Cytochemical bioassay of circulating concentrations of rat parathyroid hormone: application to a study of age and sex

Radiometrical, hormonal and biological correlates of skeletal growth in the female rat from birth to senescence

OBJECTIVE

We investigated the skeletal growth profile of female rats from birth to senescence (100weeks) on the basis of sequential radiometrical, hormonal and biochemical parameters.

DESIGN

Weaning rats entered the study which was divided into two sections: a) sequential measurements of vertebral and tibial growths and bone mineral density (BMD), estimation of mineral content of the entire skeleton (BMC) and chemical analysis of vertebral Ca; and b) determination of basal and pulsatile growth hormone (rGH), insulin-like growth hormone (IGF-I), estradiol (E2), parathyroid hormone (PTH), osteocalcin (OC) and urinary d-pyridinoline (dp) throughout the experimental period.

RESULTS

Vertebral and tibial growths ceased at week 25 whereas BMD and BMC as well as total vertebral Ca exhibited a peak bone mass at week 40. rGH pulsatile profiles were significantly higher in younger animals coinciding with the period of active growth and IGF-I peaked at 7weeks, slowly declining thereafter and stabilizing after week 60. OC and dp closely paralleled IGF-I coinciding with the period of enhanced skeletal growth, remaining thereafter in the low range indicative of reduced bone turnover. E2 increased during reproductive life but the lower values subsequently recorded were still in the physiological range, strongly suggesting a protective role of this steroid on bone remodeling. PTH followed a similar profile to E2, but the significance of this after completion of growth remains unclear.

CONCLUSIONS

Mechanisms governing skeletal growth in the female rat appear similar to those in humans. Bone progression and attainment of peak bone mass are under simultaneous control of rGH, IGF-I and calciotropic hormones and are modulated by E2. This steroid seems to protect the skeleton from resorption before senescence whereas the role of PTH in this context remains uncertain.

Rat parathyroid hormone (rPTH) ELISAs specific for regions (2-7), (22-34) and (40-60) of the rat PTH structure: influence of sex and age

Rat (r) PTH ELISAs were used to study the influence of age and sex on rPTH levels and circulating PTH molecular forms separated by HPLC. Standard curves and saturation analysis were undertaken to define epitopes. Rats were sacrificed at approximately 27, 47 and 75days. Relevant biochemical parameters and 25(OH) vitamin D were measured. Differences between sexes were analyzed by Kruskal-Wallis ANOVA, followed by Dunn's test. Epitopes were localized in regions 2-7, 22-34 and 40-60 of rPTH structure for whole (W), total (T) and carboxyl (C) rPTH ELISAs. The W-rPTH assay only detected rPTH(1-84) and N-PTH in circulation while the T-PTH assay further detected large C-rPTH fragments. The C-rPTH assay detected all circulating rPTH molecular forms including smaller C-rPTH fragments. In both sexes, weight (p<0.001), ionized calcium, creatinine, albumin and 25(OH)D values (p<0.001) increased with age, while phosphate and alkaline phosphatase decreased (p<0.001). In male rats, W-rPTH remained unchanged, while T-rPTH rose slightly (p<0.05) and C-rPTH declined by half with time (p<0.001). In female rats, W-rPTH (p<0.05), T-rPTH (p<0.001) and C-rPTH (p<0.01) all increased in older animals. In both sexes, C-rPTH/W-rPTH and C-rPTH/T-rPTH ratios decreased between 25 and 47 days, to rise again between 47 and 75 days. The initial decrease may represent an adaptation to weaning and a change of diet between 25 and 47 days while the rise corresponds to higher calcium and 25(OH)D levels between 47 and 75 days. These changes were more pronounced in female rats, indicating an influence of sex on PTH molecular form secretion or metabolism.

