Chloride transport in mitochondrion-rich cells of euryhaline tilapia (Oreochromis mossambicus) larvae

A noninvasive scanning ion-selective electrode technique (SIET) was applied to measure Cl^- transport at individual mitochondrion-rich cells (MRCs) in the skin of euryhaline tilapia (Oreochromis mossambicus) larvae. In seawater (SW)-acclimated larvae, outward Cl^- gradients (20-80 mM higher than the background) were measured at the surface, indicating a secretion of Cl^- from the skin. By serial probing over the surface of MRCs and adjacent keratinocytes (KCs), a significant outward flux of Cl^- was detected at the apical opening (membrane) of MRCs. Treatment with 100 μM ouabain or bumetanide inhibited the Cl^- secretion by ∼75%. In freshwater (FW)-acclimated larvae, a lower level of outward Cl^- gradients (0.2-1 mM) was measured at the skin surface. Low-Cl^- water (<0.005 mM) acclimation increased the apical Na⁺-Cl^- cotransporter (NCC) immunoreactivity of MRCs in the larval skin. An inward flux of Cl^- was detected when probing the exterior surface of a group of MRCs (convex-MRCs) that express the NCC. An NCC inhibitor (100 μM metolazone) reduced the flux by ∼90%. This study provides direct and convincing evidence for Cl^- transport by MRCs of SW- and FW-acclimated euryhaline tilapia and the involvement of an apical NCC in Cl^- uptake of MRCs of FW-acclimated fish.