CORRELATION OF STRUCTURE AND FUNCTION IN THE HANDLING OF GLUCOSE BY THE NEPHRONS OF THE CANINE KIDNEY*

Abstract

Correlation of the structure and the function of the glomeruli and their attached proximal convoluted tubules has been investigated in 3 normal female dogs. The activity of the glomeruli in providing by filtration a load of glucose for reabsorption by the proximal convoluted tubules was evaluated in each animal over a wide range of arterial glucose concentrations. The "glucose titration curve" (or the relationship between tubular reabsorption and filtered load) so obtained is determined by the distribution of glomerular activity (the filtration rate of a glomerulus relative to the maximal rate of glucose reabsorption of which its proximal convoluted tubule is capable) in the nephron population. When sufficient functional data had been obtained, one kidney was removed from each dog and a sample of from 45 to 47 nephrons obtained by microdissection. The values for glomerular surface (calculated as for a sphere from the average of the greatest and least diameters) and for the volume of the attached proximal convoluted tubule (calculated as for a cylinder using the length and the average of 10 diameters measured at regular intervals) were more closely correlated (p < 0.001 in 2 dogs, Cora and Dali, and p< 0.05 in one, Elena) than any of the other glomerulotubular dimensions. Taking glomerular surface (gs) as a "structural equivalent" of filtration, and proximal volume (pv) as that of the maximal glucose reabsorptive capacity of the attached proximal convolutions, the glomerular activity for each nephron could be computed in anatomical terms as gs/pv. "Glucose titration curves" derived from the anatomical data closely fitted the functional data in Cora and Dali and, if allowance were made for a small number of very "inactive" nephrons, also in Elena. The agreement between the data appears to be consistent with the validity of the basic assumptions that glomerular filtration rate is determined by the area of the external surface of the capillary tuft and that maximal glucose reabsorptive capacity depends upon the mass of proximal convoluted tubular cells. The findings also indicate that the tendency for the reabsorptive mechanism in all nephrons to be saturated at approximately the same plasma glucose concentration, with minimal "splay" in the titration curve, can be ascribed in part, as previously predicted by Smith and coworkers to a structural balance between glomeruli and tubules. Splay is also minimized, however, by a central tendency of the anatomical data that is expressed as a relatively small coefficient of variation.