Different effects of estradiol and progesterone on the secretion of calcitonin in ovariectomized rats

The basal and calcium-stimulated secretion of calcitonin (CT) was studied in ovarian steroid hormone-injected ovariectomized (Ovx) rats. Ovx rats were injected subcutaneously once daily for 3 days with estradiol benzoate (EB, 25 micrograms/kg), progesterone (P, 50 mg/kg), EB and P, or sesame oil. All rats were then infused with 10% CaCl2 at a rate of 2 ml/h for 30 min via a femoral vein catheter connected to a peristaltic pump. Blood samples (0.5 ml each) were collected from the right jugular vein at 0, 30, 60, and 120 min. The concentration of plasma CT was measured by a human calcitonin radioimmunoassay. The level of plasma calcium was determined by an automatic calcium analyzer. The basal plasma CT level was significantly higher (p less than 0.01) in Ovx rats injected with P and the combination of EB and P than that in rats injected with oil. Ovarian steroid hormones injection did not alter plasma CT concentrations at 30 and 60 min following calcium infusion when compared with those injected with oil. Ninety min following termination of calcium infusion, plasma CT levels were higher (p less than 0.01) in EB- and both EB and P-injected Ovx rats than in oil-injected animals. EB or P injection had no effect on the basal plasma calcium concentration. Plasma calcium concentration at 30 min following calcium challenge was higher (p less than 0.01) in EB-injected Ovx rats than in control (oil-injected). These results suggest that (1) progesterone stimulates the basal secretion of CT, and (2) CT secretion in response to calcium infusion is stimulated in estradiol-treated ovariectomized rats.(ABSTRACT TRUNCATED AT 250 WORDS)