Abstract

Phenytoin is a hydantoin derivative that is used clinically for treatment of epilepsy and has been reported to have antiarrhythmic actions on the heart. In failing heart, the elevated diastolic Ca2+ leak from the sarcoplasmic reticulum can be normalised by the RyR2 inhibitor, dantrolene without inhibiting Ca2+ release during systole or affecting Ca2+ release in normal healthy hearts. Unfortunately, dantrolene is hepatotoxic and unsuitable for therapeutic use. Since phenytoin and dantrolene belong to the hydantoin class of compounds, we test the hypothesis dantrolene and phenytoin have similar inhibitory effects on RyR2 using single channel recording of RyR2 activity in artificial lipid bilayers. Phenytoin produced a reversible inhibition of RyR2 channels from sheep and human failing hearts. It followed a hyperbolic dose-response with maximal inhibition of ~ 50%, Hill coefficient ~ 1 and IC50 ranging from 10 to 20 μM. It caused inhibition at diastolic cytoplasmic [Ca2+] but not at Ca2+ levels in the dyadic cleft during systole. Notably, phenytoin inhibits RyR2 from failing human heart but not from healthy heart indicating that phenytoin may selectively target defective RyR2 channels in humans. We conclude that phenytoin could effectively inhibit RyR2 mediated release of Ca2+ in a manner paralleling that of dantrolene. Moreover, the IC50 of phenytoin in RyR2 is at least 3- fold lower for other ion transporters and clinically used serum levels, pointing to phenytoin as a more human-safe alternative to dantrolene for therapies against heart failure and cardiac arrythmias.