Platelet Ca2+ handling in essential hypertension: Role of a plasma ouabain-like factor

A humoral ouabain-like plasma factor has been observed in patients with ess ential hypertension (EHT). In the present study, we hypothesized that this humoral factor might be responsible for the elevated cytosolic free calcium concentrations [Ca2+](i) seen in these patients. Patients with mild to mod erate EHT and their normotensive first degree blood relatives (NTBR) partic ipated in the study. Platelet Na+, K+-ATPase activity was assayed in EHT pa tients and their NT first-degree relatives. To confirm the ouabain-like act ivity in plasma from EHT patients, control platelets were incubated with EH T and NTBR plasma and their Na+, K+-ATPase activity was measured. In additi on, the effect of EHT plasma on platelet Ca-45-uptake was studied. The in v itro effects of ouabain (10 mu M) on (i) Ca-45-uptake and (ii) [Ca2+](i) re sponse in control platelets were also observed. A decreased Na+K+-ATPase ac tivity (P < 0.05) was observed in platelet membranes from EHT patients. Inc ubation of control platelets with EHT plasma decreased their Na+, K+-ATPase activity (P < 0.01) and increased their Ca-45-uptake (P < 0.05). C-18 Sep- Pak filtered hypertensive plasma extracts (containing the ouabain-like frac tion) also decreased Na+, K+-ATPase activity (P < 0.01) in control platelet membranes. In vitro incubation of control platelets with ouabain increased Ca-45-uptake (P < 0.05) and [Ca2+](i) response (P < 0.05) in these platele ts. Thus it appears that an ouabain-like factor in the EHT plasma may contr ibute to the elevated platelet [Ca2+](i) observed in EHT patients.