Characterization of ion and acid-base transport in the fresh water adapted mummichog (Fundulus heteroclitus)

Abstract

We examined whether ionoregulatory mechanisms of fresh water Fundulus heteroclitus in vivo are similar to those of typical freshwater species (e.g., rainbow trout, goldfish, and catfish). Under control conditions ([NaCl]ext ˜1 mmol/l), the mummichog exhibits very large Na+ influx and efflux rates but virtually no Cl– influx and a small Cl– efflux component. External NaCl levels were varied to reveal a saturable, low affinity (Km = 1,723 ± 223 μmol/l), high capacity (Jmax = 2,258 ± 288 nEq/g/h) Na+ uptake system that was independent of both Na+ efflux and ammonia excretion. A measurable Cl– influx did not occur until NaCl levels surpassed 2 mmol/l and did not saturate within the freshwater range, suggesting a completely different uptake mechanism. Cl– efflux was also independent of Cl– influx. A systemic acidosis (intraperitoneal Hcl injection) was induced in order to investigate the connection between ionoregulation and acid-base balance. The acidosis did not affect influx rates but induced an elevated Cl– efflux and an attenuated Na+ efflux. This resulted in an excess of net Cl– loss over Na+ loss which effected a net acid excretion by strong ion difference theory. These results concur with the measured acid-base fluxes which indicate that over 50% of the acid load was excreted within 4 h by differential efflux modulation. Therefore an ion/acid-base link does exist in the mummichog but differs in nature from that of other freshwater fish. Indeed, virtually all of these findings differ from the current model for most other teleosts, indicating that alternate models of ionoregulation in fresh water exist. J. Exp. Zool. 279:208–219, 1997. © 1997 Wiley-Liss, Inc.