Joint synchrony of reciprocal hormonal signaling in human paradigms of both ACTH excess and cortisol depletion

Sleep and Circadian Regulation of Cortisol: A Short Review

The central circadian pacemaker (CCP) located in the suprachiasmatic nucleus (SCN) of the hypothalamus drives the 24-hour pattern in cortisol, which functions as the main central synchronizing signal that coordinates peripheral clocks in organs that control whole body metabolism. A superimposed pulsatile pattern of cortisol allows rapid responses that fine tune the body's reaction to changes in the environment. In addition to coordinating metabolic processes to predictable environmental events, cortisol is the main catabolic signal which acts with testosterone, the quintessential male anabolic hormone, to maintain catabolic-anabolic homeostasis in men. Sleep restriction, when sufficiently substantial, increases late afternoon/early evening cortisol, but does not alter 24-hour cortisol; whereas even maximal acute circadian misalignment only slightly delays the cortisol rhythm. Prolonged circadian misalignment decreases overall cortisol exposure. The implications of these regulatory changes on health and disease requires further evaluation.

Reduced nocturnal ACTH-driven cortisol secretion during critical illness

Recently, during critical illness, cortisol metabolism was found to be reduced. We hypothesize that such reduced cortisol breakdown may suppress pulsatile ACTH and cortisol secretion via feedback inhibition. To test this hypothesis, nocturnal ACTH and cortisol secretory profiles were constructed by deconvolution analysis from plasma concentration time series in 40 matched critically ill patients and eight healthy controls, excluding diseases or drugs that affect the hypothalamic-pituitary-adrenal axis. Blood was sampled every 10 min between 2100 and 0600 to quantify plasma concentrations of ACTH and (free) cortisol. Approximate entropy, an estimation of process irregularity, cross-approximate entropy, a measure of ACTH-cortisol asynchrony, and ACTH-cortisol dose-response relationships were calculated. Total and free plasma cortisol concentrations were higher at all times in patients than in controls (all P < 0.04). Pulsatile cortisol secretion was 54% lower in patients than in controls (P = 0.005), explained by reduced cortisol burst mass (P = 0.03), whereas cortisol pulse frequency (P = 0.35) and nonpulsatile cortisol secretion (P = 0.80) were unaltered. Pulsatile ACTH secretion was 31% lower in patients than in controls (P = 0.03), again explained by a lower ACTH burst mass (P = 0.02), whereas ACTH pulse frequency (P = 0.50) and nonpulsatile ACTH secretion (P = 0.80) were unchanged. ACTH-cortisol dose response estimates were similar in patients and controls. ACTH and cortisol approximate entropy were higher in patients (P ≤ 0.03), as was ACTH-cortisol cross-approximate entropy (P ≤ 0.001). We conclude that hypercortisolism during critical illness coincided with suppressed pulsatile ACTH and cortisol secretion and a normal ACTH-cortisol dose response. Increased irregularity and asynchrony of the ACTH and cortisol time series supported non-ACTH-dependent mechanisms driving hypercortisolism during critical illness.

Pathophysiology of hypercortisolism in depression: pituitary and adrenal responses to low glucocorticoid feedback

OBJECTIVE

To test three theories of hypercortisolemia in depression-hypothalamic overdrive, impaired glucocorticoid feedback, or autonomous cortisol production.

METHOD

We applied an overnight low-cortisol feedback strategy by administering metyrapone to hypercortisolemic depressed in-patients and control subjects.

RESULTS

Under metyrapone, the increases of plasma adrenocorticotropic hormone (ACTH) concentrations and of basal and pulsatile ACTH secretion were not exaggerated in hypercortisolemic depressed patients compared with control subjects. ACTH approximate entropy (ApEn) did not differ at baseline or under metyrapone. Thus, neither hypothalamic overdrive nor irregular ACTH secretion was seen. We did not detect impaired cortisol feedback: the ACTH response was not reduced, and ApEn measures that are sensitive to feedback changes were comparable in both groups. Metyrapone disrupted cortisol secretory regularity in depressed and control subjects. On the baseline day, basal cortisol secretion was significantly increased and was highly irregular (high ApEn), and ACTH-cortisol cross-ApEn was markedly elevated in high-cortisol patients.

CONCLUSION

Classical feed-forward overdrive and impaired feedback theories of hypercortisolemia in depression were not supported. Depressive hypercortisolemia may result from alternative pathophysiological mechanisms involving irregular basal hypersecretion of cortisol, associated with adrenal enlargement, possibly through splanchnic sympathetic activation of the adrenal cortex.

Oscillations in joint synchrony of reproductive hormones in healthy men

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.

