Effect of stress on calcium homeostasis in sheep

The acute effect of stress on the plasma and cerebrospinal fluid (CSF) immunoreactive parathyroid hormone (PTH) response was studied in 6 merino sheep. Stress was exerted by weak periodical electric square waves (PESW). In addition, the effect of intravenous injection of adrenaline was studied. Under stress conditions (PESW or adrenaline injection), plasma PTH increased up to 30% and up to 50%, respectively. Weak periodical electric square waves of 3-4 mA decreased CSF PTH concentrations by up to 50%. The effect of adrenaline injection on the CSF PTH was not significant. Total calcium and magnesium in plasma and CSF did not change. Our results showed that the effect of stress on the CSF PTH is opposite to the effect on plasma PTH, and suggest that both, PESW and adrenaline, affected PTH in plasma and CSF by a Ca-independent mechanism.     

Separation of inhibitory activity from biologically active parathyroid hormone in patients with pseudohypoparathyroidism type I

Patients with pseudohypoparathyroidism type I have the symptoms of hypoparathyroidism despite elevated levels of immunoreactive parathyroid hormone (PTH). However, the circulating levels of bioactive PTH, as measured in a cytochemical bioassay, are generally within the normal range suggesting that the high levels of immunoreactive PTH are either due to the presence of biologically inactive fragments of parathyroid hormone or to the presence of an 'inhibitor' of PTH bioactivity. Gel-permeation chromatography has been used to fractionate plasma from patients with pseudohypoparathyroidism type I and revealed the presence of high levels of bioactive PTH and of an 'inhibitor'. This inhibitory activity was absent or much lower in plasma from control subjects. These results indicate, therefore, that in pseudohypoparathyroidism type I the expression of the biological activity of PTH at the level of the kidney is affected by the presence of a circulating inhibitor which can be separated from intact PTH by gel-permeation chromatography.     

Tumour necrosis factor-alpha mediates blood-brain barrier damage in HIV-1 infection of the central nervous system

The pathogenesis of brain inflammation and damage by human immunodeficiency virus (HIV) infection is unclear. Because blood-brain barrier damage and impaired cerebral perfusion are common features of HIV-1 infection, we evaluated the role of tumour necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) in mediating disruption of the blood-brain barrier. Levels of TNF-alpha were more elevated in cerebrospinal fluid (CSF) than in serum of HIV-1 infected patients and were mainly detected in those patients who had neurologic involvement. Intrathecal TNF-alpha levels correlated with signs of blood-brain barrier damage, manifested by high CSF to serum albumin quotient, and with the degree of barrier impairment. In contrast, intrathecal IL-1beta levels did not correlate with blood-brain barrier damage in HIV-1 infected patients. TNF-alpha seems to be related to active neural inflammation and to blood-brain barrier damage. The proinflammatory effects of TNF-alpha in the nervous system are dissociated from those of IL-1beta.     

Effect of parathyroid hormone and forskolin on cytoskeletal protein synthesis in cultured mouse osteoblastic cells

Parathyroid hormone (PTH) has been shown to cause transient cell shape changes in bone cells. We have examined the effects of parathyroid hormone and forskolin on the organization and expression of cytoskeletal proteins in cultured mouse endosteal osteoblastic cells. Analysis of [35S]methionine-labeled cytoskeletal proteins isolated on two-dimensional gel electrophoresis showed that PTH treatment (24 h) stimulated the de novo biosynthesis of actin, vimentin and tubulins in confluent cells, whereas forskolin had a minor effect despite a huge stimulation of cAMP production. This PTH-induced stimulation was associated with cell respreading following a mild and transitory cell retraction. PTH increased the synthesis of monomeric subunits of actin and beta-tubulins in subconfluent bone cells, whereas both monomeric and polymeric levels of beta-tubulins were increased in confluent osteoblasts. Under conditions reducing cell spreading, osteoblastic cells had initially high levels of unpolymerized subunits. In these poorly spread cells, parathyroid hormone or forskolin had no effect on the de novo synthesis of cytoskeletal proteins despite a marked elevation in intracellular cAMP levels. It is concluded that PTH affects the biosynthesis of cytoskeletal proteins in osteoblastic cells and that cAMP production does not seem to be directly involved. In addition, the effect of PTH is modulated by cell spreading and by the initial pool of cytoskeletal subunits.     

