Model-independent quantification of measurement error: empirical estimation of discrete variance function profiles based on standard curves

Bioanalytical screening of low levels of dioxins and dioxin-like PCBs in pig meat (pork) for checking compliance with EU maximum and action levels using highly sensitive "third generation" recombinant H4L7.5c2 rat hepatoma cells

BACKGROUND

Low maximum and action levels set by the European Union for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and dioxin-like polychlorinated biphenyls (DL-PCBs) in pig meat (pork) have led to a demand for reliable and cost-effective bioanalytical screening methods implemented upstream of gas chromatography/high-resolution mass spectrometry confirmatory technology, that can detect low levels of contamination in EU-regulated foods with quick turn-around times.

RESULTS

Based on the Chemically Activated LUciferase gene eXpression (CALUX) bioassay, extraction and clean-up steps were optimized for recovery and reproducibility within working ranges significantly lower than in current bioassays. A highly sensitive "3rd generation" recombinant rat hepatoma cell line (H4L7.5c2) containing 20 dioxin responsive elements was exposed to pork sample extracts, and their PCDD/Fs and DL-PCBs levels were evaluated by measuring luciferase activity. The method was validated according to the provisions of Commission Regulation (EU) 2017/644 of 5 April 2017 with spiking experiments performed selectively for PCDD/Fs and DL-PCBs and individual calibration for PCDD/Fs, DL-PCBs and the calculated sum of PCDD/Fs and DL-PCBs. The resulting performance parameters met all legal specifications as confirmed by re-calibration using authentic samples. Cut-off concentrations for assessing compliance with low maximum levels and action levels set for PCDD/Fs and DL-PCBs within a range of 0.50-1.25 pg WHO-TEQ/g fat were derived, ensuring low rates of false-compliant results (ß-error < 1%) and keeping the rate of false-noncompliant results well under control (α-error < 12%).

CONCLUSIONS

We present a fast and efficient bioanalytical routine method validated according to the European Union's legal requirements on the basis of authentic samples, allowing the analyst to reliably identify pork samples and any other EU-regulated foods of animal origin suspected to be noncompliant with a high level of performance and turn-around times of 52 h. This was facilitated in particular by a quick and efficient extraction step followed by selective clean-up, use of a highly sensitive "3rd generation" H4L7.5c2 recombinant rat hepatoma cell CALUX bioassay, and optimized assay performance with improved calibrator precision and reduced lack-of-fit errors. New restrictions are proposed for the calibrator bias and the unspecific background contribution to reportable results. The procedure can utilize comparably small sample amounts and allows an annual throughput of 840-1000 samples per lab technician. The described bioanalytical method contributes to the European Commission's objective of generating accurate and reproducible analytical results according to Commission Regulation (EU) 2017/644 across the European Union.

Molecular characterization of the RNA-protein complex directing -2/-1 programmed ribosomal frameshifting during arterivirus replicase expression

Programmed ribosomal frameshifting (PRF) is a mechanism used by arteriviruses like porcine reproductive and respiratory syndrome virus (PRRSV) to generate multiple proteins from overlapping reading frames within its RNA genome. PRRSV employs -1 PRF directed by RNA secondary and tertiary structures within its viral genome (canonical PRF), as well as a noncanonical -1 and -2 PRF that are stimulated by the interactions of PRRSV nonstructural protein 1β (nsp1β) and host protein poly(C)-binding protein (PCBP) 1 or 2 with the viral genome. Together, nsp1β and one of the PCBPs act as transactivators that bind a C-rich motif near the shift site to stimulate -1 and -2 PRF, thereby enabling the ribosome to generate two frameshift products that are implicated in viral immune evasion. How nsp1β and PCBP associate with the viral RNA genome remains unclear. Here, we describe the purification of the nsp1β:PCBP2:viral RNA complex on a scale sufficient for structural analysis using small-angle X-ray scattering and stochiometric analysis by analytical ultracentrifugation. The proteins associate with the RNA C-rich motif as a 1:1:1 complex. The monomeric form of nsp1β within the complex differs from previously reported homodimer identified by X-ray crystallography. Functional analysis of the complex via mutational analysis combined with RNA-binding assays and cell-based frameshifting reporter assays reveal a number of key residues within nsp1β and PCBP2 that are involved in complex formation and function. Our results suggest that nsp1β and PCBP2 both interact directly with viral RNA during formation of the complex to coordinate this unusual PRF mechanism.

