The functions of ion cells have been reviewed by various researchers. Usually, the function of ion cells in dehydration is to release ions into seawater, but in freshwater where fish are exposed to hydration from their surroundings, they are responsible for absorbing ions [11]. In Anadromous and Catadromous fish, the function of these cells temporarily changes from secretion to absorption and vice versa [2]. Ion cells have been shown to be responsible for the secretion of chlorine ions in seawater-adapted bony fish, whereas in freshwater bony fish, these cells are the place of calcium and chlorine ions flow into the epithelial tissue. Ion cells also play a separate role in the regulation of acid / alkali [11]. In Euryhaline species of bony fish, certain morphological changes occur in ionic cells during adaptation from freshwater to saline or seawater. In terms of properties, the number and size of these cells decrease during adaptation to seawater. Under special conditions where ion regulation in fresh water is imposed on fish, proliferation of these cells may also occur [12]. Ionic cells do not appear to be confined to bony fish, as even primitive fish, such as hagfish that have a highly permeable skin to water and as a result their blood plasma osmolarity is equivalent to that of seawater fish, have mitochondrial-rich cells in their gills that are morphologically similar to the ionic cells of bony fish. These cells have also been found in the gills and skin of lampreys that migrate between freshwater and marine environments, and the cartilaginous fish [11].
Ionic movements in these cells are performed by various enzymes, the most important of which are NA+ / K+ / 2CL and NA+ / K+ / ATPase cotransporter [4]. In other words, NA+ / K+ / 2CL and NA+ / K+ / ATPase cotransporter proteins are known as ion transporters [5].
The distribution of NKCC protein in chloride cells of freshwater-adapted Atlantic salmon is completely overlapped with NA+ / K+ / ATPase protein in the basolateral membrane [9].
These findings support the chloride secretion model by bone fish chloride cells, in which the presence of two ion transporting proteins of NA+ / K+ / 2CL and NA+ / K+ / ATPase cotransporter in the basolateral membrane of chloride cells as well as a CFTR chloride channel in the apical membrane of these cells, is shown.
The distribution of NA+ / K+ / ATPase and NKCC ion transporting proteins in the gills of Hawaiian goby was the same, so that except for the nucleus and apical of chloride cells, the rest of the chloride cells that include basolateral membranes responded positively to the simultaneous localization of these proteins [13].
NKCC is essential for the secretion of NaCl in saltwater fish, but its gene has not yet been cloned in bony fish.
Previous studies using electronic microscope have shown that NA+ / K+ / ATPase is widely present in the basolateral membrane but not in the most apical part of the cell [8].
Therefore, in the present study, high level of NA+ / K+ / ATPase and NKCC in chloride cells is likely to indicate their distribution in the basolateral membrane.
Marshall et al. (2002) recently found that NKCC immunity occurs in seawater-adopted fish throughout the chloride cell, but in freshwater-adopted fish it has a more limited distribution, which is restricted to the basolateral membrane [14].
Pelis et al. (2001) found that in seawater-adopted fish and freshwater-adopted fish, NKCC immunity occurred throughout the chloride cell, but in freshwater-adapted fish, the number of stained chloride cells and the NKCC immunity was lower [9].
In most bony fish adapted to saltwater, chloride cells appear to contain NA+ / K+ / 2CL and NA+ / K+ / ATPase ion transporting proteins in the basolateral membrane.
Cotransporter occurs in two important isoforms; secretory isoform (NKCC1) and adsorbent isoform (NKCC2). There is a tendency for NKCC immunity in the chloride cells of Hawaiian goby gills to be secretory [13]. In most tissues, the secretory form is found only in the basolateral membrane of chloride cells, while the absorbent form is found only in the apical membrane [13]. The only exception to this general rule is the choroid plexus where both the NKCC1 and NA+ / K+ / ATPase are found in the apical membrane [15].
In the present study, the NKCC immunity throughout chloride cells indicates a basolateral distribution, which shows that this isoform is of secretory type. The large number of NKCCs in fish adapted to seawater indicates that the amount of this ion-carrying protein increases with the adaptation of fish to seawater, similar to the results found in other bony fish.
Wilson et al., hypothesized that NA+ / K+ / ATPase and NKCC may be involved in ammonia excretion by the gills of mudskipper through the replacement of NH4+ by K+ [8].
In the model developed by Shigefumi Yokotu et al., for the secretion of Cl throughout the mudskipper skin, the presence of NKCC1 and NA+ / K+ / ATPase in the basolateral membrane and CFTR in the apical membrane of the skin chloride cells was implied [17].