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 3H-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 alpha 2-autoinhibition. All 10 antagonists increased the evoked overflow of tritium. pEC30% 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) alpha 2-autoinhibition, and antagonist effects against the alpha 2-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. pKd 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), pKa values (negative logarithms of dissociation constants of agonist-alpha 2-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.(ABSTRACT TRUNCATED AT 250 WORDS)