Binding of the calcium antagonist [3H] nitrendipine to human myometrial plasmalemma

Membrane fragments were prepared from nonpregnant human myometrium and fractionated by differential and sucrose gradient centrifugation. Reversible high-affinity binding sites of the dihydropyridine calcium antagonist nitrendipine were identified in highest density in that membrane fraction most enriched in markers for plasmalemma. In determinations done on several preparations, the equilibrium dissociation constant for nitrendipine at 37 degrees C was found to be 0.3 to 0.8 nM and was the same for each fraction. The maximum binding capacity was found to be 400-500 fmol/mg protein in the plasmalemma fraction.