Rapid development of renal resistance to low doses of synthetic bovine parathyroid hormone fragment 1-34. Dissociation of urinary cyclic adenosine monophosphate, phosphaturic, and calciuric responses

Parathyroid hormone-PTH1R signaling in cardiovascular disease and homeostasis

Primary hyperparathyroidism (pHPT) afflicts our aging population with an incidence approaching 50 per 100 000 patient-years at a female:male ratio of ~3:1. Decisions surrounding surgical management are currently driven by age, hypercalcemia severity, presence of osteoporosis, renal insufficiency, or hypercalciuria with or without nephrolithiasis. Cardiovascular (CV) disease (CVD) is not systematically considered. This is notable since the parathyroid hormone (PTH) 1 receptor (PTH1R) is biologically active in the vasculature, and adjusted CV mortality risk is increased almost threefold in individuals with pHPT who do not meet contemporary recommendations for surgical cure. We provide an overview of epidemiology, pharmacology, and physiology that highlights the need to: (i) identify biomarkers that establish a healthy 'set point' for CV PTH1R signaling tone; (ii) better understand the pharmacokinetic-pharmacodynamic (PK-PD) relationships of PTH1R ligands in CV homeostasis; and (iii) incorporate CVD risk assessment into the management of hyperparathyroidism.

Safety and tolerability of subcutaneous PTHrP(1-36) in healthy human volunteers: a dose escalation study

Parathyroid hormone-related protein (PTHrP) is an anabolic skeletal agent in mice, rats and humans. In previous studies, we have demonstrated that PTHrP can be administered to osteoporotic postmenopausal women at a dose of 6.56 microg/kg/day (or approximately 400 microg/day) for 3 months to yield a 4.7% increase in lumbar spine BMD. This regimen was free of hypercalcemia or adverse effects. Moreover, PTHrP appeared to stimulate bone formation selectively, without stimulating bone resorption. This efficacy in the absence of adverse effects, as well as the apparent "pure anabolic" action of PTHrP, prompted us to attempt to define the complete therapeutic window for PTHrP. In this study, we gradually escalated the dose of PTHrP(1-36) from 9 to 28 microg/kg (or approximately 570 microg to 1,946 microg) administered as a single subcutaneous dose to 22 healthy young adult subjects. PTHrP(1-36) was well tolerated even at the highest dose, just under 2.0 mg, some five times higher than we have previously demonstrated to be effective in increasing bone mass, and some 100 times higher than the maximal approved dose of PTH(1-34). Despite the large dose of PTHrP, the highest serum calcium achieved was 10.6 mg/dl, and this was observed in only one subject at the highest dose. The mean serum calcium in subjects receiving the highest dose was 9.6 mg/dl. Only one subject experienced adverse symptoms/signs, and this was at the highest dose. We conclude that subcutaneous PTHrP(1-36) is safe when administered in single doses approaching 2.0 mg. These findings indicate that the therapeutic window for PTHrP(1-36) in humans is wide and permit the design and implementation of longer safety and efficacy trials.

Intraoperative Parathyroid Hormone Assay in Patients with Graves’ Disease for Prediction of Postoperative Tetany

We measured intraoperative parathyroid hormone (IOPTH) levels before and after thyroidectomy in a large group of patients to test whether changes in IOPTH can predict postoperative tetany. Subjects were 111 consecutive patients (94 females and 17 males) with Graves' disease undergoing subtotal thyroidectomy. Blood samples for IOPTH assay were obtained after anesthesia (basal) and following skin closure (postoperative). Data were compared between patients who developed tetany (n = 9) and those who did not (n = 102). There was no significant difference in sex, age, period of antithyroid drug administration, or the weight of the thyroid between the two groups. The preoperative serum calcium level was significantly lower (p < 0.05) and the basal IOPTH significantly higher (p < 0.05) in the tetany group than in the non-tetany group. The IOPTH level was significantly lower (p < 0.005) and the average percent decrease in IOPTH levels was higher (p < 0.001) in the tetany group than in the non-tetany group. A decrease in IOPTH of more than 70% was shown to be 78% sensitive, 94% specific, and 93% accurate, and it has 78% positive predictive value and 94% negative predictive value for the development of tetany. Our study shows that a postoperative decrease of IOPTH level is the most predictive of postoperative tetany of the clinical risk factors investigated. We recommend IOPTH measurement as an adjunct to postoperative management of patients with Graves' disease to assist in preventing hypocalcemia and determining the earliest time for safe discharge.

Reelevation of parathyroid hormone level after parathyroidectomy in patients with primary hyperparathyroidism: importance of decreased renal parathyroid hormone sensitivity

BACKGROUND

We hypothesized that impaired peripheral sensitivity to parathyroid hormone (PTH) may play a role in reelevation of PTH after successful operation for primary hyperparathyroidism (pHPT).

METHODS

Factors affecting reelevation of PTH were determined in 90 patients who underwent parathyroidectomy for pHPT. PTH/nephrogenous cyclic adenosine monophosphate ratio, as an index of renal resistance to PTH, was examined in relation to factors shown to influence reelevation of PTH.

RESULTS

Serum PTH levels were elevated above the upper limit of normal in 23 patients (26%) at 1 week and in 39 patients (43%) at 1 month after parathyroidectomy. These 39 normocalcemic patients with elevated serum PTH at 1 month after parathyroidectomy had a higher preoperative serum level of PTH and lower serum phosphate and 25-hydroxyvitamin D (25OHD) concentrations than those with normal PTH (n = 59). Elevated PTH and low 25OHD were shown by multivariate analysis to be significant predictors of reelevation of PTH. Renal resistance to PTH was higher in patients with vitamin D deficiency or renal insufficiency than in patients with normal serum vitamin D concentrations or normal renal function, and it increased according to increases in levels of PTH.

