Short-term continuous infusion of human parathyroid hormone 1–34 fragment is catabolic with decreased trabecular connectivity density accompanied by hypercalcemia in C57BL/J6 mice

Abstract

Parathyroid hormone (PTH) stimulates bone resorption as well as bone formation in vivo and in organ culture. The catabolic actions of PTH have been recognized in patients with hyperparathyroidism, or with acute infusion of the N-terminal 1-34 fragment of human PTH (hPTH1-34). Whereas the anabolic actions of daily injection with PTH have been well studied in both humans and mice, the catabolic actions of PTH on murine bone remain to be defined. To do this we sought to create a model with short-term, sustained hyperparathyroidism using osmotic infusion pumps. We treated 10-week-old female C57BL/J6 mice with continuous infusion of hPTH1-34 (8·1 pmol/0·25 µl per h, equivalent to 40 µg/kg per day) or vehicle for 2 weeks, using Alzet osmotic pumps. Bone mineral density (BMD), serum total calcium, hPTH1-34, mouse intact PTH (mPTH1-84), osteocalcin and mouse tartrate-resistant acid phosphatase (mTRAP) activity, and microarchitectural variables of the distal femur were measured. Separately, we compared the effects of intermittent daily injection of hPTH1-34 (40 µg/kg per day) with continuous infusion of hPTH1-34 on BMD and bone markers. Exogenous hPTH1-34 was detected only in the PTH-infused mice. Both intermittent and continuous treatment with hPTH1-34 markedly suppressed endogenous mPTH1-84, but only the latter induced hypercalcemia. Daily PTH injection significantly increased both serum osteocalcin and mTRAP, while continuous PTH infusion showed a strong trend to stimulate mTRAP, with a slight but non-significant increase in osteocalcin. There were significant differences in BMD at all sites between animals treated with the same daily dose of intermittent and continuous hPTH1-34. Microcomputed tomography (µCT) analysis of the distal femurs revealed that hPTH1-34 infusion significantly decreased trabecular connectivity density (P<0·05). Thus, the murine bone response to continuous PTH infusion was quite different from that seen with daily PTH injection. Short-term infusion of hPTH1-34 appears to be a good model to study the mechanisms underlying the catabolic action of PTH in mice.

Effects of Cyclic Versus Daily hPTH(1-34) Regimens on Bone Strength in Association With BMD, Biochemical Markers, and Bone Structure in Mice

“…Serum levels of osteocalcin and mTRACP were measured in duplicate as bone formation and resorption markers, respectively, according to the manufacturer's instructions. (16) Mean CV% for intra-assay variations for osteocalcin and mTRACP was 2.4% and 6.7%, respectively.…”

“…(16) Mean CV% for intra-assay variation was 7.9%. Serum levels of osteocalcin and mTRACP were measured in duplicate as bone formation and resorption markers, respectively, according to the manufacturer's instructions.…”

“…(15) Moreover, we recently showed that by 2 weeks, intermittent daily PTH injection markedly increased the bone resorption marker, mouse TRACP (mTRACP), to a level produced by continuous PTH infusion. (16) Thus, in consideration of these data, as well as the differences between mice and humans in life span, body size, bone growth, and responsiveness to PTH based on our previous murine studies, (15)(16)(17)(18) we postulated that 1-week on-and off-PTH cycles would best achieve the specified aims and would serve as a reasonable model of our clinical trial. (9) Therefore, we treated mature mice with daily versus weekly on and off cyclic PTH regimens for 7 weeks and compared the effects of these PTH regimens on BMD, bone markers, bone structure, and bone strength.…”

Short-term intermittent PTH 1-34 administration enhances bone formation in SCID/Beige mice

“…On day 21, the animals were anesthetized with 5µL/g ketamine tion. However, with administration of higher doses (≥ 40 μg/kg) and/or longer durations (≥ 4 weeks), bone turnover is enhanced by elevations in cellular activities supporting both formation and resorption [20,23,24]. Interestingly, not all skeletal sites respond equally to PTH 1-34.…”

“…Studies employing murine models suggest the skeletal response to exogenous PTH 1-34 varies depending on the dose and duration of administration, as well as the mouse strain, age, and skeletal site investigated. Investigations of PTH 1-34 action on bone have employed doses ranging from 3-800 μg/ kg of body weight [10,11,[13][14][15][17][18][19] and durations lasting a couple of weeks to several months [14,[18][19][20][21][22]. In general, low dose (≤ 40 µg/kg), short term (≤ 3 weeks) intermittent PTH 1-34 administration enhances bone formation without altering bone resorp-Enzyme-linked immunosorbent assay kits for serum osteocalcin, mouse specific tartrate resistant acid phosphatase 5b (TRACP-5b), and type I collagen teleopeptides were purchased from Biomedical Technologies Inc. (Stoughton, MA), Immunodiagnostics Systems Inc. (Fountain Hills, AZ), and Nordic Bioscience Diagnostics (Herlev, Denmark), respectively.…”