Acid regulation of NaDC-1 requires a functional endothelin B receptor

Metabolically generated acid is the major physiological stimulus for increasing proximal tubule citrate reabsorption, which leads to a decrease in citrate excretion. The activity of the Na-citrate cotransporter, NaDC-1, is increased in vivo by acid ingestion and in vitro by an acidic pH medium. In opossum kidney cells the acid stimulatory effect and the ability of endothelin-1 (ET-1) to stimulate NaDC-1 activity are both blocked by the endothelin B (ET_B) receptor antagonist, BQ788. Acid feeding had no effect on brush border membrane NaDC-1 activity in mice in which ET_B receptor expression was knocked out, whereas a stimulatory effect was found in wild-type mice. Using ET A/ET_B chimeric and ET_B C-terminal tail truncated constructs, ET-1 stimulation of NaDC-1 required a receptor C-terminal tail from either ET A or ET_B. The ET-1 effect was greatest when either the ET_B transmembrane domain and C-terminal tail were present or the ET_B C-terminal tail was linked to the ET A transmembrane domain. This effect was smaller when the ET_B transmembrane domain was linked to the ET A C-terminal tail. Thus, the acid-activated pathway mediating stimulation of NaDC-1 activity requires a functional ET_B receptor in vivo and in vitro, as does acid stimulation of NHE3 activity. Since increased NaDC-1 and NHE3 activities constitute part of the proximal tubule adaptation to an acid load, these studies indicate that there are similarities in the signaling pathway mediating these responses.