Isolated perfused head of rainbow trout. II. Ionic fluxes

Branchial ionic transport processes were evaluated and compared in Ringer-perfused and blood-perfused heads of rainbow trout. Rates of Na+ and Cl- influx were similar to in vivo levels at the onset of perfusion. Na+ uptake deteriorated more rapidly over time during Ringer perfusion, whereas Cl- uptake was relatively stable regardless of the perfusate employed. Transepithelial potentials (TEP) were similar to values reported for intact freshwater teleosts and were unstable only during the initial period of blood perfusion. The TEP was extremely dependent on the external concentration of NaCl. Ammonia excretion (JNH4+net) was normal during both types of perfusion, although blood did have a stabilizing effect on JNH4+net over time of perfusion. JNH4+net was stimulated by the addition of carbonic anhydrase to Ringer, without a concomitant increase in Na+ uptake. Branchial net fluxes of Na+ and Cl- were stable but slightly negative and in this regard represented slightly stressed intact animals. Net acid flux (JH+net), however, was similar to in vivo rates. Na+ and Cl- uptake displayed Michaelis-Menten kinetics. The affinity constant (Km) for Na+ uptake was typical of intact fish during both Ringer and blood perfusion, whereas Km for Cl- uptake was abnormally high in both cases. The results are discussed with reference to the suitability of the perfused trout head preparation for studying gill ionic fluxes in vitro.