Evidence for prejunctional alpha- and beta-adrenoceptors in the canine saphenous vein: influence of frequency of stimulation and external calcium concentration.

Most studies of adrenergic neurotransmission have utilized 3H-labeled transmitter-preloaded isolated tissues. The present study has employed a radioenzymatic assay to measure field-stimulated endogenous norepinephrine overflow from the canine saphenous vein preparation. The presence of prejunctional alpha- and beta-adrenoceptors was documented in experiments conducted at two frequencies of stimulation (1 and 5 Hz) and three external calcium concentrations (1.25, 2.5, and 5 mM). There was a direct relationship between frequency of stimulation or external calcium concentration and per pulse norepinephrine overflow. Diminution of stimulated norepinephrine overflow in the presence of clonidine (10(-6) M) was taken as evidence for prejunctional alpha-adrenoceptors. Enhanced overflow in the presence of isoproterenol (10(-6) M) was evidence for prejunctional beta-adrenoceptors. The effects of these alpha- and beta-adrenergic agonists were partially reversed by antagonists, phentolamine (10(-6) M) and propranolol (10(-6) M), respectively. The influence of clonidine or isoproterenol on stimulated norepinephrine overflow was inversely related to frequency of stimulation and external calcium concentration. Enhanced stimulated norepinephrine overflow in the presence of phentolamine is evidence for negative feedback prejunctional alpha-adrenoceptor tone in the canine saphenous vein.