Cl-channel activation in epithelia

It has been known for some time that the Cl -conductance of epithelial cells can, in addition to its regulation via cAMP, be enhanced by increases in cytosolic Ca (cf. Fig. 3). This has been shown with Ca -ionophores [120,121] or with hormones increasing cytosolic Ca such as carbachol, neurotensin, ATP, etc. [50,103,104]. Usually these agonists have dual effects. They increase the Cl - as well as the K -conductance [104]. Stubs et al. [122] have shown that CF cells still increase their Cl -conductance in response to ATP. Another mechanism of Cl -channel activation has been described in whole-cell patches of colonic carcinoma and RE cells [123,124] when the cells are exposed to hypotonic media they swell and increase their Cl -conductance. This is a rather general phenomenon which is present in a lot of cells [11]. In their effort to reduce cell volume in hypotonic media (regulatory 

Frizzell and coworker s laboratory has examined the properties of the whole-cell currents generated by cAMP, increases in cytosolic Ca , and increases in cell volume [12,124], They found that the properties of all three Cl -currents are different. The cAMP induced Cl -current is linear and conducts Cl better than I. These findings do not fit with the properties of the ICOR channel, which is outwardly rectifying and conducts 1 better than CP (cf. above). On the other hand, the CP-current induced by hypoosmolar solutions had characteristics similar to the ICOR channel. Comparable results have meanwhile been obtained in another laboratory (Fromter, personal communication). 

The work from the author s laboratory has been supported by DFG Gr 480/10. 

Wischmeyer, E., Weber-Schiirholz, S., Jockusch, H. and Schurholz, T. (1991) Pfiugers Arch. 418, R45. 

Greger, R. (1991) In Methods in Enzymology (Fleischer, B. and Fleischer, S., Eds.), pp. 793-810, Academic Press, Orlando. 

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