Ryanodine activation and inhibition of the Ca2+ release channel of sarcoplasmic reticulum.
The effect of the plant alkaloid ryanodine on the skeletal muscle sarcoplasmic reticulum Ca2+ release channel was studied by determining the Ca2+ permeability of "heavy" vesicles passively loaded with 45Ca2+ in the presence or absence of ryanodine. Depending on the experimental conditions, ryanodine either stimulated or inhibited Ca2+ efflux. Vesicles were rendered permeable to 45Ca2+ at a ryanodine concentration of 0.01 microM when diluted into a medium containing the two Ca2+ release channel inhibitors Mg2+ and ruthenium red. At ryanodine concentrations greater than 10 microM, 45Ca2+ efflux was inhibited in channel-activating (5 microM Ca2+) or -inhibiting (10 mM Mg2+ plus 10 microM ruthenium red) media. An optimal stimulatory effect was observed when vesicles were incubated with ryanodine at 37 degrees C and in media that caused partial opening of the channel. Similar results to those described above were obtained using cardiac sarcoplasmic reticulum vesicles that were capable of rapid 45Ca2+ efflux. Use of the slowly permeating molecule L-[3H]glucose allowed measurement of channel-mediated efflux rates from vesicles in the presence and absence of ryanodine. At low activating concentrations, ryanodine did not appreciably change the regulation of L-glucose efflux rates by external Ca2+, Mg2+, and adenine nucleotide. These results suggested two possible modes of action of ryanodine: 1) a change in the gating mechanism of the channel which is not readily detected using the slowly permeating molecule L-glucose or 2) a change in channel structure which prevents its complete closing.
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D000227:Adenine Nucleotides / Q000378:metabolism; D000470:Alkaloids / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D005947:Glucose / Q000378:metabolism; D007473:Ion Channels / Q000187:drug effects* / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism; D009132:Muscles / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism; D010539:Permeability; D011817:Rabbits; D012430:Ruthenium Red / Q000378:metabolism; D012433:Ryanodine / Q000494:pharmacology*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*
D000227:Adenine Nucleotides; D000470:Alkaloids; D007473:Ion Channels; D012430:Ruthenium Red; D012433:Ryanodine; D008274:Magnesium; D005947:Glucose; D002118:Calcium