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Functional coupling of the beta(1) subunit to the large conductance Ca(2+)-activated K(+) channel in the absence of Ca(2+). Increased Ca(2+) sensitivity from a Ca(2+)-independent mechanism.

Nimigean CM, Magleby KL - J. Gen. Physiol. (2000)

Bottom Line: The effect of the beta(1) subunit on increasing the durations of the gaps between bursts in 0 Ca(2+)(i) was preserved over a range of voltage, but was switched off as Ca(2+)(i) was increased into the activation range.The Ca(2+)-dependent effect of the beta(1) subunit on the gaps between bursts accounted for the remaining 20% of the leftward shift.Our observation that the major effects of the beta(1) subunit are independent of Ca(2+)(i) suggests that the beta(1) subunit mainly alters the energy barriers of Ca(2+)-independent transitions.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biophysics, University of Miami School of Medicine, FL 33101-6430, USA. cnimigea@chroma.med.miami.edu

ABSTRACT
Coexpression of the beta(1) subunit with the alpha subunit (mSlo) of BK channels increases the apparent Ca(2+) sensitivity of the channel. This study investigates whether the mechanism underlying the increased Ca(2+) sensitivity requires Ca(2+), by comparing the gating in 0 Ca(2+)(i) of BK channels composed of alpha subunits to those composed of alpha+beta(1) subunits. The beta(1) subunit increased burst duration approximately 20-fold and the duration of gaps between bursts approximately 3-fold, giving an approximately 10-fold increase in open probability (P(o)) in 0 Ca(2+)(i). The effect of the beta(1) subunit on increasing burst duration was little changed over a wide range of P(o) achieved by varying either Ca(2+)(i) or depolarization. The effect of the beta(1) subunit on increasing the durations of the gaps between bursts in 0 Ca(2+)(i) was preserved over a range of voltage, but was switched off as Ca(2+)(i) was increased into the activation range. The Ca(2+)-independent, beta(1) subunit-induced increase in burst duration accounted for 80% of the leftward shift in the P(o) vs. Ca(2+)(i) curve that reflects the increased Ca(2+) sensitivity induced by the beta(1) subunit. The Ca(2+)-dependent effect of the beta(1) subunit on the gaps between bursts accounted for the remaining 20% of the leftward shift. Our observation that the major effects of the beta(1) subunit are independent of Ca(2+)(i) suggests that the beta(1) subunit mainly alters the energy barriers of Ca(2+)-independent transitions. The changes in gating induced by the beta(1) subunit differ from those induced by depolarization, as increasing P(o) by depolarization or by the beta(1) subunit gave different gating kinetics. The complex gating kinetics for both alpha and alpha+beta(1) channels in 0 Ca(2+)(i) arise from transitions among two to three open and three to five closed states and are inconsistent with Monod-Wyman-Changeux type models, which predict gating among only one open and one closed state in 0 Ca(2+)(i).

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The β1 subunit alters the single-channel gating kinetics in both the absence and presence of Ca2+i. (A) Currents recorded from single α and α+β1 channels in 0 Ca2+i. A continuous record in each case was cut into the five presented traces. The β1 subunit increases both burst duration and the duration of the gaps between bursts. The average Pos for the entire record from which each excerpt was obtained were: 0.00056 for the α channel and 0.0039 for the α+β1 channel. (B) Bursts from α and α+β1 channels presented on a faster time base. (C) Currents recorded from α and α+β1 channels in 1.8 μM Ca2+i. The traces were filtered at 4 kHz for display in this and subsequent figures, while the filtering for the analysis carried out in this paper was typically 10 kHz. Membrane potential: +30 mV; α channel, C92; α+β1 channel, C87.
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Figure 1: The β1 subunit alters the single-channel gating kinetics in both the absence and presence of Ca2+i. (A) Currents recorded from single α and α+β1 channels in 0 Ca2+i. A continuous record in each case was cut into the five presented traces. The β1 subunit increases both burst duration and the duration of the gaps between bursts. The average Pos for the entire record from which each excerpt was obtained were: 0.00056 for the α channel and 0.0039 for the α+β1 channel. (B) Bursts from α and α+β1 channels presented on a faster time base. (C) Currents recorded from α and α+β1 channels in 1.8 μM Ca2+i. The traces were filtered at 4 kHz for display in this and subsequent figures, while the filtering for the analysis carried out in this paper was typically 10 kHz. Membrane potential: +30 mV; α channel, C92; α+β1 channel, C87.

Mentions: To investigate whether the β1 subunit requires Ca2+i for its action, we used single-channel analysis to examine the gating of α and α+β1 channels in the effective absence of Ca2+i (<1 nM), which will be referred to as 0 Ca2+i. (It will be shown in a later section that effective 0 Ca2+i was achieved.) Fig. 1 A shows single-channel currents recorded in 0 Ca2+i at +30 mV from an α channel and also from an α+β1 channel. The occasional openings and bursts of openings are separated by the long closed intervals of many seconds that form the gaps between bursts. The long gaps between bursts in 0 Ca2+i give very low open probability. The average Pos for the entire records from which each excerpt was obtained were 0.00056 for the α channel and 0.0039 for the α+β1 channel, for a sevenfold increase in Po. The mean Po for 15 α channels and 21 α+β1 channels at 0 Ca2+i is plotted in Fig. 2 A (left-most points), where the presence of the β1 subunit increased Po ∼10-fold on average, from ∼0.0002 to 0.002.


