Organic inhibitors of calcium influx prevent outward as well as inward current through cardiac calcium channels but do not slow current activation. Although block is antagonized by raising external calcium or barium concentrations, the competitive effect of permeant cations does not occur at the same cation binding site at which inorganic blockers act. Organic drugs show varying degrees of use-dependent block, due in part to blockade of open channels. Nitrendipine blockade of calcium currents requires doses >100-fold higher than expected from radioligand binding to isolated membranes.