Decreased uptake of 3H-serotonin and endogenous content of serotonin in blood platelets in hypertensive patients.

The uptake and content of serotonin in blood platelets were studied in patients with essential hypertension and in five families in which at least one member was hypertensive. Blood was obtained from male and female normotensive volunteers and hypertensive patients who were free of medication. Lineweaver-Burk plots of 3H-serotonin uptake from both control subjects and hypertensive patients were linear, which suggested simple Michaelis-Menten uptake kinetics. The maximal uptake velocity (Vmax) in hypertensive patients was significantly lower than in control subjects (control = 41.7 +/- 3.3 pmol/min/10(8) platelets, n = 17; hypertensive = 26.6 +/- 3.0 pmol/min/10(8) platelets, n = 16; p less than 0.005). The affinity constant (Km) was slightly but significantly lower in hypertensive patients (control = 0.70 +/- 0.08 microM; hypertensive = 0.46 +/- 0.08 microM; p less than 0.05). The serotonin content in blood platelets determined by high pressure liquid chromatography with electrochemical detection was significantly lower in hypertensive patients (control = 165.0 +/- 12.9 nmol/10(11) platelets, n = 29; hypertensive = 105.9 +/- 10.4 nmol/10(11) platelets, n = 27; p less than 0.001). In the five families investigated, the lowered serotonin content was observed in some normotensive members. The reduced number of carriers of serotonin uptake and the slight decrease in the affinity constant observed in platelets of patients with essential hypertension suggest that serotonin metabolism is altered in essential hypertension and that blood platelets may be a useful model in studying the serotonergic modifications at the molecular level.