Modulation of Adrenoceptor-Mediated Cardiovascular Effects by Short-Term In Vivo Infusion of Isoproterenol in Rats

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Abstract
After a 16-h in vivo infusion period of isoproterenol (400 .mu.g/kg/h) from a minipump implanted subcutaneously (s.c.) in rats, we observed an increase in heart weight due to tissue edema. In isolated perfused heart preparations, the EC50s of isoproterenol to induce positive inotropy and increase in coronary flow were increased .apprx.7 and 4 times, respectively. Isoproterenol dose-response curves performed in trachea preparations, were shifted to the right by about fivefold as compared with the control group. In the presence of phentolamine, the isoproterenol induced maximal relaxation in the isolated aorta from isoproterenol-pretreated animals was reduced from 46.5 to 9.2% as compared with saline-pretreated rats. In the absence of phentolamine, the epinephrine (EPI) and norepinephrine (NE) cumulative dose-response curves in this preparation were not affected by isoproterenol pretreatment with respect to EC50 and maximal effect. However, in the presence of phentolamine, the contractile response to a supramaximal dose of NE (10 .mu.M) amounted to 30 and 70% in the saline- and isoproterenol-pretreated rats, respectively. In the presence of both phentolamine and propranolol, a similar response of 70% was observed by this supramaximal dose of agonist in the saline-pretreated rats as well. In anesthetized rats, 120 min after removal of the minipumps, sodium nitroprusside induced increase in heart rate (HR) was reduced after isoproterenol pretreatment, whereas for salbutamol the decrease in diastolic blood pressure (DBP) was also reduced. As compared with salbutamol, a marked increase was observed in the ratio of mean arterial blood pressusure (MAP) to HR for SNP after isoproterenol pretreatment. The phenylephrine-induced increase in MAP was increased after isoproterenol pretreatment. We conclude that in the pathogenesis of heart failure the .beta.-adrenoceptor-mediated effects of catecholamines are attenuated and .alpha.-adrenoceptor-mediated effects become progressively important.