Effects of prolonged ethinyl estradiol treatment on calcium channel binding and in vivo calcium-mediated hemodynamic responses in ovariectomized rats.

Ethinyl estradiol (EE2), administered orally to ovariectomized (ovex) rats, has been shown to prevent loss of bone mineral density and to decrease serum cholesterol levels. Radioligand binding studies with the dihydropyridine (DHP) [3H]PN200-110 were undertaken to characterize calcium (Ca++) channels in cardiac and aortic tissues from ovex rats treated for 35 days with EE2 (0.1 mg/kg day p.o.) or vehicle, and from vehicle-treated sham-operated controls (sham). Cardiac tissues from EE2-treated rats displayed significant increases in the density (Bmax) of high-affinity DHP binding sites (505 +/- 46 fmol/mg) compared with vehicle-treated ovex rats (303 +/- 35 fmol/mg); DHP Bmax values from EE2-treated cardiac tissues were not significantly different from vehicle-treated shams (385 +/- 76 fmol/mg). Cardiac Ca++ efflux channels from sarcoplasmic reticulum were assessed with [3H]ryanodine. [3H]Ryanodine Bmax values were not affected by EE2 treatment. However, [3H]ryanodine Kd values in preparations from EE2-treated rats were significantly decreased (10 +/- 2 nM) compared with ovex rats (35 +/- 11 nM) and were similar to values in sham rats (8 +/- 2 nM). Cardiac beta adrenoceptors were not affected by EE2 treatment, which thus confirmed the selective regulation of DHP receptors by EE2. Aortic preparations from EE2-treated rats exhibited significant increases in DHP receptors (125 +/- 37 fmol/mg) compared with both vehicle-treated ovex rats (32 +/- 3 fmol/mg) and vehicle-treated shams (24 +/- 9 fmol/mg). There were no differences in the binding affinity (Kd) of [3H]PN200-110 for cardiac or aortic sites among the three groups. To ascertain if EE2-mediated increases in Ca++ channel density and ryanodine binding affinity affected in vivo responses to agonists that use extracellular and intracellular Ca++ stores, responses to BAY k 8644 and norepinephrine were examined in pithed rats from the same three groups. No significant differences in hemodynamic responses occurred among the three groups to BAY k 8644 or norepinephrine. Thus, in female ovex rats, prolonged treatment with EE2 resulted in increased density of cardiac and aortic calcium channels which did not translate into increased calcium-mediated inotropic, rate or pressor responses. Conversely, EE2 treatment in ovex rats prevented the decrease in cardiac [3H]ryanodine binding affinity evident in vehicle-treated ovex rats. These data suggest that EE2 treatment in normotensive ovex rats resulted in modulation of both the L-type and sarcoplasmic reticulum Ca++ channels, and these alterations maintained cardiovascular homeostasis.