Renal energy metabolism and regulation of sodium reabsorption

A relationship between glomerular filtration rate (GFR) and tubular reabsorption during acute changes in GFR has been recognized since adequate methods for estimation of GFR became available [1]. Early micropuncture studies [2] have shown that the relationship between GFR and reabsorption rate already applied in the first part of the proximal tubules. It is now well established that not only the water permeable proximal tubules but also the virtually water impermeable thick ascending limb of Henle's loop (the diluting segment) participate in the regulation of sodium and water reabsorption so that not more than 10 to 15% of the filtered load is delivered to the distal convoluted tubules in the cortex.A simple hypothesis is that changes in composition of the tubular fluid, during variations in GFR, alter sodium reabsorption. In this review, evidence, mainly from experiments on dogs, is presented to show that bicarbonate concentration of the tubular fluid regulates sodium reabsorption in the proximal tubules during reduction in GFR (glomerulotubular balance). In the diluting segment, sodium reabsorption seems to depend on tubular sodium concentration