Further study of the adrenoceptors of the saphenous vein of the dog: influence of factors which interfere with the concentrations of agonists at the receptor level

Abstract

In the present study the affinities of some sympathomimetic amines for alpha- and beta-adrenoceptors of the dog saphenous vein tissue were determined after all known factors interfering with the concentration of these agonists at the receptor level had been assessed and excluded. It was observed that in control experiments the relative potencies of sympathomimetic agonists for inducing contractions were: adrenaline (1.6) greater than noradrenaline (1.0) greater than phenylephrine (0.38) greater than isoprenaline (0.009). The elimination of neuronal uptake by cocaine, 4 X 10(-6) M, enhanced predominantly the effects of noradrenaline (by a factor of 7.5), whereas block of catechol-O-methyl transferase (COMT) by U-0521, 10(-4) M, only enhanced those of adrenaline (by a factor of 2.6) and block of beta-adrenoceptors by propranolol, 5 X 10(-7) M, enhanced those of isoprenaline (by a factor of 3) and adrenaline (by a factor of 1.8). Block of COMT enhanced the effects of adrenaline approximately as much as did the blockade of neuronal uptake; this seems to indicate that the affinity of adrenaline for extraneuronal and neuronal uptake processes is approximately the same. Regarding the relaxation-inducing capacity of sympathomimetic agents it was observed that isoprenaline, adrenaline and noradrenaline are full agonists, whereas phenylephrine was not able to produce relaxation amounting to more than 5% of the maximum. Denervation did not modify the relaxant effects of isoprenaline. After elimination of all known factors interfering with the concentration of the sympathomimetic agonists in the biophase, the ratios between the ED50's of each amine for alpha- and beta-adrenoceptors were: adrenaline = 34, noradrenaline = 109 and isoprenaline = 0.0041.