Age-related differences in basal and calcium-stimulated plasma calcitonin levels in female rats.

The effects of aging on calcitonin (CT) secretion in female rats were investigated. Old (24 mo) at constant diestrus status and young (2 mo) at diestrus status rats were either ovariectomized (Ovx) or left intact as controls. Ovx rats were injected subcutaneously with estradiol benzoate (25 micrograms/kg body wt) or sesame oil one time per day for 3 days. All rats were infused with CaCl2 (10 mg/ml) at a rate of 2 ml/h for 30 min via a jugular catheter connected to a peristaltic pump. Blood samples (0.5 ml each) were collected at 0, 30, 60, and 120 min. The basal and post-CaCl2 levels of plasma Ca measured with radioimmunoassay were significantly higher (P less than 0.05-0.01) in old than in young female rats. The pre- and post-CaCl2 levels of plasma Ca and CT in young rats were not altered by Ovx or estradiol replacement. In old rats, Ovx caused a higher (P less than 0.01) level in plasma CT at 0 and 30 min after CaCl2 infusion. Both basal and stimulated levels of plasma CT were higher (P less than 0.01) in old Ovx than in young Ovx rats. These results demonstrated that 1) the increase of plasma CT in response to Ca challenge was greater in old than in young female rats, 2) the influence of estradiol and ovarian function on plasma CT concentration increases as a function of age, and 3) estradiol reduced the plasma CT in response to hypercalcemia in old Ovx rats. The sensitivity of the target tissue of young rats may be lower in response to the modulation of estrogen during hypercalcemia without compromising the secretion and hypocalcemic effect of CT in young rats. All suggested an age-related relationship between estrogen and CT secretion in minute-to-minute regulation during Ca infusion in rats.