Ca2+ activated Cl channels (CaCCs) play an important role in many physiological processes and diseases. Although bestrophin1 has been proposed to form a new family of Cl channels, there are still many controversial issues to be addressed. For instance, ATP, CCH, ADP and Ionomycin induced short circuit current (Isc) in mouse trachea and colon from mbest 1 knockout mice (VMD2-/-) showed no differences when compare to wild type (VMD2+/+). We examined the contribution of the three different mouse bestrophins for the Ca2+ activated Cl secretion in primary trachea epithelial cells (TEC). ATP-induced whole cell currents in the VMD2-/- TEC, expressing only bestrophin 2 and 3, were two times smaller than that obtained in the VMD2+/+ TEC. Furthermore, short interfering RNA (siRNA) for mbest2 suppressed ATP induced whole cell currents in VMD2-/- TEC by

70% but only by 38% in the VMD2+/+ TEC. ATP-activated currents were also significantly inhibited in VMD2+/+ TEC, treated with either siRNA targeting mbest1 or a combination of siRNA for mbest 1+2. Immunostaining analysis confirmed reduced expression of bestrophin in siRNA transfected cells. These results suggest a role of mbest1 and mbest2 for the CaCCs in the trachea. It is also likely that these proteins form dimers or they may be part of a large complex together with other membrane proteins. Supported by DFG SFB699A7 and Else- Kröner-Fresenius Stiftung