EFFECTS OF TETRODOTOXIN AND LOW-SODIUM ON ACTION-POTENTIAL PLATEAU OFVENTRICULAR MYOCARDIUM FROM NEONATAL AND ADULT GUINEA-PIGS

We examined the effects of tetrodotoxin and reduced extracellular sodi um concentrations on the myocardial action potential plateau in neonat al and adult guinea-pigs. Control action potential duration at 50% rep olarization (APD(50)) was significantly shorter in the neonate than in the adult. Tetrodotoxin and low sodium shortened APD(50) more promine ntly in the adult than in the neonate. Since a late sodium current wit h slow inactivation kinetics is reported in the ventricular myocytes, the results suggest that the late sodium current might contribute to t he developmental prolongation of the action potential duration observe d in the guinea-pig myocardium.