Peptide probe of ryanodine receptor function. Imperatoxin A, a peptide from the venom of the scorpion Pandinus imperator, selectively activates skeletal-type ryanodine receptor isoforms.

We have used [3H]ryanodine binding experiments and single channel recordings to provide convergent descriptions of the effect of imperatoxin A (IpTxa), a approximately 5-kDa peptide from the venom of the scorpion Pandinus imperator (Valdivia, H. H., Kirby, M. S., Lederer, W. J., and Coronado, R. (1992) Proc. Ntl. Acad. Sc. U.S.A. 89, 12185-12189) on Ca2+ release channels/ryanodine receptors (RyR) of sarcoplasmic reticulum (SR). At nanomolar concentrations, IpTxa increased the binding of [3H]ryanodine to skeletal SR and, to a lesser extent, to cerebellum microsomes. The activating effect of IpTxa on skeletal SR was Ca(2+)-dependent, synergized by caffeine, and independent of other modulators of RyRs. However, IpTxa had negligible effects on tissues where the expression of skeletal-type RyR isoforms (RyR1) is small or altogether absent, i.e. cardiac, cerebrum, and liver microsomes. Thus, IpTxa may be used as a ligand capable of discriminating between RyR isoforms with nanomolar affinity. IpTxa increased the open probability (Po) of rabbit skeletal muscle RyRs by increasing the frequency of open events and decreasing the duration of the closed lifetimes. This activating effect was dose-dependent (ED50 = 10 nM), had a fast onset, and was fully reversible. Purified RyR from solubilized skeletal SR displayed high affinity for [3H]ryanodine with a KD of 6.1 nM and Bmax of approximately 30 pmol/mg of protein. IpTxa increased [3H]ryanodine binding noncompetitively by increasing Bmax to approximately 60 pmol/mg of protein. These results suggested the presence of an IpTxa-binding site on the RyR or a closely associated regulatory protein. This site appears to be distinct from the caffeine- and adenine nucleotide-regulatory sites. IpTxa may prove a useful tool to identify regulatory domains critical for channel gating and to dissect the contribution of skeletal-type RyRs to intracellular Ca2+ waveforms generated by stimulation of different RyR isoforms.