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Spontaneous transient outward currents arise from microdomains where BK channels are exposed to a mean Ca(2+) concentration on the order of 10 microM during a Ca(2+) spark.

Zhuge R, Fogarty KE, Tuft RA, Walsh JV - J. Gen. Physiol. (2002)

Bottom Line: Using tight seal, whole-cell recording, we have analyzed the voltage-dependence of the STOC conductance (g((STOC))), and compared it to the voltage-dependence of BK channel activation in excised patches in the presence of different [Ca(2+)]s.The Ca(2+) sparks did not change in amplitude over the range of potentials of interest.Moreover, given the constraints imposed by the estimated channel density and the Ca(2+) current during a spark, the BK channels do not appear to be uniformly distributed over the membrane but instead are found at higher density at the spark site.

View Article: PubMed Central - PubMed

Affiliation: Biomedical Imaging Group, Department of Physiology, University of Massachusetts Medical School, Worcester, MA 01655, USA.

ABSTRACT
Ca(2+) sparks are small, localized cytosolic Ca(2+) transients due to Ca(2+) release from sarcoplasmic reticulum through ryanodine receptors. In smooth muscle, Ca(2+) sparks activate large conductance Ca(2+)-activated K(+) channels (BK channels) in the spark microdomain, thus generating spontaneous transient outward currents (STOCs). The purpose of the present study is to determine experimentally the level of Ca(2+) to which the BK channels are exposed during a spark. Using tight seal, whole-cell recording, we have analyzed the voltage-dependence of the STOC conductance (g((STOC))), and compared it to the voltage-dependence of BK channel activation in excised patches in the presence of different [Ca(2+)]s. The Ca(2+) sparks did not change in amplitude over the range of potentials of interest. In contrast, the magnitude of g((STOC)) remained roughly constant from 20 to -40 mV and then declined steeply at more negative potentials. From this and the voltage dependence of BK channel activation, we conclude that the BK channels underlying STOCs are exposed to a mean [Ca(2+)] on the order of 10 microM during a Ca(2+) spark. The membrane area over which a concentration > or =10 microM is reached has an estimated radius of 150-300 nm, corresponding to an area which is a fraction of one square micron. Moreover, given the constraints imposed by the estimated channel density and the Ca(2+) current during a spark, the BK channels do not appear to be uniformly distributed over the membrane but instead are found at higher density at the spark site.

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The “oasis” model of the spark-STOC microdomain. Top panel, the view of the BK channel distribution pattern as seen from the membrane surface. In the case of the oasis model, the channels at the spark site are confined to a region within the dotted circle. This region is less than one square micron in area (see text). In the case of the “extended oasis” model, there would be a ring of BK channels lying just outside the dotted circle (not depicted), which would not be activated by a Ca2+ spark. Bottom, cross-sectional view of the region marked by the black solid line in the top panel. Note that the distance between release site and plasma membrane is 25 nm. These depictions are qualitative representations and not drawn to scale.
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fig5: The “oasis” model of the spark-STOC microdomain. Top panel, the view of the BK channel distribution pattern as seen from the membrane surface. In the case of the oasis model, the channels at the spark site are confined to a region within the dotted circle. This region is less than one square micron in area (see text). In the case of the “extended oasis” model, there would be a ring of BK channels lying just outside the dotted circle (not depicted), which would not be activated by a Ca2+ spark. Bottom, cross-sectional view of the region marked by the black solid line in the top panel. Note that the distance between release site and plasma membrane is 25 nm. These depictions are qualitative representations and not drawn to scale.

Mentions: What then is the size of the zone of activated BK channels within such an extended oasis? The zone of activation will depend on the membrane potential, expanding at more positive potentials and constricting at more negative potentials. Hence, for example, at −40 mV, often used as an estimate of resting potential, 95% of the conductance in an STOC will come from a zone with a radius of 470 nM (area = 0.7 μm2) over which the mean [Ca2+] is 15 μM, and the [Ca2+] at the periphery is 3.6 μM. At 0 mV, 95% of the conductance in an STOC will come from a zone of 490 nm (area = 0.75 μm2) with a mean [Ca2+] of 14 μM and a peripheral [Ca2+] of 3.4 μM. This set of values is not much different at the two potentials, 0 and −40 mV, as might be expected from the fact that the value for g(STOC) is unchanged at the two potentials. At −80 mV, the values are: radius, 390 nm (area = 0.73 μm2) from which 95% of the STOC conductance derives; mean [Ca2+], 21 μM; peripheral [Ca2+], 4 μM. Fig. 5 provides a depiction of the oasis and extended oasis models.


