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Original Studies |
Departments of Intensive Care Medicine (G.V.d.B., P.W.) and Medicine, Division of Endocrinology (R.B.), University Hospital Gasthuisberg, University of Leuven, B-3000 Leuven, Belgium; Research Center for Endocrinology and Metabolism (L.C.), Department of Internal Medicine, Sahlgrenska University Hospital, 5-41345 Göteborg, Sweden; Kolling Institute (R.C.B.), University of Sydney, SW2065 St. Leonards, Australia; and Department of Medicine (C.Y.B.), Division of Endocrinology, Tulane University Medical Center, New Orleans, Louisiana 70112-2699
Address all correspondence and requests for reprints to: Greet Van den Berghe, M.D., Ph.D., Department of Intensive Care Medicine, University Hospital Gasthuisberg, University of Leuven, B-3000 Leuven, Belgium. E-mail: greta.vandenberghe{at}uz.kuleuven.ac.be
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Abstract |
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To further unravel this paradoxical metabolic condition, and in search of potential therapeutic strategies, we measured serum concentrations of leptin; studied the relationship with body mass index, insulin, cortisol, thyroid hormones, and somatomedins; and documented the effects of hypothalamic releasing factors, in particular, GH-secretagogues and TRH.
Twenty adults, critically ill for several weeks and supported with normocaloric, continuously administered parenteral and/or enteral feeding, were studied for 45 h. They had been randomized to receive one of three combinations of peptide infusions, in random order: TRH (one day) and placebo (other day); TRH + GH-releasing peptide (GHRP)-2 and GHRP-2; TRH + GHRH + GHRP-2 and GHRH + GHRP-2. Peptide infusions were started after a 1-µg/kg bolus at 0900 h and infused (1 µg/kg·h) until 0600 h the next morning. Serum concentrations of leptin, insulin, cortisol, T4, T3, insulin-like growth factor (IGF)-I, IGF-binding protein-3 and the acid-labile subunit (ALS) were measured at 0900 h, 2100 h, and 0600 h on each of the 2 study days.
Baseline leptin levels (mean ± SEM: 12.4 ± 2.1 µg/L) were independent of body mass index (25 ± 1 kg/m2), insulin (18.6 ± 2.9 µIU/mL), cortisol (504 ± 43 mmol/L), and thyroid hormones (T4: 63 ± 5 nmol/L, T3: 0.72 ± 0;08 nmol/L) but correlated positively with circulating levels of IGF-I [86 ± 6 µg/L, determination coefficient (R2) = 0.25] and ALS (7.2 ± 0.6 mg/L, R2 = 0.32). Infusion of placebo or TRH had no effect on leptin. In contrast, GH-secretagogues elevated leptin levels within 12 h. Infusion of GHRP-2 alone induced a maximal leptin increase of +87% after 24 h, whereas GHRH + GHRP-2 elevated leptin by up to +157% after 36 h. The increase in leptin within 12 h was related (R2 = 0.58) to the substantial rise in insulin. After 45 h, and having reached a plateau, leptin was related to the increased IGF-I (R2 = 0.37).
In conclusion, circulating leptin levels during protracted critical illness were linked to the activity state of the GH/IGF-I axis. Stimulating the GH/IGF-I axis with GH-secretagogues increased leptin levels within 12 h. Because leptin may stimulate oxidation of fatty acids, and because GH, IGF-I, and insulin have a protein-sparing effect, GH-secretagogue administration may be expected to result in increased utilization of fat as preferential substrate and to restore protein content in vital tissues and, consequently, has potential as a strategy to reverse the paradoxical metabolic condition of protracted critical illness.
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Introduction |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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We have previously shown that the wasting syndrome of protracted critical illness is associated with a uniformly reduced pulsatile secretion of GH, TSH, and PRL related to the low serum concentrations of insulin-like growth factor (IGF)-I and thyroid hormones, in the presence of relatively high circulating cortisol levels (3, 4, 5, 6).
In a search for novel strategies to reverse the catabolic state of patients treated in intensive care units (ICUs), we previously studied the effects of combined infusion of GH-secretagogues and TRH, and we found that pulsatile GH and TSH secretion could be jointly reactivated and that target organs were readily responsive to the amplified GH and TSH secretion (4, 5, 6).