A new rapid and reproducible homologous immunoradiometric assay for amino-terminal parathyroid hormone in the rat

Measurement of parathyroid hormone (PTH) in the rat is most often performed with competitive ligand radioimmunoassays (RIA) utilizing heterologous antibodies. We report here the validation of a newly developed homologous immunoradiometric assay (IRMA) for rat PTH. Two different goat antibodies to the amino-terminal sequence of rat PTH are utilized; one is immobilized onto plastic beads to capture the PTH molecules and the other is radiolabeled for detection. To test this new IRMA, 30 Sprague-Dawley rats were randomized into three treatment groups to receive by intraperitoneal injection: (1) saline 1 ml/kg (control); (2) calcium chloride 40 mg/kg (hypercalcemic); and (3) EDTA 300 mg/kg (hypocalcemic). Blood samples were taken at 0, 30, 60, 180, and 300 minutes after administration of the assigned treatment for measurement of ionized calcium (Ca2+) and serum PTH. Most of the variance in PTH levels was found to be due to changes in Ca2+ (r2 = 0.780, P < 0.0001). There was also a close temporal relationship between the two, with the highest levels of PTH occurring at the same measured time points as the lowest Ca2+, and vice versa. The measured detection limit of the IRMA was 3 pg/ml with intra- and interassay coefficients of variation of 1.74% and 3.07%, respectively. Serial dilutions with pooled rat serum, synthetic rat PTH-(1-34), and synthetic human PTH-(1-34) showed good parallelism with increased specificity for the pooled and synthetic PTH, despite a degree of crossreactivity with hPTH.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of bioactive rat parathyroid hormone (PTH) concentrations in vivo and in vitro by a 2-site homologous immunoradiometric assay

A new homologous 2-site assay for rat parathyroid hormone (IRMA), developed by Immutopics, Inc., has been evaluated and compared with a bone cell cAMP bioassay. Circulating PTH for adult rats assayed with this IRMA are in the range 10-15 pg/ml, and of the same order of magnitude as published values for biologically active PTH. The standard curve for the IRMA was linear over the range 3.4-240 pg/ml of rPTH 1-34, and serum samples diluted in parallel with the standard curve. The within-assay and between-assay coefficients of variation ranged from 5.2% (n = 18) to 7.6% (n = 24) and 8.3% (n = 16) to 26.4% (n = 10), respectively. Serum PTH values (mean +/- S.E.) for parathyroidectomized rats were 3.5 +/- 0.6 pg/ml (n = 18) versus 10.3 +/- 1.4 pg/ml (n = 16) for intact non-mated rats. Calcium injections suppressed circulating PTH by 50%. Lactating rats had serum PTH levels 5-fold higher and vitamin D deficient rats 60-fold higher than non-mated controls. PTH secreted from parathyroid cells in vitro was in the range 60-490 pg/ml as determined by the IRMA. These values represented 86.0 +/- 9.0% of the comparable bioassay values, indicating that the IRMA detects only bioactive PTH.

Comparison of two parathyroid hormone assays for the rat: the new immunoradiometric and the older competitive binding assay

A competitive binding assay for parathyroid hormone (PTH INS) has been used since 1986 to measure PTH in rats. During the past year an immunoradiometric assay for the measurement of PTH (PTH IRMA) in the rat was developed. The purpose of the present study was to compare results obtained with the PTH INS and IRMA and to provide a framework for comparison for investigators who have used the PTH INS in previous studies. A total of 99 rats were studied; 27 rats had normal renal function, and 72 rats had surgically induced renal failure. In the azotemic rats, the magnitude of hyperparathyroidism was varied by changing the calcium and phosphorus composition of the diet. The correlation between the two PTH assays in the 99 rats was r = 0.98, p < 0.001. For the 27 rats with normal renal function, the correlation even within the narrow range of normal PTH values was significant, r = 0.71, p < 0.001. In the 72 azotemic rats, in which the highest INS PTH value was approximately 17 times normal, the correlation between the two PTH assays was r = 0.98, p < 0.001. The PTH IRMA provides distinct advantages, such as extended standard range, shortened incubation time, increased sensitivity, and technical simplicity, but our results indicate that the PTH INS provided an accurate measurement of PTH. Furthermore, our results should provide investigators who have used the PTH INS in previous studies with a framework for comparison with studies in which the PTH IRMA was used.

