Mechanism of block by 4-aminopyridine of the transient outward current in rat ventricular cardiomyocytes

The effects of 4-aminopyridine (4-AP) on the transient outward current (I(to)) were investigated in rat ventricular cardiomyocytes at different values of intracellular pH (pH(i)) and extracellular pH (PH(o)). The 4-AP was administered either extracellularly (bath application) or intracellularly (diffusion from the intrapipette solution). The 4-AP diminished I(to) given either from inside or outside the cell membrane. The block by extracellularly applied 4-AP (4-AP(o)) of the peak amplitude of I(to) was decreased by external acidification but increased by external alkalinization; conversely, the block by 4- AP(o) was decreased by internal alkalinization but increased by internal acidification. Intracellularly applied 4-AP (3 mM) was more effective at low pH(i). Because 4-AP is a tertiary amine and exists in protonated and unprotonated forms, these results are in agreement with the assumption that one major mechanism for 4-AP to block I(to) is to penetrate the cell membrane in its uncharged form and to reach intracellular binding sites in its protonated form.