Inhibition of calcium uptake during hypoxia in developing zebrafish is mediated by hypoxia-inducible factor

The present study investigated the potential role of hypoxia-inducible factor (HIF) in calcium homeostasis in developing zebrafish (Danio rerio). It was demonstrated that zebrafish raised in hypoxic water (30 mmHg; control, 155 mmHg P ) until 4 days post-fertilization exhibited a substantial reduction in whole-body Ca levels and Ca uptake. Ca uptake in hypoxia-treated fish did not return to pre-hypoxia (control) levels within 2 h of transfer back to normoxic water. Results from real-time PCR showed that hypoxia decreased the whole-body mRNA expression levels of the epithelial Ca channel (ecac), but not plasma membrane Ca -ATPase (pmca2) or Na /Ca -exchanger (ncx1b). Whole-mount in situ hybridization revealed that the number of ecac-expressing ionocytes was reduced in fish raised in hypoxic water. These findings suggested that hypoxic treatment suppressed the expression of ecac, thereby reducing Ca influx. To further evaluate the potential mechanisms for the effects of hypoxia on Ca regulation, a functional gene knockdown approach was employed to prevent the expression of HIF-1αb during hypoxic treatment. Consistent with a role for HIF-1αb in regulating Ca balance during hypoxia, the results demonstrated that the reduction of Ca uptake associated with hypoxic exposure was not observed in fish experiencing HIF-1αb knockdown. Additionally, the effects of hypoxia on reducing the number of ecac-expressing ionocytes was less pronounced in HIF-1αb-deficient fish. Overall, the current study revealed that hypoxic exposure inhibited Ca uptake in developing zebrafish, probably owing to HIF-1αb-mediated suppression of ecac expression.