The cystic fibrosis transmembrane conductance regulator attenuates the endogenous Ca 2+ activated Cl - conductance of Xenopus oocytes

Abstract

Oocytes from Xenopus laevis activate a Ca2+ dependent Cl- conductance when exposed to the Ca2+ ionophore ionomycin. This Ca2+ activated Cl- conductance (CaCC) is strongly outwardly rectifying and has a halide conductivity ratio (GI- / GCl-) of about 4.4. This is in contrast to the cystic fibrosis transmembrane conductance regulator (CFTR)-Cl- conductance, which produces more linear I/V curves with a GI- / GCl- ratio of about 0.52. Ionomycin enhanced CaCC (DeltaG) in water injected and CFTR expressing ooyctes in the absence of 3-isobutyl-1-methylxanthine (IBMX, 1 mmol/l) by (microS) 23 +/- 1.9 (n=9) and 23.6 +/- 2.3 (n=11). Stimulation by IBMX did not change CaCC in water injected oocytes. CaCC was inhibited in CFTR-expressing ooyctes after stimulation with IBMX or a membrane permeable form of cAMP and was only 5.1 +/- 0.48 microS (n=18) and 6. 9 +/- 0.6 (n=3), respectively. Inhibition of CaCC was correlated to the amount of CFTR-current activated by IBMX. DeltaF508-CFTR which demonstrates only a small residual function in activating a cAMP dependent Cl- channel in oocytes inhibited CaCC to a lesser degree (DeltaG=12.1 +/- 1.1 microS; n=7). Changes of CFTR and CaCC-Cl- whole cell conductances were also measured when extracellular Cl- was replaced by I-. The results confirmed the reduced activation of CaCC in the presence of activated CFTR. No evidence was found for inhibition of CFTR-currents by increase of intracellular Ca2+. Moreover, intracellular cAMP was not changed by ionomycin and stimulation by IBMX did not change the ionomycin induced Ca2+ increase in Xenopus oocytes. Taken together, these results suggest that activation of CFTR-Cl- currents is paralleled by an inhibition of Ca2+ activated Cl- currents in ooyctes of Xenopus laevis. These results provide another example for CFTR-dependent regulation of membrane conductances other than cAMP-dependent Cl- conductance. They might explain previous findings in epithelial tissues of CF-knockout mice.

Bestrophin-2 mediates bicarbonate transport by goblet cells in mouse colon

“…The differences may be explained by changes that occur as a secondary consequence of disruption of the gene encoding CFTR. CFTR is known to regulate a variety of ion channels and transporters, including CaCCs (46), and there may simply be compensatory changes in expression of other channels, including Best2. Finally, the idea that membrane hyperpolarization is required for a cholinergic response is not universal: cholinergic agonists can stimulate anion secretion when the membrane potential is clamped at a depolarized level (6) and when K Ca 3.1 channels are blocked pharmacologically (9).…”

Additional Disruption of the ClC-2 Cl- Channel Does Not Exacerbate the Cystic Fibrosis Phenotype of Cystic Fibrosis Transmembrane Conductance Regulator Mouse Models

“…CFTR mediates chloride transport in the apical membranes of many epithelia. In addition, CFTR was suggested to have several other functions, including the regulation of various transport proteins such as the epithelial Na ϩ channel ENaC and certain other Cl Ϫ channels (2)(3)(4). If the pathological changes in cystic fibrosis were predominantly due to defective chloride transport, the pharmacological activation of other Cl Ϫ channels present in the same membranes would potentially prove beneficial in patients.…”

Characterization of SLC26A9, Facilitation of Cl^- Transport by Bicarbonate

“…ORCC (outward rectified chloride channel), CaCC (Calcium-activated chloride channel) or ClC-2 are examples of proposed and potential "alternative CFTR Cl -channels" [15][16][17][18][19]. Mutual interactions exist between CFTR and these reported Cl -channels, yet the precise mechanisms of interaction remain unclear [20][21][22][23][24].…”