Tubular abnormalities seen in SCD
In addition to proximal tubular hyperfunctioning, what other tubular abnormalities may be seen in patients with SCD?
• Diminished concentrating ability: Red blood cell sickling and congestion in the vasa recta leads to ischemia and associated impairment of solute reabsorption by the ascending limb of Henle loop and the vasa recta function as countercurrent exchangers. The suboptimal maintenance of the high interstitial osmolality in the inner medulla reduces effective water reabsorption across the collecting ducts, hence the reduced kidney concentrating ability. A diminished concentrating ability leads to hypo- or isosthenuria where urine osmolality typically does not exceed 450 mosm/Kg. Affected adults present with polyuria, nocturia, and volume depletion and children with enuresis. Blood transfusions of HbA-containing red blood cells can improve concentrating ability in children younger than age 15, but not thereafter due to permanent injury.
• Renal tubular acidosis: Patients may develop incomplete distal renal tubular acidosis via reduced H+-ATPase activity due to hypoxemia, selective aldosterone deficiency, distal nephron resistance to aldosterone, reduced ammonium availability, or, in rare cases, hyporenin hypoaldosteronism.
