M. J. Atkinson

Pulsatile secretion of parathyroid hormone and its action on a type I and type II PTH receptor: A hypothesis for understanding osteoporosis

Parathyroid hormone (PTH) acts on bone to modulate bone turnover [1] but we still do not know how it exerts its action at the organ-specific and cellular milieu. In the last few years we have learned much about the hierarchical order of hypothalamic-pituitary networks which modulate the function of peripheral glands in order to express their biological signals at the organ and tissue level. It became obvious that this dynamic regulation occurs through pulse amplitude and frequency modulation of hormonal signals transporting biological information at various levels in the hypothalamus, pituitary, and peripheral glands. This has been demonstrated for gonadotropins [2], growth hormone [3], thyroidstimulating hormone (TSH) [4], and ACTH [5]. The expression of the biological function of these hormonal systems depends ultimately upon the pulse amplitude and frequency modulation of the hormone receptor unit. It has been known for many years that the dynamic up and down regulation of the hormone receptor unit by the delivery of dynamically encoded hormone signals to the hormone receptor unit will determine the subsequent signal transduction to the main cellular pathways [6]. Two main such pathways are to be considered: a receptor type I-operated pathway which acts through stimulation of adenylate cyclase to produce cAMP and subsequent phosphorylation of regulatory cellular proteins by protein kinases [7, 11]; and a second pathway operated by type II receptors wherein, after binding of the hormone to its receptor, a regulatory Gp-protein is activated in a way that inositolphosphates are cleaved by phospholipase-C (PLC) to produce inositoltriphosphate (IP-3) and diacylglycerol (DAG) [8]. IP-3 acts to control intracellular-free calcium either by intracellular exchange of calcium between various corn-

Homologous radioimmunoassay for human mid-regional parathyroid hormone

A radioimmunoassay, selective for the clinically important mid region of human parathyroid hormone (hPTH), is reported. The synthetic44–68 amino acid sequence (h44–68PTH) was used as both the standard and tracer material. This eliminated many of the undesirable characteristics associated with PTH assays that employ hormone of biological origin. These components allowed the detection of mid regional fragments present in both intact and fragmented hPTH, with a working range between 50 and 2500 pg/ml h44–68PTH. There was no interference from serum proteins and no significant cross reactivity to a range of N-terminal, C-terminal and other mid regional hPTH peptides. The assay proved to be rapid (total time 24 h) and was extremely reproducible, with an intraassay and interassay variation of 2.8% and 5.6% respectively (expressed as percentage SE of mean). The plasma concentration of normal subjects was established as 129 ± 6 pg/ml (h44–68PTH) with a range of 50–300 pg/ml (n = 42). This assay, using fully synthetic hPTH peptides, was able to distinguish between euparathyroid and hyperparathyroid subjects, which suggests that the assay is of diagnostic value.

Parathyroid hormone in coronary artery disease — Results of a prospective study —

Parathyroid hormone (PTH) influences the calcium metabolism of many different mammalian cell types; indeed, hypertension due to changes in muscle tone is a frequent symptom of hypercalcemic hyperparathyroidism. In a blind study of 81 patients with various forms of heart disease undergoing coronary angiography, the plasma concentrations of the midcarboxyl regional PTH immunoreactivity were determined. PTH concentrations were elevated in 26 of the 56 patients exhibiting organic coronary artery disease (CAD). The plasma PTH levels were highest in those patients with CAD affecting three vessels and in patients with evidence of myocardial infarction. PTH levels were not influenced by previous drug treatments, and did not correlate to stress hormone levels. We propose that increased PTH levels may be a marker for initiation or potentiation of calcium-dependent changes in vascular smooth muscle behavior inducing coronary functional and anatomic lesions typical of CAD.