Leptin is the protein hormone that is expressed in adipocytes and encoded for by the ob gene (7, 8). Circulating levels of leptin vary considerably among normal individuals, with men presenting with lower leptin levels than women and with high age being associated with relatively lower leptin concentrations for body mass index (BMI) (9). Circulating leptin is known to correlate positively with fat mass and body weight in healthy human volunteers, as well as in subjects with obesity and with conditions of chronic undernutrition (8, 10). Adipocytes have been shown to release leptin in a pulsatile fashion, following a marked circadian rhythmicity with elevated nocturnal values (11, 12). The factors currently known to influence leptin release from adipocytes in man are insulin, GH, IGF-I, thyroid hormones, SRIF, glucocorticoids, cytokines, and ß-adrenoreceptor agonists (13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25).
Leptin has central effects, playing a role in appetite control (in part, through its effect on neuropeptide Y) and in the regulation of energy expenditure (26, 27, 28, 29, 30, 31). In rodents, a potential role in the neuroendocrine response to starvation has been suggested (32, 33, 34), as well as a direct effect on fat metabolism. The latter consists of the ability of leptin to increase intracellular oxidation of fatty acids and to reduce the triglyceride content of adipocytes, hepatocytes, skeletal myocytes, and pancreatic islets (35, 36), hereby counteracting the fat-storing effect of insulin.
In view of the metabolic and endocrine alterations present during prolonged critical illness, we here report on leptin serum concentrations in this condition; and we studied the relationship with BMI, insulin, cortisol, thyroid hormones, and somatomedins. In addition, we describe the leptin response to the infusion of hypothalamic releasing factors (in particular, GH-secretagogues and TRH) (37, 38, 39).
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Subjects and Methods |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Patients depending on intensive care (including mechanical ventilatory support) for at least 12 days had been eligible for participation in this study. Further inclusion criteria were: 1) a stable condition without dopamine treatment for at least 48 h, because dopamine infusion has been shown to profoundly affect pituitary function in this condition (40, 41); and 2) an expected stay in the ICU for at least another 48 h.
Exclusion criteria were: age less than 18 yr; preexisting neurologic, psychiatric, metabolic, or endocrine disease; intracranial lesions; important liver failure [prothrombine time > 2.9 International Normalized Ratio (INR)]; renal failure, requiring dialysis or hemofiltration; and concomitant treatment with glucocorticoids, estrogens, SRIF, thyroid hormones, Ca2+-reentry blockers, clonidine, amiodarone, dopamine agonists, or antagonists.
A total of 20 patients (7 women, 13 men) were included (Tables 1
and 2).
Patient characteristics are described as mean ± SD.
The age was 68 ± 13 yr (range, 32–87 yr). The Apache II score on
the ICU-admission day, an indicator of severity of illness (with higher
values reflecting a more critical condition) (42), was 14 ± 6
(range, 5–28).
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12
mmol/L (n = 11) (44). Plasma glucose levels were elevated at the
time of study start: 8.3 ± 2.1 mmol/L (range, 5.7–14 mmol/L).
Human albumin was continuously infused when serum levels were low (mean
serum albumin concentration at inclusion was 2.8 ± 0.1 g/dL). The
mean serum level of triglycerides was 188 ± 116 mg/dL (range,
66–557 mg/dL), and C-reactive protein concentration was elevated
(12.5 ± 6.2 mg/dL). Continuous hemodynamic monitoring included
electrocardiogram (n = 20), intraarterial blood pressure
(n = 20), central venous pressure (n = 20), and core and
peripheral temperature (n = 20). During the study period of 45 h, the concomitant ICU therapy, including the nutritional intake, remained virtually unaltered in all patients.
The final outcome of these patients was a mean total ICU stay of 54 ± 39 (range, 24–181) days. Eleven patients died in the ICU (55%), a mean 38 ± 39 (range, 12–133) days after the study. Nine patients were discharged to the ward and left the hospital subsequently (45%).