Motivations and methods for analyzing pulsatile hormone secretion

Endocrine glands communicate with remote target cells via a mixture of continuous and intermittent signal exchange. Continuous signaling allows slowly varying control, whereas intermittency permits large rapid adjustments. The control systems that mediate such homeostatic corrections operate in a species-, gender-, age-, and context-selective fashion. Significant progress has been made in understanding mechanisms of adaptive interglandular signaling in vivo. Principal goals are to understand the physiological origins, significance, and mechanisms of pulsatile hormone secretion. Key analytical issues are: 1) to quantify the number, size, shape, and uniformity of pulses, nonpulsatile (basal) secretion, and elimination kinetics; 2) to evaluate regulation of the axis as a whole; and 3) to reconstruct dose-response interactions without disrupting hormone connections. This review will focus on the motivations driving and the methodologies used for such analyses.

The five-parameter logistic: a characterization and comparison with the four-parameter logistic

Improvements in assay technology have reduced the amount of random variation in measured responses to the point where even slight asymmetry of the assay data can be more significant than random variation. Use of the five-parameter logistic (5PL) function to fit dose-response data easily accommodates such asymmetry. The 5PL can dramatically improve the accuracy of asymmetric assays over the use of symmetric models such as the four-parameter logistic (4PL) function. Until recently, however, the process of fitting the 5PL function has been difficult, with the result that the 4PL function has continued to be used even for highly asymmetric data. Various ad hoc modifications of the 4PL method have been developed in an attempt to address asymmetric data. However, recent advances in numerical methods and assay analysis software have rendered easier the fitting of the 5PL routine. This paper demonstrates how use of the 5PL function can improve assay performance over the 4PL and its variants. Specifically, the improvement in the accuracy of concentration estimates that can be obtained using the 5PL over the 4PL as a function of the asymmetry present in the data is studied. The behavior of the 5PL curve and how it differs from the 4PL curve are discussed. Common experimental designs, which can lead to ill-conditioned regression problems, are also examined.

Age and testosterone feedback jointly control the dose-dependent actions of gonadotropin-releasing hormone in healthy men

Healthy older men manifest combined declines in testosterone concentrations, LH secretory burst mass (amount of LH released per pulse), and feedback-sensitive regularity of unknown cause. To test a unifying hypothesis of simultaneous reductions in GnRH outflow, gonadotrope responsiveness to GnRH, and androgenic negative feedback, we monitored LH secretion 1) after bolus iv injection of a 1000-fold range of randomly ordered individual doses of GnRH on separate mornings, 2) during unmodified (eugonadal) or testosterone-withdrawn (hypoandrogenemic) negative feedback, and 3) in 16 young (age, 18-35 yr) and 15 older (age, 60-85 yr) healthy men. LH secretory burst mass and pattern regularity were quantitated by intensive blood sampling, high specificity LH beta-subunit-directed immunoradiometric assay, deconvolution analysis, and approximate entropy. GnRH dose responsiveness was assessed by four-parameter nonlinear regression analysis. We demonstrated that older men exhibit 1) delayed attainment of GnRH-evoked maximal LH secretion; 2) enhanced potency of GnRH stimulation in both the feedback-intact and feedback-withdrawn states; 3) elevated gonadotrope sensitivity to GnRH, unmasked by experimental testosterone depletion; 4) comparable young adult-like GnRH efficacy, independent of testosterone feedback milieu; and 5) diminished regularity of GnRH-induced LH release evident only during unmodified androgenic feedback. We conclude that a 3-fold interaction among GnRH dose, testosterone concentration, and age governs GnRH action, and age determines both testosterone-modulated and testosterone-independent actions of GnRH.

