Abstract

The adrenergic nerve terminals of the rabbit aorta are located at the medial-adventital border. Thus it is quite likely that the concentration of endogenously released norepinephrine progressively declines as it diffuses from the nerve terminals toward the lumen. In contrast, phentolamine is probably equally distributed throughout the smooth muscle. Under these circumstances equilibrium competitive antagonism deviates from the usual kinetics of agonist-antagonist interactions. In rabbit aortic strips phentolamine is a more effective antagonist of tyramine and nerve stimulation than it is of exogenously administered norepinephrine. At a concentration of 1.7 x 10^-7 M phentolamine preferentially antagonizes the effects of endogenously released norepinephrine without affecting either the uptake of H³-norepinephrine and H³-tyramine or antagonizing the chronotropic effects of tyramine on rabbit atria. These results indicate that concentrations of phentolamine which effectively block alpha adrenergic receptors have no significant presynaptic effects.