The study was approved by the Institutional Review Board of the University of Leuven School of Medicine. Informed consent from a first-degree relative was obtained before patient inclusion.
Study design and peptide administration
Patients were studied during a total time span of 45 h. They were randomly allocated to one of three study groups: 1) group I (n = 8) received TRH infusion (1 µg/kg bolus at 0900 h, followed by a 1 µg/kg·h continuous infusion until 0600 h the next morning) vs. placebo during the other day; 2) group II (n = 6) received TRH + GHRP-2 (1 + 1 µg/kg bolus at 0900 h, followed by a 1 + 1 µg/kg·h continuous infusion until 0600 h the next morning) vs. GHRP-2 infusion the other day (1 µg/kg bolus at 0900 h, followed by a 1 µg/kg·h continuous infusion until 0600 h); and 3) group III (n = 6) received TRH + GHRH + GHRP-2 infusion (1 + 1 + 1 µg/kg bolus at 0900 h, followed by a 1 + 1 + 1 µg/kg·h continuous infusion until 0600 h) vs. GHRH+GHRP-2 infusion (1 + 1 µg/kg bolus at 0900 h, followed by a 1 + 1 µg/kg·h continuous infusion until 0600 h).
Within these three groups, patients were randomized for the order of peptide infusion. This randomized, cross-over design was applied to minimize possible interference by order of peptide administration or by spontaneous recovery.
Placebo (NaCl 0.9%), TRH (200 µg/mL NaCl 0.9%; UCB Pharma, Brussels, Belgium), GHRP-2 (50 µg/mL NaCl 0.9%; Kaken Pharmaceutical Co. ltd., Tokyo, Japan), and human GHRH (50 µg/mL NaCl 0.9%; Ferring, Kiel, Germany) infusions were given through a separate lumen of a central venous catheter, inserted for clinical purposes. A PERFUSOR secura FT pump with a 50-ml PERFUSOR syringe (B. Braun, Melsungen, Germany) permitted precise infusions of small volumes at a constant rate. Inadvertent interruption of the infusion or unanticipated bolus injections of the peptides were hereby avoided.
During each of the two consecutive study days (at 0900 h, 2100 h, and 0600 h), serum concentrations of leptin, IGF-I, IGF-binding protein (IGFBP)-3, ALS, insulin, cortisol, T4, and T3 were determined. Blood sampling
All blood samples were collected through an arterial line, inserted for clinical purposes independently of this study. The Edwards VAMP system (Baxter Healthcare Corporation, Irvine, CA) was used, permitting withdrawal of undiluted blood samples from an indwelling catheter, without undue blood loss. Blood was collected into glass tubes; after clotting and centrifugation, the serum was kept frozen at -20 C until assay.
Assays
For determination of leptin, insulin, IGFBP-3, ALS, and cortisol concentrations, all samples were processed in duplicate in the same assay run. For measurement of IGF-I, T4, and T3 levels, all samples of the same patient were processed in duplicate in the same assay run.
The serum leptin concentrations were determined by a human leptin RIA (Linco Research, St. Charles, MO). The detection limit was 0.5 µg/L. The intraassay coefficient of variation was 6.3% at a leptin concentration of 15.6 µg/L.
The serum insulin levels were determined by a human insulin immunoradiometric assay (Medgenix INS-IRMA, Biocource, Fleurus, Belgium). The detection limit was 1 µIU/mL. The intraassay coefficient of variation was 4.5% at 6.6 µIU/mL and 2.1% at 53 µIU/mL.
The plasma concentrations of total IGF-I were measured by RIA, after acid-ethanol extraction. The intraassay coefficient of variation was 10.1% at 95 µg/L and 5.5% at 474 µg/L. The between-assay coefficient of variation was 14.8% at 109 µg/L and 10.1% at 389 µg/L. The detection limit was 10 µg/L. The normal range in healthy adults is 100–300 µg/L.
The serum IGFBP-3 concentrations were measured by RIA, as previously described (45), using antiserum R-100. The intraassay coefficient of variation was 6.2% at 2.5 mg/L and 5.5% at 5.7 mg/L, and the between-assay coefficient of variation was 11.9% at 2.9 mg/L and 14.5% at 6.3 mg/L. Normal ranges are 2.2–4.6 mg/L.
