The effects of programmed administration of human parathyroid hormone fragment (1-34) on bone histomorphometry and serum chemistry in rats

PTH treatment can result in dramatic increases in cancellous bone volume in normal and osteopenic rats. However, this potentially beneficial response is only observed after pulsatile treatment; continuous infusion of PTH leads to hypercalcemia and bone abnormalities. The purpose of these studies was to determine the optimal duration of the PTH pulses. A preliminary study revealed that human PTH-(1-34) (hPTH) is cleared from circulation within 6 h after sc administration of an anabolic dose of the hormone (80 microg/kg). To establish the effects of gradually extending the duration of exposure to hPTH without increasing the daily dose, we programmed implanted Alzet osmotic pumps to deliver the 80 microg/kg x day dose of the hormone during pulses of 1, 2, and 6 h/day, or 40 microg/kg x day continuously. Discontinuous infusion was accomplished by alternate spacing of external tubing with hPTH solution and sesame oil. After 6 days of treatment, we evaluated serum chemistry and bone histomorphometry. As negative and positive controls, groups of rats received pumps that delivered vehicle only and 80 microg/kg x day hPTH by daily sc injection, respectively. Dynamic and static bone histomorphometry revealed that the daily sc injection and 1 h/day infusion dramatically increased osteoblast number and bone formation in the proximal tibial metaphysis, whereas longer infusion resulted in systemic side-effects, including up to a 10% loss in body weight, hypercalcemia, and histological changes in the proximal tibia resembling abnormalities observed in patients with chronic primary hyperparathyroidism, including peritrabecular marrow fibrosis and focal bone resorption. Infusion for as little as 2 h/day resulted in minor weight loss and changes in bone histology that were intermediate between sc and continuous administration. The results demonstrate that the therapeutic interval for hPTH exposure is brief, but that programmed administration of implanted hormone is a feasible alternative to daily injection as a route for administration of the hormone.

Effect of age on the response to parathyroid hormone

Serum phosphate (PO4) levels and the tubular threshold for PO4 corrected for glomerular filtration (TP/GF) are age-dependent, being higher in children than in adults. We evaluated the effect of age on the response to infusion of parathyroid hormone(1-34) (PTH) in healthy children (n = 8) and adults (n = 12). In addition, six patients with pseudohypoparathyroidism (PHP) and two with PTH deficiency (hypoparathyroidism [HP]) were also studied. At baseline, TP/GF in normal subjects was inversely correlated with urinary cyclic adenosine monophosphate corrected for glomerular filtration (UcAMP/GF) (P < .0359). After PTH administration in the controls, UcAMP/GF was inversely correlated with TP/GF (P < .0007) and directly correlated with maximal fractional extraction of PO4 (FEP) (P < .0002). The slope of the regression of TP/GF (P < .0076) and FEP (P < .0034) with UcAMP/GF was steeper in children than in adults. Two HP patients had high PTH-stimulated UcAMP/GF levels, but stimulated TP/GF and FEP were not changed commensurate with levels that would expected from the normative data. In six patients with PHP, PTH-stimulated TP/GF was also correlated with peak UcAMP/GF (r = .96, P < .002). PHP patients could be distinguished from normal controls based on the combination of low peak FEP or high TP/GF together with low peak UcAMP/GF. Thus, in normal subjects, the phosphaturic response to PTH is correlated with the increase in urinary cAMP and is age-dependent, with a greater decrease of TP/GF in children than in adults.

Vascular effects of PTHrP (1-34) and PTH (1-34) in the human fetal-placental circulation

The aim of this study was to examine the vasodilatory effects of parathyroid hormone-related protein (PTHrP) (1-34) and parathyroid hormone (PTH) (1-34) on the human fetal-placental circulation utilising an in vitro placental perfusion model. In all experiments, the vasculature of an isolated human placental cotyledon was pre-constricted with the thromboxane A2 mimetic U46619. A simple dose of PTHrP (1-34) or PTH (1-34) (1.7-300 nM) was then infused into the fetal-placental circulation of the cotyledon. In other experiments, cotyledons were repeatedly infused with PTHrP (1-34) or PTH (1-34) (51.3 nM). Vasodilatory responses were significantly reduced in response to repeated exposure to PTHrP (1-34) (P < 0.001), indicating that this peptide desensitizes the fetal-placental vasculature. PTHrP (1-34) and PTH (1-34) equipotently stimulated a significant vasodilation of the fetal-placental circulation (P < 0.0001). The PTHrP receptor antagonist [Asn10, Leu 11]PTHrP (7-34) (102 nM) was infused in U46619-constricted placentae in the presence and absence of PTHrP (1-34) (10.2 nM). The PTHrP antagonist alone had no significant effect in the fetal-placental circulation. The antagonist significantly attenuated the response to PTHrP (1-34) (P < 0.015). Based on the data obtained in this study it is suggested that locally produced PTHrP (1-34) may be involved in the regulation of normal human fetal-placental vascular tone in autocrine and/or paracrine fashion.