Estradiol supplementation enhances submaximal feed-forward drive of growth hormone (GH) secretion by recombinant human GH-releasing hormone-1,44-amide in a putatively somatostatin-withdrawn milieu

To test the clinical hypothesis that an estrogen-enriched milieu enhances GHRH action, we administered placebo (Pl) and estradiol-17 beta (E(2)) orally for 23 d to six postmenopausal women in a prospectively randomized, double-masked, within-subject crossover design with 6 wk intervening. The GHRH stimulation protocol entailed consecutive i.v. infusion of L-arginine and a single i.v. pulse of saline or one of five randomly ordered doses of recombinant human GHRH-1,44-amide (0.03, 0.1, 0.3, 1.0, or 3.0 microg/kg) in a total of 12 separate morning, fasting sessions. GH secretion was monitored by sampling blood every 10 min for 6 h; chemiluminescence assay of GH concentrations; deconvolution analysis of stimulated GH release; and nonlinear dose-response reconstruction. Supplementation with E(2), compared with Pl: 1) increased (mean +/- SEM) E(2) concentrations from 18 +/- 3 (Pl) to 164 +/- 12 pg/ml (to convert to picomoles per liter, multiply by 3.57) (P < 0.001); 2) decreased IGF-I concentrations from 181 +/- 14 to 120 +/- 11 microg/liter (P < 0.01); 3) elevated mean GH concentrations from 0.27 +/- 0.06 to 0.59 +/- 0.08 microg/liter (P = 0.014); 4) potentiated GH secretion stimulated by L-arginine alone by 1.43-fold (P = 0.012); 5) reduced the ED(50) of GHRH from 0.27 +/- 0.02 to 0.13 +/- 0.01 microg/kg (P < 0.01), denoting enhanced GHRH potency; and 6) heightened the maximal slope of the dose-response function from 1.1 +/- 0.1 to 1.4 +/- 0.05 [( microg/liter) ( microg/kg)(-1)] (P < 0.05), signifying augmented pituitary sensitivity. The foregoing facilitative mechanisms were specific because E(2) replacement did alter maximal L-arginine/GHRH-induced GH secretion, indicating unchanged secretagogue efficacy. In conclusion, inasmuch as E(2) also attenuates inhibition of GH secretion by infused somatostatin and potentiates stimulation of GH secretion by GH-releasing peptide-2, we postulate that estrogenic steroids drive pulsatile GH production in part via mechanisms that include all three of GHRH, somatostatin, and putatively GH-releasing peptide/ghrelin signaling.

Neuroendocrine facets of human puberty

The unfolding of pubertal growth and maturation entails multisystem collaboration. Most notably, the outflow of gonadotropins and growth hormone (GH) proceeds both independently and jointly. The current update highlights this unique dependency in the human.

Impact of experimental blockade of peripheral growth hormone (GH) receptors on the kinetics of endogenous and exogenous GH removal in healthy women and men

Organs that respond to and metabolize GH are enriched in cognate high-affinity receptors. However, whether isologous receptors mediate the de facto access of ligand to cellular degradative pathways is not known. To address this query, we assessed the distribution and whole-body elimination kinetics of (endogenous and exogenous) GH before and after administration of a novel, potent, and selective recombinant human (rh) GH receptor antagonist peptide, pegvisomant. Sixteen healthy young adults (nine men and seven women) participated in a double-blind, prospectively randomized, within-subject cross-over study. The intervention comprised a single sc injection of placebo vs. a high dose of pegvisomant (1 mg/kg sc) timed 62 and 74 h before the overnight sampling and daytime infusion sessions, respectively. The half-life, metabolic clearance rate (MCR), and distribution volume of GH were quantitated by way of: 1) deconvolution analysis of serum GH concentration time series collected every 10 min for 10 h; 2) exponential regression analysis of the decay of GH concentrations after a 6-min iv pulse of rhGH (1 and 10 micro g/kg); 3) calculation of the MCR during constant iv infusion of rhGH (0.5 and 5.0 micro g/kg every 2 h); and 4) exponential fitting of the elimination time-course of GH concentrations following cessation of each constant infusion. Concentrations of GH and pegvisomant were measured in separate, noncross-reactive, two-site monoclonal, immunofluorometric assays. Pegvisomant concentrations averaged 4860 +/- 480 micro g/liter (+/-SEM) across the infusion interval, thus exceeding low steady state GH concentrations by 3000-fold. Inhibitory efficacy of the GH receptor antagonist peptide was affirmed by way of a 34% reduction in the serum total IGF-I concentration, i.e., from 257 +/- 37 (placebo) to 170 +/- 24 (drug) micro g/liter (P < 0.001); and a reciprocal 77% elevation of the (10-h) mean GH concentration, i.e., from 1.3 +/- 0.23 (placebo) to 2.3 +/- 0.42 (drug) micro g/liter (P = 0.003). ANOVA disclosed that prior administration of pegvisomant (compared with placebo) did not alter: 1) the calculated half-life (minutes) of secreted GH, which averaged 15 +/- 1.3 (placebo) and 14 +/- 0.69 (drug); 2) the half-time of disappearance (minutes) of an iv pulse of rhGH, 15 +/- 1.0 (placebo) and 13 +/- 0.5 (drug) (for the 10 micro g/kg dose); 3) the distribution volume (milliliters per kilogram) of rhGH, 59 +/- 6.2 (placebo) and 58 +/- 3.5 (drug); 4) the steady state GH concentration (micrograms per liter) attained during constant iv infusion of rhGH (at a rate of 5 micro g/kg every 2 h), 18.2 +/- 2.4 (placebo) and 18.3 +/- 2.3 (drug); 5) the half-life (minutes) of elimination of GH from equilibrium, 16 +/- 0.98 (placebo) and 16 +/- 1.8 (drug); and 6) the steady state MCR (liters per kilogram per day) of rhGH, 3.8 +/- 0.32 (placebo) and 3.5 +/- 0.31 (drug). In ensemble, the present data refute the a priori postulate that vascular-accessible GH receptors determine the in vivo pseudoequilibrium kinetics of GH disappearance in the human.