The serum ALS concentrations were assessed by RIA, as described elsewhere (46). The intraassay coefficient of variation was 3.4%, and the between-assay coefficient of variation was 10.5% at 5.3 mg/L and 5.4% at 24 mg/L. Normal ranges are 17–34 mg/L.
The serum concentrations of cortisol had been measured by RIA after extraction with dichloromethane. The intraassay coefficient of variation was 3.1% at 417 nmol/L. Normal ranges are 276–607 nmol/L at 0800 h; 0–276 nmol/L at 2000 h; and less than 50 nmol/L at 2400 h, if asleep.
The serum concentrations of T4 were measured by RIA using the Tetrabead-125 Diagnostic Kit (Abbott Laboratories, North Chicago, IL). The intraassay coefficient of variation was 4.4% at 44 nmol/L and 2.8% at 94 nmol/L. The between-assay coefficient of variation was 14.6% at 44 nmol/L and 4.3% at 94 nmol/L. Normal values range from 71–154 nmol/L.
The serum concentrations of T3 were measured by RIA using the T3 Riabead Kit (Abbott Laboratories). The intraassay coefficient of variation was 4.6% at 0.97 nmol/L and 3.8% at 2.49 nmol/L. The between-assay coefficient of variation was 2.1% at 0.97 nmol/L and 2.8% at 2.49 nmol/L. Normal values range from 1.20–2.90 nmol/L.
Data analysis
At inclusion, groups were compared using factorial ANOVA
[post hoc testing using Fisher’s protected
least-significant difference (PLSD)], two-tailed unpaired t
test, Mann-Whitney U-test, and 
-square test. Results were analyzed
using repeated-measures ANOVA, with post hoc testing using
Fisher’s PLSD, when appropriate.
Patient characteristics are expressed as mean ± SD; results are expressed as mean ± SEM, unless indicated otherwise.
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Results |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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and 2. The three study groups were matched at baseline, except for
age: patients randomized to receive placebo ± TRH (group I) were
found to be slightly younger (mean ± SD of 58 ±
14 yr), compared with those receiving GHRP-2 ± TRH (group II)
(mean ± SD of 78 ± 8 yr) (P =
0.003) but not significantly different from those receiving GHRH +
GHRP-2 ± TRH (group III) (mean ± SD of 70
± 8 yr). Patients randomized to one of the groups receiving
GH-secretagogues were of the same age. The severity of illness for
which patients were treated in the ICU, as indicated by the Apache II
score (P = 0.5, with ANOVA), as well as the nutritional
support (P = 0.4, with ANOVA for the daily caloric
intake) were comparable among the three study groups; the apparent
difference in ultimate survival did not reach statistical significance
(not more survivors in the group receiving GHRP-2 alone,
P = 0.06 with -square).
The baseline serum concentrations of leptin, insulin, cortisol, IGF-I,
IGFBP-3, ALS, T4, and T3 measured are
delineated in Table 2. There was no difference in these parameters
among the three groups. Furthermore, it was impossible to distinguish
patients receiving exogenous insulin and patients without insulin
treatment, or ultimate survivors and nonsurvivors, by means of these
parameters.
The normal positive correlation between BMI and leptin
was not significantly present during prolonged critical illness
[determination coefficient (R2) = 0.14, P
= 0.1] (Fig. 1). Even after leaving out
the highest and outlying BMI of 40.4 kg/m2, the correlation
did not reach significance (P = 0.08). Moreover, there
was no significant correlation between inclusion leptin levels and
nutritional intake nor with circulating cortisol, T4,
T3, insulin, or IGFBP-3; and the absence of correlation was
independent of outliers. However, serum leptin levels at inclusion did
correlate positively with serum IGF-I (R2 = 0.25,
P = 0.02) and with ALS (R2 = 0.32,
P = 0.009). Leaving out the highest and outlying IGF-I
value strengthened the correlation between IGF-I and leptin
(R2 = 0.32, P = 0.01).
