INFLUENCE OF THE RENAL MEDULLARY CIRCULATION ON THE CONTROL OF SODIUM-EXCRETION

Although the role of the renal medullary circulation in the control of urinary concentrating ability is well established, its potential infl uence on tubular sodium reabsorption is not generally recognized. Near ly 30 years ago, changes in the intrarenal distribution of blood flow were first proposed to contribute to the natriuretic response to volum e expansion. However, the lack of reliable methods for studying medull ary blood flow limited progress in this area. The recent development o f laser-Doppler flowmetry and videomicroscopic techniques for the stud y of the vasa recta circulation has renewed interest in the role of me dullary hemodynamics in the control of sodium reabsorption. Results of these studies indicate that changes in renal medullary hemodynamics a lter renal interstitial pressure and the medullary solute gradient and play an important role in the natriuretic response to elevations in r enal perfusion pressure, intravenous infusion of saline, and changes i n tubular sodium reabsorption produced by vasoactive compounds. What i s emerging from these studies is the view that changes in renal medull ary hemodynamics represent an important but misunderstood and long-ign ored factor in the control of tubular sodium reabsorption.