Evidence for dual signaling pathways for V-2 vasopressin receptor in rat inner medullary collecting duct

Previous studies have demonstrated that both the V-2-receptor agonist, 1-desamino-8-D-arginine vasopressin (dDAVP), and the V-1a-receptor agonist, [Phe(2),Orm(8)]vasotocin (PO-VT), increase intracellular calcium concentration( [Ca2+](i)) in the rat inner medullary collecting duct (IMCD). The present studies were done to clarify the receptor subtype(s) responsible for calcium mobilization. Measurements of [Ca2+](i) using fura 2 in microdissected IMCD segments, confirmed that arginine vasopressin (AVP), dDAVP, and PO-VT stimua late an increase in [Ca2+](i) and that the response to all three agents could be blocked by the specific V-2-receptor antagonist, [d(CH2)(5)(1),D-Ile(2),Ile(4),Arg(8)]vasopressin (II-VP). These results would suggest that all three agents acted through the V-2 receptor. Furthermore, we showed that PO-VT increased cAMP production in IMCD suspensions and water permeability in isolated perfused tubules. These responses were also blocked by II-VP, indicating that PO-VT is also a V-2 agonist in the IMCD. Finally, we utilized the quantitative reverse transcription-polymerase chain reaction technique of Wiesner (Nucleic Acids Res. 20: 5863-5864, 1992) to evaluate V-1a and V-2 mRNA levels in rat collecting duct. In terminal IMCD, we estimated >30 copies/cell for V-2 receptor mRNA but less than 1 copy/cell of V-1a receptor mRNA, thus there is little or no V-1a mRNA expression in the terminal IMCD. These results suggest that calcium mobilization in response to vasopressin analogues is associated with the V-2 receptor and that the V-2 receptor is linked to both adenylyl cyclase and calcium mobilization in the rat IMCD.