Loss of inverse relationship between pulsatile insulin and glucagon secretion in patients with type 2 diabetes

OBJECTIVE

In patients with type 2 diabetes, glucagon levels are often increased. Furthermore, pulsatile secretion of insulin is disturbed in such patients. Whether pulsatile glucagon secretion is altered in type 2 diabetes is not known.

RESEARCH DESIGN AND METHODS

Twelve patients with type 2 diabetes and 13 nondiabetic individuals were examined in the fasting state and after mixed meal ingestion. Deconvolution analyses were performed on insulin and glucagon concentration time series sampled at 1-min intervals.

RESULTS

Both insulin and glucagon were secreted in distinct pulses, occurring at ∼5-min intervals. In patients with diabetes, postprandial insulin pulse mass was reduced by 74% (P < 0.001). Glucagon concentrations were increased in the patients during fasting and after meal ingestion (P < 0.05), specifically through an increased glucagon pulse mass (P < 0.01). In healthy subjects, the increase in postprandial insulin levels was inversely related to respective glucagon levels (P < 0.05). This relationship was absent in the fasting state and in patients with diabetes.

CONCLUSIONS

Glucagon and insulin are secreted in a coordinated, pulsatile manner. A plausible model is that the postprandial increase in insulin burst mass represses the corresponding glucagon pulses. Disruption of the insulin-glucagon interaction in patients with type 2 diabetes could potentially contribute to hyperglucagonemia.

Pituitary-hormone secretion by thyrotropinomas

Hormone secretion by somatotropinomas, corticotropinomas and prolactinomas exhibits increased pulse frequency, basal and pulsatile secretion, accompanied by greater disorderliness. Increased concentrations of growth hormone (GH) or prolactin (PRL) are observed in about 30% of thyrotropinomas leading to acromegaly or disturbed sexual functions beyond thyrotropin (TSH)-induced hyperthyroidism. Regulation of non-TSH pituitary hormones in this context is not well understood. We there therefore evaluated TSH, GH and PRL secretion in 6 patients with up-to-date analytical and mathematical tools by 24-h blood sampling at 10-min intervals in a clinical research laboratory. The profiles were analyzed with a new deconvolution method, approximate entropy, cross-approximate entropy, cross-correlation and cosinor regression. TSH burst frequency and basal and pulsatile secretion were increased in patients compared with controls. TSH secretion patterns in patients were more irregular, but the diurnal rhythm was preserved at a higher mean with a 2.5 h phase delay. Although only one patient had clinical acromegaly, GH secretion and IGF-I levels were increased in two other patients and all three had a significant cross-correlation between the GH and TSH. PRL secretion was increased in one patient, but all patients had a significant cross-correlation with TSH and showed decreased PRL regularity. Cross-ApEn synchrony between TSH and GH did not differ between patients and controls, but TSH and PRL synchrony was reduced in patients. We conclude that TSH secretion by thyrotropinomas shares many characteristics of other pituitary hormone-secreting adenomas. In addition, abnormalities in GH and PRL secretion exist ranging from decreased (joint) regularity to overt hypersecretion, although not always clinically obvious, suggesting tumoral transformation of thyrotrope lineage cells.

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.

Postprandial suppression of glucagon secretion depends on intact pulsatile insulin secretion: further evidence for the intraislet insulin hypothesis

Type 2 diabetes is characterized by an approximately 60% loss of beta-cell mass, a marked defect in postprandial insulin secretion, and a failure to suppress postprandial glucagon concentrations. It is possible that postprandial hyperglucagonemia in type 2 diabetes is due to impaired postprandial insulin secretion. To address this, we studied eight adult Goettingen minipigs before and after an approximately 60% reduction in beta-cell mass induced by alloxan. Pigs were studied fasting and after ingestion of a mixed meal. Insulin and glucagon secretion were determined by deconvolution of blood hormone concentrations measured at 1-min intervals. The relationship between insulin and glucagon release was analyzed using cross-correlation and forward versus reverse cross-approximate entropy. We report that glucagon and insulin were secreted in approximately 4-min pulses. Prealloxan, postprandial insulin secretion drove an approximately 20% suppression of glucagon concentrations (P < 0.01), through inhibition of glucagon pulse mass. The alloxan-induced approximately 60% deficit in beta-cell mass lead to an approximately 70% deficit in postprandial insulin secretion and loss of the postprandial insulin-driven suppression of glucagon secretion. We conclude that postprandial hyperglucagonemia in type 2 diabetes is likely due to loss of intraislet postprandial suppression of glucagon secretion by insulin.

