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 de crease serum cholesterol levels. Radioligand binding studies with the dihydropyridine (DHP) [H-3]PN200-110 were undertaken to characterize c alcium (Ca++) channels in cardiac and aortic tissues from over rats tr eated for 35 days with EE2 (0.1 mg/kg day p.o.) or vehicle, and from v ehicle-treated sham-operated controls (sham). Cardiac tissues from EE2 -treated rats displayed significant increases in the density (B-max) o f high-affinity DHP binding sites (505 +/- 46 fmol/mg) compared with v ehicle-treated over rats (303 +/- 35 fmol/mg); DHP B-max values from E E2-treated cardiac tissues were not significantly different from vehic le-treated shams (385 +/- 76 fmol/mg). Cardiac Ca++ efflux channels fr om sarcoplasmic reticulum were assessed with [H-3]ryanodine. [H-3]Ryan odine B-max values were not affected by EE2 treatment. However, [H-3]r yanodine K-d values in preparations from EE2-treated rats were signifi cantly decreased (10 +/- 2 nM) compared with over rats (35 +/- 11 nM) and were similar to values in sham rats (8 +/- 2 nM). Cardiac beta adr enoceptors were not affected by EE2 treatment, which thus confirmed th e selective regulation of DHP receptors by EE2. Aortic preparations fr om EE2-treated rats exhibited significant increases in DHP receptors ( 125 +/- 37 fmol/mg) compared with both vehicle-treated over rats (32 /- 3 fmol/mg) and vehicle-treated shams (24 +/- 9 fmol/mg). There were no differences in the binding affinity (K-d) of [H-3]PN200-110 for ca rdiac or aortic sites among the three groups. To ascertain if EE2-medi ated increases in Ca++ channel density and ryanodine binding affinity affected in vivo responses to agonists that use extracellular and intr acellular Ca++ stores, responses to BAY k 8644 and norepinephrine were examined in pithed rats from the same three groups. No significant di fferences in hemodynamic responses occurred among the three groups to BAY k 8644 or norepinephrine. Thus, in female over rats, prolonged tre atment with EE2 resulted in increased density of cardiac and aortic ca lcium channels which did not translate into increased calcium-mediated inotropic, rate or presser responses. Conversely, EE2 treatment in ov er rats prevented the decrease in cardiac [H-3]ryanodine binding affin ity evident in vehicle-treated over rats. These data suggest that EE2 treatment in normotensive over rats resulted in modulation of both the L-type and sarcoplasmic reticulum Ca++ channels, and these alteration s maintained cardiovascular homeostasis.