Intracellular free (cytosolic)
calcium has been reported to be increased in the platelets of patients with
essential hypertension. We investigated the possibility that the high cytosolic
calcium concentration may be caused by a
circulating plasma factor, by incubating platelets from normotensive subjects with plasma ultrafiltrates from patients with
essential hypertension. The cytosolic
calcium concentration in normal platelets increased after incubation with plasma from patients with untreated
hypertension (80 +/- 15 percent [+/- SEM]) or from patients in whom
hypertension was well controlled by
calcium-influx blockers (129 +/- 33 percent). In contrast, the cytosolic
calcium concentration was unchanged after incubation with plasma from normotensive subjects. When platelets from the patients were incubated with plasma from the controls, cytosolic
calcium in platelets decreased by more than 30 percent, into the normal range (P less than 0.01). These data demonstrate that plasma from patients with
essential hypertension contains a substance that increases the cytosolic
calcium concentration in platelets. Cytosolic
calcium is a trigger for vascular smooth-muscle-cell contraction, and if the plasma factor acts on these cells as it acts on platelets, it may be responsible for the increased peripheral vascular resistance associated with
hypertension.