Block of Na,K-ATPase and Induction of Hybrid Death by 4-Aminopyridine in Cultured Cortical Neurons

K channel blockers such as 4-aminopyridine (4-AP) can be toxic to neurons; the cellular mechanism underlying the toxicity, however, is obscure. In cultured mouse cortical neurons, we tested the hypothesis that the toxic effect of 4-AP might result from inhibiting the Na,K-ATPase (Na,K-pump) and there-after induction of a hybrid death of concomitant apoptosis and necrosis. The Na,K-pump activity, monitored as whole-cell membrane currents, was markedly blocked by 4-AP in concen-tration- and voltage-dependent manners in low millimolar ranges. At similar concentrations, 4-AP induced a neuronal death sensitive to attenuation by the caspase inhibitor Z-VAD-FMK (Z-Val-Ala-Asp(OMe)-fluoromethyl ketone) or Ca2 chela-tor BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tet-raacetic acid-acetoxymethyl ester). Electron microscopy confirmed hybrid ultrastructural features of coexisting apopto-tic and necrotic components in same cells. We suggest that 4-AP is a potent antagonist of the Na,K-ATPase and an inducer of the hybrid death of central neurons. Aminopyridines, particularly 4-aminopyridine (4-AP), have been investigated as a means of beneficial symptomatic treatment in a variety of neurological conditions of demyeli-nation diseases including multiple sclerosis, myasthenia gra-vis, and spinal cord injury (Murray and Newsom-Davis