Characterization of alpha adrenoceptors in pial arteries of the bovine brain

Summary

When attempting to characterize the nature of adrenoceptors in bovine pial arteries, we found specific ³H-yohimbine binding was saturable, reversible and of high affinity (K _D=18.3±1.2 nM) with a B _max of 687±27 fmol/mg protein (N=4). On the other hand, there was no specific ³H-prazosin binding in these tissues. Scatchard and Hill plot analyses of specific ³H-yohimbine binding indicated one class of binding sites. From kinetic analyses of the data, association and dissociation rate constants of 1.6±0.3×10⁷ M⁻¹ min⁻¹ and 0.51±0.04 min⁻¹, respectively, were calculated (N=3). The dissociation constant from the equation K _D=K ₋₁/K ₊₁ was 35.7±7.6 nM, such being in good agreement with the K _D value estimated from Scatchard plots. Specific binding of ³H-yohimbine was displaced effectively by alpha₂ adrenergic agents and less effectively by alpha₁ adrenergic agents or beta adrenergic agents. K _i values for adrenergic drugs of ³H-yohimbine binding were as follows: yohimbine, 25 nM; clonidine, 260 nM; methoxamine, 6.8 μM; propranolol, 8.7 μM; prazosin, 21 μM; phenylephrine, 22 μM; noradrenaline, 27 μM; adrenaline, 66 μM; isoproterenol, 3,300 μM. These results indicate that alpha adrenoceptors in the bovine cerebral arteries can be classified as the alpha₂ subtype.