Kinetics of 1,25-dihydroxyvitamin D metabolism in the aging rat

To determine whether postmaturational aging influences the kinetics of 1,25-dihydroxyvitamin D [1,25(OH)2D] metabolism in the rat, we measured the metabolic clearance (MCR) and production (PR) rates of 1,25(OH)2D in 6-, 12-, 18-, and 24-mo-old Fischer 344 rats using the constant infusion method. Plasma calcium, phosphorus, and parathyroid hormone (PTH), urinary calcium and phosphorus, and glomerular filtration rate (GFR) were also measured. MCR and PR increased 57 and 91%, respectively (when expressed per rat), and 32 and 39%, respectively (when expressed per kg body wt), between 6 and 24 mo of age, with the greatest increase occurring between 18 and 24 mo. The plasma concentration of 1,25(OH)2D remained unchanged. Plasma PTH, when compared with 6-mo-old animals, was significantly elevated at 18 mo (147%) and even higher at 24 mo (240%). GFR (51Cr-labeled EDTA clearance) remained unchanged through 18 mo but tended to be reduced at 24 mo. Plasma phosphorus tended to decrease with age, whereas plasma calcium, urinary calcium, and urinary phosphorus did not differ among the age groups. These data indicate that both clearance and production of 1,25(OH)2D increase with postmaturational aging in the rat. They also suggest that the early age-related rise in plasma PTH is neither a consequence of low plasma calcium nor of renal insufficiency.

A new sensitive homologous radioimmunoassay for amino-terminal parathyroid hormone in the rat

The determination of circulating biologically active PTH in the rat has been difficult due at least in part to the inability to develop an antibody suitable for RIA of rat PTH. However, since the amino acid sequence of the rat PTH molecule has been deduced by molecular techniques, corresponding synthetic peptides have made it possible to produce such an antibody. A total of 12 roosters were immunized with synthetic rat PTH-(1-34), and one animal, RD1, developed a sensitive antibody against this amino-terminal region of the rat PTH molecule. To further increase the sensitivity of the RIA, we utilized an analog of rat PTH, Tyr1 rat PTH-(2-34), as the radioligand, which can be iodinated to high specific activity (450 microCi/micrograms). The iodinated peptide was purified by HPLC using a C18 Nova Pak HPLC column and a 20-60% acetonitrile gradient in 0.1% TFA. Synthetic rat PTH-(1-34) was used as the standard. To validate the RIA, we measured PTH under a variety of metabolic conditions. Normal values for PTH were 55.6 +/- 3.9 pg/ml (n = 26). Levels in parathyroidectomized (PTX) rats (n = 9) were undetectable, but renal insufficiency and vitamin D deficiency increased PTH to 587.4 +/- 141.3 pg/ml (n = 73) and 1662.0 +/- 137.8 (n = 27), respectively. Intraperitoneal (IP) administration of ethylenediaminetetraacetic acid (EDTA), 200 mg/kg, was used to decrease ionized calcium (ICa) from 4.75 +/- 0.07 to 3.55 +/- 0.10 mg/dl, which increased PTH from 51.3 +/- 5.9 to 109.3 +/- 13.4 pg/ml (n = 12).(ABSTRACT TRUNCATED AT 250 WORDS)

