Measurement of single nephron filtration rate in the kidney of the river lamprey, Lampetra fluviatilis L.

1. The reliability of [3H]inulin as a reference substance for the measurement of glomerular filtration rate (GFR) in river lampreys was investigated. 2. Simultaneously measured renal clearances of [3H]inulin and [14C]-polyethylene glycol (PEG) were not significantly different. 3. Recoveries of [3H]inulin in the urine following its injection into the proximal tubule and urinary duct averaged 83.5+/-4.0% (n = 14) and 93.0+/-4.6% (n = 9) respectively. 4. No evidence was obtained to suggest penetration of the tubular wall by by [3H]inulin following its introduction into peritubular capillaries. 5. Gel-filtration of lamprey plasma and urine produced no signs of any significant degradation of the inulin to fructose. 6. [3H]inulin recoveries, though significantly below 100% (P less than 0.001), were considered adequate to justify its use after allowing for possible sources of error. 7. A mean single nephron filtration rate (SNGFR) value of 7.02+/-0.27 nl/min (n = 89) was obtained from the kidneys of anaesthetized freshwater lampreys. 8. [51 Cr]EDTA was found to be totally unsuitable for the measurement of GFR in this species.