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.
D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D002118:Calcium / Q000378:metabolism; D004971:Estrus; D005260:Female; D005711:Gallopamil / Q000378:metabolism; D007700:Kinetics; D008297:Male; D009543:Nifedipine / Q000031:analogs & derivatives* / Q000378:metabolism; D009568:Nitrendipine; D010903:Pituitary Gland, Anterior / Q000378:metabolism*; D011247:Pregnancy; D051381:Rats
D005711:Gallopamil; D009568:Nitrendipine; D009543:Nifedipine; D002118:Calcium