Homologous amino-terminal radioimmunoassay for rat parathyroid hormone

Existing radioimmunoassays for parathyroid hormone (PTH) in rat plasma are based on cross-reactivity of rat PTH (rPTH) with heterologous antisera. We used the synthetic NH2-terminal fragment of rPTH [rPTH-(1-34)] to develop a homologous radioimmunoassay for circulating PTH. An antiserum to rPTH-(1-34) was raised in a goat (G-813), and the same peptide was used as radioligand (125I) and standard. Purification of the label by high-performance liquid chromatography (HPLC) increased specific binding greater than twofold and sensitivity by 50-100%. With a final antiserum dilution of 1:70,000, maximum specific binding of 30-33%, nonspecific binding of 1-5%, and 50-microliters sample additions, the assay detection limit was 1.8-2.5 pmol/l. A midregional fragment of human PTH did not displace 125I-labeled rPTH-(1-34). HPLC of extracts of rat parathyroid glands and hyperparathyroid plasma showed only a single peak of immunoreactivity that eluted 2 min after rPTH-(1-34). Dose dilution curves for rat parathyroid gland extracts, rPTH-(1-34) added to rat plasma, and endogenous rat plasma PTH all paralleled the standard curve. Immunoreactive PTH (irPTH) was detectable in greater than 90% of fasting normal rat plasma and changed appropriately in response to hyper- and hypocalcemia induced by low-calcium and vitamin D-deficient diets, injections of calcium and EDTA, and after thyroparathyroidectomy. The normal range for rat plasma irPTH was less than 2.0-12 pmol/l, in general agreement with bioassay results of others. Thus rPTH-(1-34) is an excellent immunogen for raising antisera to rPTH, and assays incorporating it may be of great value in studying rat parathyroid physiology.

Regulation of parathyroid hormone secretion by plasma calcium in aging rats

Plasma immunoreactive parathyroid hormone (irPTH) levels increase with aging. This study determined 1) whether NH2-terminal irPTH secretory responses to induced hypocalcemia differ between adult (6-mo-old) and aged (24- to 26-mo-old) male rats and 2) whether a higher set point for irPTH release by Ca is responsible for the elevated irPTH levels with aging. Basal irPTH levels were 68% higher and 1,25-dihydroxyvitamin D3 levels were 44% lower in aged rats. An acutely induced, constant hypocalcemic stimulus [0.32 mM decrement in ionized Ca (Ca2+) for 2 h] was developed in catheterized conscious adult and aged rats by ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) infusion using the Ca clamp technique. The initial irPTH secretory response to acute hypocalcemia (5-10 min) was reduced in aged rats (1.9- vs. 3.1-fold increase), suggesting reduced hormone stores. However, higher sustained irPTH levels (30 min to 2 h) were maintained in aged rats, indicating increased irPTH synthesis and release. The EGTA infusion rate necessary to maintain constant hypocalcemia was less in aged rats, suggesting skeletal resistance to PTH. Slow EGTA and Ca infusions were used to determine irPTH secretion at plasma Ca2+ levels from 0.7 to 1.5 mM. In aged rats, irPTH levels were higher at all Ca2+ concentrations, but the set point for irPTH release by Ca2+ was the same as in adult rats. Thus the elevated irPTH secretion in aged rats is not caused by a change in the set point for irPTH release but does result in decreased irPTH stores.

The effect of tartrate on bone cell acid phosphatase activity: a quantitative cytochemical study

Tartrate-resistant acid phosphatase activity (TRAPase) is widely used as a cytochemical marker to distinguish osteoclasts from macrophages and other related cell types. The degree of tartrate resistance, however, may depend on which reaction methods, tissues, or species are used. To investigate this further, we have measured the amount of cytochemical reaction product by microdensitometry. We compared osteoclast acid phosphatase (APase) activity in fresh frozen sections of neonatal rat calvaria using two different reaction methods; one is commonly employed for qualitative histochemistry and includes alpha naphthyl phosphate as substrate, simultaneous coupling to the chromagen Fast Garnet, and a 30-minute reaction time (method A). The other may be used to measure enzyme reaction rates in cells in situ and employs conditions suitable for initial velocity kinetics, namely naphthol-ASBI phosphate as substrate, post coupling to Fast Garnet, and a 2-minute reaction time. Although enzyme reaction rates differed greatly between the two methods, significant inhibition of APase activity by tartrate was observed in calvarial osteoclasts (69% and 59% with methods A and B, respectively), osteoblasts, and spleen macrophages. Using method B, mouse calvarial osteoclasts had similar APase activity to that seen in the rat. Tartrate produced little inhibition in these mouse cells, in contrast to the observations made with rat tissue, but murine spleen macrophages were significantly tartrate sensitive (40% inhibition with tartrate). On this basis, conclusions regarding the cell specificity of TRAPase should be treated cautiously.

