PHARMACOLOGICAL CHARACTERIZATION OF ANGIOTENSIN-II BINDING-SITES IN THE CANINE PANCREAS

High affinity I-125-angiotensin II (Ang II) binding sites were characterized in the canine pancreas. Total binding increased with protein concentration and equilibrium was reached within 60-90 min at 22-degrees-C. Specific binding was saturable and averaged 70% of total. Scatchard analysis of binding yielded a K(D) of 0.48 +/- 0.18 nM with a B(max) of 32.8 +/- 6.5 fmol/mg protein (mean +/- SEM, n = 6). The addition of the reducing agent dithiothreitol increased specific binding two-fold. The rank order of displacement of I-125-Ang II binding by native angiotensin peptides was Ang II greater-than-or-equal-to Ang III > AngI > Ang(1-7) > > Ang(1-6). The use of the specific Ang II antagonists CGP 42112A, PD 123177, and DuP 753 revealed that the pancreas expresses two receptor subtypes. The majority of Ang II binding sites in the pancreas could be classified as type 2 (AT2), although type 1 (AT1) sites were also detected. In vitro autoradiography revealed binding sites localized over islet cells, acinar and duct cells, as well as the pancreatic vasculature. In addition, the autoradiographic studies confirmed the predominance of the AT2 receptor subtype throughout the pancreas.