PHARMACOLOGICAL CHARACTERIZATION OF V1A VASOPRESSIN RECEPTORS IN THE RAT CORTICAL COLLECTING DUCT

Vasopressin receptors in distal segments of the rat nephron were identified in isolated tubules using two labeled ligands: the [1-(beta-mercapto-beta,beta-cyclopentamethylenepropionic acid),2-(O-methyl)tyrosine,4-threonine,8-ornithine,9-I-125-tyrosylamide]-vasotocin {I-125-d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH29]OVT} and the linear analogue, Phaa1,D-Tyr(Me)2,Phe3,Gln4,Asn5,Arg6, Pro7,Arg8,I-125-Tyr-NH29 [I-125-Tyr-NH29-linear antagonist (LA)-V1a)]. Specific I-125-d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH29]-OVT binding to cortical collecting ducts (CCD) was saturable with incubation time and dose, reversible after elimination of free ligand, and characterized by the following rank order for recognition of vasopressin analogues: desGly9-d-(CH2)5[Tyr(Et)2,Val4]arginine vasopressin (AVP) greater-than-or-equal-to d(CH2)5[Tyr(Et)2,Val4]AVP greater-than-or-equal-to AVP greater-than-or-equal-to d(CH2)5[Tyr(Me)2]AVP = 1-desamino-8-D-arginine vasopressin (DDAVP) greater-than-or-equal-to Tyr-NH29-LA-V1a > [8-arginine]vasotocin (AVT) > d(CH2)5[Tyr(Me)2,Thr4,TyrNH29]OVT > oxytocin (OT) > [Phe2,Orn8]VT >> [Thr4,Gly7]-OT. Scatchard plots of dose-dependent I-125-Tyr-NH29-LA-V1a binding to medullary thick ascending limbs (MTAL), CCD, and outer medullary collecting ducts (OMCD) revealed the presence of high- and low-affinity binding sites corresponding to V1a and V2 vasopressin receptors, respectively; the densities of V1a receptors are approximately 20% of the total number of vasopressin receptors in CCD and 5% in MTAL and OMCD.