Study of the Ca2+ ATPase activity and intracellular electrolyte concentration in red blood cells and platelets of essential and secondary hypertensives

Abstract

Changes in transmembrane Ca2+ transport causing an increase in cytoplasmic free Ca2+ concentration may play a role in the pathogenesis of hypertension. Since cellular Ca2+ ATPase activity is crucial in the regulation of cytoplasmic free Ca2+, in the present study, changes in cellular Ca2+ ATPase activity in normotensives (NT, n=12), essential (EH, n=15) and renal hypertensives (RH, n=10, serum creatinine 6.7 ± 2.3 mg/dl) were studied. Measurements were performed in hemolysates, which were either untreated, or incubated with trifluperazine to inhibit calmodulin (maximal Ca2+ ATPase activity). Furthermore in the hemolysate free Ca2+ concentration was measured with a Ca2+ selective electrode. In platelets of normotensives and essential hypertensives cytosolic free Ca2+ concentration was measured using fura 2.Tab. 1 shows mean values and standard deviations of Ca2+ ATPase activity in mmol phosphate/min and 106 cells, and of free intracellular Ca2+ concentration (Ca2+, in µmot/I *p< 0.01, +p<0.05) and the cytosolic free Ca2+ in platelets (Ca2+ nmol/l, •p<0.02). Tab.1 Ca2+ ATPase act. untreated basal act. max. act. free Ca2+ (red cells) free Ca2+ (platelets) NT EH RH 15,1±2,1 15,3±4,7 13,8±7,1 7,1±3,6 9,0±5,3 6,1±5,2 16,0±2,3 35,4±14,4* 15,2±8,2 2,4±0,6 6,3±4,5* 3,6±0,9+ 72,2±41,1 100,2±56,3• The results thus show an increased maximal Ca2+ ATPase activity in essential hypertensives. This points to an increase in total number of Ca2+ ATPase molecules. The increased capacity for Ca2+ extrusion may be a compensatory reaction to an increased cytoplasmic free Ca2+ concentration in essential hypertensives. Conversely, in renal hypertensives no change in Ca2+ ATPase was observed. The measurements of maximum Ca2+ ATPase activity could therefore be useful for biochemical characterization of essential hypertensives.