High-dose oral intake of serotonin induces valvular heart disease in rabbits.

Carcinoid tumors are rare neuroendocrine malignancies, often originating from enterochromaffin cells in the gastrointestinal tract. They can secrete serotonin (5-hydroxytryptamine, 5-HT), which is largely inactivated by the liver. Carcinoid heart disease occurs when tumor cells metastasize to the liver, as the vasoactive substances produced are able to reach the systemic circulation via the hepatic vein,
causing deposition of fibrous tissue on the endocardial surfaces of the heart. It is predominantly manifested by right-sided valvular heart
disease (VHD). Scavenging enzymes in the pulmonary endothelium may explain why left-sided cardiac involvement is unusual. The
severity of cardiac damage is correlated with the plasmatic levels of serotonin, but the lowspecificity of serotonin for cardiac damage suggests that serotonin may be necessary but not sufficient to induce cardiac lesions. Therefore, other factors combined with serotonin might be required to induce VHD. However, recent animal studies confirmed the development of carcinoid-like valvular deposits in rats after 3 months of daily subcutaneous/intraperitoneal serotonin injections to avoid the liver first-pass clearance.Whether oral administration of serotonin can also induce VHD is unknown. We hypothesized that long-term oral serotonin overload in rabbits can lead to VHD, mimicking serotonin-induced lesions of carcinoid heart disease. We demonstrate, for the first time that high dose long-term oral administration of serotonin can lead to VHD in rabbits.