Effects of Growth Hormone Administration on Bone Mineral Metabolism, PTH Sensitivity and PTH Secretory Rhythm in Postmenopausal Women With Established Osteoporosis

Authors


  • The authors state that they have no conflicts of interest.

Abstract

Introduction: Growth hormone (GH) replacement improves target organ sensitivity to PTH, PTH circadian rhythm, calcium and phosphate metabolism, bone turnover, and BMD in adult GH-deficient (AGHD) patients. In postmenopausal women with established osteoporosis, GH and insulin like growth factor-1 (IGF-1) concentrations are low, and administration of GH has been shown to increase bone turnover and BMD, but the mechanisms remain unclear. We studied the effects of GH administration on PTH sensitivity, PTH circadian rhythm, and bone mineral metabolism in postmenopausal women with established osteoporosis.

Materials and Methods: Fourteen postmenopausal women with osteoporosis were compared with 14 healthy premenopausal controls at baseline that then received GH for a period of 12 mo. Patients were hospitalized for 24 h before and 1, 3, 6, and 12 mo after GH administration and half-hourly blood and 3-h urine samples were collected. PTH, calcium (Ca), phosphate (PO4), nephrogenous cyclic AMP (NcAMP), β C-telopeptide of type 1 collagen (βCTX), procollagen type I amino-terminal propeptide (PINP), and 1,25-dihydroxyvitamin D [1,25(OH)2D] were measured. Circadian rhythm analysis was performed using Chronolab 3.0 and Student's t-test and general linear model ANOVAs for repeated measures were used where appropriate.

Results: IGF-1 concentration was significantly lower in the women with established osteoporosis compared with controls (101.5 ± 8.9 versus 140.9 ± 10.8 μg/liter; p < 0.05) and increased significantly after 1, 3, 6, and 12 mo of GH administration (p < 0.001). Twenty-four-hour mean PTH concentration was higher in the osteoporotic women (5.4 ± 0.1 pM) than in healthy controls (4.4 ± 0.1 pM, p < 0.001) and decreased after 1 (5.2 ± 0.1 pM, p < 0.001), 3 (5.0 ± 0.1 pM, p < 0.001), 6 (4.7 ± 0.1 pM, p < 0.001), and 12 mo (4.9 ± 0.1 pM, p < 0.05) of GH administration compared with baseline. NcAMP was significantly lower in osteoporotic women (17.2 ± 1.2 nM glomerular filtration rate [GFR]) compared with controls (21.4 ± 1.4 nM GFR, p < 0.05) and increased after 1 (24.2 ± 2.5 nM GFR, p < 0.05), 3 (27.3 ± 1.5 nM GFR, p < 0.001), and 6 mo (32.4 ± 2.5 nM GFR, p < 0.001) compared with baseline. PTH secretion was characterized by two peaks in premenopausal women and was altered in postmenopausal women with a sustained increase in PTH concentration. GH administration also restored a normal PTH secretory pattern in the osteoporotic women. The 24-h mean adjusted serum calcium (ACa) concentration increased at 1 and 3 mo (p < 0.001) and PO4 at 1, 3, 6, and 12 mo (p < 0.001). 1,25(OH)2D concentration increased after 3, 6, and 12 mo of GH (p < 0.05). An increase in urine Ca excretion was observed at 3 and 6 mo (p < 0.05), and the renal threshold for maximum tubular phosphate reabsorption rate (TmPO4/GFR) increased after 1, 3, 6, and 12 mo (p < 0.05). βCTX concentration increased progressively from 0.74 ± 0.07 μg/liter at baseline to 0.83 ± 0.07 μg/liter (p < 0.05) at 1 mo and 1.07 ± 0.09 μg/liter (p < 0.01) at 3 mo, with no further increase at 6 or 12 mo. PINP concentration increased progressively from baseline (60 ± 5 μg/liter) to 6 mo (126 ± 11 μg/liter, p < 0.001), with no further increase at 12 mo. The percentage increase in PINP concentration was significantly higher than βCTX (p < 0.05).

Conclusions: Our study shows that GH has a regulatory role in bone mineral metabolism. GH administration to postmenopausal osteoporotic women improves target organ sensitivity to PTH and bone mineral metabolism and alters PTH secretory pattern with greater increases in bone formation than resorption. These changes, resulting in a net positive bone balance, may partly explain the mechanism causing the increase in BMD after long-term administration of GH in postmenopausal women with osteoporosis shown in previous studies and proposes a further component in the development of age-related postmenopausal osteoporosis.

