Prejunctional beta-adrenoceptors in human and canine saphenous veins.

Experiments were designed to compare the functional importance of prejunctional beta-adrenoceptors in human and canine blood vessels. Rings of saphenous veins were mounted for isometric tension recording in organ chambers filled with physiological salt solution. Isoproterenol caused concentration-dependent relaxations of comparable magnitude during contractions of canine veins caused by electrical stimulation or norepinephrine. In the human veins, isoproterenol caused smaller relaxations during norepinephrine-induced contractions, and augmented the contractile response to electrical stimulation. Helical strips of veins were incubated with [3H]norepinephrine and mounted for superfusion and isometric tension recording. Isoproterenol augmented the release of intact labeled transmitter during electrical stimulation more in the human than in the canine vein. In veins from both species the effects of isoproterenol were inhibited by propranolol. These experiments demonstrate that the adrenergic nerves of the human saphenous vein have a greater responsiveness to prejunctional beta-adrenergic activation than those of the corresponding canine vessel. As a consequence, in the human vein, beta-adrenergic agonists augment, rather than depress the contractile response to activation of the adrenergic nerve endings.