Studies on the Elasmobranch Kidney. Ii. Reabsorption of Urea by the Smooth Dogfish, Mustelus Canis

Water, urea and glucose reabsorption have been studied simultaneously using the inulin clearance method.2. Variations in filtration rate appear to be due to changes in the number of functional glomeruli, rather than to changes in the rate of function of individual units.3. Glucose reabsorption is not correlated in any way with that of urea.4. The main factor controlling the urea reabsorption is the concentration of urea occurring normally in the plasma. Attempts to increase the reabsorption of urea by raising the plasma level result in a depression of reabsorption not only of urea but of glucose and water as well. To some extent this reduced water reabsorption is offset by a decreased filtration rate, with the result that only a very moderate diuresis ensues.5. The percentage of the filtered urea which is reabsorbed varies with the concentration in the plasma, ranging from 70% to 99.5%, comparing quite favorably with the reabsorption of glucose.6. On the average, the actual amount of unabsorbed urea left in each unit volume of filtrate is quite constant over a wide range of normally occurring urea levels.7. Variations in the amount of unabsorbed urea are correlated in a presumably causal fashion with the reabsorption of water, the rate of urine flow and the concentration of urea in the urine.8. Choice cannot be made at this time between alternate explanations of the data which would localize the site of urea reabsorption in relation to the site of water reabsorption.9. The data show clearly that the reabsorption of urea cannot be an isosmotic one.