INTRODUCTION

Bone loss and the increasing incidence of osteoporosis is an accompaniment of aging. Women undergo two phases of bone loss—a slow phase with a linear decrease in bone, continuing into old age, and a superimposed, accelerated transient phase beginning at menopause caused by estrogen deficiency.math image The slow phase in the development of osteoporosis has been attributed to alteration of age-related factors resulting in impaired osteoblast function and bone formation. These include growth hormone (GH) and insulin-like growth factor 1 (IGF-1), both major determinants of adult bone mass,math image that decrease with advancing agemath image and are lower in women with established osteoporosis.math image

The beneficial effects of GH on bone metabolism and BMD have been shown in, adult GH-deficient (AGHD) patients.math image Target organ insensitivity to the effects of PTH resulting in increased circulating PTH and abnormal PTH secretionmath image contributes to the development of osteoporosis in AGHD. GH replacement (GHR) in AGHD patients has been shown to increase bone and renal PTH receptor or target cell sensitivity to the effects of PTH and simultaneously restore PTH secretory rhythm, increase bone turnover markers, 1,25-dihydroxy vitamin D [1,25(OH)2D] concentration, and Ca absorption/reabsorption, thus contributing to the positive effects of GH on bone.math image

Postmenopausal women with osteoporosis have high circulating PTH concentrations with abnormal PTH circadian rhythmmath image and may consequently be insensitive to the effects of PTH,math image although this has not been shown conclusively. The decline in GH/IGF-1 with aging may contribute to these PTH related abnormalities through mechanisms similar to that observed in untreated AGHD.math image GH has been previously administered to healthy elderly women and women with postmenopausal osteoporosis and increases in bone turnover and BMD have been shown.math image However, the mechanisms by which GH exerts its beneficial effects on bone in established postmenopausal osteoporosis remain unexplained. We therefore studied the effects of 12 mo of GH administration on PTH secretory pattern, PTH sensitivity, and bone mineral metabolism in postmenopausal women with established osteoporosis.

MATERIALS AND METHODS

Patients

Women from a community osteoporosis screening program, with newly diagnosed osteoporosis, were recruited to the study. All patients had undergone bone densitometric evaluation using a Prodigy Oracle Fan-Beam bone densitometer (GE Medical Systems, Giles, Buckinghamshire, UK). T-scores were calculated against a reference population of UK subjects 20–39 yr of age. Osteoporosis was defined according to the WHO criteria with a T-score ≤ −2.5math image of either the lumbar spine (LS) or femoral neck (FN). Patients with diabetes, ischemic heart disease, heart failure, renal disease, cancer, chronic illness, vertebral fracture, or any disease or medication such as corticosteroids, affecting bone metabolism were excluded. Subjects were excluded if they were receiving hormone replacement therapy (HRT) or had received HRT in the year before start of the study, were on Ca and vitamin D supplements, or had ever been exposed to bisphosphonate therapy.

Table Table 1.. Demographic Characteristics of Patients and Controls
original image

Fourteen postmenopausal women with osteoporosis (63.4 ± 2.1 [SD] yr; range, 52–79 yr) were recruited. The mean BMD T-score ± SE in the lumbar spine (LS; L2–L4) and femoral neck (FN) was −3.3 ± 0.2 and −2.0 ± 0.2, respectively. For baseline comparison 14 healthy premenopausal control women (33.9 ± 2.2 yr; range, 25–39 yr) with normal BMD (LS and FN T-score was 0.3 ± 0.3 and 0.6 ± 0.2, respectively) were recruited from a database of volunteers willing to participate in medical research (Table 1).

Methods

Study visits involved admission to the Metabolic Bone Unit of the Royal Liverpool University Hospital at 1:00 p.m. for a period of 25 h. An indwelling venous cannula was inserted in the antecubital fossa of each patient at the time of admission, and blood samples were collected every half hour from 2:00 p.m. on the day of admission to 2:00 p.m. the following day. Samples were centrifuged immediately at −4°C, and serum/plasma was separated to be frozen at −70°C for later analysis. Subjects were provided with 1.5 liters of water and encouraged to drink at fairly frequent intervals to maintain hydration and urine samples were collected at 3-h intervals between 2:00–11:00 p.m. and 8:00 a.m. and 2:00 p.m., and aliquots of these samples were stored at −20°C for later analysis. A 24-h urine volume was measured to estimate fluid balance and no significant variability in the hydration of the individuals was observed. Subjects remained recumbent from 11:00 p.m. to 8:00 a.m. and slept during this period. Each subject was served with standardized hospital meals at 8:00 a.m., 12:00 p.m., 6:00 p.m., and 10:00 p.m. The serving sizes and combinations of foods contained recommended daily allowances of all nutrients including Ca and PO4.