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The addition of TRH to the infusion of either placebo, GHRP-2, or GHRH + GHRP-2, during one of both study days, which was previously shown to increase circulating levels of T4 and T3 within 24 h (6), did not alter leptin, insulin, cortisol, IGF-I, IGFBP-3, or ALS concentrations (data not shown).
Consequently, the infusion of TRH during one of two study days was considered not to play a role in the observations regarding these parameters during the infusion of placebo or GH-secretagogues. Thus, the effects of placebo, GHRP-2, and GHRH + GHRP-2 are, from here on, reported over the entire studied time course of 45 h.
Effect of placebo, GHRP-2, and GHRH + GHRP-2 (Figs. 2 and 3)
Insulin. During placebo infusion, insulin levels remained unaltered over the entire studied episode of 45 h.
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The increases in insulin levels, observed over the studied 45 h during infusion of GHRP-2 alone, were not different from those during GHRH + GHRP-2 infusion (P = 0.9 with ANOVA and Fisher’s PLSD); and both were significantly different, compared with placebo (both P = 0.004, with ANOVA and Fisher’s PLSD).
IGF-I. During placebo infusion, IGF-I levels remained unaltered over the entire studied episode of 45 h.
Within 12 h of infusion of either GHRP-2 or GHRH + GHRP-2, IGF-I levels had increased 17% and 30%, respectively. During GHRP-2 infusion, IGF-I levels further increased up to a maximum of +76% after 36 h; and during GHRH + GHRP-2 infusion, the maximal increase in IGF-I of +105% was reached after 45 h. These changes in IGF-I concentrations were independent of exogenous insulin infusion.
The increases in IGF-I levels observed over the studied 45 h during infusion of GHRP-2 alone were not different from those during GHRH + GHRP-2 infusion (P = 0.1, with ANOVA and Fisher’s PLSD); and both were significantly different, compared with placebo (P = 0.005 and P < 0.0001, respectively, with ANOVA and Fisher’s PLSD).
ALS. During placebo infusion, ALS levels remained unaltered over the entire studied episode of 45 h.
Within 12 h of infusion of either GHRP-2 or GHRH + GHRP-2, ALS levels had increased 17% and 20%, respectively. With both GHRP-2 and GHRH + GHRP-2, ALS-I levels continued to increase progressively up to a maximum of +56% and +97%, respectively, after 45 h. These changes in ALS concentrations were independent of exogenous insulin infusion.
The increases in ALS levels observed over the studied 45 h during infusion of GHRP-2 alone were not different from those during GHRH + GHRP-2 infusion (P = 0.1, with ANOVA and Fisher’s PLSD); and both were significantly different, compared with placebo (P = 0.0002 and P = 0.007, respectively, with ANOVA and Fisher’s PLSD).
IGFBP-3. During placebo infusion, IGFBP-3 levels remained unaltered over the entire studied episode of 45 h.
Within 12 h of infusion of either GHRP-2 or GHRH + GHRP-2, IGFBP-3 levels had increased 16% and 17%, respectively. With both GHRP-2 and GHRH + GHRP-2, IGFBP-3 levels continued to increase progressively up to a maximum of +50% and +65%, respectively, after 45 h. These changes in IGFBP-3 concentrations were independent of exogenous insulin infusion.
The increases in IGFBP-3 levels observed over the studied 45 h during infusion of GHRP-2 alone were not different from those during GHRH + GHRP-2 infusion (P = 0.7, with ANOVA and Fisher’s PLSD); and both were significantly different, compared with placebo (P = 0.03 and P = 0.01, respectively, with ANOVA and Fisher’s PLSD).
Cortisol. The infusion of placebo, GHRP-2, or GHRH + GHRP-2 did not alter the moderately elevated serum concentrations of cortisol.
Leptin. During placebo infusion, leptin levels remained unaltered over the entire studied episode of 45 h.
Within 12 h of infusion of either GHRP-2 or GHRH + GHRP-2, leptin levels had increased 72% and 56%, respectively. During GHRP-2 infusion, leptin rose to a maximum of +87% 24 h after initiation of the infusion, and subsequently decreased to +62% at study end. During the infusion of GHRH + GHRP-2, leptin levels continued to increase up to a maximum of +157% after 36 h infusion. These changes in leptin were independent of exogenous infusion of insulin.