Increase of nocturnal melatonin levels in hemodialyzed patients after parathyroidectomy: a pilot study

In hemodialyzed patients hormonal disturbances are known to occur. However, melatonin levels have not been completely studied. The aim of the study was to find whether changes in calcaemia affect melatonin secretion. For this reason we followed the nocturnal serum concentrations of melatonin and parathyroid hormone (PTH) in 9 hemodialyzed patients (6 women and 3 men, aged 37-65 years) both before and 1-3 months after parathyroidectomy at 6 p.m., 9 p.m., 11 p.m., 2 a.m., 5 a.m. and 7 a.m. At 6 p.m. blood samples to evaluate the levels of calcium and phosphate were also collected. Parathyroidectomy resulted in an increase in nocturnal melatonin levels. As expected, the parathyroidectomy was followed by considerable PTH decrease. PTH showed no nocturnal variation before or after parathyroidectomy. Calcium levels significantly decreased after the operation while phosphate levels increased. In summary, in hemodialyzed patients with hyperparathyroidism, parathyroidectomy significantly increases the nocturnal secretion of melatonin. Relationships between the pineal gland and parathyroid glands have yet to be elucidated.     

Parathyroid hormone release is not associated with acute sympathetic arousal in goats.

Physiological activation of the sympathetic adrenomedullary (SAM) axis following both thermal and non thermal stress was assessed by changes in serum norepinephrine, glucose and/or protein as well as indices of peripheral blood flow. The occurrence of elevated serum parathyroid hormone (PTH) did not reliably reflect SAM activation as might be predicted from pharmacological studies that document a beta adrenergic receptor mechanism in the parathyroid gland that mediates catecholamine stimulated release of PTH into the circulation. The beta agonist isoproterenol at 1 microgram/min for 60 min did produce a transient increase in serum PTH at 20 min. Overall, the data raise doubts about the physiological significance of the adrenergic receptor in the parathyroid gland. Significant increases in serum PTH of 67% and 109% above basal respectively were seen following ruminal loading with cold and thermoneutral water. Associated with the PTH change were increased serum phosphorus and elevated or constant serum protein and serum total calcium.     

Parathyroid function in acromegaly.

Parathyroid function was assessed in six acromegalic subjects by measurement of serum levels of parathyroid hormone (PTH). Only one patient had increased PTH levels. These findings suggest that hyperparathyroidism does not uniformly occur in acromegaly.     

The Vitamin D,Ionised Calcium and Parathyroid Hormone Axis of Cerebral Capillary Function: Therapeutic Considerations for Vascular-Based Neurodegenerative Disorders

