Relative location of alpha- and beta-adrenoceptors to sites of release of sympathetic transmitter in the rabbit facial vein.

Abstract

We studied alpha- and beta-adrenoceptor-mediated responses of rabbit facial vein rings to adrenergic stimulation to determine the location of the two types of receptors in relation to the sympathetic nerve terminals. Transmural electrical nerve stimulation (TNS) at low frequency elicited large beta-receptor-mediated relaxation responses in rings pretreated with phentolamine (6 x 10(-7)M). These responses were significantly greater than the corresponding alpha-receptor-mediated contractions in rings pretreated with propranolol (10(-6)M). Blockade of neuronal uptake with desmethylimipramine (DMI, 10(-7)M) increased significantly the neurogenic relaxation but had little effect on neurogenic contractility. DMI pretreatment caused a shift to the left (x 5.6) in the relaxant dose-response curve to exogenous l-norepinephrine (NE). The NE contractile dose-response curve was also shifted to the left after DMI pretreatment but by a significantly smaller amount (x 3.1). Neurogenic activation of beta-receptors evoked almost maximal relaxations in facial vein rings (85% at 2 Hz), whereas the maximum neurogenic contraction was approximately half the maximum contraction with exogenous NE (40% at 6 Hz). These results imply that the beta-adrenoceptors in the rabbit facial vein are located in close proximity to sites of adrenergic transmitter release and neuronal reuptake, whereas the alpha-receptors are more distant.