EFFECT OF RYANODINE ON CARDIAC CALCIUM CURRENT AND CALCIUM-CHANNEL GATING CURRENT

The effects of 100 mu M ryanodine on the L-type calcium channel were s tudied using the pacth-clamp technique in isolated guinea pig ventricu lar myocytes. The inactivation kinetics of the calcium current were sl owed down in the presence of ryanodine in agreement with the blockade of the release of calcium from the sarcoplasmic reticulum by the drug. The I-V and steady-state inactivation curves of the calcium current w ere shifted to negative values by ryanodine. A similar shift was obser ved in the activation and inactivation curves of the intramembrane cha rge movement associated with the calcium channel. Due to this shift, r yanodine slightly reduced the maximal amount of displaced charge altho ugh it did not modify the transition from the inactivated to the activ ated state (i.e., charge movement repriming). This result is in notabl e contrast with that obtained in skeletal muscle, where it has been fo und that ryanodine interferes with charge movement repriming. These re sults provide additional evidence of the postulated differences betwee n the architecture of the excitation-contraction coupling system in ca rdiac and skeletal muscle.