While studying isolated, perfused proximal tubules (PTs), we noticed that adding CO2/HCO3− to the PT's basolateral (BL or blood) side causes pHi to increase, reflecting stimulated H+ extrusion across the apical membrane (facing lumen). To elucidate the mechanism, we invented out-of-equilibrium (OOE) CO2/HCO3− solutions, which allow us to vary [CO2]BL, [HCO3−]BL, and pHBL—one at a time. We found that isolated increaes in [CO2]BL or isolated decreases in [HCO3−]BL stimulate H+ secretion into the tubule lumen (JH) . Surprisingly, acute changes in pHBL have no effect on JH! Thus, the tubule regulates blood pH by sensing not pH but the two major blood buffers.
The signal-transduction system linking basolateral CO2 and HCO3− to apical H+ extrusion involves: (1) Apical AT1 (angiotensin II, ANG II) receptors and ANG II that the tubule secretes into the lumen. (2) Tyrosine phosphorylation. Inhibitors of ErbB receptor tyrosine kinases block responses to basolateral CO2 or HCO3−. The same is true for knockout of receptor protein tyrosine phosphatase γ (RPTPγ), in which the extracellular ligand-binding region is an inactive carbonic-anhydrase-like domain (CALD). We are testing the hypothesis that the CALD is the sensor for extracellular CO2 and HCO3−. We are also using biochemical approaches to explore signal transduction
