Single nephron filtration rates (SNGFR) in the trout,Salmo gairdneri

1. Trout kidney slices incubated in the presence of¹⁴C-sodium ferrocyanide accumulated radioactivity. Uptake by individual nephroi increased with time, stabilizing after 30 min. The greater the concentration of radioactivity the greater the uptake per mm of tubule, while reduced specific activity inhibited¹⁴C uptake.
2. The renal excretory pattern of¹⁴C-sodium ferrocyanide was compared with that of³H-inulin in fresh water adapted trout. The renal clearance rates of the substances were similar and linearly related over a range or urine flows, providing plasma ferrocyanide concentrations were between 1.5 and 2.5 mM.¹⁴C-ferrocyanide is thus a reliable marker for glomerular activity in this species.
3. A protocol for the determination of single nephron glomerular filtration rates (SNGFR) in trout is described. The method exploits the ability to visualize ferrocyanide as Prussian Blue within nephroi after a single pulse injection superimposed upon a constant infusion of¹⁴C-ferrocyanide. The effects of surgical procedure and extraluminal contamination are assessed.
4. The SNGFR of freshwater trout, 1.31±0.2 nl/min is approximately 65% less than that observed in sea water adapted fish (3.74±1.12 nl/min). The overall, total kidney glomerular filtration rate, (GFR) is 142.6±17.4 μl/min/kg in fresh water animals and 20.1±0.9 μl/min/kg in sea water fish.
5. It is concluded that whilst SNGFRs of sea water animals are higher than those of fresh water animals, filtration is distributed to nephron populations selected to meet the homeostatic demands of the organism.