The increases in leptin levels observed over the studied 45 h during infusion of GHRP-2 alone were different from those during GHRH + GHRP-2 infusion (P = 0.02, with ANOVA and Fisher’s PLSD); and both were significantly different, compared with placebo (P = 0.005 and P = 0.006, respectively, with ANOVA and Fisher’s PLSD).
The initial change in leptin concentration (after 12 h of
infusion of placebo, GHRP-2, or GHRH + GHRP-2) correlated
positively with change in insulin concentration (R2 = 0.58,
P = 0.0001) (Fig. 3). The change in leptin after
24 h correlated positively with the alterations in insulin
(R2 = 0.36, P = 0.005), IGF-I
(R2 = 0.41, P = 0.002), and ALS
(R2 = 0.22, P = 0.04). The final change
in leptin (after 45 h) correlated positively only with the change
in IGF-I concentration (R2 = 0.37, P =
0.004), because the correlation with ALS alterations did not reach
significance (R2 = 0.17, P = 0.07). At all
times, leptin was independent of thyroid hormones and cortisol.
In the 12 patients randomized to receive GH-secretagogue treatment (GHRP-2 or GHRH + GHRP-2), the initial serum leptin levels were independent of BMI, whereas the leptin concentration after 24 h of infusion (R2 = 0.51, P = 0.009) and also at the end of the treatment (R2 = 0.46, P = 0.01) correlated positively with BMI. However, it should be noted that the significance of this correlation was highly dependent on the highest BMI in this group.
The changes in insulin, IGF-I, ALS, IGFBP-3, and leptin observed in response to GH-secretagogues at any time point during the study were independent of ultimate survival.
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Discussion |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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In healthy humans, leptin levels reflect percentage body fat and,
accordingly, correlate positively with BMI (9, 21). Within a few weeks
and despite feeding, critically ill patients lose total body water and
protein, whereas fat stores are preserved or even built up (1, 2), so
that BMI is no longer an accurate indicator of body fat and conceivably
underestimates the latter after weeks or months of intensive care.
Consequently, relatively high leptin levels for a given BMI during
critical illness would be anticipated. An activated inflammatory
cascade, as present during critical illness, may contribute to leptin
increase, mediated by cytokines such as IL-1 and TNF-
(22, 23). In
the acute phase of sepsis, the expected rise in leptin was recently
confirmed (24). Thus, it was rather surprising to find leptin levels no
longer substantially (if at all) elevated in the chronic phase of
critical illness. The absence of variability in the leptin levels
during the time course of this study is in contrast with the higher
leptin concentrations observed during early morning hours in healthy
subjects. However, it is in line with loss of diurnal variability of
other hypothalamic-pituitary-dependent hormones in critical illness,
the cause of which also remains unclear. Together, these findings
suggest that an inhibitor of the release of leptin from the adipocytes
could be present in the chronic phase of critical illness.
A plausible candidate to exert this inhibitory effect is the activated adrenergic system, characterized by high circulating levels of norepinephrine, epinephrine, and dopamine during prolonged critical illness. Activation of the ß3 adrenergic receptor on the adipocytes in rodents was found to impair leptin release (47, 48). Recently, infusion of isoproterenol has also been shown to decrease leptin secretion in humans (25). This catecholamine effect could partly explain the absence of correlation between BMI and leptin during critical illness. Alternatively, because leptin levels at study inclusion were found to correlate only with the low serum concentrations of IGF-I and ALS, the low activity status of the somatotropic axis could play a role. A positive correlation between leptin and IGF-I was previously reported to be present in anorexia nervosa (10). However, unlike in the fed, critically ill patient, the leptin-IGF-I correlation in malnourished anorexia patients was explained by the concomitantly reduced amount of body fat. The situation of the critically ill patient is more one of high age, because a reduced serum IGF-I and relatively lower leptin levels despite an increase in percentage body fat have been documented with aging, a constellation that has been attributed to a reduced activity status of the anterior pituitary (9, 49).
