PRESYNAPTIC ALPHA(2)-AUTORECEPTORS IN MOUSE HEART ATRIA - EVIDENCE FOR THE ALPHA(2D) SUBTYPES

Presynaptic alpha(2)-autoreceptors in mouse atria were characterized i n terms of the alpha(2A), alpha(2B), alpha(2C) and alpha(2D) subtypes. Segments of the atria were preincubated with H-3-noradrenaline and th en superfused and stimulated electrically. The affinity of up to 16 an tagonists for the autoreceptors was assessed as (1) pEC(30%) values, i .e. concentrations that increased previously autoinhibited release of H-3-noradrenaline (120 pulses, 3 Hz) by 30%, and (2) pK(d) values agai nst the release-inhibiting effect of 5-bromo-6-(2-imidazolin-2-ylamino )-quinoxaline (UK 14,304) under conditions of no or little autoinhibit ion (2 trains of 20 pulses, 50 Hz, train interval 120 s). The pK(d) va lues correlated well with the pEC(30%) values (r=0.98; P<0.001; slope of regression line 0.93), indicating that UK 14,304 and released norad renaline modulated the release of noradrenaline through pharmacologica lly 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, a nd (3) previously classified presynaptic alpha(2)-autoreceptors - all taken from the literature - indicated that the mouse atrial autorecept ors corresponded to the alpha(2D) subtype. For example, the pK(d) valu es at mouse atrial autoreceptors correlated closely with pK(d) values at native alpha(2D) binding sites in the bovine pineal gland (r=0.96; P<0.001); with pK(d) values at alpha(2D) binding sites in COS cells tr ansfected with the rat alpha(2D) gene (r=0.85; P<0.01); and with pK(d) values at guinea-pig cerebral and atrial and mouse cerebral alpha(2D) -autoreceptors (r=0.96-0.98; P<0.001). The antagonist pK(d) values at mouse atrial autoreceptors correlated less with pK(d) values at alpha( 2A), alpha(2B) and alpha 2C sites. It is concluded that the presynapti c alpha(2)-autoreceptors in mouse atria are alpha(2D). This identifica tion supports the hypothesis that at least the majority of alpha(2)-au toreceptors belong to the alpha(2A/D) pair of orthologous alpha(2)-adr enoceptors.