Characterization of adrenergic receptors in pial arteries of the bovine brain

Abstract

Isolated pial arteries contract by the administration of adrenergic agonists in a dose dependent manner,and the contraction is blocked by alpha adrenergic antagonists. This response is mediated by alpha adrenergic receptors. We attempted to characterize the nature of adrenergic receptors in bovine pial arteries, performing radioligand binding assay. We found that the specific binding of 3H-yohimbine, a selective alpha 2 antagonist, was saturable, reversible and of high affinity (KD= 18 nM, Bmax = 690 fmol/mg protein) (N=4). Scatchard and Hill plot analyses of 3H-yohimbine binding indicated one class of binding site. On the contrary, there was no specific binding of ^H-prazosin, a selective alpha 1 antagonist, in these tissues. These results indicate that alpha adrenergic receptors in the bovine pial arteries are classified as the alpha 2 subtype and that the contraction is mediated by alpha 2 subtype receptors. Using 3H-dihydroalprenolol (3 H-DHA), a beta antagonist, we detected beta adrenergic receptors in the same tissues. 3H-DHA binding sites were revealed to have relatively high values of KD(83 nM) and Bmax (3,200 fmol/mg protein) (N=3). Scatchard and Hill analyses also indicated one class of binding site.These values may explain the low responsiveness of the pial arteries to stimulation mediated by beta adrenergic receptors.