Parathyroid hormone secretion in normal man and in primary hyperparathyroidism: role of histamine H2 receptors

The syndrome of gastric carcinoid and hyperparathyroidism: a family study and literature review

OBJECTIVE

To present evidence supporting the hypothesis that the coexistence of gastric carcinoids (GCs) and hyperparathyroidism may represent a distinct clinical entity, not related to multiple endocrine neoplasia type 1 (MEN1).

METHODS

We studied a cohort of five young siblings (age range 26-42 years), one of whom had been found to have GC and hyperparathyroidism. All siblings underwent serial gastroscopies for the assessment of gastric neuroendocrine cell proliferations over a mean follow-up period of 31.2 months. Imaging, biochemical and hormonal as well as molecular genetic investigations were performed in the direction of MEN1 syndrome. The literature was searched for cases with coexistence of GCs and hyperparathyroidism not associated with MEN1.

RESULTS

Four of the siblings, all male, were found to have GCs in a background of Helicobacter pylori-associated chronic atrophic gastritis and pernicious anaemia, with no serological evidence of gastric autoimmunity. In two of them, asymptomatic hyperparathyroidism was also present. Screening for MEN1 gene mutations or large deletions was negative, and hormone and imaging investigations did not support a diagnosis of familial MEN1 syndrome. A literature search revealed sporadic reports of cases with GC and hyperparathyroidism not attributable to MEN1.

CONCLUSIONS

The association of GCs and hyperparathyroidism appears to constitute a distinct syndrome that can be encountered in genetically predisposed individuals, and should not be regarded as 'atypical' or 'incomplete' expression of MEN1. Its prevalence and aetiology should be the subject of future studies. Screening for hyperparathyroidism seems to be justified in patients with GC of any type.

Morphological and histochemical characteristics of mast cells and the content of in-tissue histamine in various pathological parathyroids: do mast cells participate in hormone secretion in human parathyroids?

The possibility of the participation of mast cells in human parathyroid hormone secretion was studied with regard to the frequency, distribution, and sub-types of mast cells and the content of in-tissue histamine, a chemical mediator in mast-cell granules, in human parathyroids with various pathological conditions. The above factors were compared between those of a 'normal' parathyroid group and those of 'pathological' parathyroids associated with adenoma and hyperplasia. Specimens were scanned for the mean value of the mast cell number per field of microscopic view and for the ratio of the mast cell number in glandular parenchymal tissue to that in interstitial tissue. The activated state of the mast cells was examined through classifying the mast cells into two sub-types, mucosal mast cells and connective-tissue mast cells. The high-performance liquid chromatography (HPLC) method was used for assay of in-tissue histamine. The frequency of mast cells showed no difference between the groups, whereas the distribution of mast cells, showed a distinct difference. The occurrence rate of mast cells in glandular parenchymal tissue in the 'pathological' group presented an increase as compared with that in the 'normal' group. Furthermore, the occurrence rate of mucosal mast cells in an activated state also showed an increase. This suggests that mast cells are likely to participate in parathyroid hormone secretion. The histamine-content in the 'normal' group was significantly larger than that in the 'pathological' group, which was a different outcome from that observed in mast cells from the results of light microscopy. This may require taking into consideration the difference in the histamine content of the mast cells themselves between that of mucosal mast cells and connective-tissue mast cells.

Effect of omeprazole, an inhibitor of H+,K(+)-ATPase, on bone resorption in humans

Omeprazole is an inhibitor of gastric H+,K(+)-ATPase. Although the major proton transport of osteoclast is mediated by a vacuolar-type H(+)-ATPase which is different from the gastric H+,K(+)-ATPase, in vitro studies have demonstrated that omeprazole inhibits bone resorption. In this study, the effect of omeprazole on bone resorption was evaluated in patients who had a history of gastric ulcer and were treated with maintenance doses of H2 blocker without any gastric complaints at the study time. H2-blocker administration was changed to omeprazole treatment in the study group and to no treatment in the control group. Urinary excretion of hydroxyproline and calcium decreased after omeprazole treatment in the study group. Serum intact PTH, alkaline phosphatase, osteocalcin, and tartrate-resistant acid phosphatase (TRAP) increased in this group. In the control group, there were not any changes in these parameters. The discrepancy between serum TRAP and urinary excretion of hydroxyproline and calcium in the study group was thought to be due to the suppression of bone resorption by omeprazole, which probably interfered the acidification at resorption lacunae and resulted in the inactivation of TRAP and other lysosomal enzymes. The results of our study suggest the possibility that the specific inhibitors of the osteoclastic proton pump (such as bafilomycins) will more effectively suppress bone resorption and be useful for the treatment of metabolic bone diseases with increased bone resorption.