Samples were collected in all controls and subjects with osteoporosis at baseline, after which GH (Humatrope; Eli Lilly & Co., Basingstoke, Hampshire, UK) was started at a standard daily dose of 0.2 mg, self-injected using an automated pen device (Humatrope-Pen II, Eli Lilly & Co.) at 10:00 p.m. every night in the subjects with osteoporosis. GH was initiated at 0.2 mg/d for 4 wk and titrated by increments of 0.1 mg/d every 2 wk, according to IGF-1 concentration. We aimed for a target IGF-1 concentration within ±1 SD of the median IGF-1 for a woman 45 yr of age. The IGF-1 normal ranges were established with a cohort of 450 healthy adults (age, 18–80 yr; men = 225). The distribution of the values obtained was log normal, and consequently, the measured values were log-transformed before further calculations. Means and SD of the log IGF-1 values were calculated for age intervals (10 yr). Best fit regression curves were derived for means and mean minus SD. IGF SD scores (IGF SDS) were calculated by using the formula log IGF-1 – mean/SD. Study visits were repeated 1, 3, 6, and 12 mo after the initiation of GH and all patients completed the entire 12-mo study. The local ethics committee approved the study, and written informed consent was obtained from each patient before recruitment.

Biochemistry

Serum Ca, PO4, creatinine, and albumin were measured on all samples by standard procedures on an automated platform (Hitachi 747; Roche Diagnostics, Lewes, UK). Serum Ca was adjusted for albumin.math image Serum ACa has been shown to strongly correlate with ionized Ca and has been found to be precise in subjects with Ca and albumin within the reference range.math image Serum PTH(1–84) was measured on all samples using the Advantage automated assay platform (Nichols Institute, San Juan Capistrano, CA, USA), with a detection limit of 0.5 pM and intra- and interassay CVs of <7% across the working range.

For the 1,25(OH)2D assay, serum was treated with acetonitrile and the supernatant purified through C18-OH reverse phase column to obtain the fraction containing 1,25(OH)2D, which after evaporation was measured by radioimmunoassay (IDS, Boldon, UK). Each sample contained tritiated 1,25(OH)2D to act as a recovery. The intra-assay CV was <9% and the interassay CV was <12% across the working range, with a detection limit of 15 pM. Serum 25-hydroxyvitamin D [25(OH)D] was measured using an RIA kit (DiaSorin, Stillwater, MN) after acetonitrile extraction. The intra-assay CV was <8%, and the interassay CV was <11% across the working range, with a detection limit of 4 nM.

Serum concentration of type-I collagen-β C-telopeptide (βCTX) and procollagen type-I amino-terminal propeptide (PINP) and osteocalcin were measured on the Elecsys automated platform, which uses electrochemiluminescence assays (ECLIA; Roche Diagnostics, Lewes, UK). The intra- and interassay CVs for βCTX were <4% and <5%, respectively, across the working range, with a detection limit of 0.01 μg/liter and the intra- and interassay CVs for PINP, were <2% and <2.5%, respectively, across the working range, with a detection limit of 4 μg/liter. The intra- and interassay CVs for osteocalcin were both <5% across the working range with a detection limit of 0.5 μg/L.

Urine creatinine, Ca, and PO4 were analyzed on all samples using standard laboratory methods (Roche Diagnostics). Urine values are expressed as molar ratios to creatinine (Ca/Cr, PO4/Cr) and as excretion per liter of creatinine clearance (CCr) by multiplying the urinary ratios and the serum creatinine to yield the CaE and PO4E, respectively. The renal threshold for maximum tubular phosphate reabsorption rate (TmPO4/GFR; mM of GFR) was derived from the nomogram by Walton and Bijvoet.math image Nephrogenous cyclic AMP (NcAMP), which is a reliable index of PTH activity at the level of the kidney,math image was determined from the formula: NcAMP (nM GFR) = (SCr [μM] × UcAMP [μM]/Ucr [mM]). Plasma cyclic AMP (PcAMP) was measured by radioimmunoassay (BIOTRAC cAMP; Amersham Pharmacia Biotech, Little Chalfont, UK). The intra-assay CV was <8%, and interassay CV was <10% across the working range, with a detection limit of 5 nM. Urine cyclic AMP (UcAMP) was measured by in-house radioimmunoassay (RIA) as previously described.math image The intra- and interassay CVs were <8% and 10%, respectively, with a detection limit of 0.2 μM.

