Printed in Great Britain THE RESPONSE OF THE KIDNEY OF THE FRESHWATER RAINBOW TROUT TO TRUE METABOLIC ACIDOSIS

Infusion of lactic acid into the bloodstream of trout produced a short-lived depression of blood pH and a long-lasting elevation of blood lactate. The lactate injected was distributed in a volume of 198 ml/kg. Renal excretion of lactate anion and total acid increased by approximately equal amounts dur-ing the period of high blood lactate levels, but total renal loss over 72 h accounted for only 2 % of the lactate load and 6 % of the proton load. Com-parable differences in the time courses of blood lactate and pH changes occurred when lactacidosis was induced endogenously by normocapnic hy-poxia. The immediate response of the kidney was similar to that with lactic acid infusion, but there was a long-lasting (12-72+ h) elevation of urinary acid efflux that was not associated with lactate excretion. Following hypoxia, renal excretion over 72 h accounted for 1 % of the estimated lactate load and 12-25 % °f *he proton load. A renal lactate threshold of 4-10/iequiv/ml prevents significant urinary lactate excretion. The response of the trout kidney to true metabolic acidosis is similar to that of the ma/nmalian kidney