Pharmacological Effects Of Doxazosin Enantiomers In The Rat Isolated Mesenteric Resistance Arteries

Objective: To analyse the blocking effects of （-）doxazosin,（+）doxazosinand （±）doxazosin on α₁-adrenoceptors in the rat isolated mesenteric resistancearteries, and observe the characteristics of vasoconstrictive responses toagmatine （Agm） and α-adrenoceptor agonists in the rat isolated mesentericresistance arteries.Methods: Preparations of the second and third generation branches of therat mesenteric artery were dissected and prepared. Cumulativeconcentration-vasoconstriction curves for noradrenaline （NA）, phenylephrine（Phe）, moxonidine （Mox）, clonidine （Clo）, xylazine （Xyl） and agmatine （Agm）were recorded using DMT wire myograph system620M.Results:1Optimal preload determined in the second branch of the rat mesentericarteryNormalized preloads calculated by the DMT normalization software were3.27±0.95mN （n=32）. Under5mN preload, however, vasoconstrictiveresponses to Phe at3μmol L^-1and300μmol L^-1were significantly larger thanthat under normalized preloads （P<0.05and P<0.01）. On the other hand,vasoconstrictive response to Phe at3μmol L^-1was significantly smaller inthe preparations under10mN preloads than that under normalized preloads（P<0.05）. Under20mN preload, vasoconstrictive responses to Phe at3μmol L^-1,10μmol L^-1and300μmol L^-1were significantly smaller than that undernormalized preloads （P<0.05and P<0.01）.2Optimal preload determined in the third branch of the rat mesentericarteryNormalized preloads calculated by the DMT normalization software were1.09±0.33mN （n=7）. The values of Emaxand-LogEC50obtained from the concentration-vasoconstriction curves for Phe and the vascular responses toeach concentrations of Phe in the preparations under3,4or5mN preloadswere not significantly different from that under normalized preloads （P>0.05）.3Effects of doxazosin and its enantiomers on phenylephrine-inducedvasoconstrictive responses in the rat isolated mesenteric resistancearteriesSolvent significantly affected neither the values of Emaxand-LogEC50obtained from the concentration-vasoconstriction curves for Phe nor thevascular responses to each concentrations of Phe in the second and thirdgeneration branches of the rat mesenteric artery （P>0.05）.In the second branch of the rat mesenteric artery, Phe (0.03³000μmol L^-1) produced concentration-dependent vasoconstrictive responses, and（-）doxazosin and （+）doxazosin at0.001μmol L^-1,0.01μmol L^-1and0.1μmol L^-1shifted the concentration-response curves of Phe to the rightwithout significant change in Emaxvalues （P>0.05）. Schild plot analysisindicated that （-）doxazosin and （+）doxazosin non-competitively inhibited thevasoconstrictive responses to Phe.（±）Doxazosin at the same concentrations as（-）doxazosin and （+）doxazosin shifted the concentration-response curves ofPhe to the right without significant change in Emaxvalues （P>0.05）. Schild plotanalysis indicated that （±）doxazosin competitively inhibited thevasoconstrictive responses to Phe. The pKBvalue （8.83） of （+）doxazosin waslarger than that of （±）doxazosin （8.37）（P>0.05） and significantly larger than（-）doxazosin （8.07）（P<0.01）.In the third branch of the rat mesenteric artery, Phe (0.03³000μmol L^-1)produced concentration-dependent vasoconstrictive responses, and（-）doxazosin,（+）doxazosin or （±）doxazosin at0.001μmol L^-1,0.01μmol L^-1and0.1μmol L^-1shifted the concentration-response curves of Phe to theright without significant change in Emaxvalues （P>0.05）. Schild plot analysisindicated that （-）doxazosin,（+）doxazosin and （±）doxazosin non-competitivelyinhibited the vasoconstrictive responses to Phe. The pKBvalue （9.45） of（-）doxazosin was significantly larger than that of either （±）doxazosin （8.43） or （+）doxazosin （8.00）（P<0.01）.4Profile of the vasoconstrictive responses to agmatine and fiveα-adrenoceptor agonists in the rat isolated mesenteric resistance arteriesWe attempted to construct the concentration-vasoconstriction curves forNA (0.001³0μmol L^-1, Phe (0.01³00μmol L^-1, Xyl (0.001³00μmol L^-1, Clo (0.001³0μmol L^-1, Mox (0.001³00μmol L^-1 and Agm(0.001³000μmol L^-1 in the second and third generation branches of the ratmesenteric artery. Vasoconstrictive responses were expressed as percentage ofthe initial maximal vasoconstriction to NA.When the second branch of the rat mesenteric artery was exposed to Agmand α-adrenoceptor agonists for the first time （at the beginning of the study）,the rank order of the maximal vasoconstrictions induced by these agents wasPhe=NA>Mox, and the maximal vasoconstrictions to Clo and Agm were3.94%and0.17%respectively. Xyl did not produce obviouslyvasoconstrictive response. The rank order of the EC50values (mmol L^-1 wasNA=Phe<Mox. After multiple exposures to Agm and α-adrenoceptor agonists,the maximal vasoconstriction to the last exposure to Mox （at the end of thestudy） was significantly enhanced from88.23%（at the beginning of the study）to107.60%（P<0.05）, and the maximal vasoconstrictions to the last exposureto Clo and Xyl were increased to47.44%and18.17%（P<0.01）. The rankorder of the EC50values (mmol L^-1 was NA=Phe=Clo<Mox<Xyl in thesecond branch of the rat mesenteric artery at the end of the study.When the third branch of the rat mesenteric artery was exposed to Agmand α-adrenoceptor agonists for the first time （at the beginning of the study）,the rank order of the maximal vasoconstrictions induced by these agents wasPhe=NA=Mox, and the maximal vasoconstriction to Clo was1.07%. Agm andXyl did not evoke obviously vasoconstrictive response. The rank order of theEC50values (mmol L^-1 was NA<Phe<Mox. After multiple exposures toAgm and α-adrenoceptor agonists, the maximal vasoconstrictions to the lastexposure to Phe and Mox did not change significantly （P>0.05）, and themaximal vasoconstrictions to the last exposure to Clo and Xyl were significantly increased to33.35%and1.74%（P<0.01）. Agm did not produceobviously vasoconstrictive response. The rank order of the EC50values (mmolL^-1) was NA=Phe<Mox in the third branch of the rat mesenteric artery at theend of the study.Conclusion:1Normalized preloads calculated by the DMT normalization softwarecan be used in the third but not second branch of the rat mesenteric artery.2（±）Doxazosin has the same blocking activity at α₁-adrenoceptors in thesecond and third branches of the rat mesenteric artery.（-）Doxazosinantagonizes α₁-adrenoceptors more potently in the third branch than thesecond branch, however the α₁-adrenoceptor blocking activity of （+）doxazosinin the both branches is opposite to （-）doxazosin.3After multiple exposures to Agm and α-adrenoceptor agonists,α₂-adrenoceptor-mediated vasoconstrictive responses become much morevisible and prominent in the second branch of the rat mesenteric artery.