Plasma levels of parathyroid hormone that induce anabolic effects in bone of ovariectomized rats can be achieved by stimulation of endogenous hormone secretion

Parathyroid hormone (PTH) administration increases bone mass in normal and osteopenic animals. However, this treatment currently requires the daily injection of large amounts of PTH, and the relationship of these doses to plasma levels of PTH that are achievable physiologically is unknown. We determined in ovariectomized (ovx) rats: 1) the plasma PTH levels that occur after the subcutaneous injection of graded doses of rat PTH, 2) whether similar PTH levels can be achieved by stimulation of endogenous PTH secretion, and 3) whether a plasma PTH profile that is achievable physiologically is anabolic on bone. Injection of 1, 5, or 25 micrograms/kg rat PTH-(1-34) increased plasma PTH by 46, 164, or 520 pg/mL, respectively, above basal levels within 60 min. Infusion of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid for 2 h reduced plasma Ca2+ by 0.36 mmol/L and produced a total plasma PTH response (area under the plasma PTH curve) similar to that with the 5 micrograms/kg rat PTH injection. Then, 1, 5, or 25 micrograms/kg doses of rat PTH-(1-34) were injected subcutaneously daily for 28 days in 19-week-old rats that were ovx 7 weeks earlier. The 5 and 25 micrograms/kg doses significantly increased bone mineral density in the distal femur and trabecular bone area and average trabecular thickness in the proximal tibia. All doses of PTH significantly increased indices of trabecular connectivity and cancellous bone formation, including double-labeled surface, mineralizing surface, and surface-referent bone formation rate. In conclusion, anabolic effects on bone can be achieved with a plasma PTH profile similar to that attained following stimulation of the parathyroid gland by induced hypocalcemia. These data suggest that agents that transiently increase endogenous PTH secretion may represent a novel means to promote anabolic effects in skeletal tissues.

Subacute infusion of physiological doses of parathyroid hormone raises blood pressure in humans

BACKGROUND

Acute administration of parathyroid hormone (PTH) causes vasodilation and blood pressure decrease in experimental animals. This effect contrasts with the putative role of secondary hyperparathyroidism in the pathogenesis of hypertension of patients with renal failure. Uraemia is characterized by insulin resistance and hyperinsulinaemia. We therefore investigated whether subacute administration of physiological doses of human 1,34-PTH affects blood pressure under conditions of controlled insulin levels (euglycaemic clamp technique) in humans.

METHODS

In a double-blind cross-over design 10 healthy male subjects received, on two occasions, in random order, for 2 h, either a sham infusion or an infusion of 200 units of 1,34-PTH.

RESULTS

Mean ionized calcium concentration increased significantly (P < 0.01) within the normal range during euglycaemic hyperinsulinaemia, both with sham infusion (from 1.25 +/- 0.04 to 1.29 +/- 0.02 mmol/l) and with infusion of 1,34-PTH, but the increase was more marked with 1,34-PTH administration (from 1.26 +/- 0.05 to 1.33 +/- 0.07). In addition, mean platelet intracellular calcium concentration (by fluorescence spectroscopy) was unchanged with sham infusion (49.9 +/- 4.1 versus 50.3 +/- 5.0 nmol), but increased significantly (P < 0.05; paired t-test) after 1,34-PTH infusion (from 49.8 +/- 5.0 to 52.8 +/- 5.8). The infusion of 1,34-PTH resulted in a significant (P < 0.01) increase in mean MAP (from 84 +/- 5 to 88 +/- 5 mmHg) as compared with sham infusion (85 +/- 4 versus 86 +/- 4). The intra-individual changes in intracellular calcium concentration (delta[Ca2+]i) were significantly correlated to the changes in mean MAP (delta MAP) (r = 0.87, P < 0.001). In contrast to blood pressure, insulin sensitivity was not affected by 1,34-PTH infusion (M-value: 7.2 +/- 1.6 mg/kg per min) as compared with sham infusion (7.3 +/- 1.4).

CONCLUSION

Subacute administration of physiological doses of parathyroid hormone under hyperinsulinaemic conditions significantly affects intracellular calcium and blood pressure in healthy subjects, but does not affect the action of insulin.

