Printed in Great Britain A MICROPUNCTURE STUDY OF KIDNEY FUNCTION IN THE RIVER LAMPREY, LAMPETRA FLUVIATILIS, ADAPTED TO FRESH WATER

Micropuncture techniques have been used to investigate the role of each nephron segment in the river lamprey. The proximal segment reabsorbed no more than 10 % of filtered water and tubular fluid here was iso-osmotic to plasma, at about 250 m-osmol. Further water reabsorption occurred in later nephron segments so that 44 % °f a u filtered water was reabsorbed. Dilution of tubular fluid began in the ascending limb of the nephron loop and continued in the distal and collecting segments, so that 96 % and 97"5 % °f filtered sodium and chloride, respectively, was reabsorbed by the kidney. Lampreys produce 337 ml kg-1 day"1 of a dilute urine (31-2 m-osmol) and the urinary ducts appear not to modify the composition of this urine