Presynaptic ?2-autoreceptors in mouse heart atria: evidence for the ?2D subtype

Abstract

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.

Distinct mixtures of muscarinic receptor subtypes mediate inhibition of noradrenaline release in different mouse peripheral tissues, as studied with receptor knockout mice

“…In control experiments without carbachol the overflow peaks were similar from S 1 to S 6 , giving S n /S 1 ratios close to unity (data not shown; compare Wahl et al, 1996;Trendelenburg et al, 1999;2003a). The antagonists pirenzepine and methoctramine, when present throughout superfusion, did not change the stimulationevoked overflow of tritium (S 1 ; M 3 -knockout vas deferens).…”

“…Presynaptic muscarinic receptors in the mouse NMRI, M 2 -knockout, M 4 -knockout and their corresponding wild-type control mice (data not shown; Wahl et al, 1996;Trendelenburg et al, 1999;2003a). In control experiments without carbachol the overflow peaks were similar from S 1 to S 6 , giving S n /S 1 ratios close to unity (data not shown; compare Wahl et al, 1996;Trendelenburg et al, 1999;2003a).…”

Presynaptic α2A/D-autoreceptors in the brain cortex of Cercopithecus aethiops

“…There is now much evidence to support the suggestion (Trendelenburg et al 1993) that presynaptic a 2 -autoreceptors, the prototype a 2 -adrenoceptors (Langer 1974;Starke 1977), belong to the a 2A/D branch of the a 2 -adrenoceptor tree (recent studies: Limberger et al 1995a;Starke et al 1995;Trendelenburg et al 1996a;Wahl et al 1996). An apparent exception are the a 2 -autoreceptors in the human kidney and in human atria which were classified as a 2C Rump et al 1995), in contrast to the autoreceptors in the human saphenous vein (Molderings and Göthert 1995) and probably brain cortex (Raiteri et al 1992; see Introduction of Limberger et al 1995a) which obey the rule and are a 2A .…”

“…The genetic a 2A/D subtype represents orthologous receptors, derived from a common ancestor gene, but with sufficient inter-species differences in drug affinities to yield the two pharmacological subtypes a 2A and a 2D . The a 2A/D subtype with a 2A pharmacology occurs in humans, rabbits and pigs, whereas the subtype with a 2D pharmacology occurs in rats, mice, guinea pigs and cows (see Bylund et al 1994;Trendelenburg et al 1996 a).…”

Rilmenidine reveals differences in the pharmacological characteristics of prejunctional α2‐adrenoceptors in the guinea‐pig, rat and pig

“…However, pharmacological di erences have been revealed either by determining the rank order of potency of an extensive range of antagonists, followed by comparison with known preparations based on correlation coe cient (Renouard et al, 1994;O'Rourke et al, 1994;Trendelenburg et al, 1995), or establishing the potency ratio for pairs of antagonists (Molderings & GoÈ hert, 1995;Trendelenburg et al, 1996a). For example, phentolamine has been shown to be approximately 5 fold more potent than rauwolscine at a 2D -adrenoceptors (Funk et al, 1995;Trendelenburg et al, 1996b;Wahl et al, 1996), while the reverse is true at a 2A -adrenoceptors (Trendelenburg et al, 1994;Molderings & GoÈ hert, 1995). To date, however, there have been no studies concerning the comparative e ect of putative agonists at a 2A -and a 2D -adrenoceptors.…”