CA2-LABELLABLE PROBE HAVING NOVEL CA2+ RELEASE PROPERTIES IN SARCOPLASMIC-RETICULUM( RELEASE INDUCED BY MYOTOXIN ALPHA, A RADIO)

1 Myotoxin a (MYTX), a polypeptide toxin purified from the venom of pr airie rattlesnakes (Crotalus viridis) induced Ca2+ release from the he avy fraction (HSR) but not the light fraction of skeletal sarcoplasmic reticulum at concentrations higher than 1 mu M, followed by spontaneo us Ca2+ reuptake by measuring extravesicular Ca2+ concentrations using the Ca2+ electrode. 2 The rate of Ca-45(2+) release from HSR vesicles was markedly accelerated by MYTX in a concentration-dependent manner in the range of concentrations between 30 nM and 10 mu M, indicating t he most potent Ca2+ releaser in HSR. 3 The Ca2+ dependency of MYTX-ind uced Ca-45(2+) release has a bell-shaped profile but it was quite diff erent from that of caffeine, an inducer of Ca2+-induced Ca2+ release. 4 Ca-45(2+) release induced by MYTX was remarkable in the range of pCa between 8 and 3, whereas that by caffeine was prominent in the range of pCa, i.e., between 7 and 5.5. 5 MYTX-induced Ca-45(2+) release cons ists of both early and late components. The early component caused by MYTX at low concentrations (30-300 nM) completed within 20 s, while th e late component induced by it at higher concentrations (> 0.3 mu M) w as maintained for at least 1 min. 6 Both the components were almost co mpletely inhibited by inhibitors of Ca2+ release such as Mg2+, rutheni um red and spermine. 7 Ca-45(2+) release induced by caffeine or beta,g amma-methyleneadenosine 5'-triphosphate (AMP-PCP) was completely inhib ited by high concentrations of procaine. Procaine abolished the early component but not the late one, suggesting that at least the early com ponent is mediated through Ca2+-induced Ca2+ release channels. 8 On th e basis of these results, the character of Ca2+ release induced by MYT X was quite different from that caused by caffeine or AMP-PCP, suggest ing that MYTX induces Ca2+ release having novel properties in HSR. MYT X is the first polypeptide Ca2+ inducer and has become a useful pharma cological tool for clarifying the mechanism of Ca2+ release from skele tal muscle SR.