Effect of direction and rate of change of calcium on parathyroid hormone secretion in uraemia.

We have studied the control of amino-terminal parathyroid hormone (PTH) secretion in haemodialysis patients in response to slow or fast calcium infusion and during acute hypocalcaemia. In nine patients, fast calcium infusion (0.4 mmol/kg bodyweight per hour) for 15 min increased ionised calcium and reduced PTH, with an initial t 1/2 of 12.8 min. After the infusion had ceased, calcium decreased steadily, and PTH increased, mean PTH reaching baseline values when calcium was still significantly greater than pre-infusion values. During slow calcium infusion for 2.5 h (0.1 mmol/kg bodyweight per hour), parathyroid suppression was evident at 15 min, when the calcium increment was only 0.03 mM. After 60 min, PTH did not decrease further despite progressive hypercalcaemia. Hypocalcaemic haemodialysis led to rapid increases in PTH. After 15 min, the mean calcium decrement was 0.09 mM (P less than 0.01) and the mean PTH increment was 283 pg/ml (P less than 0.01). The parathyroid response was maximal at 30 min, and did not increase subsequently, despite progressive hypocalcaemia for a further 90 min. During recovery from hypocalcaemia, PTH reduced and, despite comparable hypocalcaemia, PTH during periods of increasing calcium was always lower at a given calcium concentration than while calcium was decreasing. This influence of the direction of change of calcium was not seen during hypocalcaemia. The results showed that even in-advanced renal disease, the parathyroid glands are highly responsive to small initial increments (0.03 mM) and decrements (0.09 mM) in blood calcium, though less so to further perturbation of blood calcium.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of preproparathyroid hormone messenger RNA and hormone synthesis in human parathyroid adenomata.

Preproparathyroid hormone (preproPTH) mRNA and PTH secretion were measured in human parathyroid adenomata (n = 8) cultured in 1.0 or 3.0 mmol calcium/l and compared with changes in bovine parathyroid glands (n = 3) as a control. Incubation of bovine glands in 3.0 mmol calcium/l for 24 h resulted in a fall in mRNA levels to 47.3 +/- 21.7% (mean +/- S.D.) compared with cells incubated in 1.0 mmol calcium/l, with a concomitant decrease in secretion to 62.6 +/- 10.8%. These values fell further to 30.1 +/- 15.5% and 42.1 +/- 18.7% respectively after 48-h incubation. One human adenoma responded to high levels of calcium in a similar manner with mRNA levels falling to 44.6 +/- 11.9% and secretion to 30.3 +/- 17.3% within 24 h. However, in the majority of adenomata (seven out of eight), after 24-h incubation in 3.0 mmol calcium/l, mRNA levels fell to 54.1 +/- 14.6% but there was no change in secretion. In two of these adenomata which were cultured for 48 h, there was no suppression of secretion despite mRNA levels having fallen to approximately 60% of control. Incorporation of [35S]methionine into PTH secreted from human adenomatous cells was quantified by densitometry. There was no difference in the amount of radiolabelled PTH secreted from cells incubated in high levels of calcium compared with those in normal levels of calcium. In similar experiments, the effects of high calcium on the synthesis of PTH in bovine cells was assessed by determination of radiolabelled intracellular PTH.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunological properties of synthetic human parathyroid hormone: effect of deamidation at position 76.

Synthetic human parathyroid hormone with asparagine at residue 76 (hPTH 1-84 (Asn 76], as seen in the native peptide, was labelled with 125I and used as radioligand in radioimmunoassays. It was found to give better binding to antibodies raised against extracted human parathyroid hormone (hPTH 1-84) than 125I-labelled native hPTH 1-84. It was also superior in the study of the antigenic determinants of these antisera by the use of calibration curves with synthetic fragments of the human parathyroid hormone. The immunological properties of synthetic hPTH 1-84 (Asn 76) were examined in four region-specific labelled antibody assays and compared with those of native hPTH 1-84 and with an analogue of parathyroid hormone 1-84 with aspartic acid at residue 76 (hPTH 1-84 (Asp 76]. The synthetic hPTH 1-84 (Asn 76) and the native hormone behaved similarly in all four assays which were specific to the amino-, mid- and carboxy-regions of human parathyroid hormone. The synthetic hPTH 1-84 (Asp 76) also behaved similarly in the amino- and both mid-region-specific assays but was markedly less reactive in the carboxy-region-specific assay. This demonstrated that deamidation at residue 76 of human parathyroid hormone affects the immunological properties of the molecule. This also provides an explanation for a previous observation that in a carboxy-terminal-specific assay the native 53-84 peptide (hPTH 53-84) was more reactive than a synthetic preparation of 53-84 with aspartic acid at residue 76. It is concluded that synthetic hPTH 1-84 (Asn 76) is a satisfactory standard for the immunoassay of human parathyroid hormone and is useful as a radioligand when labelled with 125I.

