Lidocaine effects on action potentials of Purkinje fibers from neonatal and adult dogs.

We studied the effects of lidocaine (L), 2 to 40 mg/l, on the cellular electrophysiologic properties of Purkinje fibers (PF) from neonatal (0-10 days) and adult dogs. Microelectrode impalements of neonatal and adult PF were selected so that there were no differences between the two groups in control maximum diastolic potential, action potential amplitude and maximum upstroke velocity of phase 0 depolarization (Vmax). However, control action potential duration was significantly shorter in neonates than in adults (P less than .05). At a superfusate [K+] = 4 mmol/1, the threshold concentration for L effects on action potential amplitude in neonates was 40 mg/1 and in adults, 20 mg/1; on Vmax in neonates threshold was 5 mg/1 and in adults was 2 mg/1; on action potential duration in neonates and adults threshold was 2 mg/1. For all variables studied the effect of L on adult PF action potentials was greater than on neonatal PF. The effects of L on action potential duration also suggest that potassium conductance may be greater in neonatal fibers. When superfusate [K+] was increased to 6 mM the effects of L on adult PF were potentiated more than on neonates. These results provide further experimental evidence of the age-related changes in sensitivity of the specialized conducting system to cardioactive drugs.