tokens: High doses of isoniazid increase hypotension induced by vasodilators and change the accompanying reflex tachycardia to bradycardia , an interaction attributed to decreased synthesis of brain gamma-aminobutyric acid ( GABA ). In the present study , the possible enhancement by isoniazid of bradycardia induced by beta - adrenoceptor antagonists was determined in rats anaesthetised with chloralose - urethane .