Charged local anesthetics block ionic conduction in the sheep cardiac sarcoplasmic reticulum calcium release channel

We have examined the effect of the charged local anesthetics QX314, QX222, and Procaine on monovalent cation conduction in the Ca2+ release channel of the sheep cardiac sarcoplasmic reticulum. All three blockers only affect cation conductance when present at the cytoplasmic face of the channel. QX222 and Procaine act as voltage-dependent blockers. With 500 Hz filtering, this is manifest as a relatively smooth reduction in single-channel current amplitude most prominent at positive holding potentials. Quantitative analysis gives an effective valence of approximately 0.9 for both ions and Kb(0)s of 9.2 and 15.8 mM for QX222 and Procaine, respectively. Analysis of the concentration dependence of block suggests that QX222 is binding to a single site with a Km of 491 microM at a holding potential of 60 mV. The use of amplitude distribution analysis, with the data filtered at 1 to 2 kHz, reveals that the voltage and concentration dependence of QX222 block occurs largely because of changes in the blocker on rate. The addition of QX314 has a different effect, leading to the production of a substate with an amplitude of approximately one-third that of the control. The substate's occurrence is dependent on holding potential and QX314 concentration. Quantitative analysis reveals that the effect is highly voltage dependent, with a valence of approximately 1.5 caused by approximately equal changes in the on and off rates. Kinetic analysis of the concentration dependence of the substate occurrence reveals positive cooperativity with at least two QX314s binding to the conduction pathway, and this is largely accounted for by changes in the on rate. A paradoxical increase in the off rate at high positive holding potentials and with increasing QX314 concentration at 80 mV suggests the existence of a further QX314-dependent reaction that is both voltage and concentration dependent. The substate block is interpreted physically as a form of partial occlusion in the vestibule of the conduction pathway giving a reduction in single-channel current by electrostatic means.