Presynaptic alpha 2-autoreceptors in mouse heart atria: evidence for the alpha 2D subtype.
Presynaptic alpha 2-autoreceptors in mouse atria were characterized in terms of the alpha 2A, alpha 2B, alpha 2C and alpha 2D subtypes. Segments of the atria were preincubated with 3H-noradrenaline and then superfused and stimulated electrically. The affinity of up to 16 antagonists for the autoreceptors was assessed as (1) pEC30% values. i.e. concentrations that increased previously autoinhibited release of 3H-noradrenaline (120 pulses, 3 Hz) by 30%, and (2) pKd values against the release-inhibiting effect of 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) under conditions of no or little autoinhibition (2 trains of 20 pulses, 50 Hz, train interval 120 s). The pKd values correlated well with the pEC30% values (r = 0.98; P < 0.001; slope of regression line 0.93), indicating that UK 14,304 and released noradrenaline modulated the release of noradrenaline through pharmacologically identical receptors. Comparison with antagonist affinities for (1) prototypic native alpha 2 radioligand binding sites, (2) radioligand binding sites in COS cells transfected with alpha 2 subtype genes, and (3) previously classified presynaptic alpha 2-autoreceptors-all taken from the literature-indicated that the mouse atrial autoreceptors corresponded to the alpha 2D subtype. For example, the pKd values at mouse atrial auto-receptors correlated closely with pKd values at native alpha 2D binding sites in the bovine pineal gland (r = 0.96; P < 0.001); with pKd values at alpha 2D binding sites in COS cells transfected with the rat alpha 2D gene (r = 0.85; P < 0.01); and with pKd values at guinea-pig cerebral and atrial and mouse cerebral alpha 2D-autoreceptors (r = 0.96-0.98; P < 0.001). The antagonist pKd values at mouse atrial autoreceptors correlated less with pKd values at alpha 2A, alpha 2B and alpha 2C sites. It is concluded that the presynaptic alpha 2-autoreceptors in mouse atria are alpha 2D. This identification supports the hypothesis that at least the majority of alpha 2-autoreceptors belong to the alpha 2A/D pair of orthologous alpha 2-adrenoceptors.
Wahl
C A
CA
Pharmakologisches Institut, Freiburg i.Br., Germany.
Trendelenburg
A U
AU
Starke
K
K
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology*; D000818:Animals; D017660:Autoreceptors / Q000145:classification* / Q000187:drug effects / Q000502:physiology; D000068438:Brimonidine Tartrate; D002417:Cattle; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D006321:Heart / Q000294:innervation*; D006325:Heart Atria / Q000737:chemistry / Q000294:innervation; D008297:Male; D051379:Mice; D009206:Myocardium / Q000737:chemistry*; D008771:Nordefrin / Q000037:antagonists & inhibitors / Q000494:pharmacology; D011810:Quinoxalines / Q000494:pharmacology; D051381:Rats; D018341:Receptors, Adrenergic, alpha-2 / Q000145:classification* / Q000187:drug effects / Q000502:physiology; D014316:Tritium / Q000494:pharmacology; D014662:Vasoconstrictor Agents / Q000037:antagonists & inhibitors / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D017660:Autoreceptors; D011810:Quinoxalines; D018341:Receptors, Adrenergic, alpha-2; D014662:Vasoconstrictor Agents; D014316:Tritium; D000068438:Brimonidine Tartrate; D008771:Nordefrin