FORCED SWIM STRESS - SUPERSENSITIVITY OF THE ISOLATED RAT PACEMAKER TO THE CHRONOTROPIC EFFECT OF ISOPRENALINE AND THE ROLE OF CORTICOSTERONE

1. Forced swim (three daily sessions) resulted in an increased plasma corticosterone level and supersensitivity of the isolated rat pacemake r to the chronotropic effect of isoprenaline. 2. Bilateral adrenalecto my, performed 2 days before forced swim, abolished the development of pacemaker supersensitivity to isoprenaline. 3. Administration to rats of the antiglucorticoid compound RU-38486 prevented the development of pacemaker supersensitivity to isoprenaline. Pretreatment of rats not submitted to forced swim with the synthetic glucocorticoid RU-28362 ca uses pacemaker supersensitivity to isoprenaline. 4. Pretreatment of ra ts with diazepam or imipramine which block the forced swim-induced inc rease in the plasma level of corticosterone prevented the development of pacemaker supersensitivity to isoprenaline. 5. It is concluded that corticosterone plays a critical role in the modulation of the sensiti vity to catecholamines of the pacemaker beta-adrenoceptors during adap tation to repeated stress.