Studies of glomerulotubular interaction.

"From the physiological point of view, our interest in the question of the relative distribution of glomerular activity among various nephrons in the kidney is enhanced by the circumstance that the reabsorptive tubule, which is individually fixed in size, is also limited by certain maximal capacities or rates of activity, and it is fair presumption that the quantity of filtrate delivered to each tubule is somehow conditioned by this circumstance. The relative distribution of tubular perfusion is a conjoint problem; were the distribution of peritubular blood subject to control independently of the glomerular circulation, the glomerulo-tubular balance could scarcely fail to be impaired". With these words in 1943, Homer Smith' introduced the concept of a "glomerulo-tubular balance" and called attention to a nexus of related problems (and ambiguities) that remain unsettled in 1973. Smith and his colleagues approached these questions by measurements of tubular glucose reabsorption and Diodrast excretion over a wide loading range ("titration") in resting, unanesthetized man and dog as a means of delineating the basic patterns of filtrate and perfusate distribution to tubular tissues. The results indicated that both reabsorptive and excretory loads are imposed in such a manner as to assure that maximal transport rates are achieved by all tubular cells at approximately the same plasma concentration. It was inferred from this phenomenon that both glomerular filtration (and tubular perfusion) must be proportioned to nephron functional capacity at rest and, presumably, therefore, to cell mass. The corollary implication that glomerulotubular dimensions must also be proportional was substantiated nearly 20 years later by Oliver and his associates.2 3 Now up to this point, "glomerulotubular balance" has been construed in terms of a constant relationship between filtration rate (GFR) and tubular cell mass or some presumed correlate of cell mass such as the rate of maximal tubular glucose reabsorption (glucose Tm-TmG). Presumably