The role of exocytosis in the activation of the chloride conductance in Chinese hamster ovary cells (CHO) stably expressing CFTR

The aim of this study was to examine the question of whether activation of wt-CFTR (wild-type cystic fibrosis transmembrane conductance regulator) by cAMP and the opening of a Cl- conductance is paralleled by exocytosis and corresponding increases in membrane capacitance. To this end three types of Chinese hamster ovary (CHO) cells were examined: a control group of CHO cells; a group of CHO cells stably expressing wt-CFTR at high levels (also called BQ2-CHO); and a group of CHO cells stably expressing the frequent mutation Delta F508-CFTR. Whole-cell patch-clamp studies were performed to measure the membrane voltage (V-m), the membrane conductance (G(m)) and the membrane capacitance (C-m). C-m was assessed by a two-frequency lock-in amplifier method. Forskolin (Fsk, 0.1 mu mol/l) and isobutylmethylxanthine (IBMX, 0.1 mmol/l) were used to increase cytosolic cAMP. It is shown that Fsk and IBMX had no effect on V-m and G(m), in control CHO and Delta F508-CFTR-CHO cells. Fsk and IBMX depolarized wt-CFTR expressing CHO cells significantly (from -40 +/- 1.5 to -32 +/- 1.6 mV, n = 41) and enhanced G(m) strongly from 5.0 +/- 0.9 to 36 +/- 3.9 nS (n = 65). The conductance increase was mostly for Cl-, because under stimulated conditions a reduction in bath Cl- concentration depolarized these cells further and significantly from -26 +/- 1.8 to -10 +/- 1.2 mV (n = 16). This conductance had the characteristic wt-CFTR selectivity of Br- > Cl- > I- (n = 16). Despite this large increase in the Fsk-and IBMX-induced conductance C-m was not altered significantly (15.5 versus 15.7 pF, n = 50). These data indicate that stable overexpression of wt-CFTR but not of Delta F508-CFTR in CHO cells induces a cAMP-activated Cl- conductance. The activation of this large conductance obviously proceeds with little if any exocytosis.