Factors affecting sodium reabsorption by the proximal tubule as determined during blockade of distal sodium reabsorption

Changes in the tubular reabsorption of sodium independent of aldosterone activity may play an important role in determining sodium excretion. Several studies in the dog employing clearance techniques have demonstrated that infusions of isotonic saline (1-5) or plasma-like solutions (1, 2) result in a depression of the over-all net tubu-lar reabsorption of sodium as the excretion of so-dium increases. Dirks, Cirksena, and Berliner have demonstrated by micropuncture studies in the dog that this depression of tubular reabsorption during the infusion of saline occurs specifically in the proximal tubule (6). These same authors re-ported that acute constriction of the thoracic in-ferior vena cava, a maneuver known to inhibit so-dium excretion and to lead to chronic sodium re-tention and the formation of ascites (7), prevents this depression of proximal reabsorption during saline infusion (8). Such studies indicate that nonaldosterone factors determining the rate of sodium reabsorption by the proximal tubule could play a major role in the normal physiologic regu-lation of sodium balance and also may be involved in the pathogenesis of sodium retention in dis-orders characterized by the accumulation of as-cites and edema. Although the factors that determine the rate of proximal tubular reabsorption are unknown, recent studies from our laboratory have demon-strated that renal vascular resistance and perfusion pressure may affect the over-all tubular reabsorp