OPTICAL IMAGING OF CL- PERMEABILITIES IN NORMAL AND CFTR-EXPRESSING MOUSE L CELLS

Single cell optical imaging techniques were used to compare Cl- conduc tances in cystic fibrosis transmembrane conductance regulator (CFTR)-e xpressing and control mouse L cell fibroblasts. Elevation of intracell ular cAMP levels in control cells was without effect on plasma membran e Cl- permeability, whereas cells engineered to stably express CFTR di splayed a 20-fold enhancement of plasma membrane Cl- permeability in r esponse to cAMP. Control L cells displayed Ca2+-, as well as swelling- activated Cl- permeabilities, which were small compared with cAMP-stim ulated permeability in CFTR-expressing cells. CFTR-expressing cells al so displayed a similar swelling-activated Cl- permeability. Whereas 50 % of the CFTR-expressing cells possessed a small Ca2+-activated Cl- pe rmeability similar to control cells, the other cells displayed an enha nced response which was never observed in control cells. Intracellular cAMP determinations suggested that this latter result might be explai ned by a Ca2+-induced rise of cAMP. The cAMP-activated and Ca2+-activa ted Cl- conductances had different anion selectivities, as measured by light scattering of suspended cells. Activation of protein kinase C w as without effect on Cl- permeability in CFTR-expressing cells, nor di d it modify cAMP-activation of Cl- permeability. Thus, expression of h uman CFTR in L cells does not confer cAMP-sensitivity to pre-existing, endogenous Ca2+- or swelling-activated Cl- channels, but rather confe rs a novel Cl- conductance which is regulated by cAMP. Osmotic cell sw elling and PKC activation are without specific effect in CFTR-expressi ng L cells. However, elevated [Ca2+]i may play a role in activating a Cl- conductance specifically associated with CFTR.