SUBCLASSIFICATION OF PRESYNAPTIC ALPHA(2)-ADRENOCEPTORS - ALPHA(2D)-AUTORECEPTORS IN GUINEA-PIG ATRIA AND BRAIN

The study was devised to classify, by means of antagonist and agonist affinities, the presynaptic alpha 2-autoreceptors in guinea-pig heart atria and brain cortex in terms of alpha(2A), alpha(2B), alpha(2C) and alpha(2D). A set of antagonists and agonists was chosen that was able to discriminate between the four subtypes. Small pieces of the atria and slices of the brain cortex were preincubated with H-3-noradrenalin e and then superfused and stimulated electrically. In one series of ex periments (atria only), tissue pieces were stimulated by relatively lo ng pulse trains (1 min) leading to marked alpha(2)-autoinhibition. All 10 antagonists increased the evoked overflow of tritium. PEC(30%) val ues (concentrations causing 30% increase) were interpolated from conce ntration-response curves. In a second series of experiments (atria and brain slices), tissue pieces were stimulated by brief pulse trains (0 .4 s or 40 ms) that led to Little (atria) or no (brain slices) alpha(2 )-autoinhibition, and antagonist effects against the alpha(2)-selectiv e agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) w ere examined. All 10 (atria) or 8 (brain) antagonists shifted the conc entration-inhibition curve of UK 14,304 to the right. pK(d) values of the antagonists were calculated from the shifts. In a third series of experiments (brain slices only), also with brief pulse trains (40 ms), pK(a) values (negative logarithms of dissociation constants of agonis t-alpha(2)-adrenoceptor complexes) were determined by comparison of co ncentration-inhibition curves of UK 14,304, guanoxabenz and oxymetazol ine in normal tissue and in tissue in which a fraction of the receptor s had been blocked by phenoxybenzamine. PEC(30%) values in atria corre lated with pK(d) values (r = 0.942). pK(d) values in atria correlated with pK(d) values in the brain cortex (r = 0.970). It is concluded tha t the alpha(2)-autoreceptors in atria and the brain cortex are the sam e. Comparison with antagonist affinities for prototypic native alpha(2 ) binding sites, binding sites in cells transfected with alpha(2) subt ype genes, and previously classified presynaptic alpha(2)-adrenoceptor s - all taken from the literature - indicates that both autoreceptors are alpha(2D). In atria, this identification is reached with either of the two independent estimates of autoreceptor affinity, PEC(30%) and pK(d), and in the brain cortex it is supported by the agonist pK(a) va lues. The results are compatible with the hypothesis that at least the majority of alpha(2)-autoreceptors belong to the alpha(2A/D) branch o f the alpha(2)-adrenoceptor tree.