Calcium, parathyroid hormone and phospholipid turnover of human red blood cells

PTH enhances entry of calcium into RBC, stimulates their Ca ATPase and increases osmotic fragility. The effect on the latter occurs only in the presence of calcium. The mechanisms of action of PTH and those mediating their increased calcium influx into RBC are not evident. It is possible that calcium, PTH or both affects phospholipid turnover of RBC and results in production of ionophoric compounds such as phosphatidic acid which in turn allows calcium movement. The present study examined the effect of calcium (1 mM), 1-84 PTH (50 U/ml) and calcium and PTH on phospholipid contents and 32P incorporation into phosphoinositides of human RBC and RBC ghosts. Calcium produced significant decrements in 32P incorporation into phosphatidylinositol, di- and triphosphatidylinositol and an increase into phosphatidic acid. PTH decreased the 32P into phosphatidylinositol only. Both PTH or calcium caused a significant increase in RBC content of phosphatidylserine. The data show that calcium alone alters phospholipid turnover of RBC and such an effect may mediate the PTH-induced calcium influx. Alternatively, PTH may enhance entry of calcium into RBC independent of the effect of calcium on phospholipid turnover and the increase in cytosolic calcium would then alter phospholipid turnover which in turn would further facilitate the effect of PTH on calcium influx. The increase of phosphatidylserine by PTH may increase rigidity of RBC membrane and enhance osmotic fragility.