Lack of effect of metyrapone and exogenous cortisol on early porcine conceptus development

A study was conducted to evaluate the influence of maternal cortisol on early conceptus development in pigs (Sus scrofa). The corticosteroid synthesis inhibitor metyrapone was injected daily during days 14-19 of pregnancy, without (n = 6) and with commensurate administration of cortisol (n = 6). Blood samples were taken via an indwelling jugular catheter on days 14 and 18, and conceptuses were harvested during surgery on day 20. Compared with vehicle-injected control dams (n = 7) plasma cortisol and aldosterone concentrations were decreased (P < 0.01) by 52 and 29%, respectively, by metyrapone treatment. Cortisol administration reversed decreases in plasma cortisol by day 18. There were no treatment-associated effects on conceptus survival or size. Nor were there treatment-associated effects on allantoic fluid volume or content. Trophodermal glucocorticoid receptor (GR) mRNA expression decreased by 34% (P < 0.05) in metyrapone-treated pigs, and was not further influenced by concomitant administration of cortisol, thereby suggesting an influence of aldosterone on GR mRNA expression. Also, when all pigs were considered, there were treatment-independent second-order polynomial regressions (P < 0.05) between maternal plasma cortisol concentrations and embryonic weight, allantoic size and allantoic glucose concentrations, and between plasma aldosterone concentrations and trophodermal GR mRNA expression. Such biphasic corticosteroid concentration versus tissue parameter curves are noteworthy, but difficult to interpret validly. They may suggest that an appropriate corticosteroid environment is necessary for optimal porcine embryonic development during this stage of gestation, but cannot overshadow the absence of treatment effects on the porcine embryonic measures evaluated.

Aging attenuates both the regularity and joint synchrony of LH and testosterone secretion in normal men: analyses via a model of graded GnRH receptor blockade

Testosterone (T) secretion declines in the aging male, albeit for unknown reasons. From an ensemble perspective, repeated incremental signaling among gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH), and T is required to maintain physiological androgen availability. Pattern-regularity statistics, such as univariate approximate entropy (ApEn) and bivariate cross-ApEn, provide specific and sensitive model-free measurement of altered multi-pathway control. The present study exploits partial muting of one pathway (GnRH drive) to appraise adaptive regulation of LH and T secretion in young and aging individuals. Analyses comprised 100 paired 18-h LH and T concentration time series obtained in 25 healthy men ages 20-72 yr each administered placebo and three graded doses of a specific GnRH-receptor antagonist. Graded blockade of GnRH drive increased the individual regularity of LH and T secretion and the synchrony of LH-T feedforward and T-LH feedback in the cohort as a whole (P<0.001 for each). However, age markedly attenuated ganirelix-induced enhancement of univariate T orderliness and bivariate LH-T feedback and T-LH feedback synchrony (P <or= 0.0025). In summary, the present analyses support the thesis that aging disrupts coordinate control of T secretion, LH-T feedforward, and T-LH feedback in healthy men. Thus the experimental strategy of stepwise silencing of an agonistic pathway may have utility in dissecting the bases of altered neurohormonal linkages in other systems.

Aging in healthy men impairs recombinant human luteinizing hormone (LH)-stimulated testosterone secretion monitored under a two-day intravenous pulsatile LH clamp

CONTEXT

Testosterone (Te) depletion in aging men in principle could reflect deficits in the hypothalamus, pituitary gland, or testis. Available pharmacological studies of possible failure of Leydig cell steroidogenesis remain inconclusive.

OBJECTIVE

The objective of the study was to assess Te secretion in older and young men in response to near physiological LH stimulation.

INTERVENTION

Pulsatile i.v. infusion of recombinant human LH was administered for 2 d to stimulate Te secretion during suppression of endogenous LH concentrations with a potent selective GnRH receptor antagonist (ganirelix). SUBJECTS/CONTEXT: Healthy older (aged 60-73 yr, n = 8) and young (19-30 yr, n = 13) men were studied in an academic setting.

MEASURES

Pulsatile LH and Te concentrations on the second day of exogenous LH stimulation were measured.

RESULTS

Serum ganirelix concentrations and infused LH pulse increments were similar by age. In contrast, older subjects manifested: 1) reduced mean Te concentrations (P = 0.016), Te peak heights (P = 0.014), increments (P = 0.010), summed areas (P < 0.013), and interpeak Te concentrations (P = 0.023); 2) decreased Te to LH concentration ratios (P = 0.002); 3) diminished LH-Te feed-forward synchrony (P = 0.020); and 4) a blunted amplitude (P = 0.036) and advanced phase (P = 0.013) of diurnal Te rhythms.

CONCLUSION

A novel regimen of pulsatile LH stimulation for 48 h during GnRH receptor blockade unmasks deficits in pulsatile, basal, synchronous, and nyctohemeral Te secretion in healthy older men. These findings do not exclude concomitant defects in GnRH outflow and/or Te-negative feedback in the aging male.