A Homologous Biological Probe for Parathyroid Hormone in Human Serum

A method of measuring the biological activity of parathyroid hormone (PTH) in human serum that depends on the activation of its natural target enzyme, human renal cortical adenylate cyclase, is described. Optimal sensitivity ranging in different assays from 14 to 20 pg 1-34 hPTH/ml was achieved in the presence of the GTP-analogue GppNHp (10 mumol/L), 5 mmol/L MgCl2 and 1.25 mmol/L EGTA. Basal and stimulated cAMP production was reproducible within assays (c.v. below 7%, S.E.M., n = 3) and between assays (c.v. 5 to 14%, S.E.M., n = 4). The recovery of 1-34 hPTH added to individual test sera averaged 94%. The specificity of the method was established as follows: 1.) Other tested hormones, at 100 ng/ml, were ineffective; 2.) In the majority of peripheral sera from patients with hyperfunctioning parathyroid glands elevated bio-activity was detected; 3.) The circulating bio-activity fell rapidly after removal of parathyroid adenomata; 4.) Treatment with antisera for hPTH reduced the bio-activity; 5.) A PTH-antagonist inhibited the bio-activity.

Osteoporosis: A bone turnover defect resulting from an elevated parathyroid hormone concentration within the bone-marrow cavity?

SummaryRecently the bone-marrow cavity blood concentration of parathyroid hormone (PTH) has been shown to exceed that of the peripheral blood. As PTH is a primary modulator of bone cell activity, altered levels of the hormone in the bone-marrow blood may play a significant role in the aetiology of bone disease. We therefore measured PTH concentrations in marrow cavity and venous blood of 9 osteoporotic and 14 control subjects using sequence specific radioimmunoassays for intact and mid-carboxyl (Mid-C) regional human PTH (hPTH). Intact and Mid-C PTH levels were identical in the peripheral blood of control and osteoporotic subjects. Furthermore, bone-marrow cavity blood concentrations of Mid-C PTH, whilst universally higher than those found in peripheral blood, were also comparable in the osteoporotic and control subjects. The sole difference in the PTH composition of bone-marrow cavity blood from osteoporotic subjects was an increased concentration of intact PTH. The origins and consequences of elevated levels of intact PTH within the marrow cavity blood of osteoporotic subjects are discussed.

CHARACTERISATION OF THE BINDING SITES OF ANTI-PARATHYROID HORMONE ANTISERA USING SYNTHETIC PARATHYROID HORMONE PEPTIDES

Four antisera raised against partly purified PTH preparations all showed a wide range of specificities when reacting with radioiodinated PTH peptides representing several different portions of the intact hormone sequence. In contrast, antisera raised against individual peptides were only able to cross-react with other peptides that contained all or part of their amino acid sequence in common. Cross-reacting peptides were seen to contain one or more amino acid residues having high interspecies variability in common. We have explained the antigenicity and cross-reactivity of the peptides on the basis of these common highly variable amino acid sequences. We have concluded that the selection of hormonal material in radioimmunoassays for PTH should be made on the basis of the highly variable amino acid residue content. This will allow a narrowing of the assay specificities and permit detection of a desired region of the PTH hormone.

Immunoreactive parathyroid hormone is present in subretinal fluids of the human eye.

(44-68) human Parathyroid hormone (hPTH) was studied in subretinal fluids (SRF) of 20 human eyes with rhegmatogenous retinal detachments. A (44-68) hPTH carboxyl mid-regional radioimmunoassay was used. Depending on the extent and the duration of the retinal detachments, (44-68) hPTH ranged between normal serum levels (ca. 100-300 pg/ml) up to excessive amounts of 4000 pg/ml with longstanding disease and proliferative vitreoretinopathy. Aqueous humors (AH) of one patient with bilateral iridocyclitis and complicated cataract ranged at 90-132 pg/ml (44-68) hPTH, whereas a control population undergoing surgery for senile cataracts had no detectable (44-68) hPTH in AH. The AH of one diseased eye of heterochromia complicata Fuchs showed excessive amounts of more than 4000 pg/ml (44-68) hPTH.

