Thiosulfate elimination and permeability in a sulfide-adapted marine invertebrate

Oxidation of hydrogen sulfide to thiosulfate is one of the best-characteriz ed mechanisms by which animals adapted to sulfide minimize its toxicity, bu t the mechanism of thiosulfate elimination in these animals has remained un clear. In this study, we examined the accumulation and elimination of thios ulfate in the sulfide-adapted marine worm Urechis caupo. The coelomic fluid of U. caupo exposed to 50-100 mu mol L-1 sulfide in hypoxic seawater (PO2 Ca. 10 kPa) accumulated (mean +/- SD) 132 +/- 41 mu mol L-1 thiosulfate aft er 2 h, reaching 227 +/- 113 mu mol L-1 after an additional 4 h in aerated, sulfide-free seawater. In whole-animal thiosulfate clearance studies, the rate of thiosulfate elimination from the coelomic fluid followed a single e xponential time course with a half-life of 6 h. The thiosulfate permeabilit y coefficient of isolated preparations mounted in diffusion chambers was 7. 6 x 10(-5) +/- 7.7 x 10(-5) cm s(-1) for the hindgut and 5.5 x 10(-7) +/- 2 .7 x 10(-7) cm s(-1) for the body wall. These rates were independent of the direction of net efflux (mucosal-to-serosal or serosal-to-mucosal). Using a simple mathematical model of U. caupo that incorporates the thiosulfate p ermeability coefficients, the thiosulfate half-life was calculated to be 23 h without hindgut ventilation but less than 1 h with normal hindgut ventil ation. Based on this information, we propose that passive thiosulfate diffu sion across the hindgut is adequate to explain the observed rates of thiosu lfate elimination.