Dopico A M, Anantharam V, Treistman S N
Department of Pharmacology and Molecular Toxicology, University of Massachusetts Medical School, Worcester, USA.
J Pharmacol Exp Ther. 1998 Jan;284(1):258-68.
Ethanol (EtOH) reversibly activates large conductance, Ca(++)-activated K+ (BK) channels in rat neurohypophysial terminals, an effect that probably contributes to the inhibition of vasopressin release by this drug. Heterogeneity in the terminal channel population makes it difficult to determine the mechanisms underlying this activation. Here, we report the effects of EtOH on the steady-state activity of BK channels cloned from mouse brain (mslo, alpha subunit) and expressed in Xenopus oocytes. EtOH reversibly increased mslo channel activity in excised patches, showing a potency (EC50 = 24 mM) similar to that reported using native channels. EtOH activation was observed under conditions that make it highly improbable that it is mediated by freely diffusible second messengers, or secondary to G-protein modulation. Rather, it probably results from a functional interaction between the drug and the channel alpha subunit. Activation occurred without increase in the number of functional channels present in the patch and resulted from actions that were a function of EtOH concentration: at < or = 10 mM, activation was due to a decrease in the channel mean closed time, whereas between 25 and 100 mM, activation was due to both a decrease in the mean closed time and an increase in the mean open time. The characteristic high unitary conductance and ionic selectivity of BK channels were unaltered by the drug. Whereas the voltage dependence of channel gating remained unchanged, channel activation mediated by the response of the Ca(++)-sensing site(s) to increases in the concentration of intracellular calcium, [Ca++]ic, was reduced by EtOH. This finding is consistent with EtOH and [Ca++]ic behaving functionally as partial and full agonists of mslo channels, respectively. Because the potentiation of mslo activity by the drug decreased as Ca++ levels were increased, EtOH-activation of BK channels would be most evident when [Ca++]ic is near resting levels, rather than during periods of high activity and Ca++ influx.
乙醇（EtOH）可使大鼠神经垂体终末的大电导、Ca（++）激活的K+（BK）通道发生可逆性激活，这一效应可能是该药物抑制血管加压素释放的原因之一。终末通道群体的异质性使得难以确定这种激活背后的机制。在此，我们报告了EtOH对从小鼠脑克隆（mslo，α亚基）并在非洲爪蟾卵母细胞中表达的BK通道稳态活性的影响。EtOH可使切除膜片中的mslo通道活性可逆性增加，其效力（EC50 = 24 mM）与使用天然通道时报道的相似。在极不可能由自由扩散的第二信使介导或继发于G蛋白调节的条件下观察到了EtOH激活。相反，它可能是药物与通道α亚基之间功能相互作用的结果。激活发生时，膜片中存在的功能性通道数量并未增加，且激活是EtOH浓度的函数：在≤10 mM时，激活是由于通道平均关闭时间缩短，而在25至100 mM之间，激活是由于平均关闭时间缩短和平均开放时间增加。该药物未改变BK通道特征性的高单位电导和离子选择性。虽然通道门控的电压依赖性保持不变，但EtOH降低了由Ca（++）感受位点对细胞内钙浓度[Ca++]ic升高的反应介导的通道激活。这一发现与EtOH和[Ca++]ic在功能上分别作为mslo通道的部分激动剂和完全激动剂一致。由于随着Ca++水平升高，该药物对mslo活性的增强作用减弱，因此当[Ca++]ic接近静息水平时，而非在高活性和Ca++内流期间时，EtOH对BK通道的激活最为明显。
