The relationship between cAMP, Ca2+, and transport of CFTR to the plasma membrane

The mechanism whereby cAMP stimulates Cl- flux through CFTR ion channels ill secretory epithelia remains controversial. It is generally accepted that phosphorylation by cAMP-dependent protein kinase increases the open probability of the CFTR channel. A more controversial hypothesis is that cAMP triggers the translocation of CFFR from an intracellular pool to the cell surface. We have monitored membrane turnover in Calu-3 cells, a cell line derived from human airway submucosal glands that expresses high levels of CFTR using membrane capacitance and FM1-43 fluorescence measurements. Using a conventional capacitance measurement technique, we observe an apparent increase in membrane capacitance in most cells that exhibit an increase in Cl current. However, after we carefully Correct Our recordings for changes in membrane conductance, the apparent changes ill capacitance are eliminated. Measurements using the fluorescent membrane marker FM1-43 also indicate that no changes in membrane turnover accompany the activation of CFTR. Robust membrane insertion call be triggered with photorelease of caged Ca2+ in Calu-3 cells. However, no increase in Cl- current accompanies Ca2+-evoked membrane fusion. We conclude that neither increases in cAMP or Ca2+ lead to transport of CFTR to the plasma membrane in Caln-3 cells. lit addition, we conclude that membrane capacitance measurements must be interpreted with caution when large changes in membrane conductance occur.