The effects of dietary Ca and vitamin D_3 on the Cd accumulation in the liver and the the kidney were observed and discussed in relation with the response to the intestinal Ca transport. 1. Effect of dietary factors was reflected on the concentration of rat serum Ca and 1,25-(OH)_2-D after 2 weeks feeding with experimental diets differing in Cd, Ca and vitamin D_3 level. 2. Ca transport in duodenum was significantly increased by vitamin D_3,especially in the rats raised on low Ca diet. However, this effect was reduced in Cd-exposed rats. 3. Renal Cd accumulation was influenced by neither dietary Ca nor vitamin D_3. However, hepatic Cd accumulation was significantly increased in vitamin D deficient rats, although it was not influenced by dietary Ca. These results suggest that dietary Ca modulates the inhibitory effect of Cd on intestinal vitamin D stimulated Ca transport, but intestinal Cd absorption is rather increased in vitamin D deficient animals.

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^<47>Ca-Releasing activities of some vitamin D derivatives from prelabelled mouse calvaria were comparatively determined in a system of organ culture established by Raisz and Reynold. A rather high concentrations of ergocalciferol and cholecalciferol (1μg/ml) caused a significant release of ^<47>Ca from the bone, while dihydrotachysterol_2 and dihydrotachysterol_3 had no effect at the same concentration. Each of 25-OH-D_3,24R-OH-D_3 and 24,25-(OH)_2-D_3 at 100 ng/ml showed a significant ^<47>Ca-release. 1α-OH-D_3,one of synthetic analogues of active vitamin D, had more effective ^<47>Ca-releasing activities than these three natural derivatives. A concentration of 10ng/ml was enough to release ^<47>Ca from the bone. 1α, 24-(OH)_2-D_3 had a little less activity than 1α-OH-D_3,but more potent than 24-OH-D_3. These results suggest that the presence of the hydroxyl group at 1α-position causes a more effective release of ^<47>Ca than that of the 25- or 24R-hydroxyl group. Biological activities of vitamin derivatives were compared between R-and S-form in the same experimental system. Both 24R-OH-D_3 and 1α, 24R-(OH)_2-D_3 were about ten times as active as 24S-OH-D_3 and 1α, 24S-(OH)_2-D_3 in ^<47>Ca-release, respectively. 1α, 25-(OH)_2-D_3,natural hormonal vitamin D, showed the highest activity in ^<47>Ca-release in this organ culture system showing a significant activity even at a concentration of 50 pg/ml. The effect of 1α, 25-(OH)_2-D_3 on the release of ^<47>Ca from the bone was inhibited by eel calcitonin, but not by hydrocortisone, 25-OH-D_3 or 24,25-(OH)_2-D_3. This system may be used for assaying 1α, 25-(OH)_2-D_3 in human serum, and by changing the medium more frequently, the sensitivity of the assay will be increased to 5 pg/ml.

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Intestinal mucosa cytosol obtained from 24 to 28 months old hen was applied for the radioreceptor assay, which was developed by Eisman et al^<1)> for the determination of serum 1α, 25-(OH)_2-D_3 using intestinal cytosol obtained from vitamin D deficient chick. Prior to the radioreceptor assay, serum 1α, 25-(OH)_2-D_3 was fractionated and purified essentially according to the method of Eisman et al.^<1)>. 1) The yield of intestinal cytosol protein from one normal hen was six-fold higher than one vitamin D deficient chick. Furthermore, the normal hen is commercially available. 2) The binding affinity and capacity to 1α, 25-(OH)_2-D_3 of the cytosol from normal hen were comparable to those obtained from vitamin D deficient chick. 3) Amounts of 1α, 25-(OH)_2-D_3 measured by the radioreceptor assay using intestinal mucosa cytosol obtained from normal hen and vitamin D deficient chick were well correlated. 4) With this method, the circulating level of 1α, 25-(OH)_2-D_3 was determined to be 41 pg/ml. Patients with renal failure revealed lower concentration of 1α, 25-(OH)_2-D_3. From these results, it was concluded that intestinal mucosa cytosol of normal hen would be a good source of the binding protein for 1α, 25-(OH)_2-D_3 assay.

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