IGF-1 was measured with a specific RIA in the presence of a large excess of IGF-2 (Mediagnost, Tübingen, Germany) to block the interference of IGF-binding proteins.math image Intra- and interassay CVs were 1.6% and 6.4%, respectively.

Statistical analysis

Individual and population mean cosinor analysis was used first to confirm circadian rhythmicity and determine the circadian rhythm parameters of PTH using CHRONOLAB 3.0 (Universdade de Vigo, Vigo, Spain), a software package for analyzing biological time series by least squares estimation.math image The package has previously been well validated and used to analyze PTH and bone marker circadian rhythms in various groups of patients.math image The software thus provides the following circadian parameters: (1) midline estimate statistic of rhythm (MESOR), defined as the rhythm-adjusted mean or the average value of the rhythmic function fitted to the data; (2) amplitude, defined as one half the extent of rhythmic change in a cycle approximated by the fitted cosine curve (difference between the maximum and MESOR of the fitted curve); and (3) acrophase, defined as the lag between a defined reference time (2:00 p.m. of the first day in our study when the fitted period is 24 h), and time of peak value of the crest time in the cosine curve fitted to the data. A p value for the rejection of the zero-amplitude (no rhythm) assumption is also determined for each individual series and for the group. The method used by the program allows analysis of hybrid data (time series sampled from a group of subjects, each represented by an individual series).math image Bingham's test, developed for testing cosinor parameters and part of CHRONOLAB 3.0 software, was used to determine the significance of the differences of cosinor-derived circadian rhythm parameters between subjects.

After the confirmation of concerted circadian rhythms further analysis of the more extensively studied PTH rhythm was performed as the next step. Over and above the diurnal variation, the PTH rhythm has previously been shown to have bimodal peaks in healthy individuals (early evening and nocturnal) and previous studies in pathological conditions have shown alterations during the time periods of these peaks.math image Based on these previous data, time points for further analysis were selected from the individual peaks and the time of onset was defined as the time of first occurrence of at least three consecutive samples exceeding the mean levels of PTH obtained between 8:00 a.m. and 2:00 p.m. by >1 SD.math image

General linear model ANOVA (GLM ANOVA) for repeated measures was used to analyze the data. Repeated-measures ANOVA assumes normally distributed errors, equal variances, and sphericity. The Kolmogorov-Smirnov test was used to confirm normal distribution and Levenes' test for equality of variances. Mauchly's test indicated that the sphericity assumption was violated and degrees of freedom were corrected using Greenhouse-Geisser estimates of sphericity (ϵ = 0.53). This method has been validated for similar comparisons.math image For all analyses, p < 0.05 was considered significant. Values are expressed as the mean ± SE unless otherwise stated.

RESULTS

GH dose and IGF-I levels

IGF-I concentration (Fig. 1A) was significantly lower in the women with osteoporosis compared with the controls (101.5 ± 8.9 versus 140.9 ± 10.8 μg/liter; p < 0.05). In the treated patients, the mean GH dose (Fig. 1B) was 0.2 ± 0.01 mg/d at 1 mo and increasing to 0.39 ± 0.01 mg/d at 3 mo (p < 0.001) and further titrated to 0.56 ± 0.04 mg/d at 6 mo (p < 0.001 compared with baseline; p < 0.01 compared with 3 mo). Maximum dose was achieved at 6 mo in all patients, and further increases were not tolerated, and three patients had dose reductions from 6 to 12 mo because of increased musculo-skeletal pains. The mean GH dose was 0.49 ± 0.05 mg/d at 12 mo (p < 0.001 compared with baseline, p < 0.05 compared with 3 mo, and p = not significant [NS] compared with 6 mo). Mean serum IGF-I increased significantly from 101.5 ± 8.9 to 128.3 ± 12.1 μg/liter by 1 mo (p < 0.001), 157.2 ± 15.8 μg/liter at 3 mo (p < 0.001), 172.9 ± 13.6 μg/liter at 6 mo (p < 0.001, compared with baseline; p = 0.06, compared with 3 mo), and 166.9 ± 14.8 μg/liter at 12 mo (p < 0.001, compared with baseline; p = NS, compared with 3 and 6 mo). Similarly IGF-I SDS increased from −1.26 ± 0.27 at baseline to −0.62 ± 0.29 at 1 mo (p < 0.001), −0.03 ± 0.26 at 3 mo (p < 0.001), 0.28 ± 0.26 at 6 mo (p < 0.001, compared with baseline; p = 0.07, compared with 3 mo), and 0.19 ± 0.26 at 12 mo (p < 0.001, compared with baseline; p = NS, compared with 3 and 6 mo).

