Permeability to various cations of the voltage-dependent sodium channel of rat single heart cells.

The permeability to various cations of the voltage-dependent sodium channel of rat single heart cells has been studied under the current or voltage clamp conditions. The cells, dispersed by collagenase treatment, were perfused internally using a suction pipette technique. The permeability sequence, estimated from the effects of various cations on the maximum rate of rise of action potential or from either the peak amplitude or the reversal potential of the inward current was Na+ greater than Li+ greater than hydrazine greater than guanidine greater than formamidine greater than hydroxylamine greater than methylguanidine greater than monomethylamine. The ionic permeability of a compound was markedly diminished by methylation. All inorganic and organic cation currents were reduced in the presence of tetrodotoxin, a specific Na+ channel blocker. It would appear that the ionic selectivity of the sodium channel of rat single heart cells is similar to the selectivity in the squid axon and of that in the myelinated nerve of the frog.