Absence Of Ryanodine Research Articles

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.

The effects of ryanodine on model systems derived from muscle—II: Myofibrils and natural actomyosin

ATPase activity of myosin B and myofibrils was measured in calcium-buffered solutions at various concentrations of Mg 2+ and ATP in the presence and absence of ryanodine. No effect of ryanodine on myosin B ATPase was found except when ATP was present in excess over Mg 2+, when the inhibitory effect of excess ATP was suppressed by ryanodine. A small but consistent increase in myofibrillar ATPase was produced by ryanodine at [Ca 2+] = 10 −6 − 3 × 10 −5 M; this increase was more pronounced when ATP was present in excess over Mg ++. Ryanodine may act in part by stabilizing the actomyosin complex against the dissociating effect of ATP.