Secretory process regularity monitors neuroendocrine feedback and feedforward signaling strength in humans

The present experiments examine the neuroregulatory hypothesis that the degree of sample-by-sample regularity of hormone output by an interlinked hypothalamopituitary target-organ system monitors the strength of feedback and/or feedforward signaling. To test this postulate and assess its generality, we implemented a total of nine thematically complementary perturbation experiments. In particular, we altered feedback or feedforward signaling selectively in two distinct neuroendocrine systems; namely, the growth hormone (GH) insulin-like growth factor type I (IGF-I) and the luteinizing hormone-testosterone axes. Four experimental paradigms comprised preferential reduction vs. enhancement of IGF-I or testosterone feedback signal strength; and, conversely, five others entailed selective attenuation vs. augmentation of GH-releasing hormone and gonadotropin-releasing hormone feedforward signal intensity. In these independent interventions, quantitation of subordinate (nonpulsatile) secretory pattern reproducibility via the approximate entropy statistic unmasked salient changes (P values typically <10(-3)) in the conditional regularity of serial hormone output with high consistency (96-100%). In particular, approximate entropy quantified degradation of secretory subpattern orderliness under either muted feedback restraint or heightened feedforward drive. Assuming valid interpretation of the biological constraints imposed, these experimental observations coincide with earlier reductionist mathematical predictions, wherein increased irregularity of coupled parameter output mirrors attenuated feedback and/or augmented feedforward coupling within an integrative system.

Impact of uremia on female reproductive cyclicity, ovulation, and luteinizing hormone in the rat

BACKGROUND

Impaired reproductive function accompanies chronic renal insufficiency (uremia) in both the human and experimental animal. Clinical hypogonadism occurs in both genders. The present studies were designed to investigate possible anti-ovulatory effects of uremia in the female rat, a species that produces multiple ova during the normal estrous cycle.

METHODS

Renal insufficiency (uremia) was induced by 5/6 nephrectomy. Two control groups comprised sham-operated animals fed ad libitum (sham) or pair-wise with the uremic animals (pair-fed). Estrous cycles were determined by cytology of vaginal lavage. We examined concomitant changes in the preovulatory luteinizing hormone (LH) surge by radioimmunoassay (RIA), immunoradiometric assay (IRMA), and bioassay. Repetitive LH measurements were made from blood samples taken by intra-atrial catheter throughout the afternoon and evening of proestrus. The following morning (estrus), ovaries were collected, and ova were enumerated per oviduct.

RESULTS

Experimentally uremic animals manifested a threefold elevation of plasma creatinine and urea nitrogen and concomitantly a more than 50% impoverishment of ova production. Analyses of a large group of animals (N = 83) by RIA revealed uremia-associated attenuation of the preovulatory LH surge. Further measurements of the preovulatory LH surge by independent IRMA and LH bioassay (N = 26) corroborated this attenuation. Additional experiments indicated that these hormonal changes, but not changes in ovulation, might further reflect modulation of LH release by the anesthesia used in the preparative nephrectomy and catheterization surgeries. When normalized to body weight, the ovaries of uremic rats were found to weigh more than those of either the sham or pair-fed animals.

