Ca(2+) spark sites in smooth muscle cells are numerous and differ in number of ryanodine receptors, large-conductance K(+) channels, and coupling ratio between them

Am J Physiol Cell Physiol. 2004 Dec;287(6):C1577-88. doi: 10.1152/ajpcell.00153.2004. Epub 2004 Aug 11.


Ca(2+) sparks are highly localized Ca(2+) transients caused by Ca(2+) release from sarcoplasmic reticulum through ryanodine receptors (RyR). In smooth muscle, Ca(2+) sparks activate nearby large-conductance, Ca(2+)-sensitive K(+) (BK) channels to generate spontaneous transient outward currents (STOC). The properties of individual sites that give rise to Ca(2+) sparks have not been examined systematically. We have characterized individual sites in amphibian gastric smooth muscle cells with simultaneous high-speed imaging of Ca(2+) sparks using wide-field digital microscopy and patch-clamp recording of STOC in whole cell mode. We used a signal mass approach to measure the total Ca(2+) released at a site and to estimate the Ca(2+) current flowing through RyR [I(Ca(spark))]. The variance between spark sites was significantly greater than the intrasite variance for the following parameters: Ca(2+) signal mass, I(Ca(spark)), STOC amplitude, and 5-ms isochronic STOC amplitude. Sites that failed to generate STOC did so consistently, while those at the remaining sites generated STOC without failure, allowing the sites to be divided into STOC-generating and STOC-less sites. We also determined the average number of spark sites, which was 42/cell at a minimum and more likely on the order of at least 400/cell. We conclude that 1) spark sites differ in the number of RyR, BK channels, and coupling ratio of RyR-BK channels, and 2) there are numerous Ca(2+) spark-generating sites in smooth muscle cells. The implications of these findings for the organization of the spark microdomain are explored.

Publication types

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

MeSH terms

  • Animals
  • Bufo marinus
  • Calcium / metabolism
  • Calcium Signaling / physiology*
  • Cytoplasm / metabolism
  • Large-Conductance Calcium-Activated Potassium Channels
  • Microscopy, Fluorescence / methods*
  • Myocytes, Smooth Muscle / physiology*
  • Patch-Clamp Techniques
  • Potassium Channels, Calcium-Activated / physiology*
  • Ryanodine Receptor Calcium Release Channel / physiology*
  • Sarcoplasmic Reticulum / physiology


  • Large-Conductance Calcium-Activated Potassium Channels
  • Potassium Channels, Calcium-Activated
  • Ryanodine Receptor Calcium Release Channel
  • Calcium