The effect of phentolamine (regitine) on the in vitro release of catecholamines from adrenal medulla has been studied. Firstly, a continuous flow incubation (perifusion) system was developed in which secretory responses of adrenal medulla to acetylcholine were characterized by serial fluorimetric assay of catecholamine in the effluent medium. There was an initial massive release of catecholamine, which in the absence of acetylcholine, declined to a low basal level . The basal level was stable.

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... ollowing 2.5 h preincubation, catecholamine release rose abruptly after addition of 10'M acetylcholine to the medium and returned promptly to baseline after withdrawal. Pretreatment of pig adrenal medulla with 10'M phentalamine blocked the increase of catecholamine overflow by 10-2M acetylcholine . It was also found that phentolamine, itself, caused no effect on catecholamine release from adrenal medulla. These results show that phentolamine blocked the catecholamine release from perifused pig adrenal medulla in response to acetylcholine infusion .