Acute Alcohol Tolerance Is Intrinsic to the BKCa Protein, but Is Modulated by the Lipid Environment

Abstract

We found that tolerance was observable in BK Ca channels in membrane patches pulled from HEK cells and when they are placed into reconstituted 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylserine membranes. Furthermore, altering bilayer thickness by incorporating the channel into lipid mixtures of 1,2-dioleoyl-3-phosphatidylethanolamine with phosphatidylcholines of increasing chain length, or with sphingomyelin, strongly affected the sensitivity of the channel, as well as the time course of the acute response. Ethanol sensitivity changed from a strong potentiation in thin bilayers to inhibition in thick sphingomyelin/1,2-dioleoyl-3-phosphatidylethanolamine bilayers. Thus, tolerance can be an intrinsic property of the channel protein-lipid complex, and bilayer thickness plays an important role in shaping the pattern of response to ethanol. As a consequence of these findings the protein-lipid complex should be treated as a unit when studying ethanol action.The lipid hypothesis of alcohol action (4), in which the actions of ethanol on neuronal membranes was thought to be explicable mainly in terms of actions on membrane lipids, secondarily affecting proteins, has largely been replaced by the protein hypothesis. This latter is predicated on findings that alcohol can interact directly with membrane proteins to affect function (5-11). Of course, the focus on alcohol-protein interaction does not preclude a role for membrane lipids in modulating this interaction. Indeed, recent studies have made it clear that alterations in lipid environments can alter the response of signaling proteins such as membrane ion channels (12) to alcohol (13,14). Behaviorally, tolerance to the continued presence of ethanol and other drugs of abuse plays an important role in the development of dependence and addiction. In fact, the strength of acute behavioral tolerance in naïve humans is one of the better known predictors of the development of alcohol addiction (15). The elucidation of the molecular mechanisms underlying tolerance and other forms of drug-related neuroadaptation is an important goal of current research in the addiction field.Large conductance Ca 2ϩ and voltage-gated K ϩ (BK Ca ) channels (16) play an important role in the regulation of neuronal excitability, cell volume regulation (17), excitation-contraction coupling, and hormonal secretion (18 -20). Recently, it has been demonstrated that the BK Ca channel may be a direct target for ethanol (21) and may play a central role in the behavioral response to ethanol in Caenorhabditis elegans (1) and the mediation of rapid drug tolerance in Drosophila (2, 22, 23). Clinically relevant concentrations of ethanol (10 -100 mM) can potentiate (20, 24 -26) or inhibit (27-29) the activity of BK Ca channels, depending on the origin of the channel. In addition, in supraoptic neurons, ethanol increases nerve terminal BK Ca channel activity but fails to modulate cell body BK Ca channels (26,30). Efforts are underway to determine the b...

Hydrogen sulfide increases calcium-activated potassium (BK) channel activity of rat pituitary tumor cells

“…More recently, it was reported that BK channels develop a rapid tolerance within a few minutes against ethanol present in the recording solution [38]. This mechanism is not fully understood yet but seems to be an intrinsic property of BK channels contributing to drug tolerance [61]. Such a mechanism would allow the cell to adjust to normal electrical signaling condition even in the presence of an activating drug.…”

BK Channels: mediators and models for alcohol tolerance

“…‘how much organized membrane and cell are necessary to observe acute molecular tolerance?’ The answer is ‘remarkably little’ – i.e. acute tolerance is observable in isolated BK channels transposed into reconstituted bilayers [47]. But bilayer composition has a very large effect.…”

“…But bilayer composition has a very large effect. The acute ethanol tolerance observed in stable (20:1) phosphatidylcholine–dioleoylphosphatidylethanola-mine (PC–DOPE) bilayers is not observed in sphingomyelin–DOPE bilayers [47]. One suggestion to explain this effect is that acute molecular tolerance to ethanol involves interaction with multiple sites perhaps with opposing (i.e.…”

BK Channels – Focus on Polyamines, Ethanol/Acetaldehyde and Hydrogen Sulfide (H2S)

“…BK channel activity induced by ethanol was dependent on the size and shape of the phospholipids independent of their charges (Crowley et al 2005). Altering the thickness of the bilayer into which BK channels from HEK cells (human embryonic kidney cells) were inserted changed the ethanol response from potentiation in thin bilayers to inhibition in thick bilayers which correlated with mean closed time of the channels (Yuan et al, 2008). As mechanism for the biphasic channel modulation was proposed that forces of lateral stress within the lipid bilayer combine with hydrophobic mismatch to the channel gating spring structure (Yuan et al, 2007).…”