Medical management of asymptomatic primary hyperparathyroidism

Patients with mild asymptomatic primary hyperparathyroidism who do not meet currently accepted guidelines for surgery may be followed medically. General medical management of these individuals should be directed toward maintaining adequate hydration, therapy of hypertension, and avoiding immobilization. Diuretics should be used only with caution. Moderate dietary calcium intake (500-800 mg/day) should be encouraged. Propranolol and cimetidine are not useful in the therapy of primary hyperparathyroidism. Oral phosphate is efficacious in lowering serum and urinary calcium. However, because of concerns related to ectopic calcification, phosphate is usually reserved for those patients who meet surgical guidelines but who are not to undergo surgery. Bisphosphonates, potent inhibitors of osteoclast-mediated bone resorption, have been shown to lower serum and urinary calcium in patients with primary hyperparathyroidism. However, long-term data on their efficacy in this disorder are not yet available. The use of bisphosphonates at the present time is generally restricted to the research setting. More potent bisphosphonates as well as the design of newer agents that interfere with parathyroid hormone secretion may become very useful in future approaches to the medical management of primary hyperparathyroidism.

Long-term effects of histamine H2-receptor antagonists on serum parathyroid hormone in chronic renal failure

We assessed the effects of ranitidine and cimetidine (histamine H2-receptor antagonists) on the serum concentrations of parathyroid hormone (PTH) in 22 patients with end-stage chronic renal failure. Treatment for 3 months induced a significant decrease in serum concentrations of PTH determined by radioimmunoassay directed at the mid portion of the PTH molecule. In contrast, significant increases were noted using an assay directed at the amino-terminal portion, whereas no change was noted with a carboxy-terminal assay. No significant changes in serum calcium, phosphate and alkaline phosphatase were noted after treatment with either agent, but serum concentrations of osteocalcin increased significantly. We conclude the H2-receptor antagonists do not favourably influence the natural history of hyperparathyroid bone disease in patients with end-stage chronic renal failure.

Cellular immune functions in patients with primary hyperparathyroidism: effects of histamine and cimetidine

The possibility of a link between parathyroid hyperfunction and cellular immune functions was studied in primary hyperparathyroid (1 degree HP) patients. The effect of cimetidine on lymphocyte functions in 1 degree HP patients and control subjects was also investigated. Histamine-induced suppressor activity of lymphocytes from 1 degree HP patients was significantly greater than that of controls. Cimetidine addition to both normal and 1 degree HP lymphocyte cultures abolished histamine-induced suppression. In vivo administration of cimetidine, while ineffective towards normal lymphocytes, depressed phytohemagglutinin stimulation of 1 degree HP lymphocytes, indicating possible immunological damage caused by this drug, which is frequently used in the treatment of patients with 1 degree HP.

Effect of combination therapy with cimetidine and pirenzepine on plasma parathyroid hormone and calcitonin levels in hemodialyzed patients

In this study we evaluated the effects of an oral combination therapy with cimetidine and pirenzepine on plasma parathyroidhormone (PTH) and calcitonin (CT) levels in 24 patients on maintenance hemodialysis (mean age: 50 years; mean duration of dialysis treatment: 23 months). As compared to the pre-treatment plasma levels of PTH and CT, there were no significant changes of their plasma concentrations during a 4-week administration of 800 mg cimetidine or 100 mg pirenzepine daily, and the concentrations also did not change significantly during the following 4 weeks of combination therapy with cimetidine and pirenzepine in the above mentioned dosage. Serum concentrations of calcium and phosphate and the activity of the alkaline phosphatase showed no significant changes either. Therefore, we suggest that this therapeutic approach cannot be considered for the treatment of uremic hyperparathyroidism.

