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J Gen Physiol
2013 Apr 01;1414:493-7. doi: 10.1085/jgp.201210955.
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Lack of negative slope in I-V plots for BK channels at positive potentials in the absence of intracellular blockers.
Geng Y
,
Wang X
,
Magleby KL
.
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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.
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23530138
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Figure 1. Intracellular Ca2+ and Mg2+ induce a negative slope at high positive potentials for I-V plots from BK channels that is not observed in their absence. (A) Representative single-channel current records from BK channels at the indicated voltages without and with 2.5 mM Ca2+ and Mg2+ in the intracellular solution. The solutions also contained 150 mM KCl and 10 mM HEPES, pH 7.2. The presented current recordings were filtered at 5 kHz for display, but the data were collected and analyzed with 10 kHz low-pass filtering for the I-V plots. The divalent cation blockers reduce the outward single-channel current amplitudes, with a greater fractional decrease at +160 mV than at +80 mV. (B) I-V plots of single-channel current amplitudes indicate that 2.5 mM of intracellular Ca2+ and Mg2+ induce a negative slope for potentials greater than +120 mV. A negative slope is not observed in the absence of Ca2+ and Mg2+ (open circles). The dashed lines are cubic spline fits constrained to pass through the origin. Each plotted point is the mean from five or more patches. The absence of visible error bars indicates that the SEM is less than the symbol size.
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