Over a half-century ago, the work of Gabriel (1894) demonstrated the presence of substantial amounts of sodium in the chloride-free residue of bone. In man, the skeleton contains 32 to 44 per cent of total body sodium (Forbes and Lewis, 1956), and the rat 20 per cent (Cheek et al., 1957). Harrison et al. (1936) showed that the ratio of sodium to chloride in the skeleton is greatly in excess of that found in a plasma ultrafiltrate and defined this as "extra" bone sodium. Kaltreider et al. (1941). Forbes and Perly(1951), Stern et al.(1951), Edelman et al.(1952), and Norman(1963a), using radioactive isotopes, have shown that approximately 30 to 70 per cent of bone sodium is exchangeable with circulating radiosodium within 24 hours. Furthermore, this exchange occurs more readily and is greater in the young skeleton than in that of the adult (Munroe et al., 1957; Forbes et al., 1957; Norman, 1963a).
The chemical relationships of sodium and potassium to the crystal lattice of bone are not clearly known. Neuman et al.(1950), however, consider that the sodium and potassium located in the surface layer of the crystals are in equilibrium with surrounding body fluid, thus allowing ionization between them. Flanagen et al. (1950), studying both balances and composition of tissues in dogs with adrenal insufficiency, noted marked discrepancy of sodium concentration and suggested that bone might serve as a sodium reservoir. The first indication that this indeed might be so came from the work of Bergstrom and Wallace(1954), whose rather striking results showed 29 per cent loss of bone sodium in the rat during experiments of sodium depletion and acidosis induced by intraperitoneal dialysis against ammonium chloride and low sodium diet.
Since this report, a number of workers have studied this problem with somewhat different findings regarding the magnitude of change in bone sodium. The common physiologic denominator of all external or internal stimuli resulting in sodium loss from bone, however, could not be found clearly.
The experiments so be described here were undertaken to determine the effect of sodium depletion, loading, and additional chronic metabolic acidosis in both newly weaned and adult rats for a longer duration than previously reported.
Eighty weaned albino rats(Sprague Dawley strain) were used. They were divided into 2 groups: 50 for the experiments in the weaning period and 30 for adults.
These groups again were subdivided into 5 groups as follows:
1) Low Na dietary group(0.01% NaCl in diet)
2) Normal Na dietary group(0.58% NaCl in diet)
3) High Na dietary group(1.6% NaCl in diet)
4) Low Na diet and acidosis
5) High Na diet and acidosis
All rats were provided with a vegetarian diet containing 74 per cent rice. For the weaning groups the experimental diet was fed for a duration of 6 weeks, beginning from the 4th week of age, immediately following weaning, and for the adults the diet was fed for the same duration, beginning from the 34th week of age.
Chronic acidosis was induced by the administration of ammonium chloride by mouth:
1 mEq. for the weaning group and 5 mEq. for the adult group respectively.
All rats in the experimental period were weighed once every other day and examined carefully for food intake, water consumption, urine and sodium output each in an individual balance cage. In addition, in the acidotic group, the amount of
NH^^3-N in urine was measured.
At the end of 6 week's dietary feeding, under light ether anesthesia, blood from the femoral artery was collected for analysis of serum sodium and potassium. In addition, in acidotic group, blood was collected by heart puncture for measurement
of blood pH. In a part of the adult group, the adrenal glands were weighed and the tissues were examined for pathologic changes. Both femora were removed, freed of periosteum, and weighed for wet bone weight, bone ash weight, and analyzed for
sodium, potassium and calcium.
The results of the experiment are as follows:
1) The rate of growth of weaning group was the same in all dietary groups, regardless of the concentration of NaCl in the diet as well as of acid base state. In the adult group, the body weight remained unchanged in all groups.
2) Daily food intake, water consumption and urine output per unit body weight were significantly greater in the weaned group than in the adult.
3) Daily water consumption and urine output were proportional to the concentration of NaCl in the diet. Weaning acidotic groups tended to consume lesser amounts of water than non-acidotic ones, while the reverse was true in adult rats.
4) Despite the marked difference in the concentration of NaCl in the diet, the plasma concentrations of Na and K were maintained within the normal range in all non-acidotic rats. In acidotic rats, however, the plasma concentraaaation of Na tended to be lower than in non-acidotic. The plasma concentration of K tended to be higher in acidosis than in non-acidosis in adult while ti was lower in weaning rats.
5) Regardless of the acid-base state, the adrenal glands of the adult rats showed a marked sign of atrophy in high Na dietary group and of hypertrophy in low Na group.
6) The concentration of Na and Ca in the bone were higher while the concentration of K was lower in the adult than in the weaning rats. However, the Na/Ca ratio was identical in both groups. Moreover the concentration of these electrolytes in bone were neither influenced by the concentration of Na in the diet nor by the acid base imbalance.
These results indicate that sodium stored in bone is not subject to mobilization into the the extracellular fluid compartment even in case of prolonged Na depletion or acidosis. Although the exact physiological singificance of Na in bone is not clear, the results obtained in this investigation suggest that the bone take up Na during ossification process and does not release it even when the need for Na is extreme.