Selective inhibition of [3H]nitrendipine binding to brain and cardiac membranes by bacitracin

The effects of bacitracin were investigated on 3H]nitrendipine binding to rat brain and cardiac membranes in a low ionic strength (5 mM Tris-HCl) buffer. Bacitracin inhibited 3H]nitrendipine binding to rat brain and cardiac membranes with IC50 values of 400 +/- 100 and 4600 +/- 400 micrograms/mL, respectively. Scatchard analysis in brain membranes revealed that bacitracin inhibited 3H]nitrendipine binding primarily by reducing the Bmax but also by producing a small increase in the Kd. In brain membranes, Na+ (100 mM) and Ca2+ (2 mM) reduced the potency of bacitracin to inhibit 3H]nitrendipine binding by approximately sixfold with IC50 values of 2600 +/- 300 and 2100 +/- 400 micrograms/mL observed for bacitracin in the presence of 100 mM Na+ and 2 mM Ca2+, respectively. The EC50 values for the effects of Na+ and Ca2+ were 800 +/- 200 microM and 25 +/- 5 mM. K+, Mg2+, choline, and increasing the assay buffer of Tris-HCl to 50 mM also decreased the inhibition of 3H]nitrendipine binding by bacitracin. These results suggest that bacitracin specifically modulates 3H]nitrendipine binding in a cation-dependent manner and that brain and cardiac dihydropyridine binding sites are either biochemically different or exist in a different membrane environment.