EFFECT OF EXCESS DIETARY SALT ON CALCIUM-METABOLISM AND BONE-MINERAL IN A SPACEFLIGHT RAT MODEL

High levels of salt promote urinary calcium (UCa) loss and have the po tential to cause bone mineral deficits if intestinal Ca absorption doe s not compensate for these losses. To determine the effect of excess d ietary salt on the osteopenia that follows skeletal unloading, we used a spaceflight model that unloads the hindlimbs of 200-g rats by tail suspension (S). Rats were studied for 2 wk on diets containing high sa lt (4 and 8%) and normal calcium (0.45%) and for 4 wk on diets contain ing 8% salt (HiNa) and 0.2% Ca (LoCa). Final body weights were 9-11% l ower in S than in control rats (C) in both experiments, reflecting low er growth rates in S than in C during pair feeding. UCa represented 12 % of dietary Ca on HiNa diets and was twofold higher in S than in C tr ansiently during unloading. Net intestinal Ca absorption was consisten tly 11-18% lower in S than in C. Serum 1,25-dihydroxyvitamin D was una ffected by either LoCa or HiNa diets in S but was increased by LoCa an d HiNa diets in C. Despite depressed intestinal Ca absorption in S and a sluggish response of the Ca endocrine system to HiNa diets, UCa los s did not appear to affect the osteopenia induced by unloading. Althou gh any deficit in bone mineral content from HiNa diets may have been t oo small to detect or the duration of the study too short to manifest, there were clear differences in Ca metabolism from control levels in the response of the spaceflight model to HiNa diets, indicated by depr ession of intestinal Ca absorption and its regulatory hormone.