Lower Parathyroid Hormone Concentrations Research Articles

Divergent effects of protein kinase C (PKC) inhibitors staurosporine and bisindolylmaleimide i (GF109203X) on bone resorption

Activation of protein kinase C (PKC) has been suggested to play a role in bone resorption. However, phorbol esters, which activate PKC, have been reported to have both stimulatory and inhibitory effects on bone resorption. To study the role of PKC in bone resorption further, we have measured calcium release elicited by bone-resorbing hormones from isolated bones incubated with the PKC inhibitors staurosporine (ST) and the more PKC-selective ST analog bisindolylmaleimide I (GF109203X; GF). In fetal rat limb bone organ cultures, ST (1 μM) or GF (1 μM) significantly reduced the bone resorption induced by maximal concentrations of parathyroid hormone (PTH). However, when submaximal concentrations of PTH were used, lower concentrations of the two antagonists had divergent effects. GF (20–300 nM) acted solely as an antagonist, whereas ST (10–100 nM) significantly enhanced resorptive responses to PTH. ST also enhanced the bone resorption elicited by α-thrombin, tumor necrosis factor-α (TNF-α), and thyroxin (T4). ST alone had small stimulatory effects in some experiments. GF prevented the stimulatory effects of ST alone as well as the enhancing effect of ST on PTH-stimulated resorption. The divergent effects of ST and GF on the responses of bone to low concentrations of PTH and the ability of GF to antagonize the stimulatory effects of ST suggest that PKC isozymes have complex and even antagonistic effects on bone resorption.

Effects of rat fetuin on stimulation of bone resorption in the presence of parathyroid hormone.

Rat fetuin, which is the rat counterpart of human alpha 2-HS glycoprotein and bovine fetuin, is only detectable in calcified tissues such as bone matrices and dentin, and bone cells such as osteoblasts and osteocytes immunohistochemically. The effect of this protein on bone resorption was examined to study its physiological role in bone metabolism. Rat fetuin increased bone resorption in the presence of low concentrations of parathyroid hormone (PTH), but it had no activity on bone resorption without PTH. The increase in bone resorption by PTH and PTH plus rat fetuin was inhibited by the addition of chymostatin, an inhibitor for cathepsin L. Moreover, we found that when type I collagen from rat was preincubated with rat fetuin, the digestion of rat type I collagen by cathepsin L was increased. These findings suggest that rat fetuin present in bone matrix is important in bone resorption.

Subclinical vitamin D deficiency in neonates: definition and response to vitamin D supplements

To determine the biological criteria for neonatal vitamin D deficiency, serum 25-hydroxyvitamin D (calcidiol), parathyroid hormone (PTH), calcium, phosphates, and alkaline phosphatase (ALP) activity were measured during the winter-spring period in 80 healthy neonates and their mothers 3-6 d after delivery. A longitudinal 3-mo survey of the serum biology of 52 of these neonates consuming formula was also performed to test the influence of their neonatal vitamin D status on the effects of two oral ergocalciferol supplements (500 and 1000 IU or 12.5 and 25 micrograms/d). At birth, 63.7% of the infants had calcidiol concentrations < or = 30 nmol/L. Most of them had no other biological sign evocative of vitamin D deficiency, but 14 neonates had low calcidiol concentrations and serum PTH concentrations > 60 ng/L, the upper limit of the adult normal range. They also had a significantly lower mean serum calcium concentration than did neonates with calcidiol concentrations > 30 nmol/L. On the basis of the association of low calcidiol concentrations (< or = 30 nmol/L) and high PTH concentrations (> 60 ng/L) as criteria for vitamin D deficiency, 24% of the neonates born to unsupplemented mothers were found to be vitamin D-deficient. Neonatal vitamin D status influenced the response of the infants to vitamin D supplements. Neonates with no sign of vitamin D deficiency showed similar changes in their serum calcidiol, calcium, phosphate, and PTH concentrations and ALP activity and no toxic effect (hypercalcemia or highly elevated calcidiol concentration) was observed whatever their vitamin D intake. In contrast, neonates with subclinical vitamin D deficiency had normalized serum PTH within 1 mo only when they were given 1000 IU ergocalciferol (25 micrograms)/d in addition to their formula.

Receptors for parathyroid hormone and parathyroid hormone-related peptide: from molecular cloning to definition of diseases.