CONCLUSIONS

The mechanism of PTH reelevation in patients with pHPT after successful parathyroidectomy appears to be renal resistance to PTH.

A defect in renal calcium conservation may contribute to the pathogenesis of postmenopausal osteoporosis

Although all postmenopausal women are estrogen deficient, women who have postmenopausal osteoporosis may have a defect, in addition to estrogen deficiency, that accounts for their higher rates of bone resorption and greater bone loss, relative to those who do not. To test the hypothesis that one defect is an impairment in renal calcium conservation, we measured renal calcium transport in 19 osteoporotic and 19 normal postmenopausal women, whose ages (median and 25th-75th percentile range) were 70 yr (range, 67-72) and 72 yr (range, 69-74), respectively. There was no difference between groups in values for serum ionized calcium and PTH concentrations or in renal filtered load of calcium. However, before PTH infusion, the osteoporotic women had lower (P = 0.0046) values for tubular reabsorption of calcium (TRCa) of 96.8% (range, 96.0-97.1) vs. 98.0% (range, 97.2-98.3) and higher (P = 0.0154) urinary calcium excretion of 0.194 mg/dL of glomerular filtrate (GF) (0.154-0.239) vs. 0.125 mg/dL of GF (0.103-0.173) than the normal women. After infusion of 200 U of synthetic PTH (synthetic 1-34 analog of human PTH), TRCa increased and calcium excretion decreased comparably in both groups, so that the differences between groups after intervention remained: for TRCa, 98.3% (97.7-98.6) vs. 98.9% (98.4-99.3; P = 0.0042); and for calcium excretion, 0.099 mg/dL of GF (0.080-0.138) vs. 0.066 mg/dL of GF, (0.045-0.097, P = 0.0180). In conclusion, postmenopausal women with osteoporosis have a PTH-independent defect in renal calcium conservation. This defect is of sufficient magnitude to contribute to the greater negative calcium balance in post-menopausal women with osteoporosis vs. their postmenopausal peers.

Episodic secretion of parathyroid hormone in postmenopausal women: assessment by deconvolution analysis and approximate entropy

In health young subjects, parathyroid hormone (PTH) is secreted presumptively in a dual fashion, with low amplitude pulses apparently superimposed on tonic secretion. In contrast, PTH secretion has not been as well characterized in postmenopausal women, and relationships among bone density, estrogen status, and PTH release have not been explored. It is possible that a pulsatile pattern of PTH secretion is important for bone remodeling, since exogenous PTH administered in a pulsatile manner stimulates bone formation. To assess the importance of pulsatile PTH secretion as a determinant of bone mass, we measured PTH in blood sampled every 2 minutes for 6 h in four groups of older women: (1) high bone density receiving estrogen (n = 6), (2) high bone density not receiving estrogen (n = 5), (3) low bone density receiving estrogen (n = 6), and (4) low bone density not receiving estrogen (n = 8). The plasma PTH release profiles were subjected to deconvolution analysis, which resolves measured hormone concentrations into secretion and clearance components, and to an approximate entropy (ApEn) estimate, which provides an ensemble measure of the serial regularity or orderliness of the release process. In postmenopausal subjects, PTH was secreted in a fashion similar to that observed in young adults, with significant tonic secretion and PTH pulse occurrences averaging every 18-19 minutes. Pulsatile PTH secretion accounted for approximately 25% of the total secreted PTH. There were no differences in the amplitude or frequency of pulsatile PTH secretory parameters or in ApEn values among the four groups or compared with young controls. We conclude that in postmenopausal women, PTH secretory patterns and temporal organization are similar to those in healthy young subjects and are not altered in states of low bone density or estrogen deficiency. This suggests that abnormalities in orderly pulsatile PTH secretion are unlikely to play a major role in established postmenopausal osteoporosis.

Alpha 2-adrenoceptor regulation of parathyroid hormone function in the isolated perfused kidney

We investigated physiological interactions between alpha 2-adrenoceptors and parathyroid hormone (PTH) in the isolated buffer-perfused kidney. PTH infusion (10nM) caused a rapid and significant rise in cAMP excretion which diminished despite continued hormone infusion. PTH also caused a delayed phosphaturia which peaked 20-30 minutes following the start of PTH infusion. alpha 2-Adrenoceptor stimulation with epinephrine (28nM) diminished PTH-stimulated cAMP accumulation by 68-71% (p less than 0.05) but had no effect on PTH-induced phosphaturia. None of the experimental interventions affected renal hemodynamics. In additional studies, we used lithium (1mM) as a marker of proximal tubular sodium transport. PTH caused a rapid rise in lithium excretion which was temporally distinct (maximum response in 0-10 minutes) from the phosphaturia. alpha 2-Adrenoceptor stimulation completely blocked the inhibitory effect of PTH on lithium reabsorption. These results suggest that alpha 2-adrenoceptors regulate the stimulatory effect of PTH at proximal tubular adenylate cyclase. However, alpha 2-adrenoceptors play no role in phosphate transport in this segment. alpha 2-Adrenoceptor stimulation reverses PTH-induced lithium excretion, suggesting physiological antagonism in the proximal tubule, most likely involving Na/H exchange.

Autosomal dominant hypoparathyroidism with intracranial calcification outside the basal ganglia

We describe a family with autosomal dominant hypoparathyroidism. The 3 affected individuals had no detectable serum parathyroid hormone on radioimmunoassay. The propositus presented with seizures and on CT scan had bilateral basal ganglion calcification and calcification in the frontal lobes. His similarly affected mother had even more intracerebral calcification. The latter manifestation has not been described previously in autosomal dominant hypoparathyroidism.