Gene expression and cellular localization of ROMKs in the gills and kidney of Mozambique tilapia acclimated to fresh water with high potassium concentration

Abstract

Regulation of plasma K+ levels in narrow ranges is vital to vertebrate animals. Since seawater (SW) teleosts are loaded with excess K+, they constantly excrete K+ from the gills. However, the K+ regulatory mechanisms in freshwater (FW)-acclimated teleosts are still unclear. We aimed to identify the possible K+ regulatory mechanisms in the gills and kidney, the two major osmoregulatory organs, of FW-acclimated Mozambique tilapia ( Oreochromis mossambicus). As a potential molecular candidate for renal K+ handling, a putative renal outer medullary K+ channel (ROMK) was cloned from the tilapia kidney and tentatively named “ROMKb”; another ROMK previously cloned from the tilapia gills was thus renamed “ROMKa”. The fish were acclimated to control FW or to high-K+ (H-K) FW for 1 wk, and we assessed physiological responses of tilapia to H-K treatment. As a result, urinary K+ levels were slightly higher in H-K fish, implying a role of the kidney in K+ excretion. However, the mRNA expression levels of both ROMKa and ROMKb were very low in the kidney, while that of K+/Cl− cotransporter 1 (KCC1) was robust. In the gills, ROMKa mRNA was markedly upregulated in H-K fish. Immunofluorescence staining showed that branchial ROMKa was expressed at the apical membrane of type I and type III ionocytes, and the ROMKa immunosignals were more intense in H-K fish than in control fish. The present study suggests that branchial ROMKa takes a central role for K+ regulation in FW conditions and that K+ excretion via the gills is activated irrespective of environmental salinity.