Prevalence and correlates of metabolic acidosis among patients with homozygous sickle cell disease.

BACKGROUND AND OBJECTIVES: Very few studies report acid base disorders in homozygous patients with sickle cell anemia (SCA) and describe incomplete renal acidosis rather than true metabolic acidosis, the prevalence of which is unknown and presumably low. This study aimed to assess the prevalence of metabolic acidosis and to identify its risk factors and mechanisms. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: This study retrospectively analyzed 411 homozygous patients with SCA with a GFR ≥ 60 ml/min per 1.73 m(2), referred in a single center between 2007 and 2012. Acidosis and nonacidosis groups were compared for clinical and biologic data including SCA complications and hemolytic parameters. A subgroup of 65 patients with SCA, referred for a measured GFR evaluation in the setting of sickle cell-associated nephropathy, was further analyzed in order to better characterize metabolic acidosis. RESULTS: Metabolic acidosis was encountered in 42% of patients with SCA, with a higher prevalence in women (52% versus 27% in men; P<0.001). Several hemolytic biomarkers, such as lactate dehydrogenase, were different between the acidosis and nonacidosis groups (P=0.02 and P=0.03 in men and women, respectively), suggesting higher hemolytic activity in the former group. To note, fasting urine osmolality was low in the whole study population and was significantly lower in men with SCA in the acidosis group (392 versus 427 mOsm/kg; P=0.01). SCA subgroup analysis confirmed metabolic acidosis with a normal anion gap in 14 patients, characterized by a lower urinary pH (P<0.02) and no increase in urinary ammonium. Serum potassium, plasma renin, and aldosterone were similar between the two groups and thus could not explain impaired urinary ammonium excretion. CONCLUSIONS: These results suggest that the prevalence of metabolic acidosis in patients with SCA is underestimated and related to impaired ammonium availability possibly due to an altered corticopapillary gradient. Future studies should evaluate whether chronic metabolic acidosis correction may be beneficial in this population, especially in bone remodeling.

Ureteral obstruction promotes proliferation and differentiation of the renal urothelium into a bladder-like phenotype.

The renal urothelium, the monolayered epithelium that covers the papilla, is the direct target of increased pressure during obstruction, yet most studies have mainly focused on tubules, fibroblasts, and inflammatory cells. We studied this epithelium in a unilateral ureteral obstruction mouse mode land found that it was disrupted and had broken tight junctions, enlarged intercellular space, with loss of apicaluroplakins, and marginal lumen desquamation. Shortly after obstruction these urothelial cells proliferated, peaking at day 2. By day 14, the renal urothelium was transformed into a multilayered barrier with newly synthesized uroplakins including the de novo induction of uroplakin II. This proliferation was found to be fibroblast growth factor (FGF)dependent. Renal urothelial cells constitutively express the FGF receptor 2, and obstruction activated the receptor by phosphorylation. Treatment with FGF receptor 2-antisense or vitamin A (an inhibitor of the MAP kinase in the FGFR2 pathway) decreased renal urothelial cell proliferation. Among known FGF receptor 2 ligands, only FGF7 was upregulated.Infusion of FGF7 into control mice caused the formation of a multilayered structure at 7 days, resembling the urothelium 14 days following obstruction. Thus, the pressure/stretching of renal monolayered urothelial cells is a very efficient trigger for proliferation, causing the formation of a bladder-like multistratified barrier with enhanced apical uroplakin plaques. Presumably, this ensures efficient barrier protection and repair.