Calmodulin modulates initiation but not termination of spontaneous Ca2+ sparks in frog skeletal muscle

Biophys J. 2003 Aug;85(2):921-32. doi: 10.1016/S0006-3495(03)74531-7.


Calmodulin is a ubiquitous Ca(2+) sensing protein that binds to and modulates the sarcoplasmic reticulum Ca(2+) release channel, ryanodine receptor (RYR). Here we assessed the effects of calmodulin on the local Ca(2+) release properties of RYR in permeabilized frog skeletal muscle fibers. Fluorescently labeled recombinant calmodulin in the internal solution localized at the Z-line/triad region. Calmodulin (0.05-5.0 micro M) in the internal solution (free [Ca(2+)](i) approximately 50-100 nM) initiated a highly cooperative dose-dependent increase in Ca(2+) spark frequency, with a half-maximal activation (K) of 1.1 micro M, a Hill coefficient (n) of 4.2 and a fractional maximal increase in frequency (R) of 17-fold. A non-Ca(2+) binding mutant of calmodulin elicited a similar highly cooperative dose-dependent increase in spark frequency (K = 1.0 micro M; n = 3.7; R = 12-fold). Spatiotemporal properties of Ca(2+) sparks were essentially unaffected by either wild-type or mutant calmodulin. An N-terminal extension of calmodulin, (N+3)calmodulin, that binds to but does not activate RYR at nM [Ca(2+)] in sarcoplasmic reticulum vesicles, prevented the calmodulin-induced increase in spark frequency. These data suggest that exogenous Ca(2+)-free calmodulin cooperatively sensitizes the Ca(2+) release channel to open, but that Ca(2+) binding to the added calmodulin does not play a significant role in the termination of Ca(2+) sparks.

Publication types

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

MeSH terms

  • Animals
  • Calcium / metabolism*
  • Calcium Signaling / drug effects*
  • Calcium Signaling / physiology*
  • Calmodulin / metabolism*
  • Calmodulin / pharmacology
  • Cells, Cultured
  • Dose-Response Relationship, Drug
  • Membrane Potentials / drug effects
  • Membrane Potentials / physiology
  • Muscle, Skeletal / drug effects*
  • Muscle, Skeletal / physiology*
  • Rana pipiens
  • Saponins / pharmacology*
  • Sarcoplasmic Reticulum / drug effects*
  • Sarcoplasmic Reticulum / physiology*


  • Calmodulin
  • Saponins
  • Calcium