Extracellular site of action of phenylalkylamines on L-type calcium current in rat ventricular myocytes

The effects of the phenylalkylamines verapamil, gallopamil, and devapamil on L-type calcium currents (I_Ca) were studied in ventricular myocytes from rat hearts using the whole-cell patch-clamp technique. In particular, the question was addressed, whether the pharmacological binding sites for these drugs were located at the inner and/or at the outer surface of the cell membrane. Therefore, tertiary verapamil, gallopamil, and devapamil and their corresponding quaternary derivatives were applied either from the outside or the inside of the cell membrane. Extracellular application of verapamil, gallopamil and devapamil (each at 3 M) reduced Ica to 16.1 ±8.6%, 11 ± 8.9 %, and 9.3 ± 6 % of control, respectively. Intracellular application of the same substances, via the patch pipette filled with 30 M of either verapamil, gallopamil, or devapamil, failed to depress I_Ca. The quaternary derivatives of the phenylalkylamines (30 M) were ineffective both when applied extracellularly or intracellularly. It is suggested that phenylalkylamines block I_Ca in ventricular myocytes by acting on a binding site of the calcium channel molecule located at the outer surface of the cell membrane.