A pharmacological study of the angiotensin receptor and tachyphylaxis in smooth muscle

The interaction of angiotensin with its receptor has been studied on the basis of the tachyphylaxis shown by the rat uterus towards angiotensin II when pH and Ca2+ concentration are below physiological levels. 14C-Angiotensin binding and 45Ca2+-uptake investigations suggest tachyphylaxis to be due to increased binding at low pH and Ca2+ concentration. Studies with alkylating (affinity labeled) angiotensin derivatives containing the N-mustard chlorambucil suggest a "Charnière type" inhibition at the Ca-binding site of receptor and an irreversible inhibition at an anionic site. Angiotensin inhibitors containing chlorambucil do not alkylate tissue but are competitive inhibitors suggesting that the aromatic side chain in angiotensin may induce conformational changes in the receptor. The results obtained lead to a logical model for the angiotensin receptor allowing for normal activation by the hormone as well as for production of tachyphylaxis.