Divergent effects of forskolin on 3',5' cyclic adenosine monophosphate production and parathyroid hormone secretion

Forskolin, a diterpene which directly stimulates adenylate cyclase, markedly stimulated cAMP production in intact rat parathyroid glands and dispersed cells from hyperplastic and adenomatous human parathyroid tissues. Stimulation of cAMP production in human parathyroid adenomas occurred as early as 2 min and continued for at least 2 h; furthermore, a dose-response relationship was observed, with a maximal 80-fold cAMP response occurring at 100 microM forskolin. When PTH secretion by rat or human parathyroid tissues was studied at low (0.5 mM) and high (2.5 mM) extracellular Ca2+ in either the presence or absence of forskolin, no significant stimulation by forskolin was observed at 15 min, 1 h, and 2 h. When 10 human parathyroid specimens were studied with varying concentrations of forskolin at 1 mM Ca2+, 6 failed to show stimulation of PTH secretion and 4 showed modest but detectable increases in PTH that did not appear dose-related. We conclude that (1) at low and high Ca2+ levels, marked stimulation of cAMP production by forskolin can occur without a corresponding increase in PTH secretion; (2) inhibition of PTH secretion by high extracellular Ca2+ levels continues unchanged despite stimulation of cAMP production by forskolin; and (3) at intermediate Ca2+ levels (1.0 mM), PTH secretion is affected either minimally or not at all by forskolin in human hyperparathyroid tissue preparations. The marked stimulation of parathyroid adenylate cyclase by forskolin without concomitant increases in PTH secretion in the majority of tissues suggests that the level of cAMP production is not a primary or sufficient determinant of hormone secretion.

Cimetidine does not correct circulating calcium and parathyroid hormone in primary hyperparathyroidism

Thirteen patients with primary hyperparathyroidism were treated for 4-8 weeks with the H2-blocker cimetidine at a daily dose of 1 g. The drug had no effect on the serum concentrations of calcium or parathyroid hormone, nor did any of the patients improve clinically during the period of treatment. The recently reported efficacy of cimetidine in primary hyperparathyroidism is strongly questioned by the present results.

Treatment of hypercalcemia

The indications, effectiveness, and adverse effects of the numerous treatment modalities for hypercalcemia are presented. Results of a retrospective chart review of 72 admissions (53 patients) for hypercalcemia-related diseases are presented to emphasize the therapeutic principles that should be applied in the management of these patients. Hydration, forced diuresis, oral phosphates, and glucocorticosteroid drugs are common therapeutic approaches. Combination therapy with glucocorticosteroid drugs, oral phosphates, and forced diuresis is utilized to take advantage of multiple mechanisms of action. Outpatient therapy for hypercalcemia consists primarily of glucocorticosteroid agents and oral phosphates. Most patients are treated if the serum calcium level is greater than or equal to 12 mg/dL. An algorithm is presented to assist the clinician in selecting the most efficacious treatment plan.

The effects of cimetidine on serum calcium and parathyroid hormone levels in primary hyperparathyroidism

1 Cimetidine has been reported to reduce serum calcium and parathyroid hormone (PTH) levels in primary hyperparathyroidism, but some studies have reported negative findings. 2 A double-blind, controlled trial of cimetidine in this condition was, therefore, conducted. Eight patients were given cimetidine and placebo for 2 months each, in random order. Only one patient showed a significant fall in serum calcium on cimetidine and no patient showed a fall in PTH levels. 3 In addition, eight patients were studied during a 48 h continuous intravenous infusion of cimetidine. The first five patients exhibited no effect of the drug on either calcium of PTH levels, but in the sixth there appeared to be a dose-dependent suppression of PTH. The seventh and eighth patients were, therefore, given a higher dose of the drug but it failed to suppress calcium or PTH values. 4 It is concluded that cimetidine is not of value in primary hyperparathyroidism.