Adrenergic control of serotonin release from carcinoid tumor cells in vitro and in vivo

Serotonin (5-HT)-producing human carcinoid tumors of midgut origin were transplanted to the anterior eye chamber of cyclosporine-treated rats. The release of 5-HT from in oculo transplants was studied after stimulation with adrenoceptor agonists applied locally to the eye. Chamber fluid was collected by micropuncture of the eye. 5-HT levels were determined by liquid chromatography with electrochemical detection. The release of 5-HT in cell suspensions of the same tumors was similarly studied after incubation with adrenoceptor agonists. In both experimental models activation of adrenoceptors caused release of 5-HT from carcinoid tumor cells. Individual variations in the type of response to adrenoceptor stimulation could be demonstrated for the different tumors. In two tumors there was good agreement between in vitro and in vivo findings with release of 5-HT at selective activation of beta-adrenoceptors with isoprenaline. In a third tumor there was a release of 5-HT at activation of alpha-adrenoceptors with norepinephrine in vitro. However, 5-HT release from this tumor in vivo was demonstrated at activation of beta-adrenoceptors. This finding may reflect different adrenoceptor populations on the tumor cell surface, each population activated to various degrees in the in vivo and in vitro situation.