Figure FIG. 1..

GH doses and IGF-1 concentrations.

PTH

Twenty-four-hour mean PTH concentration (Fig. 2A) was higher in the osteoporotic women (5.4 ± 0.1 pM) than in healthy controls (4.4 ± 0.1 pM, p < 0.001). After GH administration, 24-h mean PTH concentration decreased progressively from baseline (5.4 ± 0.1 pM) to 1 (5.2 ± 0.1 pM, p < 0.001), 3 (5.0 ± 0.1 pM, p < 0.001), and 6 mo (4.7 ± 0.1 pM, p < 0.001), with maximum reduction in PTH concentration at 6 mo. PTH concentrations increased significantly by 12 mo (4.9 ± 0.1 pM, p < 0.05 compared with 6 mo) but remained below baseline concentrations.

Individual and population cosinor analyses for circulating PTH (Figs. 3 and 4) showed significant circadian rhythms for all healthy controls and all osteoporotic patients at all visits (p < 0.001) but with differences between patients and controls and changes after GH administration. The mean PTH MESOR was significantly higher in the osteoporotic women than in the controls (5.4 ± 0.3 versus 4.4 ± 0.3 pM, p = 0.03), but there was no significant difference in the amplitude (0.7 ± 0.1 versus 0.5 ± 0.1 pM for osteoporotic women and controls, respectively, p = 0.22) or acrophase (−156 ± 16° versus −178 ± 16° for osteoporotic women and controls, respectively, p = 0.36). After GH administration to the osteoporotic women, mean PTH MESOR decreased by 3 mo (5.0 ± 0.3 pM, p < 0.05), with a further decrease at 6 mo (4.5 ± 0.2 pM, p < 0.001) when maximum reduction in mean PTH MESOR was observed. PTH MESOR rose significantly by 12 mo (4.9 ± 0.1 pM, p < 0.05 compared with 6 mo) but remained below baseline (5.3 ± 0.3 pM, p < 0.05). The amplitude and acrophase of the PTH circadian rhythm did not change significantly after GH administration. A reduction in mean percentage increasemath image in PTH concentration between 2:00 and 11:00 p.m., without significant change in the maximum percentage increasemath image indicated a narrower afternoon/evening peak, following 3, 6, and 12 mo of GH administration. The maximum percentage increase and the mean percentage change in PTH concentration between 11:30 p.m. and 8:00 a.m.math image was significantly lower in the osteoporotic women as compared with the controls representing a less marked nocturnal peak. The maximum and mean percentage change in PTH concentration overnight increased significantly after GH administration indicating restoration of the nocturnal peak.

Figure FIG. 2..

Biochemical parameters in osteoporotic women compared with controls and before and after GH administration. NS*, no significant difference, osteoporotic women compared with control subjects; *p < 0.05 osteoporotic women compared with control subjects; **p < 0.01 osteoporotic women compared with control subjects; ***p < 0.001 osteoporotic women compared with control subjects. NS+, no significant difference compared with baseline; +p < 0.05 compared with baseline; ++p < 0.01 compared with baseline; +++p < 0.001 compared with baseline. NS€, no significant difference compared with 1 mo; p < 0.05 compared with 1 mo; €€p < 0.01 compared with 1 mo; €€€p < 0.001 compared with 1 mo. NS£No significant difference compared with 3 mo; £p < 0.05 compared with 3 mo; ££p < 0.01 compared with 3 mo; £££p < 0.001 compared with 3 mo. NSψNo significant difference compared with 6 mo; ψp < 0.05 compared with 6 mo; ψψp < 0.01 compared with 6 mo; ψψψp < 0.001 compared with 6 mo.

Figure FIG. 3..

Cosinor-derived PTH rhythms for control subjects and osteoporotic women. Uninterrupted lines represent osteoporotic women and dotted lines represent control subjects. Arrows (uninterrupted, osteoporotic women; dotted, control subjects) represent the acrophase in degrees.

