Determinants of the steady-state vol of the dogfish shark (Squalus acanthias) rectal gland cells were studied. The cellular levels of trimethylamine oxide (TMAO) in fresh tissue and slices incubated aerobically 60 min in standard (TMAO-free) elasmobranch saline were close to those in the plasma (71 & 5 mM S.E.M.); therefore, under these conditions, the cell membrane appears to be impermeable to this solute. However, depolarization of the cells in high-Kf media produced a rapid loss of TMAO. Thus, TMAO is a major, effectively impermeant solute in the rectal gland cells. The osmolarity of cell solutes in tissue water (fresh and incubated slices) did not differ significantly from values in the plasma or incubation medium, demonstrating the absence of a n osmotic pressure gradient across the cell membrane. An analysis of a simple model of cell solutes under steady-state conditions shows that the presence of an (effectively) impermeant osmolyte decreases the cellular concentration of bulk cations. The analysis is consistent with available observations on the distribution of cell Na+ and K + in tissues containing high concentrations of (nitrogeneous) osmolytes. One simplifying assumption of the model, i.e., identity (or closeness) of the respective reflection coefficients LJ for Na' and K + passage through the cell membranes could not be verified. Compared to available data on the steady-state cellular fluxes of 42Kt in slices of the rectal gland, the uptake of "Naf by the tissue was slow (the derived rate constant k' = 0.017 min-l, i.e., about one tenth of that for Kf). Hence, L J N ~ Send correspondence to the author at the Mount Desert Island Biological Laboratory, Salsbury Cove, ME 04672. *The subscripts i and o refer to intracellular, and extracellular compartments, respectively. Square brackets denote molal concentrations.
TTrimethylamine oxide.