Covalent linkage of [Phe²,Phe(p-N₃)³]AVP to vasopressin receptors

We have shown previously (Eggena et al., Endocrinology 113: 1413-1421, 1983) that [Phe²,Phe(p-N₃)³]AVP induces a prolonged hydrosmotic response in the toad bladder when activated by ultraviolet (UV) light. To determine whether this response is due to covalent binding of the ligand with 8-arginine vasopressin (AVP) receptors, bladders were challenged with the ligand in the presence of AVP or the AVP antagonist, [Phe(p-N₃)²]AVP, during photolysis. The permeability of bladders to water was tested subsequently in the absence of hormone or analogue. Bladders with a history of exposure to AVP (or to [Phe-(p-N₃)²]AVP) during UV irradiation were considerably less permeable to water than controls, suggesting that [Phe²,Phe(p-N₃)³]AVP, AVP, and [Phe(p-N₃)²]AVP compete for the same receptor system during photolysis. Other experiments were directed at defining optimal conditions for covalent linkage of [Phe²,Phe(p-N₃)³]AVP to receptors. These studies have indicated that two 10-min cycles of UV irradiation are more effective than one and that osmotic water flow at a rate of 1 mg.min^-1.cm^-2 during irradiation does not interfere with the ligand-receptor interaction. Acidification of the serosal bath solution to pH 6.5 did not inhibit covalent binding of the ligand to receptors during photolysis. However, the capacity of the ligand-receptor complex to increase bladder permeability to water was markedly inhibited by serosal fluid acidification. These experiments have suggested that [Phe²,Phe(p-N₃)³]AVP binds covalently to AVP receptors during photolysis and generates a signal that gradually decays as a function of time. The translation of this signal into the final effector process of the AVP target cell is blocked by a low pH.