| Gill is the most important organ responsible for osmoregulation in fish by iontransport. Gill epithelial cells contain a large number of ionocytes (also known asmitochondria cells and chloride), which is the main place to ion absorption andsecretion. Na~+/K~+-ATPase (NKA; sodium–potassium pump or sodium pump), aubiquitous and membrane-bound key enzyme for ion transport, is located on theionocytes membrance. NKA actively transports3Na~+out of and2K~+into animalcells via the hydrolysis of one molecule of ATP. NKA maintain intracellularhomeostasis by providing power for other ion transport systems. Nile tilapia(Oreochromis niloticus) is an euryhaline farmed fish. Under salinity stressexperiments of transferring Nile tilapia from fresh water to brackish waterenvironments, the osmoregulation of ionocytes and NKA in gills of O. niloticusresponsed to environmental salinity changes were analyzed systematicly inbiochemical (NKA enzyme activity), molecular (transcription abundance of branchialNKA), physiological (serum osmolality and serum ion) and cellular (types, numbersand position of ionocytes) levels, which could provide some basis for domesticationand breeding of brackish fish species.By the methods of scanning electron micrograph and immunohistochemistry, thispaper studied the distribution of epithelial ionocytes in Nile tilapia gills and theeffects of different salinity (0,10,20, and30) stress on ionocytes number andmorphology. The surface scanning of the gills showed the epithelial ionocytes were distributed close to the inter-lamellar regions of gill filaments. According to the apicalsize, ionocytes could be divided into three subtypes, i.e., subtype I (>6.5um), subtypeII (3.2-6.5um), and subtype III (<3.2um). Ionocytes number in the gills at differentsalinities was in the sequence of salinity10<salinity20<salinity0<salinity30.When the Nile tilapia transferred to the waters from salinity0to salinity10, the totalnumber of ionocytes in gills decreased, mainly due to the significant decrease of thesubtype I cells. Ionocytes number at salinity20was higer than that at salinity10, butthe difference was not significant. Ionocytes number at salinity30increasedsignificantly with the increase of subtype III cells. Immunohistochemical stainingfurther confirmed that at different salinities, the Na~+-K~+-ATPase immunoreactive(NKA-IR) cells were all distributed in the inter-lamellar region of the gill filaments. Itwas suggested that Nile tilapia could adapt to the salinity change of the environmentthrough changing the quantity and morphological structure of branchial ionocytes,and the subtypes I and III cells could play a key role for the osmoregulation in thewaters with low and high salinity, respectively.To further explore the mechanism of osmoregulation in gills of Nile tilapia, weset two different salinity stress experiments. The experimental group A (salinity0â†'7.5â†'15â†'20â†'25) and the experimental group B (salinity0â†'7.5â†'15â†'7.5â†'0), detect the relationship between the serum osmolality, serum ions (Na~+, Cl-, K~+)concentration with salinity changes and relationship between Na~+-K~+-ATPase (NKA)enzyme activity and NKA a1mRNA expression levels with salinity changes. Theresults showed that in the experimental group A, the level of serum osmolalitysignificantly increased with the increasing environmental salinity, exhibting a positivecorrelation relationship; Serum Na~+, Cl-concentration increased with the increasingsalinity, however, serum K~+concentration first rose significantly as the salinity (7.5,15), then significant decreased at salinity20and25; NKA activity and NKA a1mRNA expression levels in gills presented upward trend with increasing salinity.Experimental group B, from0to15salinity, serum osmolality, serum Na~+, Cl-, K~+concentration, NKA a1mRNA expression levels and enzyme activity both showed asignificant upward trend; but when transferred from15to0salinity, all indicators were shown a downward trend. The results show that, with changes in ambientsalinity, the osmolality of Nile tilapia was basicly present positively correlatedrelationship. So does the Na~+, Cl-ion concentration change. By changing the NKAtranscriptional level and enzymatic activity in gills, Nile tilapia regulates the ionconcentration in the body to maintain osmotic relative balance. |
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