Whether leptin plays a causal role in the hypothalamic-pituitary dysfunction present in prolonged critical illness, in analogy with what has been described during starvation (32, 33, 34), is not clear. The positive correlation with the low IGF-I and ALS levels could be in favor of such a regulatory effect of leptin. However, the finding that leptin was not related in any way to thyroid hormone levels or cortisol does not corroborate a pivotal role of leptin in controlling the neuroendocrine response to critical illness.
The observed increase in leptin evoked by GH-secretagogues was striking. A first possible explanation for this increase in leptin is a direct effect of GH-secretagogues on the adipocytes. Although a specific GHRP-receptor has recently been identified on cardiomyocytes3, it is, at present, still unknown whether human adipocytes contain receptors for either GHRH or GHRP. GH deficiency is characterized by high serum leptin levels and a preserved diurnal rhythmicity, whereas during critical illness, the nocturnal rise of GH and leptin release is absent, and both GH and leptin are increased by GH-secretagogues. These findings may corroborate a direct leptin-releasing effect (and eventually a role in the normal nocturnal leptin surge) of GHRH and/or the endogenous ligand for the GHRP-receptor (50), because these hypothalamic releasing factors are abundantly present in pituitary GH deficiency and thought to be less available during prolonged critical illness (51).
Alternatively, the leptin rise observed with GH-secretagogues could be evoked, either directly or indirectly, by the amplified GH secretion (6). Addition of GH to cultured mature adipocytes of the rat has no effect on leptin release (52). Nevertheless, the finding of a larger effect on leptin with GHRH + GHRP infusion, compared with GHRP alone, in this study may be in favor of such a GH-mediated pathway, because GHRH + GHRP has been shown to synergistically increase GH secretion (4, 6). The previously observed transient increase in leptin, within 1 day after initiating GH treatment in GH-deficient adults, corroborates a GH-mediated mechanism underlying our findings (17). The acute leptin-releasing effect of GH in GH deficiency is in contrast with the reduction of high leptin levels documented with GH treatment at a later stage, after alterations in body composition have occurred (16, 17). In addition, leptin rise obtained with GH-secretagogues in the critically ill patients should perhaps be interpreted within the context of moderate hypercortisolism and increased adrenergic tone. In human volunteers, during high-dose glucocorticoid treatment for 7 days, administration of GH indeed jointly stimulated insulin and leptin release, followed by a rise in oxidation of fatty acids and in energy expenditure (18).
Because leptin rise after 12 h of GH-secretagogue infusion in critically ill patients was tightly related to the increase in insulin, an indirect GH effect, mediated by insulin, is suggested. Hyperinsulinism is indeed known to be a potent stimulator of leptin release (14). However, it takes 72 h of hyperinsulinism to increase leptin in healthy volunteers, compared with 12 h or less in the critically ill patients studied here.
The increment in leptin seemed to reach a plateau within 45 h. The leptin rise after 45 h of treatment with GH-secretagogues was again, as in the untreated state, positively correlated with IGF-I. It is unclear whether this positive correlation with IGF-I in prolonged critically ill patients suggests a direct effect of IGF-I on leptin release or, alternatively, reflects an increased insulin-sensitivity or an insulin-like effect mediated by rises in free IGF-I. Because, in GH deficient adults, IGF-I administration reduces leptin levels within the short time frame of a few days (17), a direct leptin-releasing effect of IGF-I seems indeed rather unlikely.
Increasing leptin levels in prolonged critically ill patients could be of benefit to reverse the paradoxical gain of fat stores and the ongoing protein wasting, despite feeding, in this condition. Indeed, leptin has been shown to exert a direct effect on fat metabolism, stimulating the oxidation of fatty acids and reducing triglyceride content in adipocytes, as well as in the liver, skeletal muscle, and pancreas (35, 36). Consequently, increasing circulating levels of leptin, together with GH, IGF-I, and insulin, is a constellation of endocrine changes that may improve use of fat as preferential substrate, prevent fat accumulation in organs such as the liver, and concomitantly restore the protein content in vital tissues (53, 54).
