[Effects of lidocaine and its metabolite, monoethylglycine xylidide (MEGX), on transmembrane action potentials in guinea-pig papillary muscles].

Effects of lidocaine 5, 10, 20 micrograms.ml-1 and its metabolite, MEGX 5, 10, 20 micrograms.ml-1, on transmembrane action potentials were studied in isolated guinea-pig papillary muscles, perfused with modified Tyrode's solution. The basic driving rate was 1 Hz and the rate was changed from 1 Hz to 0.25, 0.5, 2, 3 and 4 Hz in a stepwise manner. Both lidocaine and MEGX produced dose- and rate-dependent depression of maximum rate of rise of action potential (Vmax) without significant changes in resting membrane potentials. The recovery kinetics of Vmax was studied by either applying premature stimuli at basic driving rate of 0.25 Hz or by stopping the basic driving of 1 Hz for 1s to 60s. A slow component (time constant: 176-206 ms for lidocaine, 300-340 ms for MEGX) was observed in premature response. And a slower component (time constant: 3-4s for MEGX) was observed in the first response after stopping stimulation. Both drugs shortened the action potential duration. These results suggest that MEGX has a lidocaine-like class I antiarrhythmic actions and that it might modify the actions of lidocaine especially in patients with elevated plasma MEGX concentration.