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.
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.
Zhuge
Ronghua
R
Biomedical Imaging Group, Department of Physiology, University of Massachusetts Medical School, Worcester, MA 01655, USA.
Fogarty
Kevin E
KE
Tuft
Richard A
RA
Walsh
John V
JV
D000818:Animals; D002024:Bufo marinus; D002118:Calcium / Q000494:pharmacology / Q000502:physiology; D004553:Electric Conductivity; D051036:Large-Conductance Calcium-Activated Potassium Channels; D008564:Membrane Potentials / Q000502:physiology; D009130:Muscle, Smooth / Q000166:cytology / Q000378:metabolism; D009994:Osmolar Concentration; D024681:Potassium Channels, Calcium-Activated / Q000502:physiology*
D051036:Large-Conductance Calcium-Activated Potassium Channels; D024681:Potassium Channels, Calcium-Activated; D002118:Calcium