Spontaneous transient outward currents arise from microdomains where BK channels are exposed to a mean Ca(2+) concentration on the order of 10 microM during a Ca(2+) spark.

Zhuge R, Fogarty KE, Tuft RA, Walsh JV - J. Gen. Physiol. (2002)

The “oasis” model of the spark-STOC microdomain. Top panel, the view of the BK channel distribution pattern as seen from the membrane surface. In the case of the oasis model, the channels at the spark site are confined to a region within the dotted circle. This region is less than one square micron in area (see text). In the case of the “extended oasis” model, there would be a ring of BK channels lying just outside the dotted circle (not depicted), which would not be activated by a Ca2+ spark. Bottom, cross-sectional view of the region marked by the black solid line in the top panel. Note that the distance between release site and plasma membrane is 25 nm. These depictions are qualitative representations and not drawn to scale.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2311396&req=5

fig5: The “oasis” model of the spark-STOC microdomain. Top panel, the view of the BK channel distribution pattern as seen from the membrane surface. In the case of the oasis model, the channels at the spark site are confined to a region within the dotted circle. This region is less than one square micron in area (see text). In the case of the “extended oasis” model, there would be a ring of BK channels lying just outside the dotted circle (not depicted), which would not be activated by a Ca2+ spark. Bottom, cross-sectional view of the region marked by the black solid line in the top panel. Note that the distance between release site and plasma membrane is 25 nm. These depictions are qualitative representations and not drawn to scale.
Mentions: What then is the size of the zone of activated BK channels within such an extended oasis? The zone of activation will depend on the membrane potential, expanding at more positive potentials and constricting at more negative potentials. Hence, for example, at −40 mV, often used as an estimate of resting potential, 95% of the conductance in an STOC will come from a zone with a radius of 470 nM (area = 0.7 μm2) over which the mean [Ca2+] is 15 μM, and the [Ca2+] at the periphery is 3.6 μM. At 0 mV, 95% of the conductance in an STOC will come from a zone of 490 nm (area = 0.75 μm2) with a mean [Ca2+] of 14 μM and a peripheral [Ca2+] of 3.4 μM. This set of values is not much different at the two potentials, 0 and −40 mV, as might be expected from the fact that the value for g(STOC) is unchanged at the two potentials. At −80 mV, the values are: radius, 390 nm (area = 0.73 μm2) from which 95% of the STOC conductance derives; mean [Ca2+], 21 μM; peripheral [Ca2+], 4 μM. Fig. 5 provides a depiction of the oasis and extended oasis models.

Bottom Line: Using tight seal, whole-cell recording, we have analyzed the voltage-dependence of the STOC conductance (g((STOC))), and compared it to the voltage-dependence of BK channel activation in excised patches in the presence of different [Ca(2+)]s.The Ca(2+) sparks did not change in amplitude over the range of potentials of interest.Moreover, given the constraints imposed by the estimated channel density and the Ca(2+) current during a spark, the BK channels do not appear to be uniformly distributed over the membrane but instead are found at higher density at the spark site.

View Article: PubMed Central - PubMed

Affiliation: Biomedical Imaging Group, Department of Physiology, University of Massachusetts Medical School, Worcester, MA 01655, USA.

ABSTRACT
Ca(2+) sparks are small, localized cytosolic Ca(2+) transients due to Ca(2+) release from sarcoplasmic reticulum through ryanodine receptors. In smooth muscle, Ca(2+) sparks activate large conductance Ca(2+)-activated K(+) channels (BK channels) in the spark microdomain, thus generating spontaneous transient outward currents (STOCs). The purpose of the present study is to determine experimentally the level of Ca(2+) to which the BK channels are exposed during a spark. Using tight seal, whole-cell recording, we have analyzed the voltage-dependence of the STOC conductance (g((STOC))), and compared it to the voltage-dependence of BK channel activation in excised patches in the presence of different [Ca(2+)]s. The Ca(2+) sparks did not change in amplitude over the range of potentials of interest. In contrast, the magnitude of g((STOC)) remained roughly constant from 20 to -40 mV and then declined steeply at more negative potentials. From this and the voltage dependence of BK channel activation, we conclude that the BK channels underlying STOCs are exposed to a mean [Ca(2+)] on the order of 10 microM during a Ca(2+) spark. The membrane area over which a concentration > or =10 microM is reached has an estimated radius of 150-300 nm, corresponding to an area which is a fraction of one square micron. Moreover, given the constraints imposed by the estimated channel density and the Ca(2+) current during a spark, the BK channels do not appear to be uniformly distributed over the membrane but instead are found at higher density at the spark site.

Show MeSH
Related in: MedlinePlus