Estrous cycle and cold stress in iron-deficient rats

Abstract

Whether iron-deficient female rats can thermoregulate when exposed to cold was studied to assess the interactive effects of iron deficiency and the female reproductive cycle. Rats were assigned to either iron-deficient (∼3 ppm Fe, n = 60) or control (35 ppm Fe, n = 60) diets for approximately 5 weeks. The two groups were then subdivided into five groups, four based on stage of the estrous cycle (proestrus, estrus, metestrus, diestrus) and the fifth group was ovariectomized. Animals were exposed to 4° C for 6 hours. Following sacrifice, tissues were collected for analysis of thyroid hormone and iron status indices. There was an interactive effect of iron status and the estrous cycle on core temperature response to the cold ( P ⩽ 0.05). Iron-deficient animals had lower core temperatures following cold exposure than did controls, and this effect was most pronounced during proestrus. Ovariectomy resulted in significantly lower core temperatures following cold stress as well as lower plasma thyroid hormone concentrations. Plasma thyroxine concentrations were unaffected by iron status or the estrous cycle, and plasma triiodothyronine concentrations were lower ( P < 0.001) in the iron-deficient rats than in controls. Thyroxine monodeiodinase activity in the liver was lower ( P ⩽ 0.01) in iron-deficient animals than in controls; this conforms with the plasma triiodothyronine findings. Brown adipose tissue deiodinase was not affected by either iron status or the estrous cycle. In conclusion, iron deficiency impairs thermoregulation in female rats, and this effect is related to the ovarian cycle. It was not, however, attributable to estrous cycle effects on indices of thyroid hormone metabolism.