A 6-hour human parathyroid hormone (1-34) infusion protocol: studies in normal and hypoparathyroid subjects

Primary Hypoparathyroidism Resistant to Conventional Therapy

A 28 year old man with primary hypoparathyroidism failed to respond to treatment with calcium and vitamin D analogues. Despite extensive investigation no reason for this was found and he is now successfully treated with teriparatide (synthetic human PTH)

(1-34) Parathyroid hormone infusion acutely lowers fibroblast growth factor 23 concentrations in adult volunteers

BACKGROUND AND OBJECTIVES

Fibroblast growth factor 23 (FGF23) regulates phosphorus and vitamin D metabolism. Parathyroid hormone (PTH) infusion for 24 hours stimulated FGF23 secretion in healthy volunteers. The extent to which this was due to a direct stimulatory effect of PTH versus an indirect effect of increasing 1,25-dihydroxyvitamin D [1,25(OH)(2)D] levels was unclear.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Changes in FGF23 in 26 adults undergoing 6-hour (1-34) PTH infusion were examined, focusing particularly on the effects of PTH on FGF23 in the early period of infusion before sustained increases in 1,25(OH)(2)D.

RESULTS

FGF23 levels declined in parallel with serum phosphate during infusion (P<0.05 for both), with both analytes decreasing within the first hour and reaching their respective nadirs at 6 hours. These changes were observed despite no change in 1,25(OH)(2)D levels during the first hour and a significant increase in 1,25(OH)(2)D from baseline after 6 hours (P<0.001). There were no differences in these responses by race. However, modest racial differences in the phosphaturic response to (1-34) PTH were observed (P=0.04 for interaction), with a higher rate of increase in fractional phosphate excretion in blacks than in whites.

CONCLUSIONS

During short-term (1-34) PTH infusion, FGF23 levels decreased in parallel with serum phosphate levels and despite significant increases in 1,25(OH)(2)D. When coupled with the results of prior longer-term infusion studies, these findings suggest that acute increases in PTH initially act to suppress FGF23 secretion, perhaps to mitigate urinary phosphate losses, before the stimulatory effect of 1,25(OH)(2)D on FGF23 eventually begins to predominate.

Acute variations in extracellular pH modulate transduction pathways of PTH in rat proximal tubule

An increase in circulating parathyroid hormone (PTH) has been shown to enhance the capacity for the kidney to excrete an acid as well an alkaline load, which suggests that changes in systemic acid-base status may modulate the effect of the hormone on bicarbonate absorption in proximal tubule. In the present study, we tested the possibility that acute variations in extracellular pH (pHe), obtained by modifying bicarbonate concentration at constant PCO2 (40 mmHg), may modulate the responses of intracellular messengers coupled to PTH receptors in a preparation of freshly isolated proximal tubule fragments. Variations in pHe, which induced parallel variations in intracellular pH (pHi), did not affect unstimulated values for adenosine 3',5'-cyclic monophosphate (cAMP) production, inositol trisphosphate accumulation, or cytosolic free Ca2+ concentration. In contrast, reducing pHe from 7.4 to 7.2 elicited a decrease of the PTH-induced cAMP production, whereas increasing pHe from 7.4 to 7.6 enhanced it. The ability for cholera toxin and forskolin (which both bypass PTH receptors) to stimulate cAMP formation was diminished at pHe 7.2 and enhanced at pHe 7.6 (the increase did not achieve statistical significance in the presence of forskolin), suggesting that variations in pHe and/or pHi may affect per se adenylyl cyclase activity. Conversely, reducing pHe from 7.4 to 7.2 enhanced the PTH-induced inositol trisphosphate accumulation and rise in cytosolic free Ca2+ whereas increasing pHe from 7.4 to 7.6 had opposite effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic respiratory alkalosis induces renal PTH-resistance, hyperphosphatemia and hypocalcemia in humans

