Preparation and characterization of adrenocortical plasma membrane angiotensin II receptors

Abstract

A bovine adrenocortical particulate fraction prepared by zonal ultracentrifugation and banding between ρ20 1.08 and 1.101 in a linear sucrose gradient bound 7.3 times more [3H]angiotensin II (ATII) per milligram protein than the original homogenate. Enzyme marker and electron microscope studies indicated that this fraction was largely devoid of mitochondria while being enriched in smooth membranes of predominantly plasmalemmal origin. The binding of labeled ATII was maximal after 10 min incubation (22 °C) and remained at equilibrium for at least 20 min thereafter. [3H]ATII binding was completely inhibited by saturating concentrations of nonradioactive ATII. The high-affinity binding site in the preparation had a specific binding capacity of 2.38 pmol∙mg−1, with an equilibrium constant of 2.36 × 108 M−1. Inhibition–displacement studies with unlabeled ATI, ATII, ATII fragments, analogs, and antagonists show that the receptor fraction has the highest affinity for the intact native octapeptide. ACTH and bradykinin had no specific effects on [3H]ATII binding. The current study suggests that the receptor fraction may be of use in a highly sensitive ATII radioligand assay.