17β-oestradiol (E2) is responsible for the rapid activation of a number of signalling pathways in various cell types. In rat distal colonic crypts, it has been shown to activate PKCα and PKCδ and to increase intracellular calcium. This activation is associated with a number of events including inhibition of a basolateral potassium channel (Harvey 2001). Inhibition of this channel could be the key factor behind the gender-specific anti-secretory effect of E2 previously reported by our group. In this study, we clarify the interplay between oestrogen-activated signalling intermediates (PKC), a possible membrane target of estrogen (KCNQ1) and its impact of whole tissue physiology (anti-secretory effect). Colonic epithelia were isolated from humanely killed Sprague-Dawley rats; mounted in an Ussing chamber and short circuit current (I_SC) were measured as an index of chloride secretion. Bacterial toxins (cholera toxin and heat stable E coli toxin) were used to stimulate transepithelial secretion. E2 at physiological concentrations (1-100 nM) decreased I_SC in a dose dependent manner (1 nM, 85±7%; 10 nM, 68±5%; 100 nM, 47±4%). This effect was dependent on PKCδ but not PKCα activity. Bicarbonate-dependent chloride secretion was unaffected by E2 (control 25±4, E2 23±5 μA cm^-2) suggesting no involvement of CFTR channels or anion exchangers. Therefore we examined the effect of E2 on other ion transporters involved in secretion. In apically permeabilized epithelia Na⁺/K⁺-ATPase activity was unaffected by E2 (control 32±5, E2 30±3 μA cm^-2). Next we examined the effect of E2 on basolateral potassium conductances. E2 inhibited a chromanol 293B sensitive conductance (control 57±6, E2 7±3 μA cm^-2), which indicated the involvement of KCNQ1 channels. Therefore, we investigated the interaction between PKCδ and the KCNQ1 channel protein. Exposure of isolated rat colonic crypts to E2 (10nM) for 15 minutes resulted in the activation of PKCδ only in the female and not in male rats. Additionally, KCNQ1 co-immunoprecipitated with PKCδ; this association was increased in the presence of E2, but only in crypt cells isolated from female rats. In vitro kinase assays using GST-tagged KCNQ1 intracellular termini as substrates revealed that PKCδ specifically phosphorylates the N terminus of KCNQ1. Taken together these data suggest the anti-secretory effect of E2 is mediated via rapid activation of PKCδ, which in turn inhibits a potassium channel (KCNQ1) crucial for the secretory processes (Kunzelmann 2002).