| Summary: | 博士 === 國防醫學院 === 生命科學研究所 === 102 === Acid-base homoeostasis is critical for survival in all biological system. Fish living in freshwater environment often encounter the impacts of external pH change. Neuroendocrine system has long been proposed as the primary link between environmental change and physiological response, however, less is known about the endocrine control of acid-base regulation in fish so far. The aim of this study is to use zebrafish as a model to identify hormones involved in acid-base regulation and to explore their underlying mechanism in regulation of acid-base homeostasis. The investigation was initiated with identifying the hormones involved in acid-base regulation by comparing gene expression of hormone receptors in gills between pH7- and pH4-acclimated zebrafish. Based on the preliminary result, three hormones (or receptor), endothelin-1, prolactin and estrogen related receptor α, were selected to be the objects of this study.
In Chapter 1, the role of EDN1 in H+ secretion was examined. Expression of EDN1 and one of its receptors, EDNRAa, was stimulated in zebrafish acclimated to acidic water. edn1 overexpression enhanced H+ secretion in embryonic skin at 3dpf. EDNRAa loss-of-function significantly decreased EDN1- and acid-induced H+ secretion. Abrogation of EDN1-enhanced H+ secretion by a vacuolar H+-ATPase inhibitor (bafilomycin A1) suggests that EDN1 exerts its action by regulating the H+-ATPase-mediated H+ secretion. EDN1 does not appear to affect H+ secretion through either altering the abundance of H+-ATPase or affecting the cell differentiation of H+-ATPase-rich (HR) ionocytes, because the reduction in secretion upon ednraa knockdown was not accompanied by decreased expression of H+-ATPase or reduced HR cell density. These findings suggest role of EDN1 in responding to acid base disturbance is acute.
In Chapter 2, the role of PRL signaling in HR cell function was examined. While only the expression of prlra was stimulated by pH4 in adult gills, expressions of both prlra and prlrb were significantly increased in pH4-exposed embryos. Konckdown of either prlr could severely reduce the ionocyte density, suggesting that prolactin participates in acid-base regulation through modulating the differentiation of ionocytes. The expression of foxi3a and gcm2 was differentially affected by knockdown of prlra and prlrb, suggesting these two receptors may mediate distinct function on ioncyte differentiation.
In Chapter 3, the role of ERRα on acid-base regulation was examined. ERRα is highly expressed in HR cell and expression of esrra (encoding ERRα) was upregulated in acidic water. Knockdown of esrra significantly reduced ability of H+ secretion in 3dfp embryos. Expressions of H+ secreting-related transporters as well as energy metabolism genes were reduced in esrra morphants, suggesting that ERRα regulates H+ secretion through modulating the transporters expression and energy metabolism.
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