ELECTROPHYSIOLOGICAL EFFECTS OF RYANODINE DERIVATIVES ON THE SHEEP CARDIAC SARCOPLASMIC-RETICULUM CALCIUM-RELEASE CHANNEL

We have examined the effects of a number of derivatives of ryanodine o n K+ conduction in the Ca2+ release channel purified from sheep cardia c sarcoplasmic reticulum (SR). In a fashion comparable to that of ryan odine, the addition of nanomolar to micromolar quantities to the cytop lasmic face (the exact amount depending on the derivative) causes the channel to enter a state of reduced conductance that has a high open p robability. However, the amplitude of that reduced conductance state v aries between the different derivatives. In symmetrical 210 mM K+, rya nodine leads to a conductance state with an amplitude of 56.8 +/- 0.5% of control, ryanodol leads to a level of 69.4 +/- 0.6%, ester A ryano dine modifies to one of 61.5 +/- 1.4%, 9,21-dehydroryanodine to one of 58.3 +/- 0.3%, 9 beta,21 beta-epoxyryanodine to one of 56.8 +/- 0.8%, 9-hydroxy-21-azidoryanodine to one of 56.3 +/- 0.4%, 10-pyrroleryanod ol to one of 52.2 +/- 1.0%, 3-epiryanodine to one of 42.9 +/- 0.7%, CB Z glycyl ryanodine to one of 29.4 +/- 1.0%, 21-p-nitrobenzoyl-amino-9- hydroxyryanodine to one of 26.1 +/- 0.5%, beta-alanyl ryanodine to one of 14.3 +/- 0.5%, and guanidino-propionyl ryanodine to one of 5.8 +/- 0.1% (chord conductance at +60 mV, +/- SEM). For the majority of the derivatives the effect is irreversible within the lifetime of a single -channel experiment (up to 1 h). However, for four of the derivatives, typified by ryanodol, the effect is reversible, with dwell times in t he substate lasting tens of seconds to minutes. The effect caused by r yanodol is dependent on transmembrane voltage, with modification more likely to occur and lasting longer at +60 than at -60 mV holding poten tial. The addition of concentrations of ryanodol insufficient to cause modification does not lead to an increase in single-channel open prob ability, such as has been reported for ryanodine. At concentrations of greater than or equal to 500 mu M, ryanodine after initial rapid modi fication of the channel leads to irreversible closure, generally withi n a minute. In contrast, comparable concentrations of beta-alanyl ryan odine do not cause such a phenomenon after modification, even after pr olonged periods of recording (>5 min). The implications of these resul ts for the site(s) of interaction with the channel protein and mechani sm of the action of ryanodine are discussed. Changes in the structure of ryanodine can lead to specific changes in the electrophysiological consequences of the interaction of the alkaloid with the sheep cardiac SR Ca2+ release channel.