1,25-Dihydroxyvitamin D3 and 24,25-dihydroxyvitamin d3: key components of the vitamin D endocrine system

The synergistic effect of TGF beta and 24,25-(OH)2D3 on resting zone chondrocytes is metabolite-specific and mediated by PKC

Transforming growth factor-beta (TGF beta) and 24,25-(OH)2D3 play a major role in chondrocyte maturation during endochondral bone formation. TGF beta and vitamin D metabolites when added separately to resting zone (RC) or growth zone (GC) chondrocyte cultures, activate protein kinase C (PKC). The present study determined whether there is an additive or synergistic effect of vitamin D3 metabolites and TGF beta on alkaline phosphatase and PKC specific activities and whether this effect is cell maturation-dependent. GC and RC chondrocytes were isolated from rat costochondral cartilage and cultured to fourth passage. The cells were incubated with vitamin D3 metabolites and TGF beta alone or in combination, and the specific activity of alkaline phosphatase, as well as the specific activity of PKC, were measured. The addition of 24,25-(OH)2D3 with TGF beta to RC cells caused a synergistic effect on alkaline phosphatase activity; this result was not found if the vitamin D3 metabolite was replaced by 1,25-(OH)2D3. The addition of 1,25-(OH)2D3 or 24,25-(OH)2D3 with TGF beta on GC cells had no synergistic or additive effect. The addition of 24,25-(OH)2D3 and TGF beta for 12 hours caused a synergistic effect on PKC activity; this effect was also observed if TGF beta was added first for 12 hours and 24,25-(OH)2D3 for the last 90 min. However, the addition of 24,25-(OH)2D3 for 90 min followed by the addition of TGF beta for an additional 10.5 hours had no synergistic effect. This study indicates that TGF beta and 24,25-(OH)2D3 have a synergistic effect on chondrocyte differentiation as well as on PKC activity, suggesting that the synergistic effect of 24,25-(OH)2D3 and TGF beta on chondrocyte differentiation may be mediated through activation of PKC. The synergistic effect of 24,25-(OH)2D3 and TGF beta was cell maturation dependent.

Regulation of arachidonic acid turnover by 1,25-(OH)2D3 and 24,25-(OH)2D3 in growth zone and resting zone chondrocyte cultures

Previous studies have shown that phospholipase A2 activity in rat costochondral chondrocyte cultures is differentially regulated by 1,25-(OH)2D3 and 24,25-(OH)2D3. 1,25-(OH)2D3 stimulates enzyme activity in growth zone chondrocytes but has no effect on the resting zone chondrocyte enzyme activity. 24,25-(OH)2D3 inhibits the resting zone enzyme but has no effect on the growth zone chondrocyte phospholipase A2. This study examined whether the metabolites affect arachidonic acid turnover in their target cell populations. Incorporation and release of [14C]arachidonate was measured at various times following addition of hormone to the cultures. Acylation and reacylation were measured independently by incubating half of the [14C]arachidonate-labeled cultures with p-chloromercuribenzoate. The results demonstrated that the distribution of [14C]arachidonate in membrane phospholipids differed between growth zone and resting zone chondrocytes and between the plasma membranes and matrix vesicles isolated from the growth zone chondrocyte cultures. Plasma membrane phospholipids were more susceptible to the release of [14C]arachidonic acid by exogenous phospholipases than were matrix vesicle phospholipids. The effect of 1,25-(OH)2D3 on growth zone chondrocytes was observed within 5 min. Incorporation was greatest after 60 min; release was greatest after 30 min. 24,25-(OH)2D3 stimulated consistently elevated incorporation throughout the incubation period, peaking at 15 min. Peak release was at 60 min. The results confirm that resting zone chondrocytes and growth zone chondrocytes retain a differential phenotype in culture and demonstrate that matrix vesicles are distinct from the plasma membrane in terms of lipid composition and arachidonic acid incorporation. 1,25-(OH)2D3 and 24,25-(OH)2D3 appear to stimulate arachidonic acid turnover in their target cells by different mechanisms. Changes in fatty acid acylation and reacylation may be one mode of vitamin D-3 action in cartilage.

