Branchial and intestinal osmoregulatory acclimation in the four-eyed sleeper, Bostrychus sinensis (Lacepède), exposed to seawater

Abstract

Bostrychus sinensis is a facultative air breather that inhabits waters of a wide range of salinities. This study aimed to elucidate whether branchial and intestinal osmoregulatory acclimation occurred in B. sinensis transferred from 5‰ water through a progressive increase in salinities to seawater. Our results indicate that B. sinensis acted as a hyperosmotic regulator in 5‰ water, but exhibited hypoosmotic hypoionic regulation in seawater. During short- (1 day) and medium- (10 days) term acclimation to seawater, there were only minor perturbations in plasma osmolality and [Na+], which returned to control levels after 45 days of exposure to seawater. Branchial Na+/K+-ATPase activity was unaffected by 1, 10 or 45 days of exposure to seawater. However, prolonged (45 days) acclimation to seawater led to a significant increase in Na+/K+-ATPase α-subunit protein abundance. Taken together, these results indicate that there could be changes in the expression of Na+/K+-ATPase isoforms and/or post-translational modification of Na+/K+-ATPase in the gills of fish exposed to seawater. Immunofluorescence microscopy revealed that acclimation to seawater for 10 days only resulted in no change in branchial Na+/K+-ATPase protein expression, but there were increases in protein expression of cystic fibrosis transmembrane regulator (CFTR)-like chloride channel and Na+:K+:2Cl− cotransporter (NKCC; probably NKCC1). Indeed, NKCC was undetectable in gills of fish kept in 5‰ water by Western blotting, but it became weakly detectable in fish exposed to seawater for 10 days and prominently expressed in fish exposed to seawater for 45 days. Therefore, our results indicate that branchial CFTR-like chloride channel and NKCC1 were the determining factors in the transition between hyperosmotic regulation and hypoosmotic hypoionic regulation in B. sinensis. Furthermore, the intestine of B. sinensis also served as an important osmoregulatory organ, since there were significant increases in both the activity and protein abundance of intestinal Na+/K+-ATPase in fish acclimated to seawater for 45 days. The effectiveness of branchial and intestinal osmoregulatory acclimation in B. sinensis during seawater acclimation led to only a minor increase in plasma osmolality, and thus resulted in relatively unchanged free amino acid contents in muscle and liver.