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The Journal of Clinical Endocrinology & Metabolism Vol. 86, No. 5 1991-1998
Copyright © 2001 by The Endocrine Society

Original Studies

Peripheral Administration of Human Corticotropin-Releasing Hormone: A Novel Method to Increase Energy Expenditure and Fat Oxidation in Man1

S. R. Smith, L. de Jonge, M. Pellymounter, T. Nguyen, R. Harris, D. York, S. Redmann, J. Rood and G. A. Bray

Pennington Biomedical Research Center (S.R.S., L.d.J., T.N., R.H., D.Y., S.R., J.R., G.A.B.), Baton Rouge, Louisiana 70808; and Neurocrine Biosciences, Inc. (M.P.), San Diego, California 92121

Address all correspondence and requests for reprints to: Steven R. Smith, M.D., Experimental Endocrinology Laboratory, Pennington Biomedical Research Center, 6400 Perkins Road, Baton Rouge, Louisiana 70808. E-mail: smithsr{at}pbrc.edu


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
CRH increases energy expenditure and decreases food intake in experimental animals. We proposed the hypothesis that peripheral administration of CRH might increase energy expenditure in human subjects. Four men and four women (age, 19–39 yr) were randomized to a double blind, cross-over trial to test the effect of human CRH (hCRH), ovine CRH (oCRH), and placebo on resting energy expenditure measured by indirect calorimetry. CRH was administered by primed continuous infusion at progressively increasing doses of 0.5, 1.0, and 2.0 µg/kg·h at 2-h intervals.

hCRH increased resting energy expenditure by 13.9% at the end of the infusion. Respiratory quotient fell from 0.828 to 0.768 during the hCRH infusion compared with a fall from 0.836 to 0.807 during placebo infusion (P < 0.05). Fat oxidation increased by 55% compared with placebo at the highest dose of hCRH. Heart rate increased during hCRH to 10.7 bpm higher than placebo (P < 0.05). oCRH did not increase heart rate. oCRH also had no significant effect on respiratory quotient, and only a small effect on energy expenditure. During hCRH infusion, venous plasma epinephrine, norepinephrine, glycerol, and nonesterified fatty acid levels were not significantly different from those during placebo treatment. Peripheral CRH administration offers a novel strategy to increase energy expenditure.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
CRH IS A pleuripotent peptide with 41 amino acids that stimulates ACTH and cortisol secretion. When infused into the brains of animals, CRH inhibits food intake and increases sympathetic outflow (1, 2, 3). There are at least two receptors for CRH (4, 5, 6, 7) with several splice variants (7, 8). In the primate brain, CRH receptors are present in many areas of the brain, including the locus coeruleus, amygdala, cerebral cortex, and septum (9, 10). CRH receptors have also been demonstrated in skeletal and cardiac muscle (7), adrenal glands, testes, ovaries, spleen macrophages (11), mast cells (12), sympathetic ganglia (13), and aortic endothelium (14).

As alterations in central CRH have been associated with regulation of energy homeostasis in rodents, and CRH receptors are abundant outside the central nervous system, we hypothesized that peripheral administration of CRH might also increase energy expenditure in humans. To test this hypothesis, we administered CRH by short-term continuous iv infusion in a randomized, double blinded, placebo- controlled, cross-over study and measured energy expenditure and fat oxidation by indirect calorimetry.

CRH is bound to a specific binding protein (CRHBP) present in the brain and pituitary of rodents and in the brain and blood of humans (15). The source and function of circulating CRH in human is uncertain. Although the amino acid structure of CRH is highly conserved across species, human CRH (hCRH) and ovine CRH (oCRH) are different at 7 of the 41 amino acids (16). CRHBP binds hCRH, but not oCRH, with high affinity (IC50, 0.19 and 471 nmol/L, respectively) (15, 17, 18, 19). CRHBP levels fall after infusion of hCRH in man, but oCRH does not have this effect on CRHBP (20). The significance of the effect of hCRH on the clearance of CRHBP from the serum is unknown. In vitro, binding of CRH with the CRHBP blocks the interaction of CRH with its receptors (15). Because CRHBP is thought to prevent hCRH binding to the CRH receptors, and hCRH, but not oCRH, binds to CRHBP, a second aim of this experiment was to test the relative efficacies of these two CRH peptides to increase thermogenesis in vivo. We tested the hypothesis that oCRH would have greater bioavailability than hCRH and therefore would have greater effects on thermogenesis.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Subjects

Nine subjects, four women and five men, completed a comprehensive medical evaluation before participation in this study. All subjects were free of significant neurologic, metabolic, endocrine, cardiac, respiratory, and gastrointestinal disease, as evidenced by a normal physical examination and laboratory assessment. The mean body mass index was 23.4 ± 1.3 kg/m2 (±SEM). The mean age was 28 yr and ranged from 19–39 yr. One subject was excluded from further participation due to the inability to achieve stable energy expenditure data during the first study day. He did not complete the protocol, leaving four men and four women. His data were not used in the statistical analysis.

Informed consent

These experiments were performed under the authorization of FDA Investigational New Drug Permits 58677 (oCRH) and 58676 (hCRH) and with the approval of the Pennington Biomedical Research Center institutional internal review board. All subjects signed a written informed consent document. All women had a negative pregnancy test immediately before the start of each infusion.

Study overview

To test the hypothesis that peripheral administration of CRH would be thermogenic in humans, we compared oCRH or hCRH against placebo in a counterbalanced cross-over design using eight normal healthy volunteers infused over 6.5 h. Subjects were tested on three occasions: a placebo infusion day, an oCRH infusion day, and a hCRH infusion day. Treatment order was randomized, and subjects completed each of the 3 days after a washout period of at least 2 weeks.

Infusion protocol (Fig. 1Go)

Volunteers arrived at the in-patient metabolic unit after an overnight fast. Alcohol, exercise, and caffeine were prohibited in the 72-h period before the infusion studies. All volunteers were familiarized with the metabolic cart before beginning the infusion procedures. Upon presentation to the In-Patient Metabolic Unit, two iv lines were inserted: one for infusion of saline and CRH, and the other for blood sampling. After resting for 0.5 h, baseline energy expenditure was measured in the supine position using indirect calorimetry. Baseline blood measurements were made, and oCRH, hCRH, or saline was given as an iv bolus (1 µg/kg BW) over less than 5 min. oCRH, hCRH, or saline was then infused at a rate of 0.5, 1.0, or 2.0 µg/kg BW·h using a Harvard syringe pump (Harvard Apparatus, Natick, MA) piggybacked into saline at a rate of 50 mL/h. The volunteers and investigators were blinded to the treatment. Energy expenditure was measured for 30 min every hour for 6.5 