Value of plasma chloride concentration and acid-base status in the differential diagnosis of hyperparathyroidism from other causes of hypercalcaemia

Parathyroid Hormone-Related Protein, Its Regulation of Cartilage and Bone Development, and Role in Treating Bone Diseases

Although parathyroid hormone-related protein (PTHrP) was discovered as a cancer-derived hormone, it has been revealed as an important paracrine/autocrine regulator in many tissues, where its effects are context dependent. Thus its location and action in the vasculature explained decades-long observations that injection of PTH into animals rapidly lowered blood pressure by producing vasodilatation. Its roles have been specified in development and maturity in cartilage and bone as a crucial regulator of endochondral bone formation and bone remodeling, respectively. Although it shares actions with parathyroid hormone (PTH) through the use of their common receptor, PTHR1, PTHrP has other actions mediated by regions within the molecule beyond the amino-terminal sequence that resembles PTH, including the ability to promote placental transfer of calcium from mother to fetus. A striking feature of the physiology of PTHrP is that it possesses structural features that equip it to be transported in and out of the nucleus, and makes use of a specific nuclear import mechanism to do so. Evidence from mouse genetic experiments shows that PTHrP generated locally in bone is essential for normal bone remodeling. Whereas the main physiological function of PTH is the hormonal regulation of calcium metabolism, locally generated PTHrP is the important physiological mediator of bone remodeling postnatally. Thus the use of intermittent injection of PTH as an anabolic therapy for bone appears to be a pharmacological application of the physiological function of PTHrP. There is much current interest in the possibility of developing PTHrP analogs that might enhance the therapeutic anabolic effects.

Myeloperoxidase interaction with peroxynitrite: chloride deficiency and heme depletion

Myeloperoxidase (MPO) is a hemoprotein involved in the leukocyte-mediated defense mechanism and uses hydrogen peroxide (H2O2) and chloride (Cl(-)) to produce hypochlorous acid. In human saliva and in hypochloremic alkalosis syndrome occurring in breast-fed infants, the MPO-H2O2 system functions in a lower Cl(-) concentration (10-70 mM) compared to plasma levels (100 mM) as part of the antibacterial defense system. The impact of low Cl(-) concentration and exposure to high peroxynitrite (ONOO(-)) synthesized from cigarette smoke or oxidative stress on MPO function is still unexplored. Rapid mixing of ONOO(-) and MPO caused immediate formation of a transient intermediate MPO Compound II, which then decayed to MPO-Fe(III). Double mixing of MPO with ONOO(-) followed by H2O2 caused immediate formation of Compound II, followed by MPO heme depletion, a process that occurred independent of ONOO(-) concentration. Peroxynitrite/H2O2-mediated MPO heme depletion was confirmed by HPLC analysis, and in-gel heme staining showing 60-70% less heme content compared to the control. A nonreducing denaturing SDS-PAGE showed no fragmentation or degradation of protein. Myeloperoxidase heme loss was completely prevented by preincubation of MPO with saturating amounts of Cl(-). Chloride binding to the active site of MPO constrains ONOO(-) binding by filling the space directly above the heme moiety or by causing a protein conformational change that constricts the distal heme pocket, thus preventing ONOO(-) from binding to MPO heme iron. Peroxynitrite interaction with MPO may serve as a novel mechanism for modulating MPO catalytic activity, influencing the regulation of local inflammatory and infectious events in vivo.

Kidney stones as a manifestation of hypercalcemic disorders. Hyperparathyroidism and sarcoidosis

When hypercalcemia is detected in a kidney stone formation, an intact parathyroid hormone measurement should be made. Detection of hyperparathyroidism (HPT) is important to prevent further stone episodes and to avoid the complications of high serum calcium in other organ systems. Stones in patients with HPT often contain apatite salts in addition to calcium oxalate because parathyroid excess may create a renal tubular acidosis. The calculi seen in patients with sarcoidosis, another hypercalcemic state that may cause stone formation, however, are usually pure calcium oxalate. Excess generation of 1,25-dihydroxyvitamin D results in intestinal hyperabsorption of calcium and secondary hyperoxaluria.

