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Biophys J, August 2000, p. 814-827, Vol. 79, No. 2

Effects of Imperatoxin A on Local Sarcoplasmic Reticulum Ca2+ Release in Frog Skeletal Muscle

Alexander Shtifman,* Christopher W. Ward,* Jianli Wang,dagger Hector H. Valdivia,dagger and Martin F. Schneider*

 *Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, Maryland 21201, and  dagger Department of Physiology, University of Wisconsin School of Medicine, Madison, Wisconsin 53706 USA

We have investigated the effects of imperatoxin A (IpTxa) on local calcium release events in permeabilized frog skeletal muscle fibers, using laser scanning confocal microscopy in linescan mode. IpTxa induced the appearance of Ca2+ release events from the sarcoplasmic reticulum that are ~2 s and have a smaller amplitude (31 ± 2%) than the "Ca2+ sparks" normally seen in the absence of toxin. The frequency of occurrence of long-duration imperatoxin-induced Ca2+ release events increased in proportion to IpTxa concentrations ranging from 10 nM to 50 nM. The mean duration of imperatoxin-induced events in muscle fibers was independent of toxin concentration and agreed closely with the channel open time in experiments on isolated frog ryanodine receptors (RyRs) reconstituted in planar lipid bilayer, where IpTxa induced opening of single Ca2+ release channels to prolonged subconductance states. These results suggest involvement of a single molecule of IpTxa in the activation of a single Ca2+ release channel to produce a long-duration event. Assuming the ratio of full conductance to subconductance to be the same in the fibers as in bilayer, the amplitude of a spark relative to the long event indicates involvement of at most four RyR Ca2+ release channels in the production of short-duration Ca2+ sparks.

Biophys J, August 2000, p. 814-827, Vol. 79, No. 2
© 2000 by the Biophysical Society   0006-3495/00/08/814/14  $2.00


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