tokens: In isolated perfused heart preparations from isoproterenol - pretreated rats , the isoproterenol - induced maximal increase in left ventricular systolic pressure was significantly reduced , compared with saline - pretreated rats ( the EC50 of the isoproterenol - induced increase in left ventricular systolic pressure was enhanced approximately 22 - fold ). These results show that 15 - day isoproterenol pretreatment not only abolished but reversed bromocriptine - induced tachycardia to bradycardia , an effect that is mainly related to further cardiac beta - adrenoceptor desensitization rather than to impairment of autonomic regulation of the heart .