Activation of Maxi Cl^- channels by antiestrogens and phenothiazines in NIH3T3 fibroblasts

The identification of alternative estrogen actions has been accumulating steadily over the past two decades. Typically, these novel actions are not directly related to nuclear transcriptional events but related to the interaction of estrogens with sites present at plasma membrane or cytosolic locations. These alternative effects, widely known as non-genomic effects, range from the modulation of plasma membrane ion channel activity to the regulation of different intracellular signalling cascades. In the present study we have investigated the modulation of a large conductance chloride channel (Maxi Cl^-) by estrogens, non-steroidal triphenylethylene antiestrogens and phenothiazines in NIH3T3 fibroblasts and the dependence on guanosine triphosphate (GTP) of the Maxi Cl^- activation. Our data identifies the non-steroidal antiestrogens toremifene and tamoxifen, and the phenothiazines chlorpromazine and triflupromazine as activators of Maxi Cl^- channels. In contrast, 17β-estradiol and cAMP, added prior to the exposure to antiestrogens, prevent channel activation. The pure antiestrogen ICI 182780 did not activate the channel nor prevent its activation by non-steroidal antiestrogens. The activation of Maxi Cl^- channels by toremifene and tamoxifen required the presence of intracellular nucleotides and was inhibited by the stable analog, GDP β -S, suggesting the participation of a G-protein in the activation process. Little is known about the physiological relevance of Maxi Cl^- channels. However, that fact that its regulation by estrogens and antiestrogens is shared by different cell types might imply a common role which needs to be identified.