Properties of [3H]l-Desamino-8-D-Arginine Vasopressin as a Radioligand for Vasopressin V2-Receptors in Rat Kidney*

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Abstract
[3H]1-Desamino-8-D-arginine vasopressin [3H]DDAVP was assessed as a radioligand for vasopressin V2-receptors by studying its membrane-binding characteristics and in vitro autoradiographic localization in rat kidney, a rich source of V2-receptors. [3H]DDAVP bound specifically to a single class of high affinity, low capacity sites in rat medullopapillary membranes. Specific [3H]DDAVP binding at 25 C reached equilibrium after 2 h of incubation and was saturable and linear with protein concentration up to 2.2 mg/ml protein. Saturation analysis gave an equilibrium dissociation constant (Kd) of 0.76 nM. Displacement of [3H]DDAVP binding by unlabeled arginine vasopressin (AVP) and related analogs gave the following order of potency, consistent with that expected for a V2-receptor: DDAVP .simeq. AVP .simeq. 1-desamino-AVP > lysine vasopressin > oxytocin > [1-(.beta.-mercapto-.beta.,.beta.-cyclopentamethylene-propionic acid, 2-(O-methyl)tyrosine]AVP. The C-terminal metabolites of AVP, (pGlu4Cyt6)AVP-(4-9), and (pGlu4Cyt6)AVP-(4-8) did not displace [3H]DDAVP binding. No degradation of [3H]DDAVP during incubation could be detected by HPLC analysis. In vitro autoradiography of [3H]DDAVP binding to rat kidney sections showed a very dense localization of displaceable binding over inner and outer medulla, with a much lower density in cortex, consistent with the known major localization of V2-receptors on renal collecting tubules. These studies suggest that [3H)DDAVP is a suitable radioligand for labeling V2-receptors and ay be useful in the characterization of vasopressin receptor subtypes in a variety of tissues and in purification of the V2-receptor.

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