Trimethylamine oxide transport across plasma membranes of elasmobranch erythrocytes

The aim of this study was to characterize the erythrocyte cell membrane transport of trimethylamine oxide (TMAO) in the little skate, Raja erincea. Uptake of TMAO occurs by two processes, Na+‐dependent and Na+‐independent. 2,4 dinitrophenol (2,4 DNP), a known ATP synthesis inhibitor, inhibited TMAO uptake, suggesting the involvement of the Na+/K+‐ATP pump in Na+‐dependent TMAO transport. Na+‐independent TMAO uptake was stimulated by cell swelling when erythrocytes were incubated in hypotonic elasmobranch incubation medium. Swelling‐activated, Na+‐independent TMAO uptake was inhibited by the anion transport inhibitors quinine and 4,4′‐diisthiocyanostilbene‐2,2′‐disulfonic acid (DIDS), two blockers of the swelling‐activated osmolyte channel in skate erythrocytes. TMAO efflux was stimulated by hypotonic stress in the erythrocytes of the spiny dogfish, Squalus acanthias. DIDS also inhibited this efflux, indicating that TMAO is transported by the organic osmolyte channel in the erythrocytes of this elasmobranch as well. J. Exp. Zool. 284:605–609, 1999. © 1999 Wiley‐Liss, Inc.