LINEAR V(1)-VASCULAR VASOPRESSIN ANTAGONISTS SUITABLE FOR RADIOIODINATION, BIOTINYLATION, AND FLUORESCENT LABELING

We modified several linear V1-vascular arginine vasopressin (AVP) anta gonists to obtain compounds suitable for radioiodination, biotinylatio n, and fluorescent labeling. In binding competition experiments with h uman platelet V1-vascular AVP receptors, the linear V1 antagonist phen ylacetyl-D-Tyr(Et)-Phe-Gln-Asn-Lys-Pro-Arg-NH2 (PhaaGln) displayed the greatest affinity [dissociation constant (K(d)) = 0.05 +/- 0.01 nM]. The radioiodinated compound acetyl-D-Tyr(Et)-Phe-Val-Asn-Lys-Pro-I-125 -labeled Tyr-NH2 ( 1251-labeled TyrPhaa) was characterized by a high a ffinity (K(d) = 1.42 +/- 0.19 nM), a low nonspecific binding, and good stability. PhaaGIn coupled to dodecabiotin retained a good affinity f or V1-vascular AVP receptors (K(d) = 1.41 +/- 0.20 nM). The complex Ph aaGIn-dodecabiotin-avidin is bifunctional, since an avidin-agarose col umn specifically bound V1-vascular AVP receptors labeled with I-125-Ty rPhaa-dodecabiotin. In A7r5 vascular smooth muscle cells loaded with f ura-2, PhaaGIn and PhaaGIn-dodecabiotin were pure antagonists as they blocked AVP-induced calcium mobilization but did not elicit a calcium signal by themselves. V1-vascular AVP receptors of A7r5 vascular smoot h muscle cells were visualized by bound PhaaGln-dodecabiotin made fluo rescent by labeling with fluorescein avidin. Thus linear V1-vascular A VP antagonists can be used as high affinity and specificity radioiodin ated, biotinylated, and fluorescent probes to explore V1-vascular AVP receptors of human and animal origin.