AUTOANTIBODIES AGAINST PARATHYROID HORMONE IN A PATIENT WITH TERMINAL RENAL INSUFFICIENCY

An autoantibody against mid-C-regional, possibly mid-regional, parathyroid hormone was detected in the plasma of a patient with terminal renal insufficiency who was on intermittent haemodialysis. 43-68(Tyr)hPTH and 42-55(Tyr)hPTH were selectively bound by his IgG fraction. 43-68(Tyr)hPTH had the highest affinity for the autoantibody; intact parathyroid hormone did not displace radiolabelled 42-55(Tyr)hPTH or 43-68(Tyr)hPTH. As concentrations of circulating antibodies against mid-C/mid-regional parathyroid hormone fell rises in intact and mid-C-regional parathyroid hormone levels and in alkaline phosphatase activity were observed. This autoantibody directed against the middle portion of the parathyroid hormone molecule seemed to have some protective properties against the osteoblast-stimulating activity of the hormone, implying that the mid-C-region or middle region of the molecule has some biological importance.

Mitt-C-regionales Parathormon in der klinischen Routine: Diagnostische Wertigkeit beim extrarenalen (primären) und renalen (sekundären) Hyperparathyreodismus

SummaryThe selective determination of mid-C-regional parathyroid hormone (mid-C-PTH) in combination with other laboratory parameters is a reliable tool for diagnosis and treatment of extrarenal (primary) and renal (secondary) hyperparathyroidism. Early stages, which show either high-to-normal serum calcium and elevated mid-C-PTH or increased serum calcium but normal mid-C-PTH, can be distinguished from overt hyperparathyroidism. Alkaline phosphatase (AP) activity and mid-C-regional PTH provide biochemical confirmation of histologically classified renal osteodystrophy. Since the index AP×PTH signifies osseous changes in dialysis patients at an early stage, therapeutic regimens may be altered without additional invasive procedures. After renal transplantation mid-C-PTH normalizes and serum creatinine decreases. Increased mid-C-PTH in patients with normal renal graft function reflects autonomous PTH secretion, which requires careful monitoring to prevent PTH-induced hypercalciuria.The selective determination of mid-C-regional parathyroid hormone (mid-C-PTH) in combination with other laboratory parameters is a reliable tool for diagnosis and treatment of extra-renal (primary) and renal (secondary) hyperparathyroidism. Early stages, which show either high-to-normal serum calcium and elevated mid-C-PTH or increased serum calcium but normal mid-C-PTH, can be distinguished from overt hyperparathyroidism. Alkaline phosphatase (AP) activity and mid-C-regional PTH provide biochemical confirmation of histologically classified renal osteodystrophy. Since the index AP X PTH signifies osseous changes in dialysis patients at an early stage, therapeutic regimens may be altered without additional invasive procedures. After renal transplantation mid-C-PTH normalizes and serum creatinine decreases. Increased mid-C-PTH in patients with normal renal graft function reflects autonomous PTH secretion, which requires careful monitoring to prevent PTH-induced hypercalciuria.

Parathyroid hormone concentration gradients across the human bone marrow.

SummaryParathyroid hormone (PTH) concentrations were compared in blood drawn from the bone marrow and antecubital vein of patients undergoing marrow biopsy for suspected hematological neoplasia. Radioimmunological analysis revealed that the bone marrow blood had a higher PTH content than blood from the peripheral circulation. Thyroid hormone-binding globulin was not distributed asymmetrically, showing that the gradient is PTH specific. The intact PTH content of marrow blood was 65% greater than that in the venous system, whereas carboxyl regional PTH levels showed a 34% gradient in favor of the marrow. Although the majority of patients were found to have hematological malignancies, there was no discernable influence of tumor on the PTH gradients. The physiological implications and possible origins of the asymmetrical PTH distribution are discussed.