IONIC REGULATION AND TRANSEPITHELIAL POTENTIAL IN THE MUDSKIPPER, BOLEOPHTHALMUS BODDAERTI

Abstract

The mudskipper, Boleophthalmus boddaerti, was able to survive well in waters of intermediate salinities (10-80 %SW). Fish in dechlorinated tap water suffered a mortality rate of 60 % by the fifth day while 60 % of those in 100 % sea water (SW) died after the third day. After being exposed to 50 and 80 % SW, plasma osmolality, Na+ and Cl- content and in vitro gill Na+-K+ ATPase activity of B. boddaerti were significantly higher than those of fish exposed to 10 % SW. However, the in vitro activities of the gill HC03-C1- ATPase and carbonic anhydrase of the latter fish were significantly greater than those of the former ones. Such correlation suggested that Na+ -K+ ATPase might be involved in hyperosmotic adaptation in this fish while HC03--C1- ATPase and carbonic anhdrase were important for hyposmotic survival. The steady state whole body transepithelial potential (TEP) values for B. boddaerti in 10, 50 and 80 % SW were 3.33, 18.3 and 22.9 mV inside relative to outside respectively. It would appear that TEP was closer to the Nearnst equilibrium potential of the sodium gradient (ENa) when the external sodium and chloride concentrations were higher than those in the plasma of the fish (50 and 80 % SW). The fact that there was no significant difference between the TEP of fish exposed to 50 and 80 % SW,and the TEP of fish kept in 10 % SW was intermediate between the calculated ENa and EC1 values indicated that the origin of TEP might not be purely passive diffusion. Electrogenic active transport mechanisms in various epithelial membranes must, to a certain extent, be also involved. Hypophysectomy significantly decreased the TEP but increased the in vitro activities of the gill HC03--C1- ATPase in fish exposed to 10 % SW. Survival was not significantly impaired and no significant change in plasma osmolality and plasma sodium and chloride content was observed. Various doses of ovine-prolactin and salmon prolactin, a freshwater-adapting hormone of teleost, were unable to restore the TEP of hypophysectomized B. boddaerti in 10 %SW back to that of the saline-injected sham-operated control. Neither did dopamine, a well established prolactin-inhibiting factor in mammals, have any effect on the TEP of the intact mudskipper. However, daily cortisol therapy in hypophysectomized B. boddaerti for 5 days brought the TEP back to a positive value inside of the organism relative to outside, though the TEP value was still lower than that of the saline-injeted sham-operated control. Cortisol treatment also significantly lowered the plasma osmolality, sodium and chloride content and gill HC03--C1- ATPase activity, further confirming the possible relationship between the gill anion-stimulated ATPase activity and uptake of ions in this mudskipper. Overall, the hormonal control of osmoregulation in B. boddaerti appeared to be quite different from other teleosts. Such difference might possibly be due to its specific mode of living as it has not only to adapt to abrupt salinity changes in the estuary but also to cope with the stress of terrestrial survival on the mudflat-a feat unparalleled by other teleosts.