M. B. Reznikova

The rate of calcium transport and calmodulin distribution in the erythrocytes of patients with essential hypertension were studied. In erythrocyte membranes subjected to calmodulin depletion by treatment with EGTA, both the affinity of the calcium pump for Ca2+ and its maximal activity were the same in normotensive and hypertensive patients. The addition of exogenous calmodulin to calmodulin-stripped membranes from erythrocytes of patients with essential hypertension resulted in a smaller increase of the maximal activity of the calcium pump and its affinity for Ca2+. The addition of calmodulin to erythrocyte membranes obtained without EGTA treatment resulted in a smaller increase of the maximal activity of the calcium pump only. There were no significant differences of calmodulin distribution (cytoplasmic concentration and size of the membrane-bound pool) between the erythrocytes of normotensive and hypertensive patients. It is suggested that alterations in the calcium pump activity of the erythrocyte membranes of patients with essential hypertension are related to the alteration of interaction between calmodulin and Mg2+, Ca2+-ATPase.

1. Calmodulin distribution in rat erythrocytes and the interrelation between calmodulin and EGTA in regulation of the rate of 45Ca accumulation by erythrocyte-membrane inside-out vesicles were studied. 2. The total content of calmodulin in rat erythrocytes is about 24 mumol/l of cells. About 60% of calmodulin is localized in cytoplasm and 40% (10 mumol/l of cells) of calmodulin is loosely bound to membranes; after subsequent washing by hypotonic solution (membranes A) the content of membrane-bound calmodulin decreases up to 0.1 mumol/l of cells. 3. The addition of exogenous calmodulin to membranes A results in increase of the maximal activity of the Ca-pump and does not influence its affinity to Ca2+. Troponin I (30 microM) completely abolishes the calmodulin effect on the Ca-pump activity without significant alterations in its basal activity. 4. The addition of EGTA in the membrane-washing solution results in decrease of the membrane-found pool calmodulin up to 0.01 mumol/l of cells (membranes B). This procedure is accompanied by decrease of the affinity of Ca-pump to Ca2+ and does not influence its maximal rate. 5. The effect of EGTA treatment (membranes B) on the affinity of Ca-pump to Ca is abolished after addition of micromolar concentrations of calmodulin or millimolar concentration of EGTA in the incubation medium. The increase of EGTA concentration in the incubation medium results in decrease of the affinity of Ca-pump to calmodulin. 6. It is assumed that two essentially different pools of calmodulin participate in the regulation of the activity of Ca-pump. (a) The pool of calmodulin which is loosely bound to membrane (this size is dependent on calmodulin concentration in cytoplasm) determines the maximal activity of Ca-pump. The effect of this calmodulin pool is blocked by troponin I. (b) The tightly bound pool of calmodulin which is removed by EGTA treatment determines the affinity of Ca-pump to Ca. 7. In this connection the reasons for contradictory data on estimation of calmodulin effect on the kinetic parameters of plasma membrane Ca-pump are discussed.

Sialic acids of erythrocyte membranes (erythrocyte 'ghosts') and blood plasma were studied in patients with essential or chronic renal hypertension, and in spontaneously hypertensive rats (SHR). The total content of sialic acids in erythrocyte membranes (determined by thiobarbituric-acid assay) was around 100 mumol/g protein in both the hypertensive patients and controls, there being no difference between the three groups. Similarly total sialic-acid content of plasma did not differ between the hypertensive patients and controls, being around 2 mmol/l. Although total membrane sialic acid was unchanged, the sialic-acid content of glycolipids extracted from erythrocyte membranes was 17% greater (P less than 0.001) in patients with essential hypertension than in renal hypertensive patients and controls (22.1 +/- 0.5 mumol/g protein v. 18.2 +/- 0.8 and 18.9 +/- 0.8, respectively). Sialic acid in plasma glycolipid did not differ between the patient groups. The animal study revealed no differences between total--or glycolipid--sialic-acid content in plasma and erythrocyte membrane in SHR and normotensive rats. The finding of an increase in the sialic-acid content of erythrocyte membrane glycolipid in essential hypertension is in agreement with recent studies demonstrating structural abnormalities in hydrophobic regions of erythrocyte membrane, and is considered a manifestation of membrane glycolipid alteration in primary hypertension.