Ionic regulation and acid-base balance are fundamental to the physiology of vertebrates including fish. Acidification of freshwater ecosystems is recognized as a global environmental problem, and the physiological responses to acid exposure in a few fish species are well characterized. However, the underlying mechanisms promoting ionic and acid-base balance for most fish species that have been investigated remain unclear. Zebrafish (Danio rerio) has emerged as a powerful model system to elucidate the molecular basis of ionic and acid-base regulation. The utility of zebrafish is related to the ease with which it can be genetically manipulated, its suitability for state-of-the-art molecular and cellular approaches, and its tolerance to diverse environmental conditions. Recent studies have identified several key regulatory mechanisms enabling acclimation of zebrafish to acidic environments, including activation of the sodium/hydrogen exchanger (NHE) and H + -ATPase for acid secretion and Na + uptake, cortisol-mediated regulation of transcellular and paracellular Na + movements, and ionocyte proliferation controlled by specific cell-fate transcription factors. These integrated physiological responses ultimately contribute to ionic and acid-base homeostasis in zebrafish exposed to acidic water. In the present review, we provide an overview of the general effects of acid exposure on freshwater fish, the adaptive mechanisms promoting extreme acid tolerance in fishes native to acidic environments, and the mechanisms regulating ionic and acid-base balance during acid exposure in zebrafish.
KEY WORDS: Acid exposure, Acid-base balance, Ionic regulation, Zebrafish
IntroductionDeclining fish populations in acidified lakes and streams (see review by McDonald, 1983a) was a significant factor promoting intensive research over the past 40 years on the ecological and physiological consequences of aquatic acidification. In freshwater (FW) ecosystems, acidification is caused largely by atmospheric acidic deposition (i.e. acid rain). Acidification of FW is a global problem, which has been documented in several geographic regions including eastern Europe, the USA and China (Psenner, 1994;Schindler, 1988;Wright et al., 2005). In Canada, it is estimated that about 40% of lakes are in regions susceptible to acid deposition (Kelso et al., 1990). Most aquatic animals, including fish, live in a narrow range of pH near neutrality, and acute or chronic exposure to acidic water can adversely affect their physiological functions (McDonald, 1983a;Wood, 1989). Nevertheless, some species thrive in acidic environments, and thus can serve as useful models for investigating the mechanisms underlying adaptation to acid stress.
REVIEWDepartment of Biology, University of Ottawa, 30 Marie Curie, Ottawa, ON, Canada, K1N 6N5.*Author for correspondence (wkwong@uottawa.ca)The mechanisms of ionic and acid-base regulation in fish have been summarized in previous reviews (Claiborne et al., 2002;Evans et al., 2005;Gilmour and Perry, 2009;Goss et al., 19...