Structure-activity relationship of yohimbine and its related analogs in blocking alpha-1 and alpha-2 adrenoceptors: a comparative study of cardiovascular activities.

We investigated the selectivities and structure requirements for alpha-1 and alpha-2 adrenoceptor blocking activities of yohimbine (YO) and its 12 related analogs, such as beta-yohimbine (beta-YO), dihydrocorynantheine (DHC) and (-)indoloquinolizidine ((-)IQ). The affinity of YO analogs to alpha-adrenoceptor was assessed by measuring their blockade of pressor responses to epinephrine in pithed rats. Among YO structure groups, the potency order was YO greater than DHC = beta-YO greater than geissoschizine methylether greater than 14 beta-hydroxy YO greater than 14 beta-benzoyloxy YO (inactive). (-)IQ was slightly less potent than YO, but much stronger than (+)IQ. Among (+/-)IQ structure groups, the potency order was (+/-)IQ greater than (+/-)1,12b-trans-1-hydroxy IQ much greater than (+/-)1,12b-cis-1-hydroxy IQ (inactive). (+/-)Borrerine was active, but (+/-)desmethylborrerine was inactive. The alpha-1 blocking activities of the four compounds YO, beta-YO, DHC and (-)IQ, were assessed in experiments of pressor responses to methoxamine in pithed rats and contractile responses to methoxamine in the rat vas deferens. The potency order was (-)IQ greater than YO greater than DHC greater than beta-YO. Furthermore, the alpha-2 blocking activities of the four analogs were assessed in experiments of pressor responses to clonidine and inhibition of electrically driven cardioacceleration by clonidine, in pithed rats. The potency order was YO greater than beta-YO greater than (-)IQ greater than DHC. Based on the potency ratio between alpha-1 and alpha-2 blocking activities, DHC or YO was most selective for alpha-1 or alpha-2 subtype, respectively, among the four YO analogs.(ABSTRACT TRUNCATED AT 250 WORDS)