CONCLUSIONS

The present experiments take advantage of an experimental uremic model to document a consistent decrease in the number of ova released during estrus in the uremic animal. Possible disruption of hypothalamic-pituitary-ovarian regulation is further highlighted by attenuation in the preovulatory LH surge. These results provide a basis for further studies of neuroendocrine pathophysiology in a rodent model of uremia-associated ovulatory disruption.

Altered diurnal regulation of blood ionized calcium and serum parathyroid hormone concentrations during parenteral nutrition

BACKGROUND

Little is known about parathyroid gland function in patients receiving total parenteral nutrition (TPN).

OBJECTIVE

Our objective was to determine whether parathyroid gland function is abnormal in TPN recipients.

DESIGN

Six patients with a mean (+/-1 SD) age of 45.5 +/- 8.0 y who had been receiving TPN for 18.7 +/- 2. 8 y underwent bone biopsy, bone mass measurements with dual-energy X-ray absorptiometry, and dynamic tests of parathyroid gland function. Diurnal variations in blood ionized calcium (iCa(2+)) and serum parathyroid hormone (PTH) concentrations were also assessed. Results were compared with those of healthy volunteers.

RESULTS

Bone mass and bone formation were subnormal in all patients. Basal serum PTH concentrations were moderately higher in the TPN recipients than in healthy volunteers, and values obtained every 30 min over 24 h were significantly higher (P < 0.001) in TPN recipients (5.0 +/- 0.9 pmol/L) than in healthy volunteers (2.6 +/- 0.6 pmol/L). The percentage increase in serum PTH during citrate-induced hypocalcemia was lower in the TPN recipients, consistent with secondary hyperparathyroidism. Evening infusions of calcium-containing TPN eliminated the nocturnal rise in serum PTH, increased the amplitude of change for iCa(2+) and PTH over 24 h, increased the orderliness of change for iCa(2+) and PTH as measured by approximate entropy (ApEn), and enhanced the synchrony of change between iCa(2+) and PTH. Treatment for 10 d with calcium-free TPN restored the nocturnal rise in serum PTH and increased ApEn for PTH. ApEn for iCa(2+) remained low, suggesting that a component of nutrient solutions, but not calcium per se, enhances the regularity of PTH release in TPN recipients.

CONCLUSION

Parathyroid gland function is abnormal in long-term TPN recipients, which may contribute to disturbances in bone metabolism.

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.

Calcium-sensing by parathyroid glands in secondary hyperparathyroidism

Calcium-sensing by the parathyroids is abnormal in familial benign hypocalciuric hypercalcemia and in primary hyperparathyroidism (primary HPT), but the role of a calcium-sensing defect in uremic secondary hyperparathyroidism (secondary HPT) remains controversial. To study the regulation of PTH release by calcium, set point estimates were obtained using the four parameter model during in vivo dynamic tests of parathyroid gland function in 31 patients with secondary HPT, 8 patients with advanced secondary HPT studied shortly before undergoing parathyroidectomy (Pre-PTX), 3 patients with primary HPT, and 20 subjects with normal renal function (NL); the response to 2-h i.v. calcium infusions was also evaluated. Neither blood ionized calcium (iCa+2) levels nor the set point for calcium-regulated PTH release differed between secondary HPT and NL; iCa+2 levels and set point values were moderately elevated in Pre-PTX and markedly elevated in primary HPT. Compared with values obtained in NL, the lowest serum PTH levels achieved during calcium infusions, expressed as a percentage of pre-infusion values, were incrementally greater in secondary HPT, Pre-PTX, and primary HPT, whereas the slope of the relationship between iCa+2 and PTH, expressed as the natural logarithm (ln) of percent preinfusion values, decreased incrementally in secondary HPT, Pre-PTX, and primary HPT. The inhibitory effect of calcium on PTH release is blunted both in secondary HPT and primary HPT because of increases in parathyroid gland mass, but a calcium-sensing defect is a late, rather than early, consequence of renal secondary HPT.

