The recently identified Ca2+ activated Cl- channel TMEM16A (anoctamin 1, Ano1) belongs to a family of 10 proteins. Voltage- and calcium-dependent gating of Ano1 has been examined in detail, and was shown to be physically coupled to the first intracellular loop. Ano1 is essential for Cl- secretion in a number of epithelial tissues, for smooth muscle contraction, the function of nociceptive neurons and smooth muscle pacemaker cells. Ano2, the nearest relative of Ano1, has also been shown to form a CaCC in olfactory receptors. However it is entirely unclear whether all anoctamins produce Ca2+ activated Cl- currents. We examined Ano4-10, which all produced transient Cl- currents upon increase in intracellular Ca2+ with some differences regarding their biophysical properties. Ano5 and Ano6 required unphysiologically high cytosolic Ca2+ concentrations to be activated. Notably, increase in intracellular Ca2+ also causes subsequent inactivation of the channel, which may be due to activation of calmodulin dependent kinase (CAMK) II. Whole cell currents generated by Ano7-9 are activated by increase in intracellular Ca2+ only with some delay. These anoctamins produced generally current of smaller amplitudes. As anoctamins examined in the present study produce larger currents in the presence of intracellular and extracellular cations, we suggest a permeability of anoctamins also for cations. In conclusion, the anoctamins are probably true Ca2+ activated Cl- channels, but with some permeability to cations.

Work supported by DFG SFB699-A7. We thank Ms. E. Tartler and Brigitte Wild for excellent technical assistance