BK channel subunit composition modulates molecular tolerance to ethanol

Background: The large conductance calcium-activated potassium channel (also called BK channel or Slo channels) is a well-studied target of alcohol action, and plays an important role in behavioral tolerance. Methods: Using patch clamp electrophysiology, we examined human BK channels expressed in HEK293 cells to test whether tolerance to ethanol occurs in excised patches and whether it is influenced by subunit composition. Three combinations were examined: hSlo, hSlo + beta(1), and hSlo + beta(4). Results: The 2 components of BK alcohol adaptation (Component 1: rapid tolerance to acute potentiation, and Component 2: a more slowly developing decrease in current density) were observed, and varied according to subunit combination. Using a 2-exposure protocol, Component 1 tolerance was evident in 2 of the 3 combinations, because it was more pronounced for hSlo and hSlo + beta(4). Conclusions: Thus, rapid tolerance in human BK occurs in cell-free membrane patches, independent of cytosolic second messengers, nucleotides or changes in free calcium. Alcohol pretreatment for 24 hours altered subsequent short-term plasticity of hSlo + beta(4) channels, suggesting a relationship between classes of tolerance. Finally, Component 2 reduction in current density showed a striking dependency on channel composition. Twenty-four hour exposure to 25 mM ethanol resulted in a down-regulation of BK current in hSlo and hSlo + beta(4) channels, but not in hSlo + beta(1) channels. The fact that hSlo + beta(1) channels show less sensitivity to acute challenge, in conjunction with less Component 1 and Component 2 tolerance, suggests subunit composition is an important factor for these elements of alcohol response.