Cl- uptake mechanism in the gills of freshwater fish

• Main Authors: Chang Il-Chi, 張詣奇
• Other Authors: Hwang Pung-Pung
• Format: Others
• Language: en_US
• Published: 2001
• Online Access: http://ndltd.ncl.edu.tw/handle/62485590920224182985

博士 === 國立臺灣大學 === 漁業科學研究所 === 90 === Mitochondria-rich (MR) cells are highly specialized cells for ion and acid-base regulation in teleost gills, and polymorphism of MR cells were identified and were suggested to display different functions in ionic transport and acid-base regulation. However, there was no comprehension data to link the polymorphism of MR cells with the proposed functions. The purpose of the present study was to explore the relationships between the morphologies of MR cells and the ion uptake in gills of freshwater-adapted tilapia and to examine the roles of various transporters and enzyme (Na+,K+-ATPase, V-H+-ATPase, Cl-/HCO3- exchanger and carbonic anhydrase) in the Cl uptake mechanism in gill MR cells in freshwater-acclimated tilapia. (1). After 14-day acclimation to L-Ca environment, tilapia developed more shallow-basin MR cells in the gills and a higher Ca2+ influx. On the other hand, fish acclimated to L-NaCl artificial freshwater developed predominantly wavy-convex cells, and this was accompanied with the highest Na+ and Cl- influxes. These suggested that appearance of shallow-basin and wavy-convex MR cells is associated with the stimulation in the uptake of Ca2+ and Na+/Cl-, respectively. (2). Based on above data, further experiments were conducted to certify that gill wavy-convex MR cells are responsible for the Cl- uptake but not for Na+ uptake in freshwater-adapted tilapia. 7-day acclimation to L-Cl resulted in more wavy-convex MR cells and higher Cl- influx in the gills of tilapia comparing with those acclimated to H-Cl, indicating a positive correlation between the number of wavy-convex MR cells and the level of Cl- influx. On the other hand, Na+ influx did not show any positive correlation with the numbers of MR cells. Dominant MR cell type in tilapia gills changes from deep-hole to wavy-convex within 6 h after acute transfer from high Cl- to low Cl- environment, while deep-hole turns out dominant much more slowly, about 24-96 h, upon acute transfer from low Cl- to high Cl- environment. These data suggested that appearance of wavy-convex MR cells with expanded apical surface is associated with the stimulation of Cl- uptake, and rapid transformation from other types to wavy-convex MR cells may reflect the timely and efficient stimulation of Cl- uptake to recover the homeostasis of internal Cl- level upon acute challenge with low environmental Cl-. Similar to tilapia, goldfish (Carassius auratus) also showed a positive correlation between the Cl- influx and the number of type III MR cell that was characterized with expanded apical surface. (3). The effects of the inhibitors of various transporters and enzyme, Cl-/HCO3- exchanger, Na+,K+-ATPase, V-type H+-ATPase and carbonic anhydrase, on Cl- influx in freshwater-adapted tilapia were examined, and protein expression of these transporters and enzyme were compared between the tilapia acclimated to high and low Cl environments. Incubation with 2 x 10-4 M ouabain (Na+,K+-ATPase inhibitor), 10-5 M N-ethylmaleimide (NEM, V-type H+-ATPase inhibitor), 10-2 M Acetazolamide (ACTZ, carbonic anhydrase inhibitor) and 6 x 10-4 M 4, 4’- Diisothio-cyanostilbene-2, 2’-disulfonic acid (DIDS, Cl-/HCO3- exchanger inhibitor) caused 40%, 60-80%, 40-60% and 40-60% reduction, respectively, in Cl influx of freshwater-acclimated tilapia. Western blot results showed that protein levels of gill Na+,K+-ATPase, V-type H+-ATPase, Cl-/HCO3- exchanger and carbonic anhydrase in L-Cl tilapia were significant higher than those in H-Cl ones. Based on these data, we proposed a new model for Cl- uptake, in which Na+,K+-ATPase, V-type H+-ATPase, Cl-/HCO3- exchanger and carbonic anhydrase are involved in the active Cl- uptake mechanism in the gills of freshwater-acclimated tilapia.