NcAMP

NcAMP (Fig. 2B) was significantly lower in osteoporotic women (17.2 ± 1.2 nM GFR) compared with controls (21.4 ± 1.4 nM GFR, p < 0.05). NcAMP increased after 1 mo of GH administration (24.2 ± 2.5 nM GFR, p < 0.05) and remained elevated at 3 (27.3 ± 1.5 nM GFR, p < 0.001) and 6 mo (32.4 ± 2.5 nM GFR, p < 0.001) compared with baseline (17.2 ± 1.2 nM GFR) and returned to levels not significantly different to baseline at 12 mo (14.8 ± 1.6 nM GFR).

Calcium

Twenty-four-hour mean ACa concentration (Fig. 2C) was not significantly different in the osteoporotic women (2.36 ± 0.004 mM, p < 0.05) compared with controls (2.35 ± 0.004 mM). The 24-h mean ACa concentration increased progressively after 1 (2.40 ± 0.002 mM, p < 0.001) and 3 mo (2.38 ± 0.004 mM, p < 0.001) of GH compared with baseline (2.36 ± 0.004 mM) but returned to concentrations that were not significantly different from baseline at 6 (2.35 ± 0.002 mM) and 12 mo (2.34 ± 0.002 mM).

Figure FIG. 4..

Cosinor-derived PTH rhythms before and 6 mo after GH administration. Uninterrupted lines represent osteoporotic women before GH administration and dotted lines represent osteoporotic women after 6 mo of GH administration. Arrows (uninterrupted, osteoporotic women before GH administration; dotted, osteoporotic women after 6 mo of GH administration) represent the acrophase in degrees.

Phosphate

Twenty-four-hour mean PO4 concentration (Fig. 2E) was lower in osteoporotic women (1.11 ± 0.01 mM, p < 0.05) compared with controls (1.15 ± 0.01 mM). Twenty-four-hour mean PO4 concentration increased progressively after 1 (1.18 ± 0.01 mM), 3 (1.23 ± 0.01 mM), and 6 mo (1.27 ± 0.01 mM) and remained elevated with no further increase at 12 mo (1.28 ± 0.01 mM) of GH administration compared with baseline (1.11 ± 0.01 mM, p < 0.001).

Vitamin D metabolites

No significant difference in serum 25(OH)D3 concentration was observed between controls (42.0 ± 6.1 nM) and osteoporotic women (47.6 ± 6.1 nM, p = 0.53). 25(OH)D3 concentrations were not significantly different after 1 (45.1 ± 4.2 nM), 3 (49.9 ± 5.3 nM), and 12 (42.2 ± 4.8 nM) mo of GH administration. An increase was observed at 6 mo (54.5 ± 4.5 nM, p < 0.05) compared with baseline (47.6 ± 3.6 nM). No significant difference in serum 1,25(OH)2D (Fig. 2D) concentration was observed between controls (74.0 ± 8.2 pM) and osteoporotic women (78.1 ± 8.2 pM, p = 0.73; Fig. 2D). 1,25(OH)2D concentrations increased by 3 mo (99.4 ± 10.0 pM, p < 0.001) and were maintained at 6 (95.7 ± 9.5 pM, p < 0.05) and 12 mo (99.9 ± 11.6 pM, p < 0.01; Fig. 2D).

Urine calcium excretion

Ca/Cr (0.6 ± 0.03 versus 0.4 ± 0.03; p < 0.001) and CaE (Fig. 2F; 0.05 ± 0.002 versus 0.03 ± 0.002 mM CCr; p < 0.001) were higher in the osteoporotic women compared with controls. Ca/Cr increased after 3 (0.8 ± 0.04; p < 0.001) and 6 (0.9 ± 0.04; p < 0.001) mo of GH and was not significantly different at 1 (0.7 ± 0.04; p = 0.1) and 12 mo (0.7 ± 0.04; p = 0.2) compared with baseline (0.6 ± 0.03). CaE also increased similarly (baseline, 0.05 ± 0.002 mM CCr; 1 mo, 0.06 ± 0.003 mM CCr, p = NS; 3 mo 0.07 ± 0.003 mM CCr, p < 0.001; 6 mo 0.08 ± 0.003 mM CCr, p < 0.001; 12 mo 0.06 ± 0.003 mM CCr, p = NS).

