STUDIES ON THE ELASMOBRANCH KIDNEY. II. REABSORPTION OF UREA BY THE SMOOTH DOGFISH, MUSTELUS CANIS

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Abstract
The reabsorption of urea by the kidney tubules of M. canis has been studied with inulin as a measure of glomerular filtration. The blood level of urea is highly variable. Of the filtered urea 70-99% may be reabsorbed, a rate almost equal to that of glucose. The control of reabsorption appears to rest upon the amt. of urea left unabsorbed in the lumen of the tubules. While on the avg. a constant amt. of urea is left in the tubules from each ml. of filtrate, the urea concn. is variable because of differences in the rate of water reabsorption. Much of the variation in unabsorbed urea is causally related in some way to the reabsorption of water, larger amts. remaining in the tubules when there is less water reabsorption, more urine formation, and a higher concn. of urea in the urine. The data indicate that urea reabsorption is not isosmotic,but the nature of its control remains obscure. It is possible that an enzyme system is involved which functions only when there is a certain level of urea on the lumen side of the cells. The expts. do not give a definite answer as to whether urea or water is reabsorbed earlier in the tubule, or whether these substances are reabsorbed simultaneously at the same site. It seems clear that variations in the rate of filtration are due to changes in the number of functioning glomeruli rather than to differences in the rate of function of individual units. It was not possible to demonstrate any tubular maximum. Data are presented on a number of characteristics: urine flow, filtration rate, blood and urine concentration, reabsorption rates, etc.