Serotonin vascular reactivity and other clinical pharmacological investigations in two intestinal carcinoid patients

In two patients with carcinoid syndrome, a reduced venoreactivity to serotonin (5-HT) was found by means of the venoconstriction test. Parachlorophenylalanine (PCPA) treatment increased 5-HT venoreactivity. A normal noradrenaline venoreactivity was detected before and during PCPA administration. The altered platelet monoamine oxidase (MAO) activity corrected by PCPA treatment. The abnormal 5-HT turnover may alter the synthesis of MAO and provoke the production of isoenzymes. Low plasma amine oxidase and high plasma histaminase activity were found in 1 of these 2 patients. A low level of plasma prekallikrein in the two patients remained unchanged during PCPA administration.