Common P-glycoprotein inhibitors include quinidine, ritonavir, and ketoconazole. Quinidine may interact with chlorpromazine to increase myocardialdepression. These were the cardiac glycosides, quinidine, propranolol, lidocaine and diphenylhydantoin. The bark contains alkaloids, including quinine and quinidine. Pindolol also shows membrane-stabilizing effects like quinidine, possibly accounting for its antiarrhythmic effects. Quinidine is a direct derivative of quinine. Loperamide taken with quinidine was found to produce respiratory depression, indicative of central opioid action. Quinimax is a combination of four alkaloids (quinine, quinidine, cinchoine and cinchonidine). Postsynaptic slow-channel CMS is treated with quinidine or fluoxetine, which plugs the ACh receptor. Quinidine sulfate is used in the treatment of atrial fibrillation in horses. Quinidine also increases levels of amikacin in the body. As a result, procaine was discovered, which has similar cardiac effects as quinidine. Methionine is also known to increase the urinary excretion of quinidine by acidifying the urine. Other alkaloids that are extracted from this tree include cinchonine, cinchonidine and quinidine. Quinidine acts as a blocker of voltage-gated sodium channels.