Human growth hormone response to repeated bouts of aerobic exercise

We examined whether repeated bouts of exercise could override growth hormone (GH) auto-negative feedback. Seven moderately trained men were studied on three occasions: a control day (C), a sequential exercise day (SEB; at 1000, 1130, and 1300), and a delayed exercise day (DEB; at 1000, 1400, and 1800). The duration of each exercise bout was 30 min at 70% maximal O2 consumption (VO2max) on a cycle ergometer. Standard meals were provided at 0600 and 2200. GH was measured every 5-10 min for 24 h (0800-0800). Daytime (0800-2200) integrated GH concentrations were approximately 150-160% greater during SEB and DEB than during C: 1,282 +/- 345, 3,192 +/- 669, and 3,389 +/- 991 min.microgram.l-1 for C, SEB, and DEB, respectively [SEB > C (P < 0.06), DEB > C (P < 0.03)]. There were no differences in GH release during sleep (2300-0700). Deconvolution analysis revealed that the increase in 14-h integrated GH concentration on DEB was accounted for by an increase in the mass of GH secreted per pulse (per liter of distribution volume, lv): 7.0 +/- 2.9 and 15.9 +/- 2.6 micrograms/lv for C and DEB, respectively (P < 0.01). Comparison of 1.5-h integrated GH concentrations on the SEB and DEB days (30 min exercise + 60 min recovery) revealed that, with each subsequent exercise bout, GH release apparently increased progressively, with a slightly greater increase on the DEB day [SEB vs. DEB: 497 +/- 162 vs. 407 +/- 166 (bout 1), 566 +/- 152 vs. 854 +/- 184 (bout 2), and 633 +/- 149 vs. 1,030 +/- 352 min.microgram.l-1 (bout 3), P < 0.05]. We conclude that the GH response to acute aerobic exercise is augmented with repeated bouts of exercise.

Suppressive effect of calcium on parathyroid hormone release in adynamic renal osteodystrophy and secondary hyperparathyroidism

Serum parathyroid hormone (PTH) levels are markedly lower in patients with the adynamic lesion (AD) of renal osteodystrophy than in those with secondary hyperparathyroidism (2 degrees HPT), but serum PTH values are often moderately elevated in AD when compared to subjects with normal renal and parathyroid gland function (NL). To study the inhibitory effect of calcium on PTH release in AD and in 2 degrees HPT, the response to two-hour intravenous calcium infusions was examined in 6 patients with AD, in 31 patients with 2 degrees HPT and in 20 NL. Basal serum PTH levels were 88 +/- 51, 536 +/- 395, and 26 +/- 6 pg/ml, respectively, in AD, 2 degrees HPT and NL, whereas basal ionized calcium levels did not differ. When expressed as a percentage of pre-infusion values, PTH levels at the end of two-hour calcium infusions were higher both in AD (23.2 +/- 5.6%) and in 2 degrees HPT (27.8 +/- 12.3%) than in NL, (11.9 +/- 5.8%, P < 0.001). Both the amplitude of suppression (%) and the rate of decline (min-1) in serum PTH were less in AD and 2 degrees HPT than in NL, P < 0.05 for each parameter; corresponding values for each group, with 95% confidence intervals, were 77% (73 to 82) and 0.039 min-1 (0.030 to 0.048) in AD, 72% (68 to 76) and 0.031 min-1 (0.025 to 0.036) in 2 degrees HPT and 87% (84 to 89) and 0.070 min-1 (0.058 to 0.089) in NL. Neither variable differed between AD and 2 degrees HPT. Basal and nadir serum PTH levels were highly correlated: r = 0.95 and P < 0.05 in AD; r = 0.90 and P < 0.01 in 2 degrees HPT; r = 0.75 and P < 0.01 in NL. The slope of this relationship was less, however, both in AD and in 2 degrees HPT than in NL, P < 0.05 by analysis of co-variance. Thus, serum PTH levels fell below 20% of pre-infusion values in fewer subjects with AD (1 of 6) or 2 degrees HPT (9 of 31) than in NL (17 of 20) (chi 2 = 17.81, P < 0.005). The results indicate that the inhibitory effect of calcium on PTH release in vivo does not differ in AD and 2 degrees HPT despite marked differences in basal serum PTH levels. Variations in functional parathyroid gland mass rather than disturbances in calcium-sensing by the parathyroids probably account not only for the lower basal serum PTH levels in patients with AD compared to those with 2 degrees HPT, but also for the moderately elevated serum PTH values commonly seen in patients with AD.