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

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/NH ₄⁺ 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 NH ₃/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.