THE EFFECTIVENESS OF BETA-ADRENOCEPTOR STIMULATION AND THE CONTRIBUTION OF BETA-1-ADRENOCEPTORS INCREASE FROM THE PROXIMAL TO THE DISTAL PART OF THE CANINE SAPHENOUS-VEIN

The present study was undertaken to characterize beta-adrenoceptors of the canine saphenous vein and their distribution along this vessel. I n a first series of experiments, concentration-response curves to isop renaline and forskolin were compared on strips taken from proximal and distal portions of the vein. The tone of the strip was previously inc reased by phenylephrine to one of the three levels: about 85, 70 and 5 5% of the maximum, which corresponds to 3.08 +/- 0.16 and 2.96 +/- 0.1 7 (n = 6) N . g-1, for proximal and distal strips, respectively. The m aximal relaxation to isoprenaline was significantly larger in the dist al than in the proximal portion, for responses starting at 85 and 70% of the maximum tone. In contrast, forskolin caused 100% relaxation, bo th proximally and distally, irrespective of the previous tone. In a se cond series of experiments, the relaxation to dobutamine and terbutali ne was compared in proximal and distal portions after the tone had bee n elevated by adrenaline, noradrenaline and phenylephrine to about 70% of the maximum. When the tone was increased by adrenaline, the relaxa tion in response to 2.7 mumol . l-1 dobutamine was larger than that to 2.7 mumol . l-1 terbutaline both in proximal and distal portions, whi le when it was increased by noradrenaline, the relaxation to terbutali ne was larger than that to dobutamine in the proximal portion; in the distal portion, dobutamine and terbutaline were equieffective. When th e tone was increased by phenylephrine, dobutamine and terbutaline caus ed equivalent relaxations proximally, but distally the relaxation to d obutamine was larger than that to terbutaline. In a third series of ex periments, the antagonism exerted by atenolol and by the compound ICI- 118,551 on the relaxation to dobutamine and terbutaline was compared p roximally and distally. Both proximally and distally atenolol was more effective against dobutamine than against terbutaline, while the comp ound ICI-118,551 was effective only against terbutaline. It is conclud ed that beta1- and beta2-adrenoceptors exist in the canine saphenous v ein and that the effectiveness of beta-adrenoceptor stimulation increa ses from the proximal to the distal portion of this vessel due to a pr ogressive increase in the density of beta1-adrenoceptors.