Studies on the nephron segment with reduced sodium reabsorption during starvation natriuresis

The segment of the nephron where carbohydrate deprivation depresses Na transport leading to natriuresis was sought by a new clearance technique designed to measure segmental reabsorption in each portion of the human renal tubule. Experiments were performed during maximal water diuresis before and 4 days after carbohydrate withdrawal. Proximal reabsorption had fallen from 70 +/- 4 to 60 +/- 5 ml X min-1, p less than 0.05, by the 4th day of sugar deprivation, accounting for the natriuresis and the associated weight loss of 1.8 kg. By the 4th day of fasting, when Na excretion had returned to control levels, GFR had fallen nonsignificantly from 99 +/- 6 to 95 +/- 5 ml X min-1, while Na reabsorption along distal segments had risen. In fact, Na transport, expressed by the equivalent volumes of solute free-water generated, rose from 17.4 +/- 3.4 to 23.6 +/- 2.1 along the ascending limb of Henle's loop, and from 8.1 +/- 0.8 to 9.2 +/- 1.3 ml X min-1 X GFR-1 X 100 along the distal tubule. Thus, analysis of segmental Na transport by this method discloses that starvation natriuresis is a proximal tubular event, progressively counterbalanced by enhanced Na reclamation in more distal sites. Volume contraction and the attendant fall in GFR concur to curb delivery out of the proximal tubule which is matched by enhanced distal Na reabsorption till a new steady-state excretion is attained.