Interactions between Ca2+-antagonist binding sites in rat adenohypophysis: dependence on estrous cycle

The Ca2+ antagonist [3H]-nitrendipine [( 3H]-NDP) displayed high affinity binding in a saturable manner to a homogeneous population of sites when measured in rat adenohypophysis homogenates prepared from males or from females at the proestrous and estrous stages. Kd values were 0.7 +/- 0.1 nM, 0.75 +/- 0.08 nM and 1.1 +/- 0.1 nM, respectively. Maximal binding capacities (Bmax) were 11 +/- 1 fmole/mg protein for males and 20 +/- 1 fmole/mg protein for females at proestrus and 23 +/- 2 fmole/mg protein at estrus. In none of these preparations was the binding of [3H]-NDP dependent on the presence of Ca2+. The Ca2+ antagonist methoxy verapamil (also known as D-600), which belongs to a class of Ca2+-antagonists different from that of [3H]-NDP, could displace [3H]-NDP in a pattern suggesting possible allosteric interactions between the sites of these two antagonists. The displacement of [3H]-NDP by D-600 was affected by the presence of Ca2+ and varied with the estrous cycle. Our results suggest the existence of interactions between binding sites for NDP and for D-600. These interactions are affected by Ca2+, which might exert its effect through binding to a site of its own. In female adenohypophysis the interactions between these systems vary with the estrous cycle, suggesting that the coupling between them is modulated during this cycle.