Kidney function of the American eel Anguilla rostrata.

Urine formation in the eel, acclimated to freshwater (FW) and seawater (SW) was studied. SW eels maintained higher plasma and urine osmotic and ionic concentrations than FW eels. Polyethylene-1,2-glycol-14C (PEG-14C) clearance was 29% greater than methoxy-inulin-3H clearance. PEG-14C clearance was considered to be the glomerular filtration rate (GFR). The GFR of SW and FW eels did not differ. Urine flow rate in SW eels was one-third of that in FW eels. The primary urinary solutes in both groups were Na and Cl. Apparently, tubular fluid secretion occurred in FW and, to a lesser degree, in SW eels. With the assumption that water reabsorption was secondary to Na reabsorption in the kidney, the clearance data were used to evaluate all possible explanations for what appeared to be fluid secretion. The data were inconsistent with the possibility that GFR had been underestimated due to glomerular sieving of PEG or active or passive reabsorption of the marker, but consistent with tubular fluid secretion. Furosemide caused diuresis in both groups of eels apparently by inhibition of Na reabsorption in the distal tubule, but it had no effect on the apparent tubular fluid secretion. Tubular ion secretion could not be conclusively implicated as a driving force for fluid secretion. However, the possibility of K, Ca, or Mg secretion in a proximal segment followed by reabsorption in a more distal part of the nephron was not ruled out.