The cystic fibrosis transmembrane conductance regulator attenuates the endogenous Ca2+ activated Cl- conductance of Xenopus oocytes.
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.
Kunzelmann
K
K
Physiologisches Institut, Albert-Ludwigs-Universität Freiburg, Hermann-Herder-Strasse 7, D-79104 Freiburg, Germany.
Mall
M
M
Briel
M
M
Hipper
A
A
Nitschke
R
R
Ricken
S
S
Greger
R
R
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D018118:Chloride Channels / Q000502:physiology*; D000242:Cyclic AMP / Q000378:metabolism / Q000494:pharmacology; D019005:Cystic Fibrosis Transmembrane Conductance Regulator / Q000235:genetics / Q000502:physiology*; D004553:Electric Conductivity; D005260:Female; D015870:Gene Expression; D018014:Gene Transfer Techniques; D015759:Ionomycin / Q000494:pharmacology; D009865:Oocytes / Q000502:physiology*; D011994:Recombinant Proteins; D014982:Xenopus laevis
D018118:Chloride Channels; D011994:Recombinant Proteins; D019005:Cystic Fibrosis Transmembrane Conductance Regulator; D015759:Ionomycin; D000242:Cyclic AMP; D002118:Calcium; D015056:1-Methyl-3-isobutylxanthine