Studies on calcium nutrition in appropriate large animal models can be directly relevant to humans. We have examined the effect of dietary Ca deficiency on various bone and bone-related variables, including plasma markers, histomorphometry, mineral content and breaking strength in pigs. Three groups of eight 38-d-old female pigs were fed adequate (0.9%; control), low (0.4%; LCa) or very low (0.1%; VLCa) Ca diets for 32 d. Plasma Ca significantly decreased over time only in the VLCa-deficient pigs. The concentrations of the parathyroid hormones (PTH) and calcitriol increased as Ca deficiency developed, and the plasma PTH and calcitriol levels varied inversely with dietary Ca. The total bone ash contents, bending moments, trabecular bone volume and the mineral apposition rate all decreased as the calcium intake decreased. The osteoclast surface areas were greater than those of controls in both Ca-deficient groups, whereas the osteoblast surface areas were greater only in the VLCa group. The plasma osteoblast-related markers (alkaline phosphatase, carboxy-terminal propeptide of type I procollagen and osteocalcin) were either greater or unaffected in the Ca-deficient pigs. The results indicate that deficient bone mineralization combined with an increased bone resorption led to bone loss and fragility. The differences in the changes in bone cells (number and activity) between LCa and VLCa groups might be due to differences (time and extent) of circulating PTH and calcitriol. The defective mineralization in both Ca-depleted groups resulted mainly from the lack of Ca because their osteoblast activity was either maintained or stimulated. The results also underline the progressive sensitivity of pigs to Ca supply and the usefulness of this model.

We examined the effects of various extracellular calcium concentrations on DNA content, procollagen type I carboxy-terminal propeptide (PICP) release (reflects type I collagen synthesis), and alkaline phosphatase activity of porcine osteoblasts. Osteoblasts seeded in control medium (2.2 mM calcium) were transferred to low (0. 5 or 1 mM) calcium medium or to high (3, 5, 7, or 10 mM) calcium medium at different stages of the culture period and for different incubation times. When osteoblasts were transferred to low or high (3 or 5 mM) calcium medium 1 or 2 days after plating and kept in that medium until the end of the culture period, PICP release was inhibited, but DNA content and alkaline phosphatase activity were unchanged, except in 5 mM calcium, which inhibited alkaline phosphatase activity. Short-term culture of subconfluent and near-confluent osteoblasts in 7 or 10 mM calcium for 48 h inhibited DNA content. DNA content returned to normal levels when cells were transferred back to control medium, whereas alkaline phosphatase inhibition induced by 5, 7, or 10 mM calcium was not reversible. Short-term culture in high calcium media did not affect PICP release. Thus, in porcine osteoblasts, low and high extracellular calcium concentrations affect DNA content, PICP release, and the expression of osteoblastic phenotype markers (alkaline phosphatase activity). These effects are dependent on the duration of calcium treatment and the state of differentiation of the osteoblasts.

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Aspirin consumption has been reported to be able to reduce colorectal cancer risk in humans and in animal models of colon carcinogenesis. Although the mechanism involved in such an effect is not yet clear, both prostaglandin-dependent and -independent effects have been proposed. Using HT-29 Glc(-/+)cells, which originate from a human colon adenocarcinoma, we demonstrated in this study a dose-dependent effect of millimolar concentration of aspirin on cell growth that was concomitant with a rapid accumulation of the cells in the G0/G1 phase, followed by an accumulation in the G2/M phase and by a minor increase in the proportion of cells undergoing nuclear condensation. Cell membrane integrity and cell release into the culture medium were not affected by this treatment. The aspirin effects were apparently unrelated to prostaglandin biosynthesis inhibition, since although these cells were found to express high levels of cyclooxygenase 1 (COX-1) and low levels of COX-2 proteins, they did not produce any measurable net amounts of prostaglandins, based on both utilization of radiolabelled arachidonic acid and the radioimmunoassay of prostaglandins E2 and F2 alpha. In contrast, we identified polyamine biosynthesis as a cellular target of aspirin, since the treatment of HT-29 Glc(-/+) cells with aspirin reduced the flux of L-ornithine through ornithine decarboxylase, an effect that could not be explained by an acute action of the drug on the ornithine decarboxylase catalytic activity. Since polyamine biosynthesis is strictly necessary for HT-29 cell growth, our data suggest that reduced flux through ornithine decarboxylase may participate in the antiproliferative activity of aspirin towards colonic tumoral cells. It is concluded that in HT-29 Glc(-/+) cells that are not functional for prostaglandin production, aspirin can affect cell growth, cell cycle, and polyamine biosynthesis without affecting cell membrane integrity.