Abstract
The verapamil-type calcium antagonist, D600, and its charged quaternary derivative, D890, were used to assess the sidedness of blockade in single calcium channels reconstituted from purified transverse tubules of skeletal muscle. Spontaneous single channel openings were induced with the agonist Bay-K8644 and recordings were made in a two-chamber planar bilayer setup so that drugs could be delivered to either side of the channel. Micromolar drug addition resulted in a greater than 10-fold decrease in probability of open channel events (po) without a significant change in single channel currents. Changes in po occurred in parallel with changes in mean open time and both parameters could be titrated with a similar IC50. At pH 7.2, cis or trans D600 blocked with an IC50 of 5 microM but for D890 the IC50 was cis 3 microM and trans greater than 75 microM (cis is the intracellular-equivalent side as defined by the voltage-dependent activation). The asymmetry of D890 blockade indicates that the drug can readily gain access to the blocking site from the aqueous phase adjacent to the inner but not extracellular end of the channel.