Membrane currents were recorded from isolated rat venticular myocytes using the whole-cell configuration of the patch clamp technique.

Isolated cells were placed in a perfusion chamber (Warner Instruments) on the stage of an inverted microscope (Olympus IX71) and bathed in a K⁺-free extracellular solution containing (in mM) NaCl 137, CsCl 5.4, MgCl₂ 0.5, CaCl₂1.0, NaH₂PO₄ 0.33, glucose 5.5, and HEPES 5 (pH 7.4), maintained at room temperature.

Cells were patched using microelectrodes with resistances between 1 and 2 MΩ. Access resistance was compensated to obtain series resistance errors of less than 5 mV.

Cells were dialyzed with a K⁺-free microelectrode solution containing (in mM): CsCl 130, TEA-Cl 20, EGTA 5, MgATP 5, TrisGTP 0.06, and HEPES 5 (pH 7.2).

Whole cell currents were recorded under voltage-clamp conditions using a Multiclamp 700B voltage clamp amplifier, Digidata 1440A computer interface, and pClamp 11 data acquisition and analysis software (Molecular Devices). Data were lowpass filtered at 4 kHz, and sampled at 10 kHz.

The membrane potential was held at -80 mV. A 50 ms pre-pulse to -40 mV was used to inactivate Na⁺channels. This was followed by a 100 ms test pulse to 0 mV to elicit the L‑type Ca²⁺ current.

The time course of changes in the amplitude of the Ca²⁺ current was monitored by recording the amplitude of the peak inward current elicited during the test pulse to 0 mV applied once every 5 s.

A stable baseline (about 5 minutes) was obtained before application of test drugs.