Abstract
The body temperatures of teleost species fluctuate following changes in the aquatic environment. As such, decreased water temperature lowers the rates of biochemical reactions and affects many physiological processes, including active transport-dependent ion absorption. Previous studies have focused on the impacts of low temperature on the plasma ion concentrations or membrane transporters in fishes. However, very few in vivo or organism-level studies have been performed to more thoroughly elucidate the process of acclimation to low temperatures. In the present study, we compared the strategies for cold acclimation between stenothermic tilapia and eurythermic goldfish. Whole-body calcium content was more prominently diminished in tilapia than in goldfish after long-term cold exposure. This difference can be attributed to alterations in the transportation parameters for Ca2+ influx, i.e., maximum velocity (Vmax) and binding affinity (1/Km). There was also a significant difference in the regulation of Ca2+ efflux between the two fishes. Transcript levels for Ca2+ related transporters, including the Na+/Ca2+ exchanger and epithelial Ca2+ channel, were similarly regulated in both fishes. However, upregulation of plasma membrane Ca2+ATPase expression was more pronounced in goldfish than in tilapia. In addition, enhanced Na+/K+-ATPase abundance, which provides the major driving force for ion absorption, was only detected in tilapia, while upregulated Na+/K+-ATPase activity was only detected in goldfish. Based on the results of the present study, we have found that goldfish and tilapia differentially regulate gill epithelial plasma membrane Ca2+-ATPase (PMCA) expression and Na+/K+-ATPase activity in response to cold environments. These regulatory differences are potentially linked to more effective regulation of Ca2+ influx kinetics and better maintenance of whole body calcium content in goldfish than in tilapia.
Highlights
Most freshwater teleost species live and grow in environments with Ca2+ concentrations that are low compared to the concentrations in their body fluids
Based on the results of the present study, we have found that goldfish and tilapia differentially regulate gill epithelial plasma membrane Ca2+-ATPase (PMCA) expression and Na+/K+-ATPase activity in response to cold environments
Ca2+ uptake from the environment mainly occurs via transcellular transportation across the gill epithelium, and this process is dependent on facilitated diffusion through the epithelial Ca2+ channel (ECaC) on the apical membrane (Pan et al, 2005), active transport by PMCA and Na+/Ca2+ exchanger (NCX) (Flik and Verbost, 1993; Flik et al, 1995), and energyconsuming maintenance of ion gradients by Na+/K+-ATPase on the basolateral membrane (Hwang et al, 2011)
Summary
Most freshwater teleost species live and grow in environments with Ca2+ concentrations that are low compared to the concentrations in their body fluids. Despite this relatively low availability from the environment, constitutive increase of wholebody calcium content is an important factor during development that affects survival (Chou et al, 2002; Chen et al, 2003). Ionic imbalance in fish cells may occur as a result of decreased environmental temperatures because the ion transporters generally display a higher temperature coefficient (Q10 = 2–3) than does permeability (Q10 = 1–2) between fish cells (Cossins et al, 1995). Other adaptive mechanisms of Ca2+ regulation have not been studied in detail
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