Alpha-1 and alpha-2 adrenoceptor: response coupling in canine saphenous and femoral veins

The aim of the present study was to analyze alpha-1 and alpha-2 adrenoceptor response coupling in isolated canine blood vessels. Rings of saphenous and femoral veins and of femoral arteries were suspended for isometric tension recording in modified Krebs-Ringer bicarbonate solution, gassed with 95% O2-5% CO2 and maintained at 37 degrees C. Dissociation constants for the alpha-1 adrenergic agonists, phenylephrine and cirazoline, and the alpha-2 adrenergic agonist, UK 14,304, were determined by analysis of concentration-effect curves to the agonists under control conditions and after partial inactivation of alpha adrenoceptors by phenoxybenzamine. The dissociation constant of phenylephrine for alpha-1 adrenoceptors in saphenous veins was approximately 10-fold higher than that obtained for the agonist in femoral arteries or femoral veins. Similarly the dissociation constant for cirazoline in the saphenous vein was higher than that obtained in other alpha-1 adrenergic systems. Dissociation constants were used to determine alpha adrenoceptor occupancy-response relationships. The alpha-1 adrenergic responses evoked by high intrinsic-efficacy agonists (cirazoline and phenylephrine) were associated with a very large receptor-reserve in the saphenous vein, but no, or only a limited receptor-reserve in the femoral vein. The dissociation constant for UK 14,304 in saphenous veins was significantly lower than that obtained for alpha-2 adrenergic stimulation by norepinephrine. There was no alpha-2 adrenoceptor reserve in the saphenous vein for these putative high intrinsic-efficacy agonists. The differences in receptor-reserve between alpha-1 adrenoceptors in canine saphenous and femoral veins and between alpha-1 and alpha-2 adrenoceptors in saphenous veins may help to explain the differential modulation of adrenergic responses in these blood vessels.