Effects of tetrodotoxin and low-sodium on action potential plateau of ventricular myocardium from neonatal and adult guinea-pigs.

We examined the effects of tetrodotoxin and reduced extracellular sodium concentrations on the myocardial action potential plateau in neonatal and adult guinea-pigs. Control action potential duration at 50% repolarization (APD50) was significantly shorter in the neonate than in the adult. Tetrodotoxin and low sodium shortened APD50 more prominently in the adult than in the neonate. Since a late sodium current with slow inactivation kinetics is reported in the ventricular myocytes, the results suggest that the late sodium current might contribute to the development prolongation of the action potential duration observed in the guinea-pig myocardium.