX and (-)DOX on the contractile responses to carbachol, histamine and 5-HT in the isolated longitudinal muscle strips of the rabbit gastric bodyInhibition by (+)DOX and (-)DOX (30μmol·L^-1) of carbachol-induced contractile responses was similar to that by (±)DOX. (+)DOX (3³0μmol·L^-1) did not affect the contractile responses to histamine significantly (P>0.05). The results of two way ANOVA analysis indicated that 30μmol·L^-1 (-)DOX decreased the contractile responses to histamine in the isolated longitudinal muscle strips slightly but significantly (P<0.01), a multiple comparison post test, however, did not show a significant difference between two groups. The potentiation by (+)DOX or (-)DOX (3³0μmol·L^-1) of 5-HT-induced contractile responses was similar to that by (±)DOX.3. Effects of phenoxybenzamine and tetrodotoxin (TTX) on the potentiation by (±)DOX of 5-HT-induced contractile responses in the isolated longitudinal muscle strips of the rabbit gastric bodyThe concentration-response curves for 5-HT before treatments with solvent, phenoxybenzamine and (±)DOX in the isolated longitudinal muscle strips were not significantly different from each other (P>0.05). Pretreatment with phenoxybenzamine (3μmol·L^-1) or TTX (0.3μmol·L^-1) affected neither the contractile responses to 5-HT nor the potentiation by (±)DOX of 5-HT-induced contractile responses in the isolated longitudinal muscle strips of the rabbit gastric body (P>0.05).4. Effects of (±)DOX and 5-HT in the isolated longitudinal muscle strips of the rabbit gastric body precontracted with KClContractile responses were induced by KCl in the longitudinal muscle strips in a concentration-dependent manner. In the preparations precontracted with KCl (30 mmol·L^-1), papaverine (100μmol·L^-1) produced obviously relaxant responses. (±)DOX (3³0μmol·L^-1) and 5-HT (0.03³0μmol·L^-1) did not produce obviously relaxant responses in the isolated longitudinal muscle strips of the rabbit gastric body precontracted with KCl.5. Effects of prazosin, terazosin, alfuzosin and DOX on the contractile responses to 5-HT in the isolated longitudinal muscle strips of the rabbit gastric bodyDOX (3³0μmol·L^-1, Sigma) significantly potentiated the contractile responses to 5-HT (P<0.01) in a concentration-dependent manner. Prazosin at highest concentration of 30μmol·L^-1 decreased the contractile responses to 5-HT slightly but significantly (P<0.01). Terazosin (3³0μmol·L^-1) potentiated the contractile responses to 5-HT slightly but significantly (P<0.01), and the potentiation by terazosin was not in a concentration-dependent manner. Alfazosin (3³0μmol·L^-1) did not affect the contractile responses to 5-HT in the isolated longitudinal muscle strips of the rabbit gastric body (P>0.05).Conclusion: The present results indicate that (±)DOX at a higher concentration specifically potentiates the contractile responses to 5-HT in the isolated longitudinal muscle strips of the rabbit gastric body. There is no chiral selectivity in the potentiation of 5-HT-induced contractile responses between (+)DOX and (-)DOX. Among the quinazoline-derivedα1-adrenoceptor antagonists, prazosin, terazosin and alfazosin are not able to produce a similar potentiation effect on 5-HT-induced contractile responses like DOX in the isolated longitudinal muscle strips of the rabbit gastric body.