The giant mudskipperPeriophthalmodon schlosserifacilitates active NH4+ excretion by increasing acid excretion and decreasing NH3 permeability in the skin

Abstract

SUMMARYPeriophthalmodon schlosseri is an amphibious and obligatory air-breathing teleost, which is extremely tolerant to environmental ammonia. It actively excretes NH4+ in ammonia loading conditions. For such a mechanism to operate efficaciously the fish must be able to prevent back flux of NH3. P. schlosseri could lower the pH of 50 volumes (w/v) of 50% seawater in an artificial burrow from pH 8.2 to pH 7.4 in 1 day, and established an ambient ammonia concentration of 10 mmol l-1 in 8 days. It could alter the rate of titratable acid efflux in response to ambient pH. The rate of net acid efflux (H+ excretion)in P. schlosseri was pH-dependent, increasing in the order pH 6.0<7.0<8.0<8.5. Net acid flux in neutral or alkaline pH conditions was partially inhibited by bafilomycin, indicating the possible involvement of a V-type H+-ATPase. P. schlosseri could also increase the rate of H+ excretion in response to the presence of ammonia in a neutral (pH 7.0) external medium. Increased H+ excretion in P. schlosseri occurred in the head region where active excretion of NH4+ took place. This would result in high concentrations of H+ in the boundary water layer and prevent the dissociation of NH4+, thus preventing a back flux of NH3 through the branchial epithelia. P. schlosseriprobably developed such an `environmental ammonia detoxification' capability because of its unique behavior of burrow building in the mudflats and living therein in a limited volume of water. In addition, the skin of P. schlosseri had low permeability to NH3. Using an Ussing-type apparatus with 10 mmol l-1 NH4Cl and a 1 unit pH gradient (pH 8.0 to 7.0), the skin supported only a very small flux of NH3 (0.0095 μmol cm-2 min-1). Cholesterol content (4.5 μmol g-1) in the skin was high, which suggests low membrane fluidity. Phosphatidylcholine, which has a stabilizing effect on membranes, constituted almost 50% of the skin phospholipids, with phosphatidyleserine and phsophatidylethanolamine contributing only 13% and 15%, respectively. More importantly, P. schlosseri increased the cholesterol level (to 5.5 μmol g-1) and altered the fatty acid composition (increased total saturated fatty acid content) in its skin lipid after exposure to ammonia (30 mmol l-1 at pH 7.0) for 6 days. These changes might lead to an even lower permeability to NH3 in the skin, and reduced back diffusion of the actively excreted NH4+ as NH3 or the net influx of exogenous NH3, under such conditions.