Site of action and active form of aminopyridines in squid axon membranes

Aminopyridines, potent potassium channel blocking agents, were studied for their site of action and active form in the nerve membrane. Voltage clamped, internally perfused squid giant axons were used. 3,4-Diaminopyridine, one of the most potent aminopyridine derivatives, blocked the potassium current much faster with internal application than with external application. When applied externally to the internally perfused axon, the onset of 3,4-diaminopyridine block was accelerated by suspending the internal flow. 4-Aminopyridine methiodide, a quaternary derivative of 4-aminopyridine, blocked the potassium current more effectively by internal application than by external application. These observations support the notion that aminopyridines act on a site more easily accessible from inside the nerve membrane than from outside. The block of the potassium current caused by internal 4-aminopyridine methiodide, similar to that caused by 4-aminopyridine or 3,4-diaminopyridine, was voltage-, time- and frequency-dependent, becoming less with longer and prolonged depolarization and with repetitive depolarizations. Low internal pH, which accelerated block re-establishment by the tertiary derivative, did not affect the quaternary derivative. Furthermore, when perfused internally at different total concentrations and different internal pH values, 2,3-diaminopyridine exhibited the same degree of block as long as the internally present cationic form concentration was kept constant. These results indicate that aminopyridines act in the cationic form.