HCO3--dependent soluble adenylyl cyclase activates cystic fibrosis transmembrane conductance regulator in corneal endothelium

cAMP-dependent activation of the cystic fibrosis transmembrane conductance regulator ( CFTR) regulates fluid transport in many tissues. Secretion by the corneal endothelium is stimulated by cAMP and dependent on HCO3-. We asked whether HCO3- can secondarily increase CFTR permeability in bovine corneal endothelial cells (BCEC) by activating soluble adenylyl cyclase (sAC). Immunofluorescence suggests that sAC is distributed throughout the cytoplasm. HCO3- (40 mM) increased cAMP concentration 42% in the presence of 50 muM rolipram ( a phosphodiesterase 4 inhibitor), and a standard HCO3- Ringer solution (28.5 mM) increased apical Cl- permeability by 78% relative to HCO3--free solution. The HCO3--dependent increase in Cl- permeability was reduced 60% by 20 mM NaHSO3 ( a weak agonist of sAC). NaHSO3 alone increased apical Cl- permeability by only 13%. The HCO3--dependent increase in Cl- permeability was reduced 57% in the presence of 50 muM Rp-adenosine 3',5'-cyclic monophosphorothioate, and 86% by 50 muM 5-nitro-2-(3-phenylpropyl-amino) benzoic acid but unaffected by 200 muM apical H2DIDS. CFTR phosphorylation was increased 23, 150, and 32% by 20 mM HSO3-, 28.5 mM HCO3-, and 28.5 mM HCO3- + 20 mM HSO3-, respectively. Activation of apical Cl- permeability by 5 muM genistein was increased synergistically by HCO3- over that due to genistein and HCO3- alone. We conclude that HCO3--stimulated sAC is a form of autocrine signaling that contributes to baseline cAMP production, thereby affecting baseline CFTR activity in BCEC. This form of autocrine signaling may be important in tissues that express sAC and exhibit robust HCO3- influx ( e. g., ocular ciliary epithelium, choroid plexus, and airway epithelium).