Keenan, Daniel M.

As a model of extreme conditions, eight healthy women, part of a 40-member Nepal mountain-climbing expedition, were monitored for dynamic endocrine adaptations. Endocrine measurements were made at frequent intervals over a 6–10-h period at four altitudes: 450 m, 4,800 m (base camp), 6,050 m, and again at 4,800 m (on descent) after an acclimatization (A) period (4,800 mA). Quantified hormones were growth hormone (GH), prolactin (PROL), cortisol (Cort), thyroid-stimulating hormone (TSH), and free thyroxine. These hormones are important to the anabolic/catabolic balance of the body, and are vital to growth, homeostasis, hypothalamic inhibition, regulation of stress, and metabolism. A key secondary question was the degree to which acclimatization can stabilize hormonal disruption. On the basis of statistical false discovery rates, the present analyses unveil marked adaptive changes in the thyroid axis at the level of pulsatile secretion of the pituitary hormone TSH and its downstream product, free thyroxine; strong effects on the mass of GH, TSH, Cort, and PROL secretion per burst; and prominent pulsatile frequency disruption and recovery for PROL and Cort. Because pulsatility changes reflect de facto perturbations in hypothalamo-pituitary control mechanisms, the present data introduce the concept of both frequency- and amplitude-dependent adaptive control of brain-pituitary neuroendocrine signals under conditions of extreme altitude exertion and exposure.

Quantification of disturbances in glucose-insulin homeostasis has been the cornerstone of appraising insulin resistance and detecting early-stage diabetes. Metabolic homeostasis arises from feedback and feed-forward interactions among (at least) all four of glucose, insulin, glucagon, and cortisol. Quantifying such tetrapartite interactions in the fasting (endogenously regulated) state overnight could elucidate very early regulatory disruption. In the present study, healthy subjects without diabetes (ND; n = 20) and patients with Type 2 diabetes (T2D; n = 21) were investigated by repeated overnight blood sampling of all four of glucose, insulin, glucagon, and cortisol concentrations. To obviate confounding by hormone-specific disappearance rates, analyses were performed at the level of production (glucose) or secretion (insulin, glucagon, and cortisol) rates estimated by regularized deconvolution analysis. Then, a novel method for quantifying the loss of homeostasis among glucose, insulin, and glucagon (and, when available, cortisol) secretion patterns was developed. Potential early stage prediabetic candidates were identified. The new methodology avoids many of the difficulties encountered in the conventional estimation of insulin-glucose sensitivity or resistance, while incorporating the dynamics of the key coregulators under fasting conditions.

Neuroendocrine systems control many of the most fundamental physiological processes, e.g., reproduction, growth, adaptations to stress, and metabolism. Each such system involves the hypothalamus, the pituitary, and a specific target gland or organ. In the quantification of the interactions among these components, biostatistical modeling has played an important role. In the present article, five key challenges to an understanding of the interactions of these systems are illustrated and discussed critically.

The long-lived glycoprotein hormone, human chorionic gonadotropin (hCG), downregulates testosterone (T) biosynthesis in vitro and in vivo in animals and humans. The degree to which short-lived pulses of pituitary luteinizing hormone (LH) do so, particularly at physiological concentrations, is not known. We test the hypothesis that continuous LH infusion compared with bolus injections of LH every 1 h or every 2 h overnight downregulates T secretory responses to a subsequent fixed template of three consecutive intravenous pulses of a physiological amount of recombinant human (rh) LH (triple stimulus). Nineteen healthy men ages 18–49 yr each underwent four separate randomly ordered overnight gonadotropin-releasing hormone-receptor antagonist treatments with superimposed intravenous infusions of saline or rhLH (1-h pulses, 2-h pulses, or continuously). Each 12-h infusion protocol was followed by the triple rhLH-pulse stimulus the next morning. During the triple stimulus, basal (nonpulsatile) as well as total (basal plus pulsatile) T secretion was higher after overnight 2- and 1-h rhLH pulses than after continuous rhLH or saline delivery. Approximate entropy, a probabilistic measure of feedforward-induced irregularity of T concentration time series, was higher after 1-h rhLH pulses than after continuous rhLH. Analytical estimation of pulsatile rhLH-T dose-response measures revealed higher T secretory sensitivity and greater rhLH potency (lower EC50) after exposure to 1-h than 2-h rhLH pulses. Collectively, these data indicate that in vivo dynamics of LH-stimulated T secretion under standardized conditions in men depend on the prior time mode of LH delivery in the bloodstream.

The present analysis tests the hypothesis that quantifiable disruption of the glucose-stimulated insulin-secretion dose-response pathway mediates impaired fasting glycemia (IFG) and type 2 diabetes mellitus (DM). To this end, adults with normal and impaired fasting glycemia (NFG, n = 30), IFG ( n = 32), and DM ( n = 14) were given a mixed meal containing 75 g glucose. C-peptide and glucose were measured over 4 h, 13 times in NFG and IFG and 16 times in DM (age range 50–57 yr, body mass index 28–32 kg/m2). Wavelet-based deconvolution analysis was used to estimate time-varying C-peptide secretion rates. Logistic dose-response functions were constructed analytically of the sensitivity, potency, and efficacy (in the pharmacological sense of slope, one-half maximal stimulation, and maximal effect) of glucose's stimulation of prehepatic insulin (C-peptide) secretion. A hysteresis changepoint time, demarcating unequal glucose potencies for onset and recovery pathways, was estimated simultaneously. According to this methodology, NFG subjects exhibited distinct onset and recovery potencies of glucose in stimulating C-peptide secretion (6.5 and 8.5 mM), thereby defining in vivo hysteresis (potency shift −2.0 mM). IFG patients manifested reduced glucose onset potency (8.6 mM), and diminished C-peptide hysteretic shift (−0.80 mM). DM patients had markedly decreased glucose potency (18.8 mM), reversal of C-peptide's hysteretic shift (+4.5 mM), and 30% lower C-peptide sensitivity to glucose stimulation. From these data, we conclude that a dynamic dose-response model of glucose-dependent control of C-peptide secretion can identify disruption of in vivo hysteresis in patients with IFG and DM. Pathway-defined analytic models of this kind may aid in the search for prediabetes biomarkers.

