Bodil Schmidt-Nielsen

Sodium-linked urea transport by the renal tubule of the spiny dogfish Squalus acanthias☆

Abstract Renal volume regulation and excretion of water, sodium, and urea was studied in the spiny dogfish, Squalus acanthias, when exposed to sea water and dilute sea water, and when extracellular fluid volume was expanded by i.v. infusion of dogfish Ringer, 0·55 M NaCl, and 1·1 M urea solutions. It was found that: 1. 1. Na and urea were reabsorbed at a fixed ratio of 1·6 moles of urea per mole of Na under all conditions studied. 2. 2. The ratios between the solutes in the plasma are maintained at quite constant values. They return to the normal ratios within 12–24hr following expansion of extracellular volume with various solutions. 3. 3. Renal excretion of water and solutes is greatly increased following transfer to dilute sea water. 4. 4. Isosmotic or hyperosmotic expansion of extracellular fluid volume leads to a quick restoration of normal body weight and body fluid composition within 24 hr, but the kidneys play an insignificant role in the excretion of the excess urea and Na.

Chemical and morphological differences in the kidney zones of the elasmobranch, Raja erinacea mitch

The histological investigation of the kidney of the skate Raja erinacea revealed a thin cap of dorsal bundles, which contain segments of single nephrons that are arranged separately in a countercurrent manner, and a large ventral zone, where the second proximal segments (PII) and parts of the lower nephron are located. This zonation is apparent in fresh, unfixed material and makes it possible to separate small tissue samples under a dissecting microscope. The osmolality in both zones does not differ. The dorsal bundle zone had a lower urea concentration and a higher sodium concentration than the ventral zone. The differences in the mean concentrations of the tissue samples indicate a gradient for the two substances along the bundles. Determinations of amounts of water and solutes per mg solute-free, dry tissue of the two zones, showed that the amounts of water, total osmolytes, Na and K were greater in the bundle zone than in the ventral zone, while the amount of urea was identical in the two zones. This indicates that the lower urea concentration in the bundle zone is established through an accumulation of Na and water in the interstitium. The countercurrent arrangement of very early and late segments of single renal tubules supports the concept of passive reabsorption of urea in the kidney of the marine elasmobranch.

Species Differences in Renal Structure and Function — Applications to Nephrotoxicity in Man

A classical example of how physiologists can take advantage of a specific renal adaptation is that of E.K. Marshall Jr. and the aglomerular goose fish (26). Marshall had shown that dog renal tubules accumulate the dye phenol red even when glomerular filtration was stopped by lowering the blood pressure. However, due to the belief held by most physiologists in the theory proposed by Cuhsny (that tubular secretion did not exist), the proofs presented by Marshall were not accepted. Then Marshall found from the literature that some fishes have renal tubules without glomeruli and therefore must form urine entirely by secretion. He came to Mount Desert Island Biological Laboratory in the summer of 1926 to work with the goose fish Lophius piscatorius. The work proved that not only is phenol red secreted by the renal tubules of the goose fish, but also a number of organic acids and bases as well as fluid and electrolytes (27). Wher these results were presented by Marshall his staunch opponent A.N. Richards was overheard saying: “at last Marshall has found one animal that fits his theories” (26).