Ionic currents contributing to the action potential in single ventricular myocytes of the guinea pig studied with action potential clamp

With the action potential clamp procedure we studied the contribution of various ionic currents to the action potential in single ventricular myocytes. Action potentials were elicited by a current pulse through the suction pipette and recorded by a computer. A representative action potential was then repetitively replayed to the same cell under voltage-clamp conditions. Successive pharmacological blocks of ionic currents allowed for the first time the measurement of the contribution of the L-type calcium current (I_Ca) and the [Ca²⁺]_i-activated currents as well as the potassium current to the action potential. Experiments using caffeine as a tool to increase calcium release from the sarcoplasmic reticulum supported the idea that I_NaCa contributes to the plateau during the second half of the action potential and even lasts into diastole, whereas strong elevation of the intracellular [Ca]_i during the action potential additionally activated the non-specific cation channel.