The effects of chronic respiratory alkalosis on divalent ion homeostasis have not been reported in any species. We studied four normal male subjects during a four-day control period (residence at 500 m), during six days of chronic respiratory alkalosis induced by hypobaric hypoxia (residence at 3450 m), followed by a six-day eucapnic recovery period (500 m) under metabolic balance conditions. Chronic respiratory alkalosis (delta PaCO2, -8.4 mm Hg, delta[H+] -3.2 nmol/liter) resulted in a sustained decrement in plasma ionized calcium concentration (delta[IoCa++]p, -0.10 mmol/liter, P less than 0.05) and a sustained increment in plasma phosphate concentration (delta[PO4]p, +0.14 mmol/liter, P less than 0.005) associated with increased fractional excretion of Ca++ (+0.5%, P less than 0.005), decreased phosphate clearance (-6.1 ml/min, P less than 0.025) and decreased excretion of nephrogenous cAMP (-1.5 nmol/100 ml GFR, P less than 0.0025). Urinary phosphate excretion decreased by 15.4 mmol/24 hr on day 1 of chronic respiratory alkalosis (P less than 0.0025), but returned to control values by day 6 despite hyperphosphatemia. Serum intact [PTH] did not change. Sustained hypomagnesuria (-0.8 mmol/24 hr, P less than 0.05) occurred during chronic respiratory alkalosis and was accounted for, at least in part, by decreased fractional excretion of Mg++ (-0.7%, P less than 0.05) in the absence of change in plasma magnesium concentration. Serum 1,25(OH)2D levels were unchanged by chronic respiratory alkalosis. In conclusion, the decrease in nephrogenous cAMP generation despite unchanged serum intact PTH concentration suggests that chronic respiratory alkalosis results in impaired renal responsiveness to PTH as manifested by alterations in PTH-dependent renal calcium and phosphate transport.(ABSTRACT TRUNCATED AT 250 WORDS)

Antiidiotypic PTH antibodies as a cause of elevated immunoreactive parathyroid hormone in idiopathic hypoparathyroidism, a second case: another manifestation of autoimmune endocrine disease?

A 69-year-old man became hypocalcemic under medical observation. The hypocalcemia occurred in the presence of circulating immunoreactive parathyroid hormone (PTH). Common causes of secondary hyperparathyroidism were excluded, as was PTH resistance using PTH infusions. The immunoreactive PTH was examined in detail. PTH immunoreactivity (1) was not retained on a C18 SPE-column, suggesting unusual molecular or physicochemical properties, unlike bona fide PTH; (2) was precipitated with 15% PEG, indicating a molecular size far in excess of native PTH; (3) had an apparent molecular size similar to immunoglobulins on size exclusion chromatography; (4) was retained on affinity chromatography with both Protein A and anti-hIgG antibodies. These data lead us to conclude that the immunoreactive PTH was due to antiidiotypic PTH autoantibodies. No significant quantities of true PTH were found in the patient's serum suggesting that his hypoparathyroidism was a result of PTH deficiency. Autoimmunity might explain the occurrence of both processes if an arrested antiidiotypic antibody cascade is assumed.

Skeletal responsiveness to parathyroid hormone in healthy females: relationship to menopause and oestrogen replacement

In order to directly evaluate the role of parathyroid hormone (PTH) and its interaction with oestrogens for postmenopausal bone loss, studies were performed where synthetic human (1-38) PTH was infused s.c. over 24 h in 15 healthy females. Measurements were made of serum electrolytes, PTH and biochemical indices of bone turnover: serum osteocalcin and alkaline phosphatase and fasting urinary hydroxyproline and calcium. During the infusion of PTH there were significant increases of serum calcium (15%), fasting urinary calcium (55%) and hydroxyproline (80%) but a reduction of the serum osteocalcin concentrations (15%). There were significant relations between the calcaemic response and the increases of urinary calcium and hydroxyproline and the two latter were also closely related. There was, however, no correlation between the responses to PTH for the formative vs the resorptive indices. Postmenopausal women displayed greater increases of serum calcium and fasting urinary hydroxyproline indicating greater skeletal sensitivity to exogenous PTH. Following treatment with oestrogens the indices of skeletal responsiveness were reversed towards the premenopausal values. The findings demonstrate that during short-term infusion of PTH there is a dissociation between bone resorption and formation and, furthermore, that the menopause is associated with an enhanced skeletal sensitivity for PTH.

Familial hypoparathyroidism due to an abnormal parathyroid hormone molecule

The case of a 53-year-old man with familial hypoparathyroidism in the presence of circulating immunoreactive PTH is discussed. The patient responded to exogenous PTH by an increase in urinary cAMP excretion and by several post cyclase parameters including an increase in serum calcium and 1,25-dihydroxyvitamin D, an increase in urinary phosphate excretion and a decrease in urinary calcium. Immunoreactive PTH was detected in this patient's serum by three separate anti-PTH antisera. This immunoreactive PTH behaved aberrantly with these antisera. Nonparallelism to the standard curve was seen in two radioimmunoassays and the material was detected by an antiserum which preferentially binds bovine PTH. No circulating PTH binding activity was detectable. Family studies confirmed the genetic nature of the abnormality. HPLC studies revealed the presence of an abnormally hydrophobic fraction containing immunoreactive PTH. We believe these findings constitute strong evidence for the presence of an abnormal PTH molecule with reduced biological activity resulting in hypoparathyroidism.