The initial goal of the present study was to elucidate the effects of 2-APB on membrane currents, which revealed the presence of novel K ؉ channels in WEHI-231 cells. Under whole-cell patch clamp conditions, 2-APB induced background K ؉ current (I K,bg ) and hyperpolarization in WEHI-231 cells. Lowering of intracellular MgATP also induced the I K,bg . The I K,bg was blocked by micromolar concentrations of quinidine but not by tetraethylammonium. In a single channel study, two types of voltage-independent K ؉ channels were found with large (346 picosiemens) and medium conductance (112 picosiemens), named BK bg and MK bg , respectively. The excision of membrane patches (inside-out (i-o) patches) greatly increased the P o of BK bg . In i-o patches, cytoplasmic MgATP (IC 50 ‫؍‬ 0.18 mM) decreased the BK bg activity, although non-hydrolyzable adenosine 5-(␤,␥-imino)-triphosphate had no effect. A pretreatment with Al 3؉ or wortmannin (50 M) blocked the inhibitory effects of MgATP. A direct application of phosphoinositide 4,5-bisphosphate (10 M) inhibited the BK bg activity. Meanwhile, the activity of MK bg was unaffected by MgATP. In cell-attached conditions, the BK bg activity was largely increased by 2-APB. In i-o patches, however, the MgATPinduced inhibition of BK bg was weakly reversed by the addition of 2-APB. In summary, WEHI-231 cells express the unique background K ؉ channels. The BK bg s are inhibited by membrane-delimited elevation of phosphoinositide 4,5-bisphosphate. The activation of BK bg would hyperpolarize the membrane, which augments the calcium influx in WEHI-231 cells.Immune responses are initiated and regulated by the physical interactions of receptors and ligands on lymphocytes and antigen-presenting cells. In addition to the external factors, intrinsic changes of lymphocytes would participate in shaping the ongoing responsiveness of cells. An important intrinsic determinant of responsiveness could be the membrane potential (V m ) that is mainly determined by the potassium permeability of cell membrane and transmembrane gradient of [K ϩ ]. K ϩ channels, besides setting resting V m , play critical roles in various cellular functions (1). Among them, the modulation of calcium signals by providing electrical driving force has been suggested as an essential role of K ϩ channels (1, 2). In human primary T cells, two kinds of K ϩ channels, the voltage-gated K ϩ channels Kv1.3 and the calcium-activated K ϩ channel IKCa1 (hSK4), are found to play such a role; Kv1.3 channels are essential for activation of quiescent cells, and signaling through protein kinase C pathway enhances expression of IKCa1 channels that are required for proliferation (3, 4). Besides V m regulation, K ϩ channels also regulate the loss of intracellular K ϩ , which is a prerequisite step in the normotonic-or the Fas-mediated apoptosis (5, 6). In contrast to the studies in T cells, the characteristics of K ϩ channels and their roles in B cells have been rarely investigated, and the types of reported K ϩ channels are restricted to the v...

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Using the planar lipid bilayer technique, we tested whether NO directly activates calcium-activated potassium (Maxi-K) channels isolated from rat brain. We used streptozotocin (STZ) as NO donor, and the NO release was controlled with light. In the presence of 100-800 uM STZ, the Maxi-K channel activity increased up to 3-fold within several tens of seconds after the light was on, and reversed to the control level several minutes after shutting off the light. Similar activation was observed with other NO donors such as S-nitroso-Af-acetylpenicillamine and sodium nitroprusside. The degree of activity increase was dependent upon the initial open probability (P\"\ t ). When the P init was lower, the activity increase was greater. These results demonstrate that NO can directly affect the Maxi-K channel activity, and suggest that the Maxi-K channel might be one of the physiological targets of NO in brain.

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