CH-38083, a selective, potent antagonist of alpha-2 adrenoceptors

The selectivity and specificity of CH-38083 [7,8-(methylenedioxi)-14-alpha-hydroxyalloberbane HCl], a berbane derivative for alpha adrenoceptors has been studied and compared with yohimbine and idazoxan in peripheral tissues and in the central nervous system. In isolated tissue experiments CH-38083 was a competitive antagonist at presynaptic alpha-2 adrenoceptors on the axon terminals of the rat vas deferens (pA2 against xylazine = 8.17 +/- 0.06) and of the longitudinal muscle strip of guinea pig ileum (pA2 against xylazine = 8.07 +/- 0.20). As far as its postsynaptic alpha-2 adrenoceptor antagonistic activity is concerned its affinity in rat vas deferens (pA2 = 4.95 +/- 0.11) against l-phenylephrine and in rabbit pulmonary artery (pA2 = 5.38 +/- 0.33 against l-norepinephrine) was markedly less than that displayed for presynaptic sites. From pA2 values obtained in rat vas deferens the calculated alpha-1/alpha-2 adrenoceptor selectivity ratios for yohimbine, idazoxan and CH-38083 were 4.7, 117.5 and 1659, respectively. CH-38083 failed to show any affinity for histamine and muscarinic receptors and it even potentiated the effect of serotonin on atropinized longitudinal muscle strip of guinea pig ileum. It enhanced the release of [3H]norepinephrine from electrically stimulated mouse vas deferens loaded previously with labeled [3H]norepinephrine. In binding studies carried out in rat brain membrane preparations using [3H]prazosin and [3H]idazoxan, the selectivity ratios (Ki alpha-1/Ki alpha-2) proved to be 32.5, 289.5 and 1368 for yohimbine, idazoxan and CH-38083, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)