Direct sampling of the human spermatic veins has disclosed concomitant LH and testosterone (T) pulses, suggesting pulsatile LH concentration-dependent stimulation of T secretion. However, studies to date have examined this hypothesis using only pharmacological stimulation with hCG. The present study tests the hypothesis that age is marked by decreased T secretory responses to repeated near-physiological iv pulses of recombinant human LH administered in a Clinical Translational Science Center. Participants included 92 healthy men aged 18–75 yr with BMI 18–34 kg/m2. The contribution of endogenous LH pulses was minimized by combined injection of a selective GnRH receptor antagonist sc and successive pulses of biosynthetic LH iv. A new analytical dose response model was applied to estimate the properties of exogenous LH's drive of T secretion. Regression of LH-T dose response potency estimates on age showed that the efficacy of pulses of biosynthetic LH progressively decreased with age ( P = 0.014, r = 0.26). Testis sensitivity to exogenous LH pulses also declined with age ( P = 0.011, r = 0.27). Moreover, estimated Leydig cell downregulation by LH pulses rose significantly with age ( P = 0.039, r = 0.22). These outcomes were selective, since the recovery potency of infused LH was not affected by age but was reduced by increasing BMI ( P = 0.011, r = 0.27). Assuming stable bioactivity of infused recombinant human LH, these novel data indicate that factors associated with age and BMI attenuate LH efficacy and testis sensitivity and augment Leydig cell downregulation in healthy men.

Background: The relationship between circulating ACTH levels and cortisol secretion in Cushing's disease is not precisely known. Hypothesis: Chronic ACTH hyperstimulation leads to decreased adrenal potency and is restored after normalization of ACTH secretion. Subjects: Subjects included 20 patients with Cushing's disease, eight patients in long-term remission, and 36 healthy controls. Outcomes: ACTH and cortisol secretion rates and analytical dose-response estimates of endogenous ACTH efficacy (maximal cortisol secretion), dynamic ACTH potency, and adrenal sensitivity (slope term) from 24-h ACTH-cortisol profiles were evaluated. Results: Both basal and pulsatile secretion of ACTH and cortisol were increased in patients with active disease but normal in cured patients. ACTH, but not cortisol pulse frequency, was amplified in patients and restored after successful surgical treatment. ACTH EC50, an inverse measure of potency, was higher during pulse onset in Cushing's disease (59 ± 7.4 ng/liter) than in controls (20 ± 3.7 ng/liter) (P < 0.0001) and remitted patients after surgery [15 ± 3.2 ng/liter, P value not significant (NS) vs. controls] and during pulse recovery phases [128 ± 18 (P <0.0001), 70 ± 8.4, and 67 ± 17 ng/liter (NS vs. controls), respectively]. Efficacy was increased in active disease and normalized after surgical treatment [patients, 38 ± 8.3 nmol/liter · min, vs. controls, 21 ± 2.3 nmol/liter · min (P <0.0001), and cured patients, 15 ± 3.2 nmol/liter · min (NS vs. controls)]. Sensitivity to endogenous ACTH did not differ among the three groups. Conclusion: The adrenal gland in Cushing's disease exhibits decreased responsiveness to submaximal ACTH drive and amplified efficacy, with unchanged sensitivity. These target-gland abnormalities are reversible in long-term remission after pituitary surgery.

Negative-feedback (inhibitory) and positive-feedforward (stimulatory) processes regulate physiological systems. Whether such processes are themselves rhythmic is not known. Here, we apply cross-approximate entropy (cross-ApEn), a noninvasive measurement of joint (pairwise) signal synchrony, to inferentially assess hypothesized circadian and ultradian variations in feedback coupling. The data comprised simultaneous measurements of three pituitary and one peripheral hormone (LH, FSH, prolactin, and testosterone) in 12 healthy men each sampled every 10 min for 4 days (5,760 min). Ergodicity, due to the time series stationarity of the measurements over the 4 days, allows for effective estimation of parameters based upon the 12 subjects. Cross-ApEn changes were quantified via moving-window estimates applied to 4-day time series pairs. The resultant ordered windowed cross-ApEn series (in time) were subjected to power spectrum analysis. Rhythmicity was assessed against the null hypothesis of randomness using 1,000 simulated periodograms derived by shuffling the interpulse-interval hormone-concentration segments and redoing cross-ApEn windows and spectral analysis. By forward cross-ApEn analysis, paired LH-testosterone, LH-prolactin, and LH-FSH synchrony maintained dominant rhythms with periodicities of 18–22.5, 18, and 22.5 h, respectively (each P < 0.001). By reverse (feedback) cross-ApEn analysis, testosterone-LH, testosterone-prolactin, and testosterone-FSH synchrony cycles were 30, 18, and 30–45 h, respectively (each P ≤ 0.001). Significant 8- or 24-h rhythms were also detected in most linkages, and maximal bihormonal synchrony occurred consistently at ∼0400–0500. Collectively, these analyses demonstrate significant ultradian (<24 h), circadian (∼24 h), and infradian (>24 h) oscillations in pituitary-testis synchrony, wherein maximal biglandular coordination is strongly constrained to the early morning hours.