Potassium excretion through ROMK potassium channel expressed in gill mitochondrion-rich cells of Mozambique tilapia.

Despite recent progress in physiology of fish ion homeostasis, the mechanism of plasma K+ regulation has remained unclear. Using Mozambique tilapia, a euryhaline teleost, we demonstrated that gill mitochondrion-rich (MR) cells were responsible for K+ excretion, using a newly invented technique that insolubilized and visualized K+ excreted from the gills. For a better understanding of the molecular mechanism of K+ excretion in the gills, cDNA sequences of renal outer medullary K+ channel (ROMK), potassium large conductance Ca(2+)-activated channel, subfamily M (Maxi-K), K(+)-Cl(-) cotransporters (KCC1, KCC2, and KCC4) were identified in tilapia as the candidate molecules that are involved in K+ handling. Among the cloned candidate molecules, only ROMK showed marked upregulation of mRNA levels in response to high external K+ concentration. In addition, immunofluorescence microscopy revealed that ROMK was localized in the apical opening of gill MR cells, and that the immunosignals were most intense in the fish acclimated to the environment with high K+ concentration. To confirm K+ excretion via ROMK, K+ insolubilization-visualization technique was applied again in combination with K+ channel blockers. The K+ precipitation was prevented in the presence of Ba2+, indicating that ROMK has a pivotal role in K+ excretion. The present study is the first to demonstrate that the fish excrete K+ from the gill MR cells, and that ROMK expressed in the apical opening of the MR cells is a main molecular pathway responsible for K+ excretion.