Alteration of Ca2+ fluxes in brain microsomes by K+ and Na+: Modulation by sulfated polysaccharides and trifluoperazine

Rat brain microsomes accumulate Ca2+ at the expense of ATP hydrolysis. The rate of transport is not modulated by the monovalent cations K+, Na+, or Li+. Both the Ca2+ uptake and the Ca2+-dependent ATPase activity of microsomes are inhibited by the sulfated polysaccharides heparin, fucosylated chondroitin sulfate, and dextran sulfate. Half-maximal inhibition is observed with sulfated polysaccharide concentrations ranging from 0.5 to 8.0 mu g/ml. The inhibition is antagonized by KCl and NaCl but not by LiCl. As a result, Ca2+ transport by the native vesicles, which in the absence of polysaccharides is not modulated by monovalent cations, becomes highly sensitive to these ions. Trifluoperazine has a dual effect on the Ca2+ pump of brain microsomes. At low concentrations (20-80 mu M) it stimulates the rate of Ca2+ influx, and al concentrations > 100 mu M it inhibits both the Ca2+ uptake and the ATPase activity. The activation observed at low trifluoperazine concentrations is specific for the brain Ca2+-ATPase; for the Ca2+-ATPases found in blood platelets and in the sarcoplasmic reticulum of skeletal muscle, trifluoperazine causes only a concentration-dependent inhibition of Ca2+ uptake. Passive Ca2+ efflux from brain microsomes preloaded with Ca2+ is increased by trifluoperazine (50-150 mu M), and this effect is potentiated by heparin (10 mu g/ml), even in the presence of KCl. It is proposed that the Ca2+-ATPase isoform from brain microsomes is modulated differently by polysaccharides and trifluoperazine when compared with skeletal muscle and platelet isoforms.