Methylene blue counteracts cyanide cardiotoxicity: Cellular mechanisms

In adult left ventricular mouse myocytes, exposure to sodium cyanide (NaCN) in the presence of glucose dose-dependently reduced contraction amplitude, with ∼80% of maximal inhibitory effect attained at 100 μM. NaCN (100 μM) exposure for 10 min significantly decreased contraction and intracellular Ca²⁺ concentration ([Ca²⁺]_i) transient amplitudes, systolic but not diastolic [Ca²⁺]_i, and maximal L-type Ca²⁺ current (I_Ca) amplitude, indicating acute alteration of [Ca²⁺]_i homeostasis largely accounted for the observed excitation-contraction abnormalities. In addition, NaCN depolarized resting membrane potential (E_m), reduced action potential (AP) amplitude, prolonged AP duration at 50% (APD₅₀) and 90% repolarization (APD₉₀), and suppressed depolarization-activated K⁺ currents but had no effect on Na⁺-Ca²⁺ exchange current (I_NaCa). NaCN did not affect cellular adenosine triphosphate levels but depolarized mitochondrial membrane potential (Δψ_m) and increased superoxide (O₂^-) levels. Methylene blue (MB; 20 μg/ml) added 3 min after NaCN restored contraction and [Ca²⁺]_i transient amplitudes, systolic [Ca²⁺]_i, E_m, AP amplitude, APD₅₀, APD₉₀, I_Ca, depolarization-activated K⁺ currents, Δψ_m, and O₂^- levels toward normal. We conclude that MB reversed NaCN-induced cardiotoxicity by preserving intracellular Ca²⁺ homeostasis and excitation-contraction coupling (I_Ca), minimizing risks of arrhythmias (Em, AP configuration, and depolarization-activated K⁺ currents), and reducing O₂^- levels. NEW & NOTEWORTHY Cyanide poisoning due to industrial exposure, smoke inhalation, and bioterrorism manifests as cardiogenic shock and requires rapidly effective antidote. In the early stage of cyanide exposure, adenosine triphosphate levels are normal but myocyte contractility is reduced, largely due to alterations in Ca²⁺ homeostasis because of changes in oxidation-reduction environment of ion channels. Methylene blue, a drug approved by the U.S. Food and Drug Administration, ameliorates cyanide toxicity by normalizing oxidation-reduction state and Ca²⁺ channel function.