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

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Abstract
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 μg/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 μg/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 μM 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.Key words: bacitracin, [3H]nitrendipine, brain, heart.