The early proximal tubule: a high-capacity delivery-responsive reabsorptive site.

The proximal convoluted tubule is responsible for reclaiming almost all of the filtered bicarbonate, glucose, and amino acids, as well as 40% or more of the filtered sodium, fluid, chloride, and phosphate. Walker and co-workers demonstrated the importance of this nephron segment as a high-capacity transport site in the first mammalian micropuncture studies, and they suggested that the first portion of the proximal tubule played a particularly important role in the ability of the nephron to adapt to variations in filtered load. Since then, many studies using micropuncture and in vivo and in vitro microperfusion techniques have confirmed that the early proximal tubule has a higher transport capacity than the late proximal tubule for a number of solutes. Moreover, at least for bicarbonate, fluid, and chloride, the transport capacity is not static, but is in a dynamic state, adapting in response to changes in filtration. In this review we have focused on the high capacity and load dependence of early proximal bicarbonate and fluid reabsorption. In addition, we summarize the evidence for axial heterogeneity along the proximal convoluted tubule for transport of a variety of other solutes.