In conclusion, in protracted critical illness, circulating leptin levels were related to the suppressed GH/IGF-I axis. Reactivating the GH/IGF-I axis with GH-secretagogues for 2 days concomitantly increased leptin levels within 12 h. Whether this finding reflects a direct leptin-releasing effect of GH-secretagogues or, rather, points towards a role for GH in the physiological regulation of leptin secretion remains to be determined. Increasing levels of leptin, together with GH, IGF-I, and insulin, may switch substrate use from protein to fat, enabling protein anabolism to restore, which is crucial for the onset of recovery from prolonged critical illness. The data further support future exploration of the therapeutic potential of GH-secretagogues in intensive care-dependent, critically ill patients.
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Acknowledgments |
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Footnotes |
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2 A Clinical Research Investigator of the Fund for Scientific
Research, Flanders, Belgium (G.3c05.95N).
3 Ong H. GHRP receptors: are these binding
sites specific to pituitary or cardiac tissue? Oral communication at
the 24th International Symposium on GH and Growth Factors in
Endocrinology and Metabolism, Antwerp, Belgium, October 3–4,
1997.
Received February 11, 1998.
Revised April 27, 1998.
Revised June 8, 1998.
Accepted June 12, 1998.
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References |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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increases serum leptin concentrations in humans. J Clin Endocrinol Metab. 82:3084–3086.) subunit of the high molecular
weight insulin-like growth factor-binding protein complex in normal
subjects. J Clin Endocrinol Metab. 70:1347–1353.[Abstract]
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 P. Marzullo, C. Buckway, K. L. Pratt, A. Colao, J. Guevara-Aguirre, and R. G. Rosenfeld Leptin Concentrations in GH Deficiency: The Effect of GH Insensitivity J. Clin. Endocrinol. Metab., February 1, 2002; 87(2): 540 - 545. [Abstract] [Full Text] [PDF]
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R. Coutant, F. Boux de Casson, S. Rouleau, O. Douay, E. Mathieu, M. Audran, and J. M. Limal Body Composition, Fasting Leptin, and Sex Steroid Administration Determine GH Sensitivity in Peripubertal Short Children J. Clin. Endocrinol. Metab., December 1, 2001; 86(12): 5805 - 5812. [Abstract] [Full Text] [PDF]
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L. Ghizzoni, G. Mastorakos, M. E. Street, G. Mazzardo, A. Vottero, M. Vanelli, and S. Bernasconi Leptin, Cortisol, and GH Secretion Interactions in Short Normal Prepubertal Children J. Clin. Endocrinol. Metab., August 1, 2001; 86(8): 3729 - 3734. [Abstract] [Full Text] [PDF]
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 N. LAMEIRE and R. VANHOLDER Pathophysiologic Features and Prevention of Human and Experimental Acute Tubular Necrosis J. Am. Soc. Nephrol., February 1, 2001; 12(90001): 20S - 32. [Abstract] [Full Text]
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M. S. Gill, A. A. Toogood, J. Jones, P. E. Clayton, and S. M. Shalet Serum Leptin Response to the Acute and Chronic Administration of Growth Hormone (GH) to Elderly Subjects with GH Deficiency J. Clin. Endocrinol. Metab., April 1, 1999; 84(4): 1288 - 1295. [Abstract] [Full Text]
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G. Van den Berghe, P. Wouters, F. Weekers, S. Mohan, R. C. Baxter, J. D. Veldhuis, C. Y. Bowers, and R. Bouillon Reactivation of Pituitary Hormone Release and Metabolic Improvement by Infusion of Growth Hormone-Releasing Peptide and Thyrotropin-Releasing Hormone in Patients with Protracted Critical Illness J. Clin. Endocrinol. Metab., April 1, 1999; 84(4): 1311 - 1323. [Abstract] [Full Text]
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%) in serum
concentrations of leptin, insulin, cortisol, IGF-I, IGFBP-3, and ALS,
with infusion of either placebo (triangles), GHRP-2 (1
µg/kg·h, squares), or the combination of GHRH (1
µg/kg·h) and GHRP-2 (1 µg/kg·h) (circles) during
prolonged critical illness. The P values indicate the
level of significance of the between-group-difference in these changes,
as calculated with repeated-measures ANOVA.