Impaired Renal HCO3 - Excretion in Cystic Fibrosis

Abstract

BackgroundPatients with cystic fibrosis (CF) do not respond with increased urinary HCO3− excretion after stimulation with secretin and often present with metabolic alkalosis.MethodsBy combining RT-PCR, immunohistochemistry, isolated tubule perfusion, in vitro cell studies, and in vivo studies in different mouse models, we elucidated the mechanism of secretin-induced urinary HCO3− excretion. For CF patients and CF mice, we developed a HCO3- drinking test to assess the role of the cystic fibrosis transmembrane conductance regulator (CFTR) in urinary HCO3-excretion and applied it in the patients before and after treatment with the novel CFTR modulator drug, lumacaftor-ivacaftor.Resultsβ-Intercalated cells express basolateral secretin receptors and apical CFTR and pendrin. In vivo application of secretin induced a marked urinary alkalization, an effect absent in mice lacking pendrin or CFTR. In perfused cortical collecting ducts, secretin stimulated pendrin-dependent Cl−/HCO3− exchange. In collecting ducts in CFTR knockout mice, baseline pendrin activity was significantly lower and not responsive to secretin. Notably, patients with CF (F508del/F508del) and CF mice showed a greatly attenuated or absent urinary HCO3−-excreting ability. In patients, treatment with the CFTR modulator drug lumacaftor-ivacaftor increased the renal ability to excrete HCO3−.ConclusionsThese results define the mechanism of secretin-induced urinary HCO3− excretion, explain metabolic alkalosis in patients with CF, and suggest feasibility of an in vivo human CF urine test to validate drug efficacy.

Impaired renal HCO3− secretion in CFTR deficient mice causes metabolic alkalosis during chronic base‐loading

“…Noteworthy, we see a significantly lower pendrin function in CFTR KO β‐ICs after a two‐day 100 mmol/L or 200 mmol/L base‐load, although pendrin expression was tentatively higher in CFTR KO mice at this time point. We have previously shown, that even during resting conditions, loss of CFTR blunts apical Cl − /HCO 3 − function in β‐ICs of the CCD, 6 which indicates that CFTR may regulate pendrin function. This discrepancy between expression and function could be attributed to a decreased trafficking of pendrin to the apical membrane in CFTR KO mice, as suggested by others during base‐loading 5 .…”

“…This effect requires functionally intact β‐ICs as it is absent in pendrin and CFTR KO mice 6 . One of the hallmark findings of that study was the critical dependence of CFTR in permitting acutely stimulated urine base excretion in mice and humans 6 …”

“…After a meal or in response to an alkaline challenge secretin plasma levels increase. In the kidney, secretin triggers a marked urinary HCO 3 − excretion that is seen as an acute increase in urine pH, an increase in urine [HCO 3 − ], and an increase of urine HCO 3 − excretion rates 6 . This effect requires functionally intact β‐ICs as it is absent in pendrin and CFTR KO mice 6 .…”

“…CFTR is widely expressed in epithelial tissues and loss of CFTR results in impaired Cl − and HCO 3 − secretion, leading to pancreatic insufficiency, recurring lung infections, gradual decline of lung function, and eventually death 1 . Moreover, CF patients are known to have a higher incidence of metabolic alkalosis 2‐4 that is likely caused by a decreased capacity to excrete base‐equivalents secondary to an altered renal HCO 3 − handling 3,5,6 …”

“…In the kidney, secretin triggers a marked urinary HCO 3 − excretion that is seen as an acute increase in urine pH, an increase in urine [HCO 3 − ], and an increase of urine HCO 3 − excretion rates 6 . This effect requires functionally intact β‐ICs as it is absent in pendrin and CFTR KO mice 6 . One of the hallmark findings of that study was the critical dependence of CFTR in permitting acutely stimulated urine base excretion in mice and humans 6 …”