Ammonium-dependent sodium uptake in mitochondrion-rich cells of medaka (Oryzias latipes) larvae

Abstract

In this study, a scanning ion-selective electrode technique (SIET) was applied to measure H+, Na+, and NH4+gradients and apparent fluxes at specific cells on the skin of medaka larvae. Na+uptake and NH3/NH4+excretion were detected at most mitochondrion-rich cells (MRCs). H+probing at MRCs revealed two group of MRCs, i.e., acid-secreting and base-secreting MRCs. Treatment with EIPA (100 μM) blocked 35% of the NH3/NH4+secretion and 54% of the Na+uptake, suggesting that the Na+/H+exchanger (NHE) is involved in Na+and NH3/NH4+transport. Low-Na+water (<0.001 mM) or high-NH4+(5 mM) acclimation simultaneously increased Na+uptake and NH3/NH4+excretion but decreased or even reversed the H+gradient at the skin and MRCs. The correlation between NH4+production and H+consumption at the skin surface suggests that MRCs excrete nonionic NH3(base) by an acid-trapping mechanism. Raising the external NH4+significantly blocked NH3/NH4+excretion and Na+uptake. In contrast, raising the acidity of the water (pH 7 to pH 6) enhanced NH3/NH4+excretion and Na+uptake by MRCs. In situ hybridization and real-time PCR showed that the mRNAs of the Na+/H+exchanger ( slc9a3) and Rhesus glycoproteins ( Rhcg1 and Rhbg) were colocalized in MRCs of medaka, and their expressions were induced by low-Na+acclimation. This study suggests a novel Na+/NH4+exchange pathway in apical membranes of MRCs, in which a coupled NHE and Rh glycoprotein is involved and the Rh glycoprotein may drive the NHE by generating H+gradients across apical membranes of MRCs.