Characterization of [3H]‐nitrendipine binding to uterine smooth muscle plasma membrane and its relevance to inhibition of calcium entry

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Abstract
1 Specific, high affinity (KD = 164 pM) binding of the Ca channel inhibitor [3H]-nitrendipine was identified in plasma membrane-enriched fractions from the rat myometrium. 2 Although dihydropyridines effectively competed for [3H]-nitrendipine binding sites, both verapamil and D600 were poor competitors. Diltiazem (10 μM) increased [3H]-nitrendipine binding by about 40%, but had no effect on binding affinity. Among several other drugs tested, diethylstilboestrol (DES) caused a considerable inhibition of binding, with an IC50 value of 4 μM. 3 Both La3+ and EDTA (or EGTA) inhibited binding. The inhibition by the latter could be overcome by the addition of Ca2+ or Mg2+. 4 A clear relationship was found between [3H]-nitrendipine binding and 5′-nucleotidase activity in the various subcellular fractions. 5 Data on K+-stimulated Ca2+ influx in the intact uterine strips showed a good agreement between the inhibition by both nitrendipine and DES of stimulated Ca influx and their inhibitory effect on [3H]-nitrendipine binding to plasma membrane. This type of correlation was lacking in the case of D600. 6 These results suggest that Ca channels in the myometrial membrane possess multiple sites at which different drugs can act to block these channels.