Functional coupling of the beta(1) subunit to the large conductance Ca(2+)-activated K(+) channel in the absence of Ca(2+). Increased Ca(2+) sensitivity from a Ca(2+)-independent mechanism.

Nimigean CM, Magleby KL - J. Gen. Physiol. (2000)

The β1 subunit alters the single-channel gating kinetics in both the absence and presence of Ca2+i. (A) Currents recorded from single α and α+β1 channels in 0 Ca2+i. A continuous record in each case was cut into the five presented traces. The β1 subunit increases both burst duration and the duration of the gaps between bursts. The average Pos for the entire record from which each excerpt was obtained were: 0.00056 for the α channel and 0.0039 for the α+β1 channel. (B) Bursts from α and α+β1 channels presented on a faster time base. (C) Currents recorded from α and α+β1 channels in 1.8 μM Ca2+i. The traces were filtered at 4 kHz for display in this and subsequent figures, while the filtering for the analysis carried out in this paper was typically 10 kHz. Membrane potential: +30 mV; α channel, C92; α+β1 channel, C87.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2232893&req=5

Figure 1: The β1 subunit alters the single-channel gating kinetics in both the absence and presence of Ca2+i. (A) Currents recorded from single α and α+β1 channels in 0 Ca2+i. A continuous record in each case was cut into the five presented traces. The β1 subunit increases both burst duration and the duration of the gaps between bursts. The average Pos for the entire record from which each excerpt was obtained were: 0.00056 for the α channel and 0.0039 for the α+β1 channel. (B) Bursts from α and α+β1 channels presented on a faster time base. (C) Currents recorded from α and α+β1 channels in 1.8 μM Ca2+i. The traces were filtered at 4 kHz for display in this and subsequent figures, while the filtering for the analysis carried out in this paper was typically 10 kHz. Membrane potential: +30 mV; α channel, C92; α+β1 channel, C87.
Mentions: To investigate whether the β1 subunit requires Ca2+i for its action, we used single-channel analysis to examine the gating of α and α+β1 channels in the effective absence of Ca2+i (<1 nM), which will be referred to as 0 Ca2+i. (It will be shown in a later section that effective 0 Ca2+i was achieved.) Fig. 1 A shows single-channel currents recorded in 0 Ca2+i at +30 mV from an α channel and also from an α+β1 channel. The occasional openings and bursts of openings are separated by the long closed intervals of many seconds that form the gaps between bursts. The long gaps between bursts in 0 Ca2+i give very low open probability. The average Pos for the entire records from which each excerpt was obtained were 0.00056 for the α channel and 0.0039 for the α+β1 channel, for a sevenfold increase in Po. The mean Po for 15 α channels and 21 α+β1 channels at 0 Ca2+i is plotted in Fig. 2 A (left-most points), where the presence of the β1 subunit increased Po ∼10-fold on average, from ∼0.0002 to 0.002.

Bottom Line: The effect of the beta(1) subunit on increasing the durations of the gaps between bursts in 0 Ca(2+)(i) was preserved over a range of voltage, but was switched off as Ca(2+)(i) was increased into the activation range.The Ca(2+)-dependent effect of the beta(1) subunit on the gaps between bursts accounted for the remaining 20% of the leftward shift.Our observation that the major effects of the beta(1) subunit are independent of Ca(2+)(i) suggests that the beta(1) subunit mainly alters the energy barriers of Ca(2+)-independent transitions.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biophysics, University of Miami School of Medicine, FL 33101-6430, USA. cnimigea@chroma.med.miami.edu

ABSTRACT
Coexpression of the beta(1) subunit with the alpha subunit (mSlo) of BK channels increases the apparent Ca(2+) sensitivity of the channel. This study investigates whether the mechanism underlying the increased Ca(2+) sensitivity requires Ca(2+), by comparing the gating in 0 Ca(2+)(i) of BK channels composed of alpha subunits to those composed of alpha+beta(1) subunits. The beta(1) subunit increased burst duration approximately 20-fold and the duration of gaps between bursts approximately 3-fold, giving an approximately 10-fold increase in open probability (P(o)) in 0 Ca(2+)(i). The effect of the beta(1) subunit on increasing burst duration was little changed over a wide range of P(o) achieved by varying either Ca(2+)(i) or depolarization. The effect of the beta(1) subunit on increasing the durations of the gaps between bursts in 0 Ca(2+)(i) was preserved over a range of voltage, but was switched off as Ca(2+)(i) was increased into the activation range. The Ca(2+)-independent, beta(1) subunit-induced increase in burst duration accounted for 80% of the leftward shift in the P(o) vs. Ca(2+)(i) curve that reflects the increased Ca(2+) sensitivity induced by the beta(1) subunit. The Ca(2+)-dependent effect of the beta(1) subunit on the gaps between bursts accounted for the remaining 20% of the leftward shift. Our observation that the major effects of the beta(1) subunit are independent of Ca(2+)(i) suggests that the beta(1) subunit mainly alters the energy barriers of Ca(2+)-independent transitions. The changes in gating induced by the beta(1) subunit differ from those induced by depolarization, as increasing P(o) by depolarization or by the beta(1) subunit gave different gating kinetics. The complex gating kinetics for both alpha and alpha+beta(1) channels in 0 Ca(2+)(i) arise from transitions among two to three open and three to five closed states and are inconsistent with Monod-Wyman-Changeux type models, which predict gating among only one open and one closed state in 0 Ca(2+)(i).

Show MeSH
Related in: MedlinePlus