Separation of Na-Ca exchange and transient inward currents in heart cells

Enzymatically dispersed single cells from rabbit ventricle were voltage clamped using the suction pipette method to investigate whether in isolated cardiac cells a recently described slow inward current (IEX) due to the electrogenic Na+-dependent Ca2+ extrusion also underlies a transient inward current (ITI), which can trigger certain cardiac arrhythmias. The cells were held at -40 mV to inactivate the fast sodium current. After depolarizing pulses (to 0 or +10 mV for 50 to 200 ms), slow inward "tail" currents were consistently recorded. Previous results indicate that this tail current IEX is generated by the Na+-Ca2+ exchanger. After loading the cells with Ca2+ by blocking the Na+-K+ pump [either with strophanthidin (10(-5) M) treatment or by reducing external K+ to 1 mM or less], ITIS appeared. These were usually spontaneous but occasionally were time locked to the clamp pulses. It was possible to separate IEX and ITI by a variety of methods. These include the following. 1) Different stimulation protocols; repolarizing to more negative potentials augmented IEX and decreased or eliminated ITI. Increasing the rate of stimulation diminished IEX and increased ITI. 2) Pharmacological methods; adding BaCl2 (0.5-2.0 mM) or caffeine (5-10 mM) decreased IEX but abolished ITI. The findings suggest that different mechanisms regulate these two currents.