Prejunctional alpha2A-autoreceptors in the canine saphenous vein.

The aim of the study was to determine the subtype of prejunctional alpha2-autoreceptors in the canine saphenous vein. Segments of the vein were incubated with 3H-noradrenaline and subsequently perifused with modified Krebs-Henseleit solution. Five periods of electrical stimulation were applied (S1-S5; each for 2 min, 1 Hz). Concentration-response curves for the inhibitory effect of the alpha2-adrenoceptor agonists oxymetazoline and UK-14,304 and for the facilitatory effect of nine antagonists were determined. Correlations between the pEC30%s for the antagonists obtained in the present study and the pKis for the same antagonists obtained in tissues expressing only alpha2A- (HT29 cells), alpha2B- (neonatal rat lung), alpha2C- (OK cells) or alpha2D-adrenoceptors (rat submaxillary gland) showed that the alpha2-autoreceptors in the canine saphenous vein resemble most closely the alpha2A-subtype. Furthermore, oxymetazoline was a highly potent agonist (pIC50% = 8.10) and prazosin was a weak antagonist (pEC30% = 6.46), confirming that the alpha2-adrenoceptors involved in the modulation of the response to electrical stimulation of the canine saphenous vein do not belong to either the alpha2B- or alpha2C-subtype. On the other hand, the EC30% ratios phentolamine/rauwolscine and idazoxan/rauwolscine were much closer to the ratios obtained at alpha2-autoreceptors of the rabbit- (alpha2A) than of the guinea-pig brain cortex (alpha2D). The results suggest that the sympathetic nerves of the canine saphenous vein are endowed with alpha2A-adrenoceptors.