A calcium reversible action of barbiturates on the leech Retzius cell

The Na salts of phenobarbital, barbital, pentobarbital, secobarbital, methohexital and thiopental in concentrations of 1 to 5 mM all caused prolonged action potentials in leech Retzius cells. The prolongation was favored by low Ca and was reversible by elevation of Ca. Barbiturate-prolonged action potentials were not affected by 50 micronm tetrodotoxin or replacement of Cl by propionate, but they were dependent on external Na. Their amplitudes were increased by steady hyperpolarization and input resistance was reduced during them. They were of shorter duration when elicited at a rapid rate. They were sometimes shortened by intracellular iontophoresis of Ca and by use of the ionophore X537A, but technical factors complicated the interpretation of both kinds of experiments. The results are consistent with the hypothesis that the barbiturates used blocked a voltage-dependent inward Ca current which activates a K conductance necessary for normal repolarization.