Tissue- and Salinity-specific Na–Cl Cotransporter (NCC) Orthologues Involved in the Sea lamprey (Petromyzon Marinus) Adaptive Osmoregulation
Abstract Two ncc orthologues (termed ncca and nccb) were found in the sea lamprey genome, whereas nkcc2 was not. In a phylogenetic comparison among other vertebrate amino acids, NCC and NKCC deduced sequences, the sea lamprey NCC’s occupied basal positions within the NCC clade. In freshwater, ncca mRNA was found only in the gill and nccb only in the intestine, whereas both were found in the kidney. Acclimation to seawater increased nccb mRNA in the intestine and kidney. Intestinal nccb mRNA also increased during late metamorphosis. The electrophysiological approach in the Ussing chamber of intestinal tissue ex vivo showed significant differences between freshwater and seawater-acclimated juveniles. Luminal application of indapamide (NCC inhibitor) resulted in 73 and 30% inhibition of short-circuit current (Isc) in the proximal and distal intestine, respectively. The luminal application of bumetanide (NKCC inhibitor) did not affect intestinal Isc. Indapamide also inhibited ex vivo intestinal water absorption. Our results indicate that NCCb is likely the key passive ion cotransporter protein for ion uptake by the lamprey intestine to facilitate water absorption in seawater. As such, the preparatory increases in intestinal nccb mRNA expression during metamorphosis are likely critical to the development of whole animal salinity tolerance.