Urine phosphate excretion and TmPO4/GFR

PO4/Cr (2.2 ± 0.12 versus 2.5 ± 0.12; p = 0.1) and PO4E (Fig. 2H; 0.18 ± 0.009 mM CCr versus 0.19 ± 0.009 mM CCr; p = 0.3) were not significantly different in the two groups. PO4/Cr increased significantly after 1 (2.8 ± 0.15; p < 0.001), 3 (3.2 ± 0.16; p < 0.001), 6 (3.3 ± 0.17; p < 0.001), and 12 mo (2.7 ± 0.13; p < 0.05) of GH compared with baseline (2.2 ± 0.12) with similar increases in PO4E (baseline, 0.18 ± 0.009 mM CCr; 1 mo, 0.22 ± 0.012 mM CCr, p < 0.001; 3 mo, 0.25 ± 0.012 mM CCr, p < 0.001; 6 mo, 0.28 ± 0.014 mM CCr, p < 0.001; 12 mo, 0.21 ± 0.012 mM CCr, p < 0.05).

TmPO4/GFR (Fig. 2G) was not significantly different between controls (0.98 ± 0.02 mM GFR) and osteoporotic women (0.96 ± 0.02 mM GFR, p = 0.6). TmPO4/GFR increased after GH administration for 1 mo (0.99 ± 0.02 mM GFR, p < 0.05) and remained elevated at 3 (0.99 ± 0.02 mM GFR, p < 0.05), 6 (1.01 ± 0.02 mM GFR, p < 0.001), and 12 (1.08 ± 0.02 mM GFR, p < 0.001) mo compared with baseline.

Markers of bone turnover

βCTX concentrations (Fig. 2I) were significantly higher in osteoporotic women (0.74 ± 0.07μg/liter) compared with controls (0.20 ± 0.07 μg/liter, p < 0.001). After GH administration βCTX concentrations increased progressively from baseline (0.74 ± 0.07μg/liter) to 1 (0.83 ± 0.07 μg/liter, p < 0.05) and 3 mo (1.07 ± 0.09 μg/liter, p < 0.001), with no further increase seen at 6 (1.18 ± 0.10 μg/liter, p < 0.001 compared with baseline) and 12 mo (1.08 ± 0.12 μg/liter, p < 0.001 compared with baseline).

PINP (Fig. 2J) concentrations were significantly higher in osteoporotic women (60 ± 5 μg/liter) compared with controls (35 ± 5 μg/liter, p < 0.01). PINP concentrations increased progressively from baseline (60 ± 5 μg/liter) to 1 (69 ± 5 μg/liter, p < 0.001), 3 (93 ± 6 μg/liter, p < 0.001), and 6 mo (126 ± 11 μg/liter, p < 0.001). The increase was maintained after 12 mo (122 ± 14 μg/liter, p < 0.001) of GH administration.

Osteocalcin concentrations increased progressively from baseline (34 ± 3 μg/liter) to 1 (37 ± 3 μg/liter, p < 0.05), 3 (48 ± 3 μg/liter, p < 0.01), and 6 mo (61 ± 4 μg/liter, p < 0.001). The increase was maintained after 12 mo (59 ± 5 μg/liter, p < 0.001 compared with baseline) of GH administration.

The percentage increase in PINP concentration was significantly higher than βCTX after 6 (76 ± 25% versus 142 ± 25%, p < 0.05) and 12 mo (61 ± 25% versus 133 ± 25%, p < 0.05) of GH administration and the percentage increase in osteocalcin concentration was significantly >12 mo (61 ± 25% versus 76 ± 25%, p < 0.05).

DISCUSSION

Postmenopausal women with osteoporosis have lower circulating IGF-1 concentration with higher 24-h mean PTH and lower NcAMP concentration compared with healthy premenopausal women with normal BMD. GH administration resulted in increased IGF-1 concentration, decreased PTH concentration, and increased NcAMP. GH administration also resulted in an increase in 1,25(OH)2D, serum ACa, serum PO4, TmPO4/GFR, and biochemical markers of bone turnover with a greater percentage increase in markers of bone formation than resorption. Our findings indicate a decrease in target organ sensitivity to PTH in postmenopausal women with osteoporosis that increased after GH administration. GH administration restored the circadian rhythm of PTH that was altered in osteoporotic women.

