3,4-diaminopyridine. A potent new potassium channel blocker

3,4-diaminopyridine has been found to act very potently in selectively blocking the potassium channels of squid axon membranes. The apparent dissociation constants for this action are estimated to be 5.8 micron and 0.7 micron for external and internal applications, respectively, the potency being about 50 times higher than that of 4-aminopyridine. The block depends upon the membrane potential, time, and stimulus frequency. 3,4-diaminopyridine shows great promise as a useful tool for the study of membrane ionic channels.