Blood-brain barrier dysfunction characterised by brain parenchymal extravasation of plasma proteins may contribute to risk of neurodegenerative disorders, however the mechanisms for increased capillary permeability are not understood. Increasing evidence suggests vitamin D confers central nervous system benefits and there is increasing demand for vitamin D supplementation. Vitamin D may influence the CNS via modulation of capillary function, however such effects may be indirect as it has a central role in maintaining calcium homeostasis, in concert with calcium regulatory hormones. This study utilised an integrated approach and investigated the effects of vitamin D supplementation, parathyroid tissue ablation (PTX), or exogenous infusion of parathyroid hormone (PTH) on cerebral capillary integrity. Parenchymal extravasation of immunoglobulin G (IgG) was used as a marker of cerebral capillary permeability. In C57BL/6J mice and Sprague Dawley rats, dietary vitamin D was associated with exaggerated abundance of IgG within cerebral cortex (CTX) and hippocampal formation (HPF). Vitamin D was also associated with increased plasma ionised calcium (iCa) and decreased PTH. A response to dose was suggested and parenchymal effects persisted for up to 24 weeks. Ablation of parathyroid glands increased CTX- and HPF-IgG abundance concomitant with a reduction in plasma iCa. With the provision of PTH, iCa levels increased, however the PTH treated animals did not show increased cerebral permeability. Vitamin D supplemented groups and rats with PTH-tissue ablation showed modestly increased parenchymal abundance of glial-fibrillary acidic protein (GFAP), a marker of astroglial activation. PTH infusion attenuated GFAP abundance. The findings suggest that vitamin D can compromise capillary integrity via a mechanism that is independent of calcium homeostasis. The effects of exogenous vitamin D supplementation on capillary function and in the context of prevention of vascular neurodegenerative conditions should be considered in the context of synergistic effects with calcium modulating hormones.     

Inhibition of PTH Secretion by Interleukin-1β in Bovine Parathyroid Glandsin VitroIs Associated with an Up-Regulation of the Calcium-Sensing Receptor mRNA

The principal regulator of parathyroid hormone (PTH) secretion is ionized calcium, but other factors are also known to modulate PTH secretion, such as vitamin D, estrogen, and recently inorganic phosphate. Interleukin-1 (IL-1) possesses a wide variety of biological activities and is produced by leukocytes as well as by various other cells including cells from endocrine tissues and might play a role as a paracrine factor in the control of PTH secretion. We investigated the effectin vitroof IL-1β on PTH release, PTHmRNA and the mRNA for the extracellular calcium-sensing receptor (CaR) levels in preparations of bovine parathyroid cells. PTH secretion from cultured parathyroid tissue slices was significantly inhibited in a medium containing IL-1β at a concentration of 2000 pg/ml (PTH in % of control: 63.5 ± 5.3), n=10 (p<0.01). The inhibitory effect of IL-1β was not found in preparations of dispersed cells. The inhibitory effect of IL-1β could be counteracted by the IL-1 receptor antagonist (IL-1ra), indicating that the inhibitory effect was mediated through the specific IL-1 receptor on the parathyroid cells. IL-1β (2000 pg/ml) up-regulated CaRmRNA levels to 180% of control, whereas no change in PTHmRNA was found. IL-1ra abolished the upregulating effect of IL-1β on the CaRmRNA. This study demonstrates a direct effectin vitroof IL-1β on PTH secretion from bovine parathyroid glands, an effect which may be mediated at least in part through the specific IL-1 receptor causing an upregulation of the calcium-sensing receptor mRNA. IL-1 might therefore play a role as a auto- and/or paracrine factor in the regulation of the PTH secretion.     

Effect of parathyroid hormone and forskolin on cytoskeletal protein synthesis in cultured mouse osteoblastic cells

Parathyroid hormone (PTH) has been shown to cause transient cell shape changes in bone cells. We have examined the effects of parathyroid hormone and forskolin on the organization and expression of cytoskeletal proteins in cultured mouse endosteal osteoblastic cells. Analysis of [³⁵S]methionine-labeled cytoskeletal proteins isolated on two-dimensional gel electrophoresis showed that PTH treatment (24 h) stimulated the de novo biosynthesis of actin, vimentin and tubulins in confluent cells, whereas forskolin had a minor effect despite a huge stimulation of cAMP production. This PTH-induced stimulation was associated with cell respreading following a mild and transitory cell retraction. PTH increased the synthesis of monomeric subunits of action and β-tubulins in subconfluent bone cells, whereas both monomeric and polymeric levels of β-tubulins were increased in confluent osteoblasts. Under conditions reducing cell spreading, osteoblastic cells had initially high levels of unpolymerized subunits. In these poorly spread cells, parathyroid hormone or forskolin had no effect on the de novo synthesis of cytoskeletal proteins despite a marked elevation in intracellular cAMP levels. It is concluded that PTH affects the biosynthesis of cytoskeletal proteins in osteoblastic cells and that cAMP production does not seem to be directly involved. In addition, the effect of PTH is modulated by cell spreading and by the initial pool of cytoskeletal subunits.     

