Effect of excess dietary salt on calcium metabolism and bone mineral in a spaceflight rat model

Abstract

High levels of salt promote urinary calcium (UCa) loss and have the potential to cause bone mineral deficits if intestinal Ca absorption does not compensate for these losses. To determine the effect of excess dietary 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 salt (4 and 8%) and normal calcium (0.45%) and for 4 wk on diets containing 8% salt (HiNa) and 0.2% C (LoCa). Final body weights were 9-11% lower in S than in control rats (C) in both experiments, reflecting lower 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 transiently during unloading. Net intestinal Ca absorption was consistently 11-18% lower in S than in C. Serum 1,25-dihydroxyvitamin D was unaffected by either LoCa or HiNa diets in S but was increased by LoCa and HiNa diets in C. Despite depressed intestinal Ca absorption in S and a sluggish response of the Ca endocrine system to HiNa diets, UCa loss did not appear to affect the osteopenia induced by unloading. Although any deficit in bone mineral content from HiNa diets may have been too 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 depression of intestinal Ca absorption and its regulatory hormone.

The Effects of Orbital Spaceflight on Bone Histomorphometry and Messenger Ribonucleic Acid Levels for Bone Matrix Proteins and Skeletal Signaling Peptides in Ovariectomized Growing Rats*

“…Studies that evaluated calcium uptake and clearance, bone histomorphometry, and resorption of a preflight fluorochrome label failed to detect changes in bone resorption in male rats during spaceflight (1,4,12,46). The present study in OVX rats clearly demonstrates increased cancellous bone resorption with net bone loss.…”

Effects of Vitamin D and Calcium Supplementation on Micro-architectural and Densitometric Changes of Rat Femur in a Microgravity Simulator Model

“…During the space flight physiologic changes occur in the bone metabolism of astronauts, in response to microgravity including a decrease in circulating concentrations of the active form of vitamin D (1,25-dihydroxyvitamin D) and a consequent decrease in intestinal calcium absorption which finally results in the release of calcium from bones (18-21). Consequently, calcium is excreted through the gastrointestinal system and urine (22, 23).…”

Plasma 25-hydroxycholecalciferol and 1,25-dihydroxycholecalciferol concentrations are decreased in hind limb unloaded Dahl salt-sensitive female rats

“…The hind limb unloaded (tail-suspended, HU) rat was used as a disuse model [18–22]. This model utilizes a special cage with an overhead pulley system to which the tails are attached by paper clip through a loop of orthopedic tape made by attaching the cut ends of the tape firmly to two-thirds the length of the tail.…”