Calcium, phosphorus, and magnesium homeostasis in ruminants

Significant advances have been made in the last 30 years in the understanding of Ca, PO4, and Mg homeostasis in ruminants. Despite these advances, the primary cellular or molecular lesions responsible for failure of homeostasis have yet to be identified. Suggested causes, such as primary hormone deficiencies, have been eliminated, and we now believe that aging and nutrition can reduce the ability of intestine, bone, and kidney to respond rapidly to the hormone signals responsible for homeostasis during rapid increases in demand for these minerals. Further research is required before these lesions can be identified and new knowledge applied to the development of economical, effective programs that prevent milk fever and hypomagnesemic tetany. Until then, diseases such as milk fever will continue to affect 8 to 9 per cent of our dairy cows, and dairy farmers will continue to spend millions of dollars a year for treatment of the primary disease and the many secondary problems that result from these diseases.

Effect of 24R,25-dihydroxyvitamin D3 in experimental diabetic rats

The results are presented of a controlled study in male Wistar rats into the effects of 24R,25-dihydroxyvitamin D3 on blood glucose levels, bone calcium content and ADP-induced platelet aggregation in streptozocin-induced diabetes mellitus. Blood glucose levels were shown to be decreased by 10 micrograms/kg 24R,25-dihydroxyvitamin D3. The reduced bone calcium content associated with diabetes mellitus was returned to normal levels with both 1 and 10 micrograms/kg 24R,25-dihydroxyvitamin D3. It was also shown to exhibit dose-dependent anti-platelet activity. The data suggest that 24R,25-dihydroxyvitamin D3 might have potential as a mild therapeutic agent in the treatment of osteoporosis and platelet hyperactivity associated with diabetes mellitus.

Regulation of calcium and phosphorus homeostasis in the dairy cow

This paper describes the mechanism of calcium and phosphorus homeostasis in dairy cows in an effort to provide a clearer understanding of the rationale behind current management and supplementation practices. Specifically addressed is the need to keep prepartum dietary calcium intake at less than or equal to 50 g/d to minimize the incidence of milk fever. Also discussed is the need to increase National Research Council recommendations for postpartum dietary calcium from 2.7 to 3.4 g/kg milk. This is particularly important during the first 1 to 2 mo of lactation to maintain calcium balance.

Unilateral nephrectomy and 1,25-dihydroxyvitamin D3

Several renal functions respond to nephron loss by a compensatory adaptation. Whether the production of 1,25(OH)2D3 also adapts to a renal mass reduction is still a matter of controversy. In the present study we have investigated in rats the influence of unilateral nephrectomy, in both the acute (48 hr) and chronic (2 to 6 weeks) state, on plasma 1,25(OH)2D3 level measured by competitive protein binding assay. In the acute state no difference in plasma 1,25(OH)2D3 level between sham-operated (SHAM) and unilateral-nephrectomized (UNI-NX) rats was found. The presence of the thyroparathyroid glands was not required for maintaining plasma 1,25(OH)2D3 at a normal level 48 hr after UNI-NX. In the chronic state in rats fed at 1.1% Ca diet, plasma 1,25(OH)2D3 (means +/- SEM) was 94 +/- 4 in SHAM and 98 +/- 8 pM in UNI-NX. In rats fed a 0.1% Ca diet it was 252 +/- 16 in SHAM and 239 +/- 20 pM in UNI-NX. Analysis of 3H-1,25(OH)2D3 plasma decay curve indicated that in UNI-NX under a high calcium diet the normalization of plasma 1,25(OH)2D3 appears to be entirely due to an increase in production, whereas under a low calcium diet part of it may also result from a moderate decrease in the elimination rate. In conclusion, this study indicates that unilateral nephrectomy does not affect the level of plasma 1,25(OH)2D3 even under a calcium restriction challenge. This compensatory adaptation appears to be independent of parathyroid hormone.