Noncompetitive blockade of vasopressin-1 receptors in rat tail artery

To study vasopressin-1 receptor-mediated contraction of the isolated rat tail artery, pseudoirreversible blockade was produced by incubating tissues with 10 nM 1-beta-mercapto,beta,beta,cyclopentamethylene propionic acid, 2(O-me)tyrosine,-8-D-arginine vasopressin and 1.2 mM magnesium. Concentration-response curves to vasopressin agonists were constructed before and during exposure to 1-beta-mercapto,beta,beta,cyclopentamethylene propionic acid, 2(O-me)tyrosine,-8-D-arginine vasopressin, and at three time points after washing out the antagonist. EC50 values (-log M) for arg-vasopressin (AVP, 8.02 +/- 0.16) and arg-vasotocin (AVT, 9.51 +/- 0.12) were unaltered after exposure to the antagonist and washing of the tissues. The control EC50 for lys-vasopressin (LVP, 8.33 +/- 0.09) was significantly lower than after blockade (P less than .05). At each time point after blockade, the intrinsic activity of LVP was greater than either AVP or AVT. Increasing the ambient magnesium ion concentration greater than 1.2 mM did not alter responses to vasopressin agonists. Reducing the ambient magnesium ion from 1.2 mM to zero increased the EC50 values for all vasopressin agonists by 6- to 20-fold, but had no effect on maximal response size, nor on the concentration-response curve for norepinephrine. Pseudoirreversible blockade performed in the absence of magnesium also increased the EC50 for LVP but not for AVP or AVT after exposure to the antagonist. We conclude that LVP has greater efficacy than AVP and AVT at vasopressin receptors in rat tail artery, and that magnesium ions affect only agonist affinity but not efficacy at this vasopressin-1 receptor.