Decrease in the basal levels of cytosolic free calcium in chondrocytes during aging in culture: possible role as differentiation-signal

Flow-induced calcium oscillations in rat osteoblasts are age, loading frequency, and shear stress dependent

Bone adaptation to mechanical loading is dependent on age and the frequency and magnitude of loading. It is believed that load-induced fluid flow in the porous spaces of bone is an important signal that influences bone cell metabolism and bone adaptation. We used fluid flow-induced shear stress as a mechanical stimulus to study intracellular calcium (Ca) signaling in rat osteoblastic cells (ROB) isolated from young, mature, and old animals. Fluid flow produced higher magnitude and more abundant [Ca(2+)](i) oscillations than spontaneous oscillations, suggesting that flow-induced Ca signaling encodes a different cellular message than spontaneous oscillations. ROB from old rats showed less basal [Ca(2+)](i) activity and were less responsive to fluid flow. Cells were more responsive to 0.2 Hz than to 1 or 2 Hz and to 2 Pa than to 1 Pa. These data suggest that the frequency and magnitude of mechanical loading may be encoded by the percentage of cells displaying [Ca(2+)](i) oscillations but that the ability to transduce this information may be altered with age.

Strontium ranelate increases cartilage matrix formation

Based on previous studies showing that strontium ranelate (S12911) modulates bone loss in osteoporosis, it could be hypothesized that this drug also is effective on cartilage degradation in osteoarthritis (OA). This was investigated in vitro on normal and OA human chondrocytes treated or not treated with interleukin-1beta (IL-1beta). This model mimics, in vitro, the imbalance between chondroformation and chondroresorption processes observed in vivo in OA cartilage. Chondrocytes were isolated from cartilage by enzymatic digestion and cultured for 24-72 h with 10(-4)-10(-3) M strontium ranelate, 10(-3) M calcium ranelate, or 2 x 10(-3) M SrCl2 with or without IL-1beta or insulin-like growth factor I (IGF-I). Stromelysin activity and stromelysin quantitation were assayed by spectrofluorometry and enzyme amplified sensitivity immunoassay (EASIA), respectively. Proteoglycans (PG) were quantified using a radioimmunoassay. Newly synthesized glycosaminoglycans (GAGs) were quantified by labeled sulfate (Na2(35)SO4) incorporation. This method allowed the PG size after exclusion chromatography to be determined. Strontium ranelate, calcium ranelate, and SrCl2 did not modify stromelysin synthesis even in the presence of IL-1beta. Calcium ranelate induced stromelysin activation whereas strontium compounds were ineffective. Strontium ranelate and SrCl2 both strongly stimulated PG production suggesting an ionic effect of strontium independent of the organic moiety. Moreover, 10(-3) M strontium ranelate increased the stimulatory effect of IGF-I (10(-9) M) on PG synthesis but did not reverse the inhibitory effect of IL-1beta. Strontium ranelate strongly stimulates human cartilage matrix formation in vitro by a direct ionic effect without stimulating the chondroresorption processes. This finding provides a preclinical basis for in vivo testing of strontium ranelate in OA.

Cytosolic [Ca(2+)] modulates basal GLUT1 activity and plays a permissive role in its activation by metabolic stress and insulin in rat epithelial cells

The aim of this work was to investigate the role of cytosolic free calcium ([Ca(2+)]c) in the stimulation of GLUT1 by metabolic stress and insulin. Chelation of [Ca(2+)]c with bapta, introduced in rat liver epithelial Clone 9 cells in the acetoxymethyl (AM) form, decreased their basal rate of 2-deoxyglucose uptake in a dose-dependent fashion. Maximal inhibition at 75 microM bapta was by 38 +/- 8% (n = 8). The effect was partially reversed by ionomycin. Basal sugar uptake was also decreased by lowering extracellular [Ca(2+)] in ionomycin-permeabilized cells. Increasing [Ca(2+)]c over its resting level of 168 +/- 32 (n = 27) had no affect on sugar uptake. Chelation of [Ca(2+)]c did not change the abundance of surface GLUT1 and had no significant effect on the affinity of GLUT1 for sugars. In addition, calcium chelation abolished the activation of GLUT1 by azide, arsenate, 2,4-dinitrophenol and insulin. However, [Ca(2+)]c did not increase in the presence of azide. We conclude that [Ca(2+)]c, near or below its resting level, modulates GLUT1 activity over a considerable range and plays a permissive role in the activation of the carrier by metabolic stress and insulin.

Age-related decreases in stimulatory G protein-coupled adenylate cyclase activity in osteoblastic cells

In this study we examined parathyroid hormone (PTH)-, forskolin (FSK)-, and cholera toxin (CTX)-stimulated adenosine 3',5'-cyclic monophosphate (cAMP) accumulation in rat osteoblastic cells (ROB) isolated from young (4 mo), mature (12 mo), and old (24-28 mo) male rats. Exposure to PTH increased cAMP accumulation in a concentration-dependent manner in all ROB cells examined. However, the maximum response in ROB from young rats was threefold greater than the maximum response in those from mature and old rats. Exposure to FSK also stimulated cAMP accumulation in a concentration-dependent manner, but there were no significant differences in responsiveness among ROB isolated from young, mature, and old rats. Exposure to CTX resulted in a dramatic concentration-dependent increase in cAMP in ROB from young rats but only a modest increase in ROB from mature and old rats. PTH binding kinetics were similar in ROB from rats in each age group. These data suggest an age-related defect in stimulatory G protein coupling to adenylate cyclase, which contributes to decreased osteoblastic responsiveness to PTH.

The effect of hydrocortisone and thyroxine on development of calcium homeostasis in embryonic intestinal epithelium

Cytoplasmic Ca2+ concentration of epithelial cells from 14-day embryonic chick duodena decreased during 72 h of organ culture to a value 54% of that found at 17 days in vivo. The ability of cells to maintain a constant Ca2+ concentration when challenged with high extracellular calcium was also significantly reduced. Addition of 1 microM hydrocortisone during culture restored both parameters of Ca2+ homeostasis to that of 16-day uncultured duodena, and rise in cytoplasmic Ca2+ was significant within 4 h of hormone treatment. Thyroxine influenced epithelial Ca2+ similarly, but to a lesser degree and only after 48-72 h of culture. These data indicate that glucocorticoids, and possibly thyroid hormones, influence the development of calcium homeostasis in intestinal epithelium.