The influence of PTH, calcium gluconate and EDTA on the parotid saliva in the goat

1. The role of exogenous parathyroid hormone (PTH) and stimulation or inhibition of endogenous hormone release, on the parotid gland of normal and thyroparathyroidectomized (t.x.p.t.x.) goats was studied. 2. The intravenous infusion of PTH and EDTA produced a transitory rise in saliva flow rate in intact animals. In t.x.p.t.x. goats the flow of saliva decreased transiently throughout the infusion. 3. The calcium levels in parotid saliva was unchanged throughout the infusion of PTH, EDTA, calcium gluconate both alone or with propranolol, in either intact or t.x.p.t.x. animals. 4. The parathyroid hormone infusion caused an increase in salivary phosphate concentration in both intact and operated goats. The effects of PTH upon the salivary flow and concentration of P are discussed.     

Fine structure of the parathyroid glands in baboons, Papio hamadryas in response to experimental hypercorticoidism

Female baboons maintained under laboratory conditions were subjected to a series of 10 weekly injections (4 mg/kg body weight) of the synthetic glucocorticoid, triamcinolone hexacetonide. In response to the treatment, serum immunoreactive parathyroid hormone (PTH) levels were raised, though blood calcium levels remained within normal physiological limits. Light and electron-microscopic studies were made on the parathyroid glands at the end of the experimental period. The baboon parathyroid glands were composed of 'light' and 'dark' forms of the chief cells in varying ratios from gland to gland even within a single animal. Glucocorticoid-induced parathyroid hyperactivity as measured by circulating PTH levels was not accompanied by cellular hypertrophy, though there was an increase in the relative number of 'light' cells. At the ultrastructural level, after treatment, many of the 'light' cells were found to contain more free ribosomes, larger profiles of granular endoplasmic reticulum and had better developed mitochondria. Interdigitations between adjacent chief cells were more complex in treated glands. Apart from these features, chief cells of treated glands were basically similar to those of untreated controls. Our study showed that functional parathyroid hyperactivity in baboons is not necessarily accompanied by significant ultrastructural changes in chief cells.     

Biosynthesis and secretion of parathyroid hormone are sensitive to proteasome inhibitors in dispersed bovine parathyroid cells

Preproparathyroid hormone (prepro-PTH) is one of the proteins abundantly synthesized by parathyroid chief cells; yet under normal growth conditions, little or no prepro-PTH can be detected in these cells. Although this may be attributed to effective cotranslational translocation and proteolytic processing, proteasome-mediated degradation of PTH precursors may be important in the regulation of the levels of these precursors and hence PTH secretion. The effects of N-acetyl-Leu-Leu-norleucinal, N-acetyl-Leu-Leu-methional, carbobenzoxy-Leu-Leu-leucinal (MG132), benzyloxycarbonyl-Ile-Glu(t-butyl)-Ala-leucinal (proteasome inhibitor I), and lactacystin on the biosynthesis and secretion of PTH were examined in dispersed bovine parathyroid cells. We demonstrate that treatment of these cells with proteasome inhibitors caused the accumulation of prepro-PTH and pro-PTH. Compared with mock-treated cells, the processing of pro-PTH to PTH was delayed, and the secretion of intact PTH decreased in proteasome inhibitor-treated cells. Relieving the inhibition of the proteasome by chasing MG132-treated cells in medium without the inhibitor led to the rapid disappearance of the accumulated prepro-PTH, and the rate of PTH secretion was restored to levels comparable to those in mock-treated cells. Furthermore, overexpression of the Hsp70 family of molecular chaperones was observed in proteasome inhibitor-treated cells, and we show that PTH/PTH precursors interact with these molecular chaperones. These data suggest the involvement of parathyroid cell proteasomes in the quality control of PTH biosynthesis.     

