Angiotensin II receptors in resistance vessels

We have studied binding of 125I-angiotensin II (AII) to a membrane fraction prepared from mesenteric arterial arcades of the rat. Two problems were encountered: heterogeneity of binding sites and appreciable degradation of tracer that could not be inhibited without adversely affecting binding capacity. Analysis of binding results was carried out using a non-linear regression method allowing correction for tracer and receptor degradation and direct estimation of binding constants. From saturation and dissociation curves it was apparent that there were two classes of binding sites. KD of the high affinity site was within the physiological range of plasma AII concentration at between 0.05 and 0.5 nM. Calcium, magnesium and manganese increased binding capacity, whereas EDTA and EGTA had the opposite effect. Potassium increased binding capacity but had a biphasic effect in the presence of sodium.