[3H]nitrendipine binding in the guinea pig left ventricle: effects of calcium and lanthanum.

The purpose of this study was to investigate the effects of Ca2+ and La3+ on [3H]nitrendipine binding in the heart. [3H]Nitrendipine binding in EDTA washed (10 mM) membrane preparations of the guinea pig left ventricle showed a single class of specific high affinity binding sites with a dissociation constant (Kd) of 90 +/- 3.4 pM (mean +/- S.E.M.; n = 5) and a maximal number of binding sites (Bmax) of 70.5 +/- 6.3 fmol/mg protein. Ca2+ (2 mM) increased the Bmax to about 154% of control. EDTA and La3+ both reduced the number of binding sites, and their effects were antagonized by Ca2+. Ca2+ (10 mM) also shifted the La3+ concentration-dependent inhibition of [3H]nitrendipine binding to the right (IC50 = 10.4 and 22.8 microM respectively). These results suggest that Ca2+ is required for [3H]nitrendipine binding in the guinea pig heart and that La3+ antagonizes the effect of Ca2+.