The effects of gossypol and chloroquine interaction on serum electrolytes of protein-malnourished rats

Serum sodium (Na), potassium (K), chloride (Cl), phosphate (Po), and calcium (Ca) were assayed in adult male Wistar rats fed a low protein diet (LP) and a normal protein diet (control, NP) and administered gossypol (GP) and chloroquine (CQ) alone and in combination (GPCQ), for 4 and 8 weeks. Gossypol was administered at 20 mg/kg body wt/rat/day in Goya oil and chloroquine at 5 mg/kg body wt/rat for 3 days/week in distilled water. The data, analyzed by two-way ANOVA, showed that the interaction had significant effects on the serum levels of Na, K and Po (P < 0.001) but not on chloride and calcium (P > 0.05). Serum levels of sodium, potassium and phosphate in LP rats were significantly lower than in NP rats. In LP and NP rats, the levels of Na, K, and Po at 8 weeks were significantly higher than at 4 weeks; and GPCQ treatment produced significantly lower sodium levels in LP and NP rats, lower phosphate in LP rats and higher phosphate in NP-fed rats than GP or CQ administered alone. Potassium level in GPCQ was between levels in Gp- and CQ-treated rats. The administration of gossypol and chloroquine together may not adversely affect serum electrolytes in protein-malnutrition state any more than gossypol or chloroquine administered alone.

Renal tubular function in hyperparathyroidism

Renal tubular function was assessed in a group of patients with mild hyperparathyroidism before and after a mean period of 2.7 years conservative management. It was also assessed, before and after a mean of 3.3 years following surgery in a group of patients with initially higher plasma calcium concentration. Mean maximum urine osmolality was within the accepted range as was the maximum urine plasma hydrogen ion gradient in both groups at the time of diagnosis. No significant change in renal tubular function was observed in either group over the periods of this study. Although deterioration after a long period cannot be excluded, we do not consider that regular assessment of renal tubular function is necessary in the conservative management of primary hyperparathyroidism.

Acid-base homeostasis during chronic PTH excess in humans

The chronic renal and systemic acid-base effects of hyperparathyroidism in humans remain controversial and unresolved. The present studies evaluated the acid-base response of normal human subjects to a 13-day intravenous infusion of synthetic b(1-34) PTH sufficient to result in sustained hypercalcemia and hypophosphatemia. The acid-base response was biphasic: an initial transient renal acidosis developed on the first day of PTH infusion, followed by a prompt increase in net acid excretion and plasma [HCO3-] of sufficient magnitude to result in a steady state of mild metabolic alkalosis. The results indicate that: 1) sustained, continuous, experimentally produced hyperparathyroidism results in a steady state of mild metabolic alkalosis; 2) the alkalosis is both generated and maintained, at least in part, by renal mechanisms; and 3) reported renal acidosis in sustained clinical conditions of primary hyperparathyroidism is not attributable to either direct or indirect effects of PTH excess when present for a 2-week period, an interval sufficient to re-establish a new steady state of renal and systemic acid-base equilibrium.

Bilateral acute corneal calcification

A 38-year-old man with brittle, juvenile onset diabetes mellitus and bilateral severe dry eyes with recurrent corneal ulcers developed atypical band-shaped calcifications of both corneas during a 24-hour period. Serum calcium, phosphate, and carbon dioxide levels all were within normal limits. The patient was mildly uremic but was not in renal failure. When EDTA chelation failed to clear the deposits, partial keratectomies were performed in both eyes and the specimens were examined by light and electron microscopy, including energy dispersive x-ray analysis. Microscopic studies revealed an atypical calcific keratopathy which involved neither Bowman's layer nor the most superficial stromal lamellae. The deposits were confined to deeper lamellae in the anterior stroma and by electron microscopy were composed of extracellular crystalline aggregates. Energy dispersive x-ray analysis of these aggregates confirmed the presence of calcium and phosphate. Corneal dessication appeared to be a major contributing factor in the rapid formation of these deposits.