Clinical heterogeneity of pseudohypoparathyroidism: from hyper- to hypocalcemia

Pseudohypoparathyroidism (PHP) is a rare inherited syndrome characterized by parathyroid hormone (PTH) resistance and is frequently associated with Albright's hereditary osteodystrophy and resistance to other cAMP-mediated hormones. The usual neonatal presentation is mild primary hypothyroidism secondary to resistance to thyroid-stimulating hormone; hypocalcemia usually develops after age 3-5 years. This work describes the diversity in the clinical expression and course of PHP, with emphasis on calcium levels by age and treatment, in 8 children under long-term follow-up at our pediatric tertiary center. The calcium levels at presentation ranged from transient neonatal hypocalcemia to infantile hypercalcemia to childhood/adolescence hypocalcemia. Interestingly, relative hypocalciuria at diagnosis and during therapy, in the presence of renal PTH resistance, was the rule. These findings indicate that transient neonatal hypocalcemia associated with other clinical features or a family history of PHP may be a flag for clinicians to screen for PTH resistance later in life. In addition, PTH resistance may be missed by surveying calcium levels only; thus the PTH levels have to be checked as well. In addition, the recommendation for patients with hypoparathyroidism that strict low-normal calcium levels be maintained during therapy in order to prevent hypercalciuria is probably not applicable in PHP.     

Effect of age and Gender on Fasting and Food-Stimulated Levels of Calcitonin,Parathyroid, and Gastrin Hormones in Healthy Adults

Objective: To investigate the effect of age and gender on basal and food-stimulated serum calcitonin (CT), parathyroid hormone (PTH), and gastrin levels among healthy adults.Methods: Ninety-six healthy adults (76 men and 20 women) aged between 21 and 43 years were recruited. Serum CT, PTH, and gastrin levels were measured after a 9-hour overnight fast, and 1 and 3 hours postprandially.Results: PTH levels decreased early and increased late after feeding. This change was significant in men but not in women. CT levels increased in response to food intake in men but not in women. Gastrin levels were significantly increased after feeding in both men and women. Mean basal and food stimulated CT, PTH, and gastrin levels did not significantly differ between genders. Fasting and post-prandial PTH levels were higher while gastrin levels were lower in older subjects (>30 years old) compared to younger subjects (≤30 years old). Fasting and postprandial CT levels were not significantly different between age groups.Conclusion: Age had a significant effect on fasting and food-stimulated PTH and gastrin hormone levels. The effect of age on PTH levels was independent of baseline vitamin D levels. Men showed significant changes in CT and PTH levels in response to feeding compared to women, although the mean hormone levels were not significantly different between men and women.Abbreviations: CT = calcitonin; MTC = medullary thyroid carcinoma; PTH = parathyroid hormone; SD = standard deviation     

Parathyroid hormone inhibition of Na+/phosphate cotransport in OK cells: generation of second messengers in the regulatory cascade

Dose-dependent inhibition of Na/phosphate cotransport by parathyroid hormone (PTH) was correlated with the generation of hormone-mediated second messengers, cAMP, 1,2-diacylglycerol and inositol 1,4,5 trisphosphate in an established epithelial cell line (opossum kidney (OK) cells). PTH results in a dose-dependent decline in Na/phosphate cotransport with a half-maximal response at about 10(-11) M. This hormone concentration is commensurate with the levels required to increase 1,2-diacylglycerol and inositol 1,4,5-trisphosphate concentrations by about half maximal but not with those needed for cAMP generation (10(-9) to 10(-8) M PTH). Accordingly, activation of phospholipase C may be physiologically more important than stimulation of adenylate cyclase at normal PTH levels.     

