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

Abstract

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.

Ionoregulatory Aspects of the Osmorespiratory Compromise during Acute Environmental Hypoxia in 12 Tropical and Temperate Teleosts

“…With respect to other moieties, K 1 flux was originally attributed to a simple diffusive loss from the gill cells via the transcellular pathway (McDonald et al 1986;Wood et al 2009;Iftikar et al 2010;Deboeck et al 2013), though recent evidence suggests that active efflux may occur through the MRCs, at least in seawater teleosts (Furukawa et al 2012). Regardless, the covering (in oscars) or uncovering (in trout) of MRCs by PVCs during hypoxia would explain the observed differences, and the same may be true of the other species that exhibited these differing response patterns ( fig.…”

The influence of dissolved organic matter (DOM) on sodium regulation and nitrogenous waste excretion in the zebrafish (Danio rerio)

“…Interpretation of the present [ 3 H]PEG-4000 results seems straightforward: the DOM effects on Na + effluxes are not paracellular. However, the K + loss rate results are more problematic; for example, cellular K + channels may respond differently from cellular Na + channels, and recent findings of active K + excretion by specific gill ionocytes in certain situations (Furukawa et al, 2012) further confound interpretation. Clearly, more work is needed to determine whether DOM reduces transcellular Na + efflux rates in zebrafish.…”

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

“…Since the chemical gradient of K ϩ is the pivotal factor that creates membrane potential of the cells, the regulation of plasma K ϩ levels is critical in all vertebrates. Recently, the K ϩ excretory mechanism was investigated in SW-acclimated tilapia, and it became clear that the gill ionocytes excrete K ϩ through an apically expressed renal outer medullary K ϩ channel (ROMK) (10), whose ortholog was originally found in the rat kidney (19). However, the mechanisms of K ϩ regulation in FW fish were still poorly understood, since FW fish possess different types of ionocytes from SW fish.…”