Ca2+ release induced by myotoxin alpha, a radio-labellable probe having novel Ca2+ release properties in sarcoplasmic reticulum.
1. Myotoxin alpha (MYTX), a polypeptide toxin purified from the venom of prairie rattlesnakes (Crotalus viridis viridis) induced Ca2+ release from the heavy fraction (HSR) but not the light fraction of skeletal sarcoplasmic reticulum at concentrations higher than 1 microM, followed by spontaneous Ca2+ reuptake by measuring extravesicular Ca2+ concentrations using the Ca2+ electrode. 2. The rate of 45Ca2+ release from HSR vesicles was markedly accelerated by MYTX in a concentration-dependent manner in the range of concentrations between 30 nM and 10 microM, indicating the most potent Ca2+ releaser in HSR. 3. The Ca2+ dependency of MYTX-induced 45Ca2+ release has a bell-shaped profile but it was quite different from that of caffeine, an inducer of Ca(2+)-induced Ca2+ release. 4. 45Ca2+ 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 45Ca2+ release consists of both early and late components. The early component caused by MYTX at low concentrations (30-300 nM) completed within 20 s, while the late component induced by it at higher concentrations (> 0.3 microM) was maintained for at least 1 min. 6. Both the components were almost completely inhibited by inhibitors of Ca2+ such as Mg2+, ruthenium red and spermine. 7. 45Ca2+ release induced by caffeine or beta,gamma-methyleneadenosine 5'-triphosphate (AMP-PCP) was completely inhibited by high concentrations of procaine. Procaine abolished the early component but not the late one, suggesting that at least the early component is mediated through Ca(2+)-induced Ca2+ release channels. 8. On the basis of these results, the character of Ca2+ release induced by MYTX was quite different from that caused by caffeine or AMP-PCP, suggesting that MYTX induces Ca2+ release having novel properties in HSR. MYTX is the first polypeptide Ca2+ inducer and has become a useful pharmacological tool for clarifying the mechanism of Ca2+ release from skeletal muscle SR.
Furukawa
K
K
Department of Pharmaceutical Molecular Biology, Pharmaceutical Institute Tohoku University, Sendai, Japan.
Funayama
K
K
Ohkura
M
M
Oshima
Y
Y
Tu
A T
AT
Ohizumi
Y
Y
D000255:Adenosine Triphosphate / Q000031:analogs & derivatives / Q000037:antagonists & inhibitors / Q000494:pharmacology; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism; D002110:Caffeine / Q000037:antagonists & inhibitors / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002132:Calcium Radioisotopes; D003435:Crotalid Venoms / Q000037:antagonists & inhibitors / Q000302:isolation & purification / Q000494:pharmacology*; D066298:In Vitro Techniques; D017736:Ion-Selective Electrodes; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism / Q000648:ultrastructure; D011343:Procaine / Q000494:pharmacology; D011817:Rabbits; D012433:Ryanodine / Q000493:pharmacokinetics; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*
D002132:Calcium Radioisotopes; D003435:Crotalid Venoms; D012433:Ryanodine; C005147:5'-adenylyl (beta,gamma-methylene)diphosphonate; D002110:Caffeine; D011343:Procaine; C018932:myotoxin A; D000255:Adenosine Triphosphate; D017301:Ca(2+) Mg(2+)-ATPase; C002630:alpha,beta-methyleneadenosine 5'-triphosphate; D002118:Calcium