Antagonists that differentiate between alpha(2A)- and alpha(2D)-adrenoceptors

Four antagonists were examined for their ability to differentiate alpha(2A)- from the orthologous alpha(2D)-adrenoceptors. The antagonists were (2S,12bS) 1', 3'-dimethylspiro(1, 3, 4, 5', 6, 6', 7, 12b-octahydro-2H-benzo-[b]furo[2, 3-a]quinolizine)-2, 4'-pyrimidin-2'-one (MK 912), 2-[2-(methoxy-1, 4-benzodioxanyl)imidazoline (RX 821002), efaroxan and benoxathian. The a,autoreceptors in rabbit brain cortex were chosen as alpha(2A)- and the alpha(2)-autoreceptors in guinea-pig brain cortex as alpha(2D)-adrenoceptors. Slices of the brain cortex were preincubated with H-3-noradrenaline and then superfused and stimulated electrically by brief pulse trains (4 pulses, 100 Hz) that led to little, if any, alpha(2)-autoinhibition. 5-Bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) was used as an alpha(2)-adrenoceptor agonist. UK 14,304 decreased the stimulation-evoked overflow of tritium. The antagonists shifted the concentration-inhibition curve of UK 14,304 to the right in an apparently competitive manner. Dissociation constants of the antagonists were calculated from the shifts. MK 912, RX 821002 and efaroxan had markedly higher affinity for (guinea-pig) alpha(2D)-adrenoceptors (pK(d) values 10.0, 9.7 and 9.1, respectively) than for (rabbit) alpha(2A)-adrenoceptors (pK(d) 8.9, 8.2 and 7.6, respectively). Benoxathian had higher affinity for alpha(2A)-(pK(d) 7.4) than for alpha(2D)-adrenoceptors (pK(d) 6.9). Ratios calculated from the K-d values of the four compounds differentiated between alpha(2A) and alpha(2D) up to 100 fold. It is concluded that MK 912, RX 821002, efaroxan and benoxathian are antagonists with high power to differentiate alpha(2A)- from alpha(2D)-adrenoceptors.