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cAMP stimulation of CFTR-expressing Xenopus oocytes activates a chromanol-inhibitable K+ conductance
M Mall1, K Kunzelmann, A Hipper
1Physiologisches Institut der Albert-Ludwigs-Universität Freiburg, Hermann-Herder-Strasse 7, D-79104 Freiburg, Germany.
Abstract:
Cystic fibrosis transmembrane conductance regulator (CFTR) functions as a Cl- channel in a large variety of cells expressing this protein. Recently evidence has accumulated that it also regulates other ion channels. A coordinated increase in Cl- and K+ conductances is necessary in many Cl--secreting epithelia. This has, for example, recently been demonstrated for the colonic crypt, for which a new type of K+ channel and a specific inhibitor of this channel, the chromanol 293B, have been described. In the present study we have examined whether the cAMP-evoked activation of CFTR, overexpressed in Xenopus oocytes, in addition to its known activation of a Cl- conductance, also upregulates endogenous K+ channels. It is shown that CFTR-cRNA-injected but not water-injected oocytes possess a cAMP-activated Cl- conductance. Of the cAMP-induced whole-cell current increase, 15-25% was due to a 293B-, Ba2+and TEA+-inhibitable K+ conductance. The cRNA of the mutated CFTR (DeltaF508 CFTR) had no such effect. We conclude that cAMP activated CFTR and an endogenous IsK-type and 293B-sensitive K+ conductance. Similar events, occurring, for example, in the colonic crypt possessing CFTR and 293B-sensitive K+ channels, might explain the coordinated cAMP-mediated increase in Cl- and K+ conductances.
Insights
Cystic fibrosis transmembrane conductance regulator (CFTR) activation by cAMP upregulates not only chloride channels but also specific potassium channels. This finding explains coordinated ion conductances in secreting epithelia.
Area of Science:
- Cellular Physiology
- Ion Channel Regulation
- Epithelial Transport
Background:
- Cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel crucial for epithelial function.
- CFTR is increasingly recognized for its role in regulating other ion channels.
- Coordinated Cl- and K+ conductances are vital for Cl--secreting epithelia, such as the colonic crypt.
Purpose of the Study:
- To investigate if cAMP-activated CFTR upregulates endogenous K+ channels in addition to Cl- conductance.
- To determine if this effect is mediated by CFTR itself.
Main Methods:
- Overexpression of wild-type CFTR and mutated DeltaF508 CFTR in Xenopus oocytes.
- Measurement of cAMP-induced whole-cell currents using electrophysiology.
- Pharmacological inhibition of K+ channels using chromanol 293B, Ba2+, and TEA+.
Main Results:
- CFTR-cRNA injected oocytes exhibited a cAMP-activated Cl- conductance.
- A significant portion (15-25%) of the cAMP-induced current was a 293B-, Ba2+, and TEA+-inhibitable K+ conductance.
- Mutated DeltaF508 CFTR did not produce this effect.
Conclusions:
- cAMP-activated CFTR upregulates an endogenous IsK-type and 293B-sensitive K+ conductance.
- This mechanism may explain the coordinated increase in Cl- and K+ conductances observed in epithelia like the colonic crypt.