Changes in PTH and bone metabolism in postmenopausal women with osteoporosismath image have previously been mainly attributed to oestrogen deficiency after the menopause, diminished response to vitamin D and to aging itself.math image GH effects on PTH in osteoporosis have only been studied in the short term and with the co-administration of other calcitropic agents with variable results including decreased,math image unchanged,math image or increased concentrationmath image attributed to increased bone turnover and enhanced mobilization of skeletal Ca or increased Ca absorption.math image Previous studies have not measured NcAMP, which reflects the activity of PTH in both physiological and pathophysiological states and is a reliable index of PTH function.math image Because NcAMP excretion parallels changes in PTH secretion,math image the reciprocal increase in NcAMP excretion with decreasing PTH concentration, observed in our study indicates increased renal sensitivity to the effects of PTH after GH administration. The observed decrease in NcAMP back to baseline levels at 12 mo may be a reflection of the mean GH dose decrease at 12 mo. However, the significantly lower PTH concentration at 12 mo compared with baseline in the presence of a NcAMP concentration similar to baseline still suggests an improvement in PTH sensitivity having achieved a new equilibrium. PTH showed a sustained increase between 2:00 and 11:00 p.m. with a reduced nocturnal rise in osteoporotic women as previously shown.math image After GH administration the PTH secretory pattern changed significantly with restoration to a rhythm resembling that similar to healthy control subjects,math image supporting a role for GH in regulating PTH secretory rhythm.

1,25(OH)2D and Ca absorption decrease with aging and the normal increase in 1,25(OH)2D in response to infusions of PTH is blunted possibly because of decreased renal 25(OH)D 1α-hydroxylase sensitivity to PTH.math image It has also been suggested that women with osteoporosis have a defect in renal calcium conservationmath image and the higher urine Ca excretion in association with the relatively higher PTH concentration in our subjects suggests renal resistance to the effects of PTH may contribute to this. Although the concentrations of 25(OH)D3 were similar in both groups studied, they were in the low normal range, and this may have affected PTH secretion. The initial increase in circulating Ca with no change in urine Ca excretion after GH administration may partly reflect increased renal sensitivity to PTH resulting in renal Ca reabsorption as a direct effect of PTH.math image A possible increase in 1-α hydroxylase activity resulting in increased 1,25(OH)2D production,math image and subsequent Ca absorption may also have contributed.math image The simultaneous increase in urine Ca excretion probably reflects renal regulation of the higher filtered Ca load. By 12 mo, however, the circulating Ca and renal Ca excretion achieve equilibrium possibly with increased Ca utilization for bone matrix formation. PO4 levels have been shown to increase or remain unchanged in aging postmenopausal women with a negative relationship between PO4 and vitamin D. In our osteoporotic subjects with normal vitamin D concentrations, we found a lower circulating PO4 concentration associated with a marginally lower TmPO4/GFR compared with controls, although the difference was not statistically significant. An initial increase in both the TmPO4/GFR and serum PO4 concentration was observed after GH administration, possibly as a result of a direct anti-phosphaturic effect of GH/IGF-1math image and increased 1,25(OH)2D3 activity.math image Urine PO4 excretion also increased in parallel with increasing serum PO4 and therefore a higher filtered PO4 load before a new equilibrium was established at 12 mo when circulating phosphate and TmPO4/GFR reached a plateau and phosphate excretion began to decrease. Increased phosphaturia could also be a reflection of the change in renal sensitivity to PTH during GH therapy, but clearly the dominant effect of these hormones on PO4 is varying with time.

GH administration to osteoporotic patients simultaneously increases markers of both bone formation and resorption and thus no increase in BMD is seen in the short term but BMD increases after prolonged GH administration.math image Our data confirm a simultaneous increase in bone resorption and formation with the increase in bone formation markers becoming significantly higher than resorption only by 6 mo, possibly explaining the delay in increase in BMD after GH administration. The sequence of changes in bone turnover markers is different from the response to exogenously administered fixed dose PTH, which results in an early increase in bone formation markers preceding any increase in resorption by about a month. The apparent difference may be a result of several individual factors or more likely a combination of these factors. This may include the gradual increase in GH dose, the subsequent gradual increase in circulating GH/IGF-1 concentrations, paracrine and autocrine effects of IGF-1 and other growth factors in the bone microenvironment, an increase in bone cell sensitivity to endogenous PTH, and changes in bone cell responsiveness to changes in the circadian rhythm of endogenous PTH.

In conclusion, our results suggest that lower GH and IGF-1 concentration are associated with target organ insensitivity to the effects of PTH and abnormal PTH circadian rhythm in postmenopausal women with osteoporosis. These differences may be a result of an age-related process alone, with osteoporosis being an epi-phenomenon or may be the result of a combination of these two processes. GH administration restores sensitivity to PTH and PTH rhythm with subsequent changes in bone turnover, Ca, and PO4 metabolism resulting in positive bone balance contributing to the delayed increase in BMD shown in previous studies. We believe our study consolidates previous studies and proposes a further component in the development of osteoporosis in postmenopausal women.

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