THE ELECTRON MICROSCOPIC APPEARANCE OF THE KIDNEY IN HYDRATED AND DEHYDRATED RATS

Abstract

Renal tissue from dehydrated rats forming a concentrated urine and from water‐loaded rats forming a very dilute urine has been compared by electron microscopy. The most striking and consistent difference between the two groups is the presence in the hydrated animals of marked thickening and fibrillary changes of the basement membrane of the pars recta of the proximal tubule and the descending limb of the loop of Henle. These changes are largely reversed by dehydration or by the administration of pitressin. It seems likely that this appearance is due to hydration of the basement membrane, which becomes swollen and possibly less permeable to the outward passage of water and to the inward passage of sodium and urea. Such a change in the permeability of the membrane would interfere with the function of the loop as a counter‐current multiplier and would effectively prevent the building up of a medullary osmotic gradient and the formation of a concentrated urine. The reduction in papillary osmolality and the more rapid turnover of sodium within the medulla which is believed to occur in water diuresis could be explained on this view. The hypothesis is also compatible with the idea that vasopressin acts primarily and possibly solely on the collecting ducts.