Activation of wild-type and ΔF508-CFTR by phosphodiesterase inhibitors through cAMP-dependent and -independent mechanisms

Abstract

 The cAMP-dependent activation of the cystic fibrosis transmembrane conductance regulator (CFTR) and its modulation through inhibition of phosphodiesterases (PDE) were studied with the cell-attached patch-clamp technique in Calu-3 cells (expressing endogenous CFTR) and NIH3T3 cells [expressing either wild-type (Wt)-CFTR or ΔF508-CFTR]. In Calu-3 cells, CFTR current was augmented by increasing concentrations of 8-(4-chlorophenylthio)-adenosine 3′,5′-cyclic monophosphate (CPT–cAMP) and reached a saturating level at ≥60 µM. Varying the forskolin concentration also modulated CFTR activity; 10 µM was maximally effective since supplemental application of 200 µM CPT–cAMP had no additional effect. Activation of CFTR by increasing the cAMP concentration occurs through an increase of the NP _o (product of the number of functional channels and the open probability) since the single-channel amplitude remains unchanged. In Calu-3 and NIH3T3-Wt cells, PDE inhibitors, milrinone (100 µM), 8-cyclopentyl-1,3-dipropylxanthine (CPX, 25 µM), and 3-isobutyl-1-methylxanthine (IBMX, 200 µM), did not enhance CFTR current initially activated with 10 µM forskolin, but each potentiated CFTR activity elicited with a submaximal forskolin concentration (e.g., 100 nM) and prolonged the deactivation of CFTR channel current upon removal of forskolin. Millimolar IBMX increased the NP _o of both Wt- and ΔF508-CFTR even under maximal cAMP stimulation. Quantitatively, these effects of millimolar IBMX on NP _o approximate those of genistein, which potentiates the cAMP-dependent CFTR activity via a mechanism that does not involve increases in cellular cAMP. Thus, depending on the concentration, PDE inhibitors may affect CFTR through different mechanisms.