Parathyroid hormone and platelet cytosolic calcium concentration in essential hypertension

To investigate the possible relationships between blood pressure,parathyroid hormone (PTH) and platelet cytosolic Ca²⁺ concentration([Ca²⁺]_i) in patients with essential hypertension, we studied17 patients with this disease aged 48±2 years and 17normotensive controls aged 44±3 years. Platelet [Ca²⁺]_iwas measured by spectrofluorimetry using the dye Fura-2 acetoxymethylester.Patients with essential hypertension displayed lower levelsof serum ionized Ca²⁺ (0.99±0.03 versus 1.1±0.01mmol/l, P<0.05) and higher serum intact PTH levels (37±3versus 26±2 pg/ml, P<0.01) than the normotensive controls.Although serum levels of intact PTH were significantly correlatedwith mean arterial pressure (MAP) in the combined group of subjects(r=0.42, P<0.05), there was no correlation when each groupwas considered separately. Resting platelet [Ca²⁺]_i was alsohigher in patients than in controls (57±3 versus 48±2nmol/l,P<0.005). When platelets were stimulated in vitro with thrombin,the increment in [ca²⁺]_i was also greater in patients than incontrols in the presence of extracellular Ca²⁺ (264±24versus 194±19 nmol/l, P<0.05) but not in its absence(123±12 versus 112±10 nmol/l). The thrombin-inducedincrement in [ca²⁺]_i was correlated with MAP in the hypertensivepatients (r=0.64, P<0.01) and in the combined group of subjects(r=0.42, P<0.05). There was no relationship between restingor thrombin-induced [Ca²⁺]_i and serum PTH in either group ofpatients or in the combined group of subjects. Furthermore,preincubation of platelets in vitro with PTH had no effect neitheron resting platelet [Ca²⁺]_i or on thrombin-induced incrementsin [ca²⁺]_i, both in the presence and in the absence of extracellularCa²⁺ in either group of subjects. Therefore, despite platelet[Ca²⁺]_i and serum PTH seem to be related to blood pressure,it appears that PTH does not participate directly in the elevatedplatelet [Ca²⁺]_i found in patients with essential hypertension.