Characterization of flow-regulated cortical collecting duct endothelin-1 production

Collecting duct (CD) endothelin-1 (ET-1) is an autocrine inhibitor of Na⁺ and water reabsorption. Salt or water loading increases CD ET-1 production; this is likely due, at least in part, to increased tubule fluid flow. The mechanisms by which flow stimulates CD ET-1 production are incompletely understood. In particular, flow induction of cortical CD (CCD) and inner medullary CD (IMCD) ET-1 synthesis may occur via different mechanisms. Since flow-mediated ET-1 production in IMCD has been more extensively characterized than in the CCD, this study was undertaken to further examine putative signaling pathways involved in flow-stimulated CCD ET-1 production. The CD cell line, mpkCCDcl4, was exposed to static or flow (2 dyne/cm² for 2 h) conditions and ET-1/GAPDH mRNA levels were assessed. Intracellular Ca²⁺, Ca²⁺-stimulated Ca²⁺ release, calcineurin, and protein kinase c α/β isoforms were all involved in the ET-1 flow response. TRPC6, but not other CD-expressed TRP channels (TRPC3, 4, and 5, or TRPV4) played a role in the ET-1 flow response. Purinergic signaling pathways and cilia were not involved in the ET-1 flow response. Based on these and previously published findings, we present a comparison of flow-stimulated CD ET-1 production between CCD and IMCD We suggest that flow-stimulated CCD ET-1 production may be more involved in responding to Na⁺ delivery, while IMCD ET-1 production may be more responsive to water and solute delivery; the responsible pathways for mediating these effects in the two regions of the CD appear to be substantially distinct from one another.