[Functional expression of vasopressin receptors V1a and V2 along the mammalian nephron]

In vitro studies on single microdissected segments have been extensively used during the 20 past years to localize V1 and V2 vasopressin receptors within the mammalian kidney, and define their role in the control of water balance. Based on vasopressin-dependent adenylate cyclase activity measurements and quantitative RT-PCR studies, it is now clear that V2 receptors are present along the whole collecting duct from cortex to papilla, and, in most species, in the ascending limb of Henle's loop (thick and thin limb); occasionally in the distal tubule but not in the other segments. The stimulation by cyclic AMP of sodium chloride reabsorption in the thick ascending limb, and of urea reabsorption in the papillary collecting duct indicates that vasopressin--in addition to its well known hydroosmotic effect--also participates in the building up of the corticopapillary gradient of osmotic pressure. As regards the V1a receptor, binding studies as well as quantitative RT-PCR, and measurements of free cytosolic calcium concentration allow us to draw the following conclusions. In the rat, the V1a receptor is absent from the glomerulus, the proximal tubule (convoluted and straight portions), the tick ascending limb of Henle's loop and the terminal portion of the papillary collecting duct. It is present in the thin ascending limb and the cortical and outer medullary portions of the collecting duct. Its presence in the thin descending limb has not, up to now, been explored. By contrast with previous data in the rabbit, the V1a receptor does not alter vasopressin-dependent sodium and water reabsorption in the rat cortical collecting duct. Further studies will be necessary to determine its functional role in that segment, as well as in the thin ascending limb. Finally, vasopressin V2 agonists have been shown to induce intracellular calcium release in the papillary collecting duct, a segment devoid of V1a receptors. This effect--which cannot be ascribed to a cross-reaction with oxytocic receptors--indicates either an unusual coupling of the V2 receptor to phospholipase C or, else, the presence of a new vasopressin receptor.