Skeletal changes in multiparous, nulliparous and ovariectomized mice fed either a nutrient-sufficient or -deficient diet containing cadmium

Abstract

As a simulation of the etiological factors known for Itai-Itai disease, a syndrome characterized by osteomalacia and renal dysfunction in its Japanese victims, female mice were subjected to the individual and combined stresses of dietary cadmium, nutrient-deficient diet, multiparity and ovariectomy; the calcium-depleting effect of each factor was evaluated by determining Ca levels in femur and lumbar vertebrae. At age 68 days, female mice were given nutrient-sufficient (+) or -deficient (−), purified diets containing either 0.25 (environmental), 5, or 50 ppm Cd as Cdcl 2; the nutritional composition of (−) diet simulated that of food consumed by Japanese victims of Itai-Itai disease. At age 70 days, half of the females began a breeding regimen of six consecutive, 42-day rounds of pregnancy/lactation (PL mice); the remainder were maintained as virgin, non-pregnant controls (NP mice). Limited numbers of PL and NP mice were sacrificed at the end of each reproductive round. PL(+) mice taken at the end of round (R)-6 had successively borne litters in all six rounds, while PL(−) counterparts had nonsuccessively borne only three. At the conclusion of the 252-day reproductive period, remaining females entered the 392-day, post-reproductive phase of the experiment. At age 546 days (mid-R-12), PL females having successfully borne at least three litters were ovariectomized (OV) to mimic human menopause; at the same time, NP females were either ovariectomized or sham-operated (SO). After surgery, all females were maintained to age 714 days (mid-R-16), then sacrificed. During the post-reproductive period, food consumption by females of the same reproductive status was unaffected by elevated levels of Cd or nutrient-deficiencies in diet. However by R-16, Cd at 50 vs. 0.25 ppm had reduced body mass by 11% in both NP and PLOV females, femur and lumbar vertebral calcium content (TCa) by 20 and 25% in the respective groups, and femur and vertebral calcium/dry weight ratios (Ca/DW) by 12 and 11%. Alternative R-16 comparisons indicated that (−) diet also diminished skeletal Ca, but that the additional factors of (prior) multiparity and ovariectomy generated only small and non-significant effects. Comparison of skeletal status between the ends of the reproductive and post-reproductive periods indicated that (1) individual NP groups, regardless of Cd exposure, generally sustained small decreases in TCa and CaDW over time (consistent with aging), but PL groups without exception secured significant gains (consistent with cessation of multiparous activity), (2) skeletal integrity of PL groups was significantly more compromised by the combination of Itai etiological factors at the end of R-6 than R-16, and (3) among those factors, the most demineralizing over lifetime were chronic exposure to Cd followed by ingestion of (−) diet. Despite these findings, skeletal degeneration characteristic of the Itai-Itai syndrome was ultimately not duplicated in this mouse model suggesting that the full-blown disease required primary and profound skeletal demineralization secondarily supported and enhanced by renal dysfunction.