CFTR-dependent membrane insertion is linked to stimulation of the CFTR chloride conductance

We examined the relation between Cl current (I-Cl) stimulation and cell membrane capacitance (C-m) when cystic fibrosis transmembrane conductance regulator (CFTR) was expressed in Xenopus oocytes. I-Cl and C-m increased in parallel when oocytes expressing CFTR were stimulated by forskolin (10 mu M) and 3-isobutyl-1-methylxanthine (1 mM). The adenosine 3',5'-cyclic monophosphate (cAMP)-induced increase in surface area detected by C-m was confirmed by morphometry in the same oocytes used for the electrical recordings. These increases in I-Cl and C-m were reversible and were absent from control oocytes not injected with CFTR cRNA. The time to reach peak I-Cl lagged slightly behind the peak in C-m. I-Cl was varied by altering CFTR expression level or agonist dose or by expressing different CFTR mutants. In all cases, there was a close correlation between I-Cl and C-m, and the kinetics of I-Cl and C-m stimulation were more rapid the larger the magnitude of the stimulated current. The C-m-I-Cl relation for wild-type CFTR saturated, consistent with a limited capacity of cells to increase their surface area. These results indicate that stimulation of the CFTR I-Cl is linked closely to increases in membrane area. This suggests that CFTR is present in the membrane vesicles whose insertion is stimulated by cAMP. The contents of these vesicles may provide a link between activation of CFTR and its cAMP-dependent regulation of other channels.