Ion and acid-base regulation in the freshwater mummichog (Fundulus heteroclitus): A departure from the standard model for freshwater teleosts

Marjorie L. Patrick, Chris M. Wood

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65 Scopus citations


Ion and acid-base balance were examined in the freshwater-adapted mummichog (Fundulus heteroclitus) using a series of treatments designed to perturb the coupling mechanisms. Unidirectional Cl- uptake (J(in)/(Cl)) was extremely low whereas J(in)/(Na) was substantial (three- to sixfold higher); comparable differences occurred in unidirectional efflux rates (J(out)/(Cl), J(out)/(Na)). J(in)/(Cl) was refractory to all treatments, suggesting that Cl-/base exchange was unimportant or absent. Indeed, no base excretion or modulation of ion fluxes occurred for acid-base balance for up to 8 h after NaHCO3- loading (injections of 1000 or 3000 nequiv. · g-). Acute environmental low pH (4.5) and amiloride (10-4 M) treatments caused concurrent inhibition of J(in)/(Na) and net H+ excretion (J(net)/(H+)), indicating the presence of Na+/H+ exchange J(in)/(Na) was elevated and J(net)/(H+) restored during recovery from both treatments, but this exchange did not appear to be dynamically adjusted for acid-base homeostasis. High external ammonia exposure (1 mmol · l-1) initially blocked ammonia excretion (J(net)/(Amm)) but had no effect on J(in)/(Na), whereas high pH (9.4) reduced both J(net)/(Amm) and J(in)/(Na). Inhibition of J(in)/(Na) by the low pH and amiloride treatments had no effect on J(net)/(Amm). These results indicate that ammonia excretion is entirely diffusive and independent of both Na+ uptake and the protons that are transported via the Na+/H+ coupling. In addition, ureagenesis served as a compensatory mechanism during high external ammonia exposure, as a marked elevation in urea excretion partially replaced the inhibited J(net)/(Amm). In all treatments, changes in the Na+-Cl- net flux differential were consistent with changes in J(net)/(H+) measured by traditional water titration techniques, indicating that the former can be used as an estimate of the acid-base status of the fish. Overall, the results demonstrate that the freshwater-adapted F. heteroclitus does not conform to the ion/acid-base relationships described in the standard model based on commonly studied species such as trout, goldfish, and catfish.

Original languageEnglish (US)
Pages (from-to)445-456
Number of pages12
JournalComparative Biochemistry and Physiology - A Molecular and Integrative Physiology
Issue number4
StatePublished - Apr 1999


  • Acid-base balance
  • Ammonia excretion
  • Freshwater ion regulation
  • Fundulus heteroclitus
  • Killifish
  • Mummichog
  • Teleost
  • Urea excretion

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Physiology


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