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The function of Maxi Cl− channels in cell physiology remains unresolved. Single-channel events were first described in skeletal muscle and subsequently in many different cell types, including epithelia (Hanrahan et al. 1985; Velasco et al. 1989; Vaca & Kunze, 1992; Brown et al. 1993; Riquelme et al. 1995), muscle (Blatz & Magleby, 1983; Saigusa & Kokubun, 1988), macrophages (Schwarze & Kolb, 1984), nerve cells (Forshaw et al. 1993; Bettendorff et al. 1993) and glial cells (Dermietzel et al. 1994). Almost all Maxi Cl− channel recordings have been made following excision of the membrane patch containing the channel. The need for membrane excision might indicate the involvement of intracellular inhibitory factors rather than artefactual activation by membrane disruption. An early indication of Maxi Cl− channel modulation came from experiments showing that protein kinase C regulates Maxi Cl− channels (Saigusa & Kokubun, 1988), and that excised Maxi Cl− channels can be modulated by GTP, GDP and some of their analogues (McGill et al. 1993). Subsequently, the observations that Maxi Cl− channels could be reversibly activated under whole-cell (Hardy & Valverde, 1994; Diaz et al. 1999) and cell-attached (Hardy & Valverde, 1994; Li et al. 2000) recording conditions by triphenylethylene antioestrogens such as toremifene or tamoxifen provided a method of studying these channels in intact cells. Oestrogens exert most of their actions by binding and activating their receptors, which function as transcription factors (Beato, 1989). Antioestrogens are generally considered to act by competing with the binding of oestrogens to the oestrogen receptors (Jordan, 1984). In addition to the classical effect of oestrogen and antioestrogen, some of their actions are related to their interaction with binding sites, which generally determine a rapid effect (seconds to minutes) as opposed to the long-term genomic effect (> 30 min). In most cases, the rapid effects represent the interaction of oestrogens with a plasma membrane target and/or the generation of intracellular signals (Ropero et al. 1999; Falkenstein et al. 2000; Nadal et al. 2000, 2001). Oestrogens have been found to exert rapid effects on the electrical activity of cells in the central nervous system (Minami et al. 1990), vascular system (Ruehlmann et al. 1998) and endocrine system (Nadal et al. 1998), and in non-excitable cells such as fibroblasts (Hardy & Valverde, 1994). Some of these actions can be either mimicked (Ruehlmann et al. 1998) or antagonised (Wong & Moss, 1991) by antioestrogens. However, the mechanisms underlying the rapid modulation of membrane excitability by oestrogens and antioestrogens are still poorly understood. Recently, both the direct interaction of steroids with the subunits forming channel structures (Valverde et al. 1999) and the generation of intracellular signals which, in turn, would modulate the activity of different ion channels have been described (Ropero et al. 1999). In the study presented here we have demonstrated that triphenylethylene antioestrogens activate Maxi Cl− currents by binding to an extracellular plasma membrane site. Such activation depends upon the activity of an okadaic acid-sensitive protein phosphatase. On the other hand, the activation of Maxi Cl− channels by antioestrogens can be prevented by the extracellular application of 17β-oestradiol, a process that appears to be dependent upon a phosphorylation step.

The effect of the non-steroidal antioestrogens tamoxifen and toremifene on voltage-gated cationic currents was examined in primary cultures of rat hypothalamic neurones and the C1300 mouse neuroblastoma cell line using the whole-cell patch clamp technique. When applied to the external bathing solution both tamoxifen and toremifene were able to inhibit TTX-sensitive sodium currents with IC50 values of 1–2 μM and delayed rectifier type potassium currents (IC50, 2–3 μM). However, only toremifene showed a significant inhibition of the IA current (IC50 3 μM). Inhibition of voltage-gated cationic currents was significantly impaired when tamoxifen was applied in a serum-containing solution. The steroidal antioestrogen ICI 182,780 did not inhibit any of the currents at 10 μM., This work was funded by the Royal Society, Cystic Fibrosis Trust and The Wellcome Trust.

Cell volume is frequently down-regulated by the activation of anion channels. The role of cell swelling-activated chloride channels in cell volume regulation has been studied using the patch-clamp technique and a non-invasive microspectrofluorimetric assay for changes in cell volume. The rate of activation of these chloride channels was shown to limit the rate of regulatory volume decrease (RVD) in response to hyposmotic solutions. Expression of the human MDR1 or mouse mdr1a genes, but not the mouse mdr1b gene, encoding the multidrug resistance P-glycoprotein (P-gp), increased the rate of channel activation and the rate of RVD. In addition, P-gp decreased the magnitude of hyposmotic shock required to activate the channels and to elicit RVD. Tamoxifen selectively inhibited both chloride channel activity and RVD. No effect on potassium channel activity was elicited by expression of P-gp. The data show that, in these cell types, swelling-activated chloride channels have a central role in RVD. Moreover, they clarify the role of P-gp in channel activation and provide direct evidence that P-gp, through its effect on chloride channel activation, enhances the ability of cells to down-regulate their volume.

Antiestrogens antagonize many genomic effects of estrogen through binding to the nuclear estrogen receptor. We report here that NIH3T3 fibroblasts grown in the presence of colchicine acquire the activation of a large conductance chloride channel upon exposure to extracellular but not intracellular antiestrogens. This effect can be prevented by extracellular 17 beta-estradiol, but not intracellular 17 beta-estradiol or extracellular 17 alpha-estradiol. This is the first demonstration of a regulatory role for antiestrogens and estrogens in the regulation of ionic channels occurring through an interaction of these compounds with a plasma membrane binding site distinct from the classical estrogen receptor and subsequent activation of intracellular second messenger pathway (or pathways).

WE read with interest the short communication reporting on an outbreak of type C botulism in laying hens by Sharpe and others (2011). It is a timely reminder of the economic and welfare costs of the disease in the chicken industry. The authors correctly point out the increased risk of botulism if laying birds have access to toxin-laden carcases. The authors have developed an argument that …

The antioestrogen, tamoxifen, blocks volume-regulated chloride channels and reduces transparency in bovine lenses maintained in vitro. In contrast to tamoxifen, the steroidal antioestrogen, ICI 182780, did not block volume-regulated chloride currents in three cultured cell lines and required 10 fold higher concentration to induce significant opacification of bovine lenses maintained in vitro. These data suggest that ocular toxic side effects will be minimized by use of the steroidal (ICI 182780) rather than nonsteroidal antioestrogens (tamoxifen).