Value of laboratory tests in the differential diagnosis of hypercalcemia

Eighteen laboratory tests were compared in the differentiation of primary hyperparathyroidism from hypercalcemia associated with malignancy. Statistical comparisons of the test results were carried out in four patient groups and two control groups. The patient groups evaluated were those with confirmed primary hyperparathyroidism, those with malignancy with hypercalcemia, those with malignancy without hypercalcemia, and those with surgically cured primary hyperparathyroidism. These groups allowed determination of the relative diagnostic values of the tests and a rationale for their value. After exclusion of patients with renal failure from the patient and control groups, these data indicated that the laboratory tests with the greatest differential diagnostic value, in order of efficacy, were: albumin, carboxy-terminal parathyroid hormone, venous pH, cholesterol, chloride, alkaline phosphatase, phosphorus, and the chloride/phosphate ratio. Hemoglobin, hematocrit, and red blood cell count also had some value, particularly in male patients. However, none of these tests individually achieved better than an 81 percent classification accuracy. With application of logistic discriminant analysis, only three tests--albumin, parathyroid hormone, and chloride--were identified as statistically significant in jointly improving the diagnostic separation between these two patient groups. Although the 94.4 percent classification accuracy achieved by use of these three variables in a logistic discriminant function was better than that obtained with any individual variable, incorrect classification was still a significant problem, particularly in the case of patients with malignancy and high concentrations of parathyroid hormone. With the exception of albumin and chloride measurements, the commonly available ancillary laboratory tests proposed to aid this differential diagnosis do not give any more information than the analysis of parathyroid hormone alone and merely add to the increased cost of medical care.

Respiratory alkalosis and reduced plasmatic concentration of ionized calcium in rats treated with 1,25 dihydroxycholecalciferol

The daily administration of supraphysiological doses of 1,25 dihydroxycholecalciferol (0.1-2.5 micrograms/d/100 g body weight) to rats, produced respiratory alkalosis. With the doses of 0.1-0.2 micrograms/d/100 g and feeding a diet with 0.7% of calcium, calcemias did not exceed 2.75 mM, and significantly reduced plasma ionized calcium levels were measured. The latter phenomenon was found associated with increased urinary excretion of cAMP, soft tissue calcium content, and polyuria with hypostenuria, all known effects of parathyroid hormone. These effects were absent in thyroparathyroidectomized rats treated in the same fashion. Present results suggest that the stimulus of low levels of plasma ionized calcium overcomes the probably inhibitory effect of the steroid on parathyroid hormone secretion.

Differential laboratory diagnosis of hypercalcemia

It is proposed that this review will adopt the following format: establishment of hypercalcemia. This demands a discussion of the problem of normal ranges, the usage of either total calcium or ionized calcium in making this decision and where total calcium is used whether adjustment of this value for serum protein concentration should be used and if so, the formulae which have been cited to perform this. Having established hypercalcemia why is it necessary to differentiate this? This will involve reviewing those clinical situations in which differentiation of hypercalcemia has been attempted and will include an attempt to produce an up to date indication of conditions in which hypercalcemia has been described. When hypercalcemia has been established the laboratory tests which have been further used to discriminate will be divided into single tests such as N- or C- terminal parathormone, 1,25- dihydroxycholecalciferol, cyclic AMP; the combination tests which have been used including phosphate clearance, chloride vs. bicarbonate etc. proceeding to those groups which have used discriminant function to help in the decision making; dynamic testing will also be discussed particularly with reference to steroid suppression but will also include other known suppressants such as Mithramycin and Calcitonin. A final section will be included attempting to assess overall the present state of art in differentiating laboratory diagnosis of hypercalcemia and will also attempt to highlight those areas which appear to be most fruitful areas of progress in the future.

Analysis of the components of the hypercalcaemia in a 14-year-old boy following prolonged immobilisation

A fourteen-year-old boy developed severe symptomatic hypercalcaemia following prolonged supine bed rest for major trauma. Treatment consisted initially of intravenous saline and frusemide together with oral phosphate, followed by intramuscular salmon calcitonin. Only after mobilisation and weight bearing was a sustained fall of plasma calcium to normal achieved. Plasma immunoreactive parathormone levels using both N-terminal and mixed terminal specific antisera were always undetectable and urinary cyclic AMP levels were within the normal range throughout. However, before mobilisation, the tubular reabsorption of phosphate was reduced and that of calcium was increased thus indirectly suggesting increased parathormone activity. The hypercalcaemia was due to a combination of increased calcium release from bone and increased tubular reabsorption. We suggest that a factor other than parathormone is responsible for altered tubular handling of calcium and phosphate which develop following prolonged immobilisation in these patients.

