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

Ion and acid-base balance were examined in the freshwater-adapted mummichog (Fundulus heteroclitus) using a series of treatments designed to perturb t he coupling mechanisms. Unidirectional Cl- uptake (J(in)(Cl)) was extremely low whereas J(in)(Na) was substantial (three- to sixfold higher); comparab le 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- l oading (injections of 1000 or 3000 nequiv.g(-1)). Acute environmental low p H (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 N a+/H+ exchange. J(in)(Na) was elevated and J(net)(H+) restored during recov ery from both treatments, but this exchange did not appear to be dynamicall y adjusted for acid-base homeostasis. High external ammonia exposure (` mmo l . l(-1)) initially blocked ammonia excretion (J(net)(Amm)) but had no eff ect 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 ent irely 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 e levation in urea excretion partially replaced the inhibited J(net)(Amm). In all treatments, changes in the Na+-Cl- net flux differential were consiste nt with changes in J(net)(H+) measured by traditional water titration techn iques, indicating that the former can be used as an estimate of the acid-ba se status of the fish. Overall, the results demonstrate that the freshwater -adapted F. heteroclitus does not conform to the ion/acid-base relationship s described in the standard model based on commonly studied species such as trout, goldfish, and catfish. (C) 1999 Elsevier Science Inc. All rights re served.