Lack of an effect of sodium zeolite A on rat tibia histomorphometry

Cell culture studies suggest that the aluminum silicate polymer sodium Zeolite A (SZA) increases bone cell proliferation and extracellular matrix production. This study in rats investigated the short-term (2 weeks) and long-term (18 weeks) in vivo effects of SZA on growth rate (weight gain) and tibia histomorphometry. In separate short-term experiments, female (experiment 1) or male (experiment 2) Sprague-Dawley rats grown and maintained on normal calcium diets were gavaged daily during a 2 week treatment period with 30 mg/kg, 100 mg/kg, or 500 mg of SZA/kg of rat body weight. In the long-term study (experiment 3) ovariectomized (OVX) rats were fed a low calcium diet containing 0, 1.80, and 9.00 g of SZA/kg of diet (0, 125, and 617 mg/kg of body weight, respectively). Short- and long-term treatment of growing rats with SZA resulted in a dose-dependent increase in bone aluminum. In the first experiment, growing intact female rats showed no significant SZA dose-dependent response in growth rate (weight gain) or histomorphometry of cortical bone in the tibial diaphysis or cancellous bone in the secondary spongiosa of the tibial metaphysis. In the second experiment, growing male rats, with right hind limbs immobilized by unilateral sciatic neurotomy, showed no SZA dose-dependent response in growth rate. The longitudinal growth of cancellous bone in the tibia of the denervated limb and the intact contralateral limb were not influenced by sciatic neurectomy and/or by treatment with SZA. Histomorphometry demonstrated that cortical bone mass and formation was reduced in the sciatic-sectioned limb when compared with the contralateral intact limb of vehicle-treated rats, as evidenced by significant reductions in static measurements of cortical bone area (-8.5%) and cross-sectional area (-4.8%) and in calculations of the periosteal formation rate (-33.8%) and mineral apposition rate (-31.6%), and the endocortical formation rate (-35.5%) and mineral apposition rate (-37.9%). The cancellous bone mass of denervated limbs of vehicle-treated rats was also deficient, as evidenced by decreased cancellous bone area (-39.1%) and perimeter (-31.9%). The bone mineral apposition rate was decreased (-26.7%) indicating reduced osteoblast activity. Treatment with SZA did not influence these indices in the tibiae of either sciatic-sectioned limbs or contralateral intact limbs. In the long-term experiment, OVX resulted in a dramatic 88% decrease in cancellous bone volume which was prevented by treatment with 17/β-estradiol and not influenced by treatment with Zeolite A. The increases in osteoblast and osteoclast number following OVX were not influenced by SZA. The results indicate that SZA treatment has no anabolic effect on cortical and cancellous bone formation and mass in normal growing female rats and that this compound does not protect against osteopenia due to reduced load bearing in the growing male rat or gonadal hormone deficiency in adult female rats.