The effects of fasting and refeeding on serum parathormone and calcitonin concentrations in young and old male rats

Although fasting and refeeding reveal the existence of age-related changes in carbohydrate and lipid metabolism, the effects of aging on mineral metab olism in refed animals are unknown. We therefore investigated hormonal regu lation of calcium metabolism in young (4 months) and old (26 months) male r ats fasted for 48 hours and then refed for 4 or 24 hours. Serum concentrati ons of total and ionized calcium and parathormone were similar in control y oung and old rats. Serum calcitonin level was higher, and the concentration s of albumin and inorganic phosphate and alkaline phosphatase activity were lower in fed old rats. In young fasted rats, the serum ionized and total c alcium was decreased, and phosphate concentration was increased. In old rat s, fasting resulted in the increase of serum parathormone level. Pasting re duced serum alkaline phosphatase activity to a similar extent in both age g roups. In young rats, refeeding for 24 h normalized serum calcium and phosp hate levels and alkaline phosphatase activity, and decreased serum concentr ations of PTH and calcitonin. In old refed rats, serum calcitonin concentra tion was raised by 77% compared to fed or fasted animals, whereas parathorm one levels were normalized. Our results indicate that old fasted or refed r ats maintain normal serum calcium concentration in a different way than you ng animals, possibly through the increase in serum levels of parathormone a nd/or calcitonin. Thus, dietary manipulations such as fasting and refeeding constitute an interesting model for the investigation of the effects of ag ing on the hormonal regulation of serum calcium level.