Osmoregulation in the mudskipper,Boleophthalmus boddaerti II. transepithelial potential and hormonal control.

Boleophthalmus boddaerti submerged in 10%, 50% and 80% seawater (sw) for 7 days, had whole body transepithelial potentials (TEP) of 3.3, 18.3 and 22.9 mV, respectively. Hypophysectomy significantly decreased the TEP ofB. boddaerti and reversed the polarity of the TEP of the fish exposed to 10% sw.Hypophysectomy also significantly decreased the branchial Na(+)-K(+) activated adenosine triphosphatase (Na(+),K(+)-ATPase) activity but increased the activity of branchial HCO3 (-)-Cl(-) stimulated adenosine triphosphatase (HCO3 (-),Cl(-)-ATPase) inB. boddaerti exposed to 10% sw. However, survival in 10% sw was not significantly impaired by hypophysectomy and no significant change in plasma osmolality and plasma Na(+) and Cl(-) concentrations was observed.Various doses of ovine-prolactin or salmon-prolactin were unable to restore the TEP of hypophysectomizedB. boddaerti in 10% sw to that of the sham-operated fish. However, cortisol increased TEP to a positive value in hypophysectomizedB. boddaerti, though it was still lower than the sham-operated control. Cortisol treatment also affected the plasma osmolality, plasma Na(+) and Cl(-) contents and branchial Na(+),K(+)-ATPase and HCO3 (-),Cl(-)-ATPase activities. Overall, the hormonal control of osmoregulation inB. boddaerti appeared to differ from that of other teleosts.