A re-investigation of questionable subclassifications of presynaptic α2-autoreceptors: rat vena cava, rat atria, human kidney and guinea-pig urethra

Abstract

It has been suggested that at least the majority of mammalian presynaptic α₂-autoreceptors belong to the genetic α_2A/D-adrenoceptor subtype. The aim of the present study was to re-examine the α₂-autoreceptors in tissues in which previous assignments conflicted with this α_2A/D rule: in the rat vena cava and rat heart atria, where the autoreceptors were classified as α_2B or similar to, but not identical with, α_2D, and in the human kidney, where they were classified as α_2C. Also re-examined were the autoreceptors in the guinea-pig urethra, where they were suggested to be α_2A, in agreement with the rule, but in contrast to indications that the α_2A/D-adrenoceptor of the guinea pig possesses α_2D pharmacological properties. Tissue pieces were preincubated with ³H-noradrenaline and then superfused and stimulated electrically under autoinhibition-free or almost autoinhibition-free conditions. The K _d values of up to 14 antagonists (including the partial agonist oxymetazoline) against the release-inhibiting effect of the α₂ agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) were determined.

EC

₅₀ between 6.3 and 13.2 nM. All antagonists (except prazosin in one case) shifted the concentration-inhibition curve of UK 14,304 to the right. Comparison of the K _d values thus obtained with K _d values at known α₂ subtypes indicated that the autoreceptors in the rat vena cava, rat atria and the guinea-pig urethra were α_2D and those in the human kidney α_2A. For example, the pK _d values of the antagonists in the rat vena cava, in rat atria and in the guinea-pig urethra were closely correlated with pK _d values at the prototypic α_2D radioligand binding sites in the bovine pineal gland (r = 0.96, P < 0.001; r = 0.92, P < 0.01; and r = 0.95; P < 0.001) and with the pK _d values at the α_2D-autoreceptors of guinea-pig atria (r = 0.99, P < 0.001; r = 0.95, P < 0.001; and r = 0.98, P < 0.001). The pK _d values at the autoreceptors in rat vena cava, rat atria and guinea-pig urethra were not significantly or more loosely correlated with pK _d values at α_2A, α_2B and α_2C binding sites and α_2A-autoreceptors. On the other hand, the pK _d values of the antagonists in the human kidney were closely correlated with pK _d values at the prototypic α_2A radioligand binding sites in HT29 cells (r = 0.95; P < 0.001) and with pK _d values at the α_2A-autoreceptors of the pig brain cortex (r = 0.97; P < 0.001), but were not significantly or more loosely correlated with pK _d values at α_2B, α_2C and α_2D binding sites and α_2D-autoreceptors.

2D

, those in the human kidney α_2A, and those in the guinea-pig urethra equally α_2D. All, therefore, conform to the rule that α₂-autoreceptors belong at least predominantly to the genetic α_2A/D subtype of the α₂-adrenoceptor. The apparent paradox of an α_2A-autoreceptor in the urethra of the guinea pig, a species in which the genetic α_2A/D-adrenoceptor otherwise has α_2D pharmacological properties, is removed.