Ionized calcium in body fluids

This article will review the methods currently available to the clinician and research worker for measuring the concentration of ionized calcium in various body fluids including whole blood, serum, plasma, urine, cerebrospinal fluid, milk, and synovial fluid. The methods to be reviewed are based on procedures involving bioluminescence, colorimetry and ion-selective electrodes. Emphasis will be given to the precision and, wherever possible, accuracy of each technique. Possible sources of error and interfering agents will be identified. Attention will be given to the recommended conditions for measuring ionized calcium in each body fluid. An assessment will be made of the theoretical and practical importance of measuring ionized calcium rather than total calcium and of its value in clinical medicine.

Hypercalcemia of immobilization in neurologically injured children: a prospective study

Seven of 20 consecutive admissions to the Children's Head Injury and Spinal Injury Services of the Rancho Los Amigos Hospital demonstrated hypercalcemia ranging from 10.7 mg % to 13.2 mg %, with an average of 11.4 mg % (normal range 8.5 mg-10.5 mg %). Return to normocalcemia coincided with mobilization in five of the seven patients. Four patients in the series developed heterotopic ossification; two of these demonstrated concurrent hypercalcemia. This association has not been previously reported. This study has helped to clarify the clinical suspicion that hypercalcemia of immobilization occurs more frequently than is suggested in the literature. Elevated alkaline phosphatase levels are associated with heterotopic ossification. The role of hypercalcemia in the development of heterotopic ossification remains speculative.

Prostaglandin-mediated hypercalcemia: a paraneoplastic syndrome

Evidence has been presented for prostaglandin-mediated hypercalcemia and bone resorption in malignancies of both, experimental animals and man. Occurence of hypercalcemia in cancer patients is known for a long time, but its pathogenesis has been poorly understood so far. Besides ectopic parathyroid hormone secretion by tumors, an osteoclast-activating factor released from leukocytes and direct bone destruction by tumor cells, prostaglandins of the E series have to be considered as one of the candicates involved in the pathomechanism of hypercalcemia and osteoclastic osteolysis in cancer patients. This new concept on the pathophysiology of cancer-associated hypercalcemia has implications for the diagnosis and management of this common complication of neoplastic disease.

Measurement of ionised calcium in body fluids-a review

This paper reviews the techniques available to the clinical biochemist for measuring ionised calcium concentrations in biological fluids with particular reference to serum. At present ionised calcium may be measured colorimetrically, using tetramethyl murexide, or potentiometrically, using a calcium ion-selective electrode. These techniques compare favourably in terms of precision with existing methods for measuring total calcium. Advantages of measuring ionised calcium in preference to total calcium are (a) that there is no effect of venous occlusion or change of posture on the ionised fraction of the effect on total calcium, (b) that ionised calcium is the physiologically active form of the element, and (c) that the ionised calcium concentration is a more reliable indicator of the calcium status of patients in certain clinical conditions. The main problem in the measurement of ionised calcium is the marked dependence of the ionised fraction of the pH of the sample. Extreme care must be taken to avoid loss of CO2 or build-up of acid during the handling of the blood sample.

Regulation of calcium metabolism

This paper reviews the regulation of calcium metabolism in man. The body's calcium economy is determined by the relationship between the intestinal absorption of calcium, the renal handling of calcium, and by the movements of calcium in and out of the skeleton. These processes are influenced by many factors, the most important of which are parathyroid hormone and the hormones derived from the renal metabolism of vitamin D, notably 1,24-dihydroxyvitamin D3. The role of endogenous calcitonin in man is still controversial, but there are several other hormones which have some influence on calcium metabolism, including thyroid hormone, growth hormone, and the adrenal and gonadal steroids. Clinical disorders of calcium metabolism and their treatment are discussed in terms of the disturbances in normal physiology they represent.

Abnormalities in mineral metabolism suggestive of parathyroid over-activity in rheumatoid arthritis

A three-part study on mineral metabolism in patients with classical rheumatoid arthritis is described. In the first two parts, biochemical abnormalities were revealed suggestive of parathyroid over-activity, and in the third part, observation on calcium absorption provides a hyperparathyroid pattern. The importance of these findings in relation to demineralisation of bone in rheumatoid arthritis is discussed.