Effects of boldine on mouse diaphragm and sarcoplasmic reticulum vesicles isolated from skeletal muscle

The effects of boldine [(S)-2,9-dihydroxy-1,10-dimethoxyaporphine], a major alkaloid in the leaves and bark of boldo (Peumus boldus Mol.), on skeletal muscle were studied using mouse diaphragm and isolated sarcoplasmic reticulum membrane vesicles. Boldine, at 10-200 μM, has little effect on the muscle-evoked twitches; however, the ryanodine-induced contracture was potentiated dose-dependently. At higher concentrations of 300 μM, boldine by itself induced muscle contracture of two phases, which were caused by the influx of extracellular Ca²⁺ and induction of Ca²⁺ release from the internal Ca²⁺ storage site, the sarcoplasmic reticulum, respectively When tested with isolated sarcoplasmic reticulum membrane vesicles,boldine dose-dependently induced Ca²⁺ release from actively loaded sarcoplasmic reticulum vesicles isolated from skeletal muscle of rabbit or rat which was inhibited by ruthenium red, suggesting that the release was through the Ca²⁺ release channel, also known as the ryanodine receptor. Boldine also dose-dependently increased apparent [³H]-ryanodine binding with the EC₅₀ value of 50 μM. In conclusion, we have shown that boldine could sensitize the ryanodine receptor and induce Ca²⁺ release from the internal Ca²⁺ storage site of skeletal muscle.