Vasopressin and oxytocin receptors coupled to Ca2+ mobilization in rat inner medullary collecting duct.

Abstract

In renal collecting duct epithelial cells, arginine vasopressin (AVP) at greater than nanomolar concentrations has been reported to transiently increase intracellular free calcium ([Ca2+]i) in a manner consistent with activation of the phosphoinositide pathway. To investigate whether any of the known neurohypophysial hormone subtypes are involved, we measured [Ca2+]i in microdissected rat terminal inner medullary collecting duct (IMCD) using fura-2. To allow quantitative comparisons of the response under different conditions, we determined the areas under the response curves (in nM.min) over 1.5 min using numerical integration. AVP, the V1b-receptor agonist [deamino1,D-3-(pyridyl)Ala2,Arg8]vasopressin, the V2-receptor agonist 1-desamino-8-D-arginine vasopressin, oxytocin, and the selective oxytocin-receptor agonist [Thr4,Gly7]oxytocin (TG-OXT), each at 10 nM, significantly increased [Ca2+]i (69.52 +/- 10.25, 27.0 +/- 11.7, 24.33 +/- 5.83, 14.75 +/- 2.81, and 14.57 +/- 3.50 nM.min, respectively). In contrast, a V1a-selective agonist ([Phe2,Ile3,Orn8]vasopressin) did not increase [Ca2+]i (0.43 +/- 2.36 nM.min). In desensitization studies, challenge with 10 nM AVP or TG-OXT completely prevented a rise in [Ca2+]i in response to immediate rechallenge with the same agent, but not the other, demonstrating homologous desensitization. The lack of cross-desensitization implies that at least two receptors are present that can trigger a rise in [Ca2+]i in response to neurohypophysial hormones. Antagonists for oxytocin ([des-glycinamide9,d(CH2)5(1),O-Me-Tyr2,Thr4,Orn8]vaso tocin), V2 ([d(CH2)5(1),D-Ile2,Ile4,Arg8]vasopressin), and V1a ([d(CH2)5(1),O-Me-Tyr2,Arg8]vasopressin) receptors partially inhibited the [Ca2+]i response induced by 10 nM AVP (89.5, 81.6, and 51.4% inhibition, respectively). These data are consistent with the view that both an oxytocin receptor and a vasopressin receptor are coupled to a [Ca2+]i mobilization response in rat terminal IMCD. This vasopressin receptor is distinct from both the V1a receptor and the V2 receptor and may be either the V1b receptor or a novel vasopressin receptor subtype.