Mechanism of Cu2+-induced elevation of [3H]cimetidine binding to membranes in rat brain

Abstract

The mechanism of increasing effect of CuCl(2) on specific [(3)H]cimetidine binding was examined in brain membranes of rats. CuCl(2)-Induced elevation of [(3)H]cimetidine binding was high in Krebs-Ringer solution (pH 7.4) compared to those in 50 mM Na, K-phosphate buffer (pH 7.4) and in 50 mM Tris-HCl buffer (pH 7.4). CaCl(2) (5-50 mM) inhibited effect of CuCl(2), but NaCl (25-200 mM), KCl (5-100 mM) or MgCl(2) (5-50 mM) did not. CuCl(2) (50 ?M) elevated 9.3- and 2.5-fold the binding in phosphate- and Tris-HCl buffer, respectively. EDTA-2Na decreased the binding elevated by 50 ?M CuCl(2) in phosphate buffer to the similar level in Tris-HCl buffer, whereas it did not affect those in Tris-HCl buffer. The absorption spectra of cimetidine and CuCl(2) mixture showed a peak at 317 nm in phosphate buffer that was not observed in Tris-HCl buffer. It is suggested that cimetidine-Cu(2+) chelate complex could be formed in phosphate buffer, resulting in higher amount of binding in phosphate buffer than in Tris-HCl buffer. PdCl(2) also caused a marked elevation in [(3)H]cimetidine binding, seeming to be due to formation of cimetidine-Pd(2+) chelate complex. There were two types of [(3)H]cimetidine binding in the presence of 20 nM PdCl(2): high affinity binding with K(d) = 0.7 +/- 0.1 nM and low affinity binding with K(d) = 44.3 +/- 3.0 nM. It is suggested that cimetidine-Cu(2+) complex binds to cimetidine binding sites in brain with higher affinity than cimetidine alone.