Identification of transcripts encoding a parathyroid hormone-like peptide in messenger RNAs from a variety of human and animal tumors associated with humoral hypercalcemia of malignancy

The syndrome of humoral hypercalcemia of malignancy (HHM) appears to be mediated in many instances by a parathyroid hormone-like peptide, which has recently been purified, sequenced, and cloned. Using a probe representing the coding region of the human PTH-like peptide, we examined by Northern analysis poly (A)+ RNA from a variety of human and animal tumors associated with HHM. Hybridizing transcripts were identified in mRNA from each of 12 human and each of four animal HHM-associated tumors, with a complex hybridization pattern observed in the human mRNAs and a relatively simple pattern observed in the animal mRNAs. Poly (A)+ RNA prepared from tumors of similar histological types unassociated with HHM failed to hybridize with the probe. Messenger RNA-dependent biological activity from the animal tumors was entirely eliminated in a hybridization-arrest experiment using a complementary oligonucleotide spanning the region of homology between human PTH and the PTH-like peptide. These findings indicate that the PTH-like peptide is associated with the syndrome of HHM in a wide spectrum of tumor types from a variety of mammalian species and that the PTH-like sequence in the proximal amino terminus of the peptide is highly conserved.

Bone status of senescent male rats: chemical, morphometric, and mechanical analysis

The bone status of male rats 6, 12, and 24 months of age (n = 10) was examined. Femur calcium (Ca), phosphorus (P), and osteocalcin contents; serum chemistry; and mechanical properties of the bone were measured and correlated. Diaphyseal Ca, P, and osteocalcin contents were not different in animals 6 and 12 months of age but decreased significantly at 24 months: -7.4% for Ca, -4.2% for P, and -24% for osteocalcin compared to 12 months. Femurs from 24-month-old (senescent) rats were characterized by a scalloped appearance of the midfemoral endosteal surface and by cortical porosities. These age-associated changes coincided with nearly two-fold increases in serum immunoreactive parathyroid hormone (PTH) and osteocalcin. Serum Ca did not change with age, whereas serum P decreased (-14.8%) from 6 to 24 months. Maximum breaking force required to fracture femurs at midshaft did not change with age. Hence, the strength of the femur as an intact organ was not compromised with age despite the loss of diaphyseal Ca and P in the senescent animal. However, ultimate stress, a parameter that normalizes for differences in bone geometry and size, decreased 35% in femurs from 12- compared with 24-month-old animals. These mechanical results might be explained by the morphometric finding that, in contrast to the small but progressive age-associated increases in femur weight and length, the cortical and medullary areas increased at least two-fold. Therefore, the strength of the intact femur was maintained by architectural compensations, even though normalized tissue strength decreased with age. These findings suggest that bone status was compromised in the aged male rat.

Identification of a cDNA encoding a parathyroid hormone-like peptide from a human tumor associated with humoral hypercalcemia of malignancy

Humoral hypercalcemia of malignancy is a common paraneoplastic syndrome that appears to be mediated in many instances by a parathyroid hormone-like peptide. Poly(A)+ RNA from a human renal carcinoma associated with this syndrome was enriched by preparative electrophoresis and used to construct an enriched cDNA library in phage lambda gt10. The library was screened with a codon-preference oligonucleotide synthesized on the basis of a partial N-terminal amino acid sequence from a human tumor-derived peptide, and a 2.0-kilobase cDNA was identified. The cDNA encodes a 177 amino acid protein consisting of a 36 amino acid leader sequence and a 141 amino acid mature peptide. The first 13 amino acids of the deduced sequence of the mature peptide display strong homology to human PTH, with complete divergence thereafter. RNA blot-hybridization analysis revealed multiple transcripts in mRNA from tumors associated with the humoral syndrome and also in mRNA from normal human keratinocytes. Southern blot analysis of genomic DNA from humans and rodents revealed a simple pattern compatible with a single-copy gene. The gene has been mapped to chromosome 12.