Development and application of a mid-region specific assay for human parathyroid hormone.

A new peptide spanning residues 28-54 of human parathyroid hormone (PTH) was synthesized and used to develop a homologous immunoradiometric assay specific for the mid-region of human PTH. The peptide was coupled to cellulose and used to absorb mid-region antibodies from a goat antiserum against intact human PTH. This assay has been applied to the measurement of circulating PTH in man: in normal subjects the concentration in serum ranged from undetectable (less than 40 pg/ml) to 70 pg/ml, the reference standard being the human PTH 28-54 peptide. In patients with primary hyperparathyroidism concentrations ranged from 120 to 1800 pg/ml. Hormone was not detected in patients with hypoparathyroidism. In normal subjects and in patients with primary hyperparathyroidism the mid-region PTH concentrations were similar to those obtained in an amino-terminal specific assay. By contrast, carboxy-terminal PTH concentrations were markedly higher being 10-fold greater in both groups studied. In patients with primary hyperparathyroidism undergoing parathyroidectomy and in chronic renal failure patients who were infused with calcium, mid-region and amino-terminal PTH disappeared much more rapidly than carboxy-terminal PTH. However, although mid-region PTH was initially cleared as quickly as amino-terminal PTH, it then reached a plateau and remained at a higher level. Thus the mid-region specific assay described here is proving to be of value in the study of the secretion and metabolism of PTH.

Effects of decreasing serum calcium on circulating parathyroid hormone and vitamin D metabolites in normocalcaemic and hypercalcaemic patients treated with APD.

The changes of parathyroid hormone (PTH) and of vitamin D metabolites after intravenous administration of the bisphosphonate APD were studied in ten patients with Paget's disease of bone and in ten patients with tumour-induced hypercalcaemia. After APD all patients with Paget's disease became hypocalcaemic and showed an increase in both N-PTH and C-PTH values. Patients with malignancies had a nearly six-fold greater decrease in serum calcium but rises in N-PTH and C-PTH were observed only in those who developed hypocalcaemia. Overall, a clear rise in PTH was found when serum calcium fell below 2.20 mmol/l. Basal 25-hydroxy- and 24,25-dihydroxyvitamin D concentrations were similar in the two groups and showed no change after APD treatment. Circulating 1,25-dihydroxyvitamin D, however, increased in all patients with Paget's disease and in six of the hypercalcaemic patients. It is concluded that the main regulator of PTH secretion is the concentration of calcium per se rather than the magnitude or the rate of its change. The production of 24,25-dihydroxyvitamin D is not affected by wide variations in serum calcium while that of 1,25-dihydroxyvitamin D is sensitive to these changes.

Comparison of intravenous (3-amino-1-hydroxypropylidene)-1, 1-bisphosphonate and volume repletion in tumour-induced hypercalcaemia.

In a group of thirty patients with tumour-induced hypercalcaemia the effects of volume repletion and intravenous (3-amino-1-hydroxypropylidene)-1, 1-bisphosphonate (APD) were examined. Volume repletion was only partially effective in lowering serum calcium and raising glomerular filtration rate and it increased the tendency towards hypomagnesaemia. In twenty-nine of the patients serum calcium, serum magnesium, and glomerular filtration rate were rapidly restored to normal by intravenous APD, in doses of 1.75-30 mg/day. Tubular reabsorption of phosphate was lower than normal in five of nine patients in whom it was studied and remained unchanged despite correction of hypovolaemia and serum and urine calcium levels or changes in parathyroid hormone. These findings suggested that tumours may produce a phosphate-lowering factor. The improvement in clinical condition with volume repletion depends on its ability to adjust calcium excretion to the abnormal production of calcium from bone. APD, in contrast, returns pathological bone destruction to normal without any undesirable side-effects.