Effect of parathyroid hormone (hPTH[1-34]) infusion on serum 1,25-dihydroxyvitamin D and parathyroid hormone in normal women

Calcium absorption declines with age. Because 1,25-dihydroxyvitamin D (1,25(OH)2D) is the major hormone controlling calcium absorption, changes in vitamin D metabolism may account for the malabsorption of aging. Serum levels of 1,25(OH)2D have been reported to either decrease or remain unchanged with age. To assess the effect of aging on renal production of 1,25(OH)2D, we evaluated the response of renal 25OHD 1 alpha hydroxylase to human parathyroid hormone (hPTH(1-34) stimulation in 119 women ages 25-83 years. In this population, baseline serum 25OHD and 1,25(OH)2D values did not significantly change with age, but serum iPTH (r = 0.44; p < 0.001) and serum creatinine (r = 0.31; p < 0.01) increased with age. However, the stimulatory activity of hPTH(1-34) on the renal production of 1,25(OH)2D declined with age (r3 = -0.36; p < 0.001) and was most apparent after age 75, being 50% less than that of younger women. Besides age, the production of 1,25(OH)2D was found to be dependent on baseline serum iPTH (r = -0.31; p < 0.0001). Administration of hPTH(1-34) led to suppression of endogenous PTH, and suppressibility of endogenous PTH declined with age (r = 0.53; p < 0.0001). The increase in serum PTH and decreased suppressibility of PTH with age could be due to mild secondary hyperparathyroidism. The increase in PTH with age is probably responsible for maintaining normal serum 1,25(OH)2D levels in elderly subjects; however, decreased metabolism of 1,25(OH)2D in the elderly could also maintain normal serum 1,25(OH)2D levels.

Effects of intermittent hPTH(1-34) alone and in combination with 1,25(OH)(2)D(3) or risedronate on endosteal bone remodeling in canine cancellous and cortical bone

Therapies utilizing intermittent human parathyroid hormone(1-34) (hPTH[1-34]) in combination with other agents have recently been proposed as possible anabolic regimens for the treatment of osteoporosis. We conducted a 24 week study in aged beagle dogs to determine the effects of intermittent hPTH(1-34) administered alone or in combination with 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) on the endosteal remodeling in cancellous and cortical bone. Additionally, we tested the interaction between hPTH(1-34) and a new potent bisphosphonate, risedronate. The three treatment groups were compared with a vehicle control group. Kinetic reconstruction of the remodeling unit revealed substantial differences between the groups in resorption and formation at the basic multicellular unit level. Although the estimates of final erosion depth were unaffected by treatment, tunneling resorption was noted in six of the eight dogs administered hPTH(1-34) alone. These qualitative morphological changes in the resorption lacunae were attenuated or absent in dogs administered hPTH(1-34) in combination with either 1,25(OH)(2)D(3) or risedronate. Functional periods for resorption were significantly increased, and the resorption rates were significantly decreased in the hPTH(1-34) + risedronate group. Analyses of the formative site demonstrated that the wall thickness was significantly increased and the bone balance significantly more positive in all three hPTH(1-34) treatment groups. The most positive bone balance was achieved in the combined hPTH(1-34) + risedronate group (+ 15.6 + or - 14.2 mm, p <0.05). Increases in the mineral apposition rate in the early phases of the formative period suggest that an increase in osteoblastic activity (number or function) may contribute to the increase in wall thickness. Treatment with hPTH(1-34) alone or in combination with 1,25(OH)(2)D(3) caused an approximately 2-fold increase in the activation frequency in cancellous bone, which was essentially normalized to control values by the coadministration of risedronate. The impact of these changes on the cancellous bone microstructure was significant only in the combined hPTH(1-34) + risedronate group where normalized bone turnover in the face of a positive bone balance effected a significant increase in the trabecular thickness. Analyses of sequential fluorochrome labels, administered to reconstruct the temporal changes in intracortical activation, demonstrated the presence of an apparent cyclic pattern of activation in the cortex of placebo-treated dogs. Generally, activation was increased throughout the study in dogs administered hPTH(1-34) alone or in combination. However, in the hPTH(1-34) + risedronate group, activation was significantly blunted toward the end of the study, and the cyclic pattern of activation was modulated. These data suggest that intermittent hPTH(1-34) in combination with risedronate may be superior to hPTH(1-34) in combination with 1,25(OH)(2)D(3) as a therapeutic regimen for osteoporosis due to the protective effect of this bisphosphonate on the cortical and endocortical envelope.