The effect of dietary salt on bone in a genetically-defined rat underloading /overloading model

One the most serious health hazards to aging and to long term space flight is the loss of bone. The most important determinant of the debilitation due to bone loss is the peak bone mass achieved during late adolescence, which itself may be influenced by gender and environmental factors such as mineral balance. Sodium intake is considered a risk factor for both hypertension and osteoporosis. It is estimated that 30--60% of the population is hypertensive and 30--40% is salt sensitive.;This study investigated the possible effects of an ad libitum 1% or 2% saline treatment on normotensive (W) and salt sensitive hypertensive (SS) young female rats. Our hypothesis was that hypertensive rats would have more severely affected bone than normotensive rats due to saline treatment and/or genotype. In addition, how these effects might be altered by immobilization/overloading stress was examined as this further burdens NASA space pioneers. The right hindlimb of each animal was immobilized by binding to the abdomen with elastic bandage tape. After sacrifice, three sites (both the underloaded and the overloaded tibiae, as well as the L-2 vertebrae) were processed for histomorphometric analysis. The wet weight (g) and length (mm) of the excised right immobilized and left overloaded femur and the ulna were measured. A 3-point bending test was applied to femurs only. Ten elements were analyzed from femurs and one ulna with Induced Coupled Plasma (ICP) spectrometry. The systolic blood pressure and heart rate were measured in the 6th week of study.;A more robust result was seen with 2% saline treatment. Both hypertensive genotype and 2% saline treatment significantly increased blood pressure and heart rate, and decreased femur magnesium. The SS rats had significant reductions in bone mass, femur cross-sectional area and zinc concentrations with simultaneous elevations in femur stiffness, strength and calcium concentrations. Two percent saline treatment decreased cancellous bone in the weight-bearing tibia bone in both strains. Immobilization increased the bone stiffness and mineral concentration, while decreasing cancellous bone volume. Our findings suggest genotype, saline treatment, and immobilization adversely affect bone in adolescent female rats.