STUDIES ON THE ELASMOBRANCH KIDNEY. IV. THE SECRETION OF PHENOL RED BY THE SMOOTH DOGFISH, MUSTELUS CANIS

¹ Department of Biology, Vassar College, Poughkeepsie, N. Y., and the Marine Biological Laboratory, Woods Hole, Mass.

1. With increasing plasma concentrations of phenol red, starting at very low levels, there is a rapid and extensive increase in its excretion, reaching an asymptote at approximately 10-11 mgm.%. This is clearest when the data are so treated as to neutralize the effect of variations in the rate of filtration.

2. Much more phenol red is secreted by the tubules than is filtered by the glomeruli. The percentage of excreted dye which was eliminated by tubular secretion varied from a maximum of 96% at the lowest measurable plasma levels to a minimum of about 60% at the highest blood levels obtained. The decrease was essentially linear.

3. At low plasma levels the ratio of the clearance of phenol red and inulin was approximately 25; at the highest plasma levels the phenol red clearance was reduced to only about three times that of inulin.

4. The U/P ratios were of a very different order of magnitude. That of inulin remained relatively constant at a level between 2 and 5; while with increasing plasma levels the phenol red U/P ratio fell from an initial 60 to a minimum of approximately 6. A fall below 1.0 has been reported for the spiny dogfish but this was at plasma phenol red levels far above those achieved in the present experiments.

5. In this series of experiments the rate of phenol red filtration and the rate of phenol red secretion both became reduced at the higher blood levels of phenol red.

6. With decreasing rates of filtrate formation, a reduction in the rate of water reabsorption prevented rates of urine formation from falling to even lower levels than they did.

7. While at higher plasma levels of phenol red there was a decrease in total water reabsorption, there was very little change in the percentage of the filtrate which was reabsorbed.

8. Data produced by these experiments indicate that the reduced filtrate formation at higher plasma levels was due primarily to a complete cessation of the function of some glomeruli rather than to a mere reduction of filtration rates of all the glomeruli.

9. Comparison of these data on Mustelus with those existing in the literature on Squalus acanthias indicates that the two kidneys behave in much the same fashion. The fact that most measurable factors were relatively lower in Mustelus suggests that the total excretory mass may be smaller in relation to the total weight in this animal. It is possible also that some of this difference might disappear if the two species were compared on the basis of body surface rather than body weight.