Modulation of P-gp transport may lead to increased drug accumulation in normal tissues including heart with resulting increases in toxicity.We aimed to investigate the influence of verapamil (a P-gp inhibitor)-bufadienolides interaction on cardiotoxicity and bufadienolide uptake by the isolated heart.The study was performed in Langendorff isolated perfused guinea-pig hearts by bufadienolides infusion in the absence and presence of verapamil (250,500 ng/ml).Arrhythmia parameters were evaluated by ECG and the content of bufadienolides in heart were measured by ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS).In the present of verapamil,the bufadienolide-induced wide QRS duration was markedly reduced,while the bufadienolide-induced ECG changes characterized by prolonged P-R interval,slow heart rate and low QRS amplitude were significantly enhanced.Furthermore,the contents of a variety of bufadienolide compounds in heart were significantly higher after verapamil treatment.Although verapamil reduced the bufadienolide-induced ventricular arrhythmias,verapamil worsened heart block and lethal bradycardia of bufadienolides partly via increasing the uptake of bufadienolides in heart tissue,which could compromise the protective effects of verapamil against bufadienolide intoxication.These results suggested that the P-gp inhibitors may produce dangerous interactions with drugs containing bufadienolides.