Functional classification of mitochondrion-rich cells in euryhaline Mozambique tilapia (Oreochromis mossambicus) embryos, by means of triple immunofluorescence staining for Na+/K+-ATPase,Na+/K+/2Cl- cotransporter and CFTR anion channel

Abstract

SUMMARY Mozambique tilapia Oreochromis mossambicus embryos were transferred from freshwater to seawater and vice versa, and short-term changes in the localization of three major ion transport proteins,Na+/K+-ATPase,Na+/K+/2Cl- cotransporter (NKCC) and cystic fibrosis transmembrane conductance regulator (CFTR) were examined within mitochondrion-rich cells (MRCs) in the embryonic yolk-sac membrane. Triple-color immunofluorescence staining allowed us to classify MRCs into four types: type I, showing only basolateral Na+/K+-ATPase staining; type II, basolateral Na+/K+-ATPase and apical NKCC; type III, basolateral Na+/K+-ATPase and basolateral NKCC; type IV, basolateral Na+/K+-ATPase,basolateral NKCC and apical CFTR. In freshwater, type-I, type-II and type-III cells were observed. Following transfer from freshwater to seawater, type-IV cells appeared at 12 h and showed a remarkable increase in number between 24 h and 48 h, whereas type-III cells disappeared. When transferred from seawater back to freshwater, type-IV cells decreased and disappeared at 48 h, type-III cells increased, and type-II cells, which were not found in seawater, appeared at 12 h and increased in number thereafter. Type-I cells existed consistently irrespective of salinity changes. These results suggest that type I is an immature MRC, type II is a freshwater-type ion absorptive cell, type III is a dormant type-IV cell and/or an ion absorptive cell (with a different mechanism from type II), and type IV is a seawater-type ion secretory cell. The intracellular localization of the three ion transport proteins in type-IV cells is completely consistent with a widely accepted model for ion secretion by MRCs. A new model for ion absorption is proposed based on type-II cells possessing apical NKCC.

Environmental factors responsible for switching on the SO42−excretory system in the kidney of seawater eels

“…However, there has been no report on the factors in SW that trigger such changes in ion regulation. For example, mitochondria-rich cells in the gills are altered from an absorptive FW type to an excretory SW type when euryhaline teleosts are transferred from FW to SW (10). The environmental factors that induce the change are most likely Na ϩ or Cl Ϫ ions that exist at high concentrations in SW and are the target of regulation by the cells, but there is no report on the factor that triggers the change.…”

Hormonal control of salt and water balance in vertebrates

“…These cells have high levels of Na + /K + -ATPase, Na + , K + , 2Cl ¡ contransporter (NKCC) and the CFTR apical chloride channel that are responsible for salt secretion by chloride cells. In most teleost Wsh these transporters increase over 1-14 days following exposure to seawater (Hiroi et al, 2005;McCormick, 2001), thereby increasing the overall capacity of the tissue to secrete sodium and chloride. Cortisol upregulates these transporters in most euryhaline teleosts, and in several model euryhaline species there is an important interaction of cortisol with the growth hormone/insulin-like growth factor I axis to increase salt secretory capacity of the gill Sakamoto and McCormick (2006, this volume).…”

Differential regulation of sodium–potassium pump isoforms during smolt development and seawater exposure of Atlantic salmon

“…In addition to providing the electrochemical gradient for chloride secretion, NKA also pumps sodium into the paracellular space where sodium leaves through leaky tight junctions on a favorable electrochemical gradient. In seawater-adapted salmon, these three ion transport proteins have been localized specifically to ionocytes (McCormick et al, 2003;Hiroi et al, 2005).…”