Glutamine transport at the blood-brain and blood-cerebrospinal fluid barriers

Glutamine has multiple physiological and pathophysiological roles in the brain. Because of their position at the interface between blood and brain, the cerebral capillaries and the choroid plexuses that form the blood-brain barriers (BBB) and blood-cerebrospinal fluid (CSF) barriers, have the potential to influence brain glutamine concentrations. Despite this, there has been a paucity of data on the mechanisms and polarity of glutamine transport at these barrier tissues. In situ brain perfusion in the rat, indicates that blood to brain L-[14C]glutamine transport at the blood-brain barrier is primarily mediated by a pH-dependent, Na(+)-dependent, System N transporter, but that blood to choroid plexus transport is primarily via a pH-independent System N transporter and a Na(+)-independent carrier that is not System L. Transport studies in isolated rat choroid plexuses and primary cultures of choroid plexus epithelial cells indicate that epithelial L-[14C]glutamine transport is polarized (apical uptake>basolateral) and that uptake at the apical membrane is mediated by pH dependent System N transporters (identified as SN1 and SN2 by polymerase chain reaction) and the Na(+)-independent System L. Blood-brain barrier System N transport is markedly effected by cerebral ischemia and may be a good marker of endothelial cell dysfunction. The multiple glutamine transporters at the blood-brain and blood-CSF barriers may have role in meeting the metabolic needs of the brain and the barrier tissues themselves. However, it is likely that the main role of these transporters is removing glutamine, and thus nitrogen, from the brain.     

Amelioration of type I diabetes-induced osteoporosis by parathyroid hormone is associated with improved osteoblast survival

Type 1 diabetic osteoporosis results from impaired osteoblast activity and death. Therefore, anti-resorptive treatments may not effectively treat bone loss in this patient population. Intermittent parathyroid hormone (PTH) treatment stimulates bone remodeling and increases bone density in healthy subjects. However, PTH effects may be limited in patients with diseases that interfere with its signaling. Here, we examined the ability of 8 and 40 μg/kg intermittent PTH to counteract diabetic bone loss. PTH treatment reduced fat pad mass and blood glucose levels in non-diabetic PTH-treated mice, consistent with PTH-affecting glucose homeostasis. However, PTH treatment did not significantly affect general body parameters, including the blood glucose levels, of type 1 diabetic mice. We found that the high dose of PTH significantly increased tibial trabecular bone density parameters in control and diabetic mice, and the lower dose elevated trabecular bone parameters in diabetic mice. The increased bone density was due to increased mineral apposition and osteoblast surface, all of which are defective in type 1 diabetes. PTH treatment suppressed osteoblast apoptosis in diabetic bone, which could further contribute to the bone-enhancing effects. In addition, PTH treatment (40 μg/kg) reversed preexisting bone loss from diabetes. We conclude that intermittent PTH may increase type 1 diabetic trabecular bone volume through its anabolic effects on osteoblasts.     

Parathyroid hormone secretion in the absence of extracellular free Ca2+ and transmembrane Ca2+ influx

The involvement of extracellular Ca2+ and Ca2+ influx across the plasma membrane in parathyroid hormone (PTH) secretion was investigated in vitro using a new preparation of bovine parathyroid cells. Incubation of these cells in the presence of 25 microM or 2.5 microM free ambient Ca2+ induced a maximal rate of PTH secretion. Low free Ca2+ secretion is not associated with changes in membrane permeability, requires metabolic energy, and is reversible. The Ca2+ channel blocker D600 had no effect on either 45Ca-influx or PTH secretion in these cells. These results, showing that extracellular Ca2+ and Ca2+ influx across the plasma membrane are not required for PTH secretion by parathyroid cells, emphasize the differences in the cellular mechanisms underlying the secretion of PTH vs that of other secretory cells.