Subclassification of presynaptic α-adrenoceptors: α-autoreceptors in guinea-pig atria and brain.

The study was devised to classify, by means of antagonist and agonist affinities, the presynaptic α-autoreceptors in guinea-pig heart atria and brain cortex in terms of α, α, α and α. 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-noradrenaline and then superfused and stimulated electrically. In one series of experiments (atria only), tissue pieces were stimulated by relatively long pulse trains (1 min) leading to marked α-autoinhibition. All 10 antagonists increased the evoked overflow of tritium. pEC values (concentrations causing 30% increase) were interpolated from concentration-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) α-autoinhibition, and antagonist effects against the α-selective agonist 5-bromo-6(2-imidazolin-2-ylamino)-quinoxaline UK 14,304 were examined. All 10 (atria) or 8 (brain) antagonists shifted the concentration-inhibition curve of UK 14,304 to the right. pK 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 values (negative logarithms of dissociation constants of agonist-α-adrenoceptor complexes) were determined by comparison of concentration-inhibition curves of UK 14,304, guanoxabenz and oxymetazoline in normal tissue and in tissue in which a fraction of the receptors had been blocked by phenoxybenzamine. pEC values in atria correlated with pK values ( r = 0.942). pK values in atria correlated with pK values in the brain cortex ( r = 0.970). It is concluded that the α-autoreceptors in atria and the brain cortex are the same. Comparison with antagonist affinities for prototypic native α binding sites, binding sites in cells transfected with α subtype genes, and previously classified presynaptic α-adrenoceptors - all taken from the literature - indicates that both autoreceptors are α. In atria, this identification is reached with either of the two independent estimates of autoreceptor affinity, pEC and pK, and in the brain cortex it is supported by the agonist pK values. The results are compatible with the hypothesis that at least the majority of α-autoreceptors belong to the α branch of the α-adrenoceptor tree. [ABSTRACT FROM AUTHOR]