Photoaffinity labeling of the rat isolated portal vein: determination of affinity constants and "spare" receptors for angiotensins II and III.

Photoaffinity labeling of the isolated rat portal vein with [azidobenzoic acid, isoleucine]angiotensin II resulted in selective partial inactivation of angiotensin receptors without affecting norepinephrine receptors. Establishment of dose-response curves to angiotensin II and III before and after photoaffinity labeling has permitted the calculation of "spare" receptors and affinity constants for angiotensins II and III. Spare receptors appear to exist for angiotensin II (greater than 60%) but not for angiotensin III. Furthermore, the data indicate that angiotensin III has a higher binding affinity (K = 6 X 10(-8) M) than angiotensin II (K = 3 X 10(-7) M), but is considerably less potent than angiotensin II in eliciting the contractile response. If angiotensins II and III act at the same receptors in the portal vein, angiotensin III could inhibit the constricting action of angiotensin II and thereby play a role in the angiotensin-mediated control of portal venous capacity.