New vanadate‐induced Ca 2+ pathway in human red cells

Vanadate is a commonly used Ca 2+ pump blocker, exerting a substantial effect on Ca 2+ extrusion at millimolar concentrations in human red cells. At such levels, vanadate also seems to open an L type‐like Ca 2+ channel in these cells (J Biol Chem 257 (1982) 7414; Gen Physiol Biophys 16 (1997) 359). Since neither a dose‐dependence effect nor a metabolic requirement for the latter action could be found in the literature, we have addressed this matter in the present work. Accordingly, vanadate action on Ca 2+ entry was systematically investigated in both young and old human red cells after metabolic depletion. Although vanadate enhanced Ca 2+ entry indifferently in either cell type, a distinct over‐all effect was paradoxically found depending on whether or not metabolic substrates that give rise to ATP were present. In ATP‐depleted cells, unlike with ATP‐containing cells, vanadate‐stimulated Ca 2+ entry was neither blocked by raising external K + nor by adding voltage‐dependent Ca 2+ channel blockers (nifedipine, calciseptine, FTX3.3) or compounds affecting polyphosphoinositide metabolism (Li + , neomycin). Likewise, full substitution of external Na + by other cations did not inhibit vanadate‐enhanced Ca 2+ entry. Regardless of the cell age, stimulation by vanadate depended strongly on internal Na + (0–30 mM). Vanadate stimulation was significantly reduced (about 55%) by heparin (10 mg/ml) only in young cells and by ryanodine (about 35%, 250 μM) in old cells. The results suggest presence of a new vanadate‐induced Ca 2+ entry pathway in ATP‐depleted cells.