Intracellular Ca2+ homeostasis, crucial for physiological functions, requires a normal functioning plasma membrane Ca2+ pump (PMCA). Several reports described inhibition of the PMCA in red cells (RBCs) exposed to high glucose concentrations in vitro, or in vivo (in diabetics), correlated with glycation of lysine residues on the purified PMCA. We discovered a broad distribution of PMCA Vmax in normal RBCs, with a minority of cells having a very reduced PMCA Vmax. In additional preliminary experiments we found (i) an inverse correlation between glycated Hb (%Hb A1c) and PMCA Vmax in normal RBCs, suggesting that the decline in pump Vmax was age-related (ii) that the fraction of high- Na+, low-K* RBCs which we discovered in sickle RBCs and in small amounts among normal RBCs (""""""""CVres"""""""" RBCs: resistant to dehydration when K-permeabilized by [Ca2+ + A23187] or Valinomycin), also had a high %Hb A1c, suggesting that they represent senescent RBCs; and (iii) that CVres RBCs show loss of membrane phospholipid asymmetry (PS-exposure), found also in diabetics' RBCs, associated with adherence to vascular endothelia. Based on these results we propose the following studies: To test (1) the hypothesis that glycation of the RBC PMCA is causative in lowering its Vmax, rather than simply reflecting cell age, by testing whether the increase in Hb A1c in RBCs from poorly controlled Type 2 diabetics correlates with increased fractions of RBCs with reduced Vmax and higher %Hb A1c, and if this is normalized by improved glycemic control; (2) whether increased glycation of diabetics' RBC transport proteins also correlates with increased numbers of CVres RBCs thus explaining their reduced life-span; and (3) whether the loss of RBC membrane phospholipid asymmetry (PS-exposure) in diabetic RBCs is related to the processes by which glycation generates low PMCA Vmax and CVres cells. If the correlation between increased Hb glycation and Vmax reduction is confirmed, future studies would test whether the PMCA purified from Iow-Vmax RBCs shows increased glycation of the lysine residues relative to that purified from high-Vmax RBC fractions. ? ?
| Bookchin, Robert M; Etzion, Zipora; Lew, Virgilio L et al. (2009) Preserved function of the plasma membrane calcium pump of red blood cells from diabetic subjects with high levels of glycated haemoglobin. Cell Calcium 45:260-3 |
