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

J Gen Physiol. 2002 Jul;120(1):15-27. doi: 10.1085/jgp.20028571.


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.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Bufo marinus
  • Calcium / pharmacology
  • Calcium / physiology
  • Electric Conductivity
  • Large-Conductance Calcium-Activated Potassium Channels
  • Membrane Potentials / physiology
  • Muscle, Smooth / cytology
  • Muscle, Smooth / metabolism
  • Osmolar Concentration
  • Potassium Channels, Calcium-Activated / physiology*


  • Large-Conductance Calcium-Activated Potassium Channels
  • Potassium Channels, Calcium-Activated
  • Calcium