Increased plasma serotonin in primary pulmonary hypertension

Pulmonary hypertension can occur in patients who have disorders associated with altered platelet serotonin storage, including collagen vascular disease and platelet storage pool disease. We tested the hypothesis that primary pulmonary hypertension (PPH) may be also associated with impaired handling of serotonin by platelets, resulting in increased plasma serotonin levels.

We used radioenzymatic assays to measure serotonin in platelets and plasma and serotonin released during in vitro platelet aggregation in 16 patients with PPH, and in 16 normal controls matched for age and sex. Six patients were restudied after heart-lung transplantation to determine whether serotonin abnormalities persisted after pulmonary arterial pressure returned to normal.

Patients had decreased platelet serotonin concentration (1.8 +/- 0.6 x 10(-18) mol/platelet versus 3.2 +/- 0.2 x 10(-18) mol/platelet in controls; P < 0.01) and increased plasma serotonin concentration (30.1 +/- 9.2 x 10(-9) mol/L versus 0.6 +/- 0.1 x 10(-9) mol/L in controls; P < 0.001). Serotonin released during in vitro platelet aggregation was higher in patients than in controls. After heart-lung transplantation, platelet serotonin concentrations remained decreased and plasma levels remained increased.

Abnormal handling of serotonin by platelets leading to an increase in plasma serotonin occurs in PPH. The persistent decrease in platelet storage of serotonin after heart-lung transplantation suggests that this platelet abnormality is not secondary to PPH.