Cortisol enhancement of PTH-stimulated cyclic AMP accumulation in cultured fetal rat long bone rudiments

Salmon calcitonin reduces vertebral bone loss in glucocorticoid-treated beagles

Glucocorticoids induce bone loss by increasing bone resorption and decreasing bone formation. In this study we have investigated the potential of salmon calcitonin to attenuate glucocorticoid-induced bone loss in a dog model. Male beagles were divided into three groups: 1) untreated controls, 2) prednisone-treated dogs (1 mg.kg-1.day-1 orally), and 3) prednisone-calcitonin-treated dogs (1.5 U.kg-1.day-1 calcitonin subcutaneously and 1 mg.kg-1.day-1 prednisone orally). Assessment of bone mass by dual energy X-ray absorptiometry demonstrated that bone density remained stable in controls throughout 48 wk. Prednisone-treated dogs lost 13.2% of their initial bone mass by 48 wk. Concomitant calcitonin treatment attenuated prednisone-induced bone loss to only 3.2% at 48 wk. Bone histomorphometry of the spine showed reduced trabecular bone volume in prednisone-treated dogs, whereas control and prednisone-calcitonin-treated animals maintained normal trabecular bone volume. Both prednisone- and prednisone-calcitonin-treated dogs acquired a defect in osteoblastic function as evidenced by a reduction in mean wall thickness and trabecular thickness. Thus calcitonin attenuates the early bone loss induced by glucocorticoids. However, calcitonin failed to prevent glucocorticoid-induced osteoblast dysfunction.

Glucocorticoids and dopamine-1 receptors on vascular smooth muscle cells

The effect of glucocorticoids on the dopamine (DA)-mediated cyclic adenosine monophosphate (cAMP) by intact vascular smooth muscle cells (VSMC) was studied in rats. Cultured VSMC were obtained from renal arteries of 14-week-old Wistar-Kyoto rats by explant method. Micromolar concentrations of dexamethasone (DEX) pretreatment for 48 hours potentiated DA-mediated response without any change of affinity constant. However, micromolar concentrations of aldosterone pretreatment for 48 hours had almost no effect on DA-mediated response. The DEX-induced facilitation began at 6 hours and reached maximum at 24 hours after DEX administration in a dose-dependent manner. Inhibitors of protein and RNA synthesis blocked this glucocorticoid effect. The basal activity of adenylate cyclase in DEX-treated cells was twofold higher than that in control cells. Treatment of VSMC with DEX increased cholera toxin-stimulated and forskolin-stimulated adenylate cyclase activity. However, pertussis toxin treatment did not augment or reduce the effect of DEX treatment. These results suggest that glucocorticoids increase DA-mediated cAMP formation by VSMC through glucocorticoid type II receptors and the induction of protein synthesis and that the activation of the catalytic unit may play some role in this facilitation.

Increase of adenylate cyclase catalytic-unit activity by dexamethasone in rat osteoblast-like cells

Glucocorticoids are known to increase the cyclic AMP response to parathyroid hormone (PTH) in cultured bone organs or bone cells. Using the osteoblast-like cell line ROS 17/2.8, which possesses receptors for both PTH and glucocorticoids, we investigated which component of the complex hormone receptor-guanine nucleotide regulatory unit--adenylate cyclase was affected by dexamethasone treatment. In response to PTH, isoproterenol or forskolin, a compound that is supposed to act directly on the catalytic unit, cyclic AMP production by intact cells and adenylate cyclase activity in purified plasma membrane were markedly increased by dexamethasone. Whereas NaF, guanosine 5'-[beta gamma-imido]triphosphate and Mn/ stimulated adenylate cyclase activity were similarly enhanced in membranes isolated from glucocorticoid-treated cells, the activity of the stimulatory guanine nucleotide regulatory unit, as assessed by reconstitution into membranes from the CYC- clone, which is genetically devoid of this component, was not altered. Thus in osteoblast-like cells dexamethasone appears to increase cyclic AMP synthesis by influencing the catalytic unit. Moreover, since it has been reported that glucocorticoids may produce changes in cell calcium metabolism, we evaluated cytoplasmic free Ca2+ concentration ([Ca2+]i) and intracellular Ca2+ stores mobilizable by the bivalent-cationophore ionomycin, by using the intracellular fluorescent indicator Quin-2. The results indicated that dexamethasone treatment did not influence [Ca2+]i but markedly decreased ionomycin-releasable Ca2+ stores.

Hypocalcaemia induced by glucocorticoids in a child with hypoparathyroidism treated with 1-alpha-hydroxyvitamin D3

Glucocorticoids tend to lower the intestinal absorption of calcium, leading to a negative calcium balance. A 7-year-old girl with hypoparathyroidism was maintained as normocalcaemic on 1-alpha-hydroxyvitamin D3 (1 alpha D3) and calcium gluconate lactate. During an episode of aplastic anaemia she was treated with prednisolone, with a subsequent dramatic fall of serum calcium despite 1 alpha D3 treatment and serum 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in the high-normal range. Glucocorticoids seem to have interfered directly with cellular events responsible for intestinal absorption of calcium.

Modulation of PTH-stimulated cyclic AMP in cultured rodent bone cells: the effects of 1,25(OH)2 vitamin D3 and its interaction with glucocorticoids

Parathyroid hormone (PTH)-stimulated cyclic adenosine monophosphate (cAMP) in rat osteoblastlike (OB) cells has been shown to be modulated by steroid hormones; glucocorticoids are known to increase the level, while the effects of 1,25(OH)2D3 are inhibitory. In the present study, we found that the PTH-stimulated cAMP responses are similar in neonatal mouse and fetal rat OB cells. Dexamethasone (0.13-13 nM) augmented PTH-stimulated cAMP in both species. Mouse cells showed a higher maximal response to dexamethasone (100% increment) than rat cells (60-70% increment) with similar sensitivity to dexamethasone (ED50 approximately 1.0 nm). On the other hand, 1,25(OH)2D3 decreased PTH-stimulated cAMP, but the effect required pharmacological levels of hormone; mouse cells responded at a lower dose (1.3 nM) and were more sensitive than rat cells (responded at 13 nM) to 1,25(OH)2D3 treatment. Introduction of physiological concentrations of 1,25(OH)2D3 (0.013-1.3 nm) in addition to dexamethasone (13 nM) resulted in a synergistic enhancement of PTH-stimulated cAMP in rat cells. In contrast, a dose-dependent antagonistic effect was observed in mouse cells. In summary, our findings demonstrate species and concentration-dependent differences in hormonal responses to 1,25(OH)2D3 and a complex interplay among PTH, dexamethasone, and 1,25(OH)2D3.

Phosphaturic response of hydrocortisone in the presence and the absence of parathyroid hormone

The effect of hydrocortisone (HC) on the renal Pi transport in the presence and in the absence of parathyroid hormone (PTH) in the rat was examined using clearance techniques. During constant infusion of PTH in parathyroidectomized (PTX) rats, HC decreased the reabsorption of Pi from 2.84 to 2.33 mumol/min (P less than 0.001), the TmPi from 4.55 to 3.83 mumol/min (P less than 0.001), the reabs. Pi/GFR values from 0.93 to 0.74 (P less than 0.001) and the TmPi/GFR from 1.73 to 1.42 mumol/ml (P less than 0.001). In the absence of PTH, HC diminished the TmPi from 8.36 to 6.58 mumol/min (P less than 0.005) and the TmPi/GFR from 3.36 to 2.51 mumol/ml (P less than 0.001). It is concluded that the phosphaturic response of hydrocortisone may be important in the homeostasis of inorganic phosphate in the body.