title: Lack of negative slope in I-V plots for BK channels at positive potentials in the absence of intracellular blockers.
abstract: Large-conductance, voltage- and Ca(2+)-activated K(+) (BK) channels display near linear current-voltage (I-V) plots for voltages between -100 and +100 mV, with an increasing sublinearity for more positive potentials. As is the case for many types of channels, BK channels are blocked at positive potentials by intracellular Ca(2+) and Mg(2+). This fast block progressively reduces single-channel conductance with increasing voltage, giving rise to a negative slope in the I-V plots beyond about +120 mV, depending on the concentration of the blockers. In contrast to these observations of pronounced differences in the magnitudes and shapes of I-V plots in the absence and presence of intracellular blockers, Schroeder and Hansen (2007. J. Gen. Physiol. http://dx.doi.org/10.1085/jgp.200709802) have reported identical I-V plots in the absence and presence of blockers for BK channels, with both plots having reduced conductance and negative slopes, as expected for blockers. Schroeder and Hansen included both Ca(2+) and Mg(2+) in the intracellular solution rather than a single blocker, and they also studied BK channels expressed from α plus β1 subunits, whereas most previous studies used only α subunits. Although it seems unlikely that these experimental differences would account for the differences in findings between previous studies and those of Schroeder and Hansen, we repeated the experiments using BK channels comprised of α plus β1 subunits with joint application of 2.5 mM Ca(2+) plus 2.5 mM Mg(2+), as Schroeder and Hansen did. In contrast to the findings of Schroeder and Hansen of identical I-V plots, we found marked differences in the single-channel I-V plots in the absence and presence of blockers. Consistent with previous studies, we found near linear I-V plots in the absence of blockers and greatly reduced currents and negative slopes in the presence of blockers. Hence, studies of conductance mechanisms for BK channels should exclude intracellular Ca(2+)/Mg(2+), as they can reduce conductance and induce negative slopes.
authors.affiliation: Department of Physiology and Biophysics, University of Miami Miller School of Medicine, Miami, FL 33136, USA. ygeng@med.miami.edu
authors.forename: Yanyan
authors.initials: Y
authors.lastname: Geng
authors.forename: Xiaoyu
authors.initials: X
authors.lastname: Wang
authors.forename: Karl L
authors.initials: KL
authors.lastname: Magleby
mesh_terms: D000200:Action Potentials / Q000187:drug effects*; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D042541:Intracellular Space / Q000737:chemistry; D015640:Ion Channel Gating / Q000187:drug effects; D051037:Large-Conductance Calcium-Activated Potassium Channel alpha Subunits / Q000037:antagonists & inhibitors* / Q000378:metabolism; D008274:Magnesium / Q000494:pharmacology; D008564:Membrane Potentials; D051379:Mice; D026902:Potassium Channel Blockers / Q000494:pharmacology*; D021122:Protein Subunits / Q000037:antagonists & inhibitors / Q000378:metabolism; D014981:Xenopus
chemical_list: C465449:Kcnma1 protein, mouse; D051037:Large-Conductance Calcium-Activated Potassium Channel alpha Subunits; D026902:Potassium Channel Blockers; D021122:Protein Subunits; D008274:Magnesium; D002118:Calcium