The parathyroid hormone/parathyroid hormone-related peptide receptor belongs to a distinct family of G protein-coupled receptors, the members of which usually signal through at least two second messenger systems, adenylate cyclase and phospholipase C. The parathyroid hormone/ parathyroid hormone-related peptide receptor is most abundantly expressed in bone, kidney and growth-plate chondrocytes, and, at lower levels, in a variety of fetal and adult tissues. To search for human diseases that are caused by parathyroid hormone/parathyroid hormone-related peptide receptor defects, genomic DNA of patients with pseudohypoparathyroidism type Ib and of patients with Jansen's metaphyseal chondrodysplasia was screened for mutations in all coding exons of the receptor gene. Inactivating parathyroid hormone/parathyroid hormone-related peptide receptor mutations were excluded in patients with pseudohypoparathyroidism type Ib. However, a receptor mutation that causes agonist-independent, constitutive cAMP accumulation was identified in a patient with Jansen's metaphyseal chondrodysplasia, a rare form of short-limbed dwarfism associated with hypercalcemia despite normal or low concentrations of parathyroid hormone and parathyroid hormone-related peptide. These findings allow the conclusion to be drawn that parathyroid hormone/parathyroid hormone-related peptide receptors mediate the endocrine actions of parathyroid hormone, which are required for the control of calcium homeostasis and the autocrine-paracrine actions of parathyroid hormone-related peptide, which are required for normal growth-plate development.

Interactions of tumor necrosis factor with local and systemic factors in fetal rat limb bones.

In many tissues the actions of tumor necrosis factor-alpha (TNF) are indirectly mediated through the production of autacoids or other cytokines. To determine the role that these factors might have in the action of TNF on bone resorption, we examined the effects of several selective inhibitors on TNF-stimulated resorption. The cyclooxygenase inhibitor indomethacin did not prevent TNF-stimulated resorption in fetal rat limb bones. Stimulation of resorption by TNF was also unaffected by the platelet activating factor antagonist WEB 2086. A 17.5 kD interleukin receptor antagonist protein, at concentrations that completely blocked the bone-resorbing actions of maximally effective concentrations of interleukin (IL)-1 beta, failed to affect the stimulatory actions of TNF. TNF-stimulated resorption was inhibited by both interferon-gamma and dexamethasone. Dexamethasone inhibited TNF-stimulated resorption more effectively than it inhibited parathyroid hormone (PTH)-stimulated resorption. When bones were treated simultaneously with low concentrations of TNF and PTH, potentiation of the bone-resorbing effects was elicited. These results suggest that TNF stimulates resorption through a pathway different from that by which PTH produces its effects. Transforming growth factor-beta (TGF-beta) enhanced responses to TNF; TGF-beta failed to inhibit the effects of TNF, even in long-term culture or when bones were pretreated with TGF-beta. Synergistic interactions between TNF and several other bone-resorbing factors have now been demonstrated. In contrast to the actions of TNF on certain other functions, the bone-resorbing effects of TNF, as determined in the fetal rat limb bone system, do not seem to be mediated by PAF, IL-1, or prostaglandins.

Cellular mechanisms in proximal tubular reabsorption of inorganic phosphate

Filtered inorganic phosphate (Pi) is largely reabsorbed in the proximal tubule. Na-Pi cotransport, with a stoichiometry of at least 2:1, mediates uphill transport at the apical membrane; at the basolateral membrane different types of transport systems can be involved in efflux and uptake of Pi from the interstitium. Regulation of transcellular Pi flux involves alteration of the apical Na-Pi cotransport; at least three different cellular control/sensing systems seem to participate in this regulation and are exemplified by parathyroid hormone (PTH)-dependent inhibition, Pi deprivation-dependent increase, and insulin-like growth factor I (IGF-I)-dependent increase in Na-Pi cotransport. For PTH inhibition, recent evidence suggests a role of the phospholipase C/protein kinase C-dependent regulatory cascade in inhibition of Na-Pi cotransport, at least at low PTH concentrations. In addition, an endocytic mechanism seems to be involved in this PTH action. Little is known of the cellular mechanisms in Pi deprivation-dependent and/or IGF-I-dependent increases in Na-Pi cotransport; they are dependent on de novo protein synthesis. Recent experiments involving an expression in Xenopus laevis oocytes led to the identification of an approximately 50 kDa membrane protein that is a good candidate for being involved in brush-border membrane Na-Pi cotransport activity.

Activation of bovine renal cortex membrane adenylyl cyclase by low concentrations of parathyroid hormone.

Under conditions designed to reduce non-specific adsorption of parathyroid hormone (PTH), the sensitivity of the renal membrane adenylyl cyclase to PTH was significantly enhanced. Stimulation of the enzyme could be observed at hormone concentrations as low as 2 x 10-11M. In addition, kinetic analysis of hormone activation revealed that under conditions where non-specific adsorption is great, downward concavity of Eadie-Hofstee plots is observed, whereas when such adsorption is reduced these plots become upwardly concave. These results suggest that in the absence of non-specific adsorption, PTH activation of kidney membrane adenylyl cyclase can occur at hormone concentrations approaching physiological. In addition, at low hormone concentrations PTH activation is probably more complex than previously recognized.

Enhancement of Parathyroid Hormone-Stimulated Bone Resorption by Poly-L-Lysine*

Poly-L-lysine (PL II; mol wt, 1000-4000) was added to fetal rat bones cultured in a chemically defined medium (BGJ) containing bovine serum albumin in the presence and absence of parathyroid hormone (PTH). Bone resorption was measured by the release of previously incorporated 45Ca. The addition of PL II at concentrations of 3-100 microgram/ml enhanced the stimulation of bone resorption by submaximal doses of PTH but had little effect on 45Ca release from control unstimulated cultures. Higher concentrations of PL II produced inhibition of 45Ca release. Dialysis of PL II did not alter enhancement or inhibition by PL II. PL II did not increase sensitivity to PTH in serum-supplemented cultures. Higher molecular weight PL II preparations were less effective. PL II did not enhance the resorptive response to 1,25-dihydroxyvitamin D, prostaglandin E2, osteoclast-activating factor, or bacterial endotoxin. The mechanism of the selective ability of PL II to enhance the response to low concentrations of PTH is unknown but may be due to the ability of this basic polypeptide to interfere with binding of PTH to sites other than the hormone receptor or to block degradation of PTH by bone.

Secretion and metabolic efficacy of parthyroid hormone in patients with severe hypomagnesemia.

ABSTRACT Three adult patients with hypocalcemia secondary to severe magnesium depletion were evaluated in order to determine the etiology of hypocalcemia in this syndrome. In each patient, the concentration of circulating parathyroid hormone was within the range of normal despite severe hypocalcemia and hypomagnesemia. In the 2 patients tested, circulating parathyroid hormone increased transiently above normal in response to treatment with magnesium. Urinary excretion of phosphate and cyclic AMP increased normally in response to an acute infusion of parathyroid extract and serum calcium increased 2 mg/100 ml and 1.4 mg/100 ml, respectively, in response to intramuscular injections of the hormone in the 2 patients who were tested. It was concluded that the normal but inappropriately low concentrations of parathyroid hormone were attributable to hypomagnesemia and could account for the perpetuation but not the onset of hypocalcemia in this syndrome. Peripheral resistance to parathyroid hormone is not operati...

Hyperparathyroidism due to clear cell hyperplasia: Serial determinations of serum ionized calcium, parathyroid hormone and calcitonin

Serial determinations of serum ionized calcium (Ca ++), serum total calcium (Ca), parathyroid hormone (PTH) and calcitonin levels were made in a patient with hyperparathyroidism due to clear cell hyperplasia. Although serum Ca declined to normal levels prior to parathyroidectomy, serum Ca ++ and PTH levels remained elevated until after operation. This suggests that the calcium ion electrode may prove valuable in the diagnosis and management of patients with hyperparathyroidism, especially when they are normocalcemic. The excised parathyroid glands yielded very low PTH concentrations upon extraction, implying that in this form of hyperparathyroidism PTH secretion is excessive whereas storage is markedly reduced. CT levels were normal in both serum and excised glands. This finding is of interest in relation to the patient's associated generalized neurofibromatosis and serves further to distinguish the present syndrome from that characterized by mucosal neuromas, hyperparathyroidism and medullary carcinoma of the thyroid.

Clinical applications of radioimmunoassay of plasma parathyroid hormone

The lack of sufficient quantities of human parathyroid hormone (PTH) for antibody production is the single most important factor responsible for the slow progress in the development of ideal radioimmunoassays of human PTH in plasma. Use of an assay which employs an antibovine PTH antiserum, however, has proved applicable to clinical problems of parathyroid dysfunction. Further progress in the characterization of heterogeneous forms of plasma PTH has been prevented by the great technical difficulties encountered in the fractionation and separation of low concentrations of PTH from plasma in systems using molecular sieve technics or zone electrophoresis.