Ryanodine is a cell permeant ryanodine receptor modulator. Ryanodine can either stimulate or inhibit Ryanodine-mediated Ca2+ release depending on its concentrations. Poisonous diterpenoid found in Ryania speciosa.
Ryanodine (100-5000 nM, 30-120 mintutes) irreversibly depresses twitch and tetanic tension of both fast and slow muscle in a dose-related manner[1]. At concentrations above 250 nM, ryanodine induces a slowly developing, dose dependent contracture which could not be blocked by 5 mMCo2+[1].
References:
[1]. Meissner G, et al. Ryanodine activation and inhibition of the Ca2+ release channel of sarcoplasmic reticulum. J Biol Chem. 1986 May 15;261(14):6300-6.
[2]. Fryer MW, et al. The action of ryanodine on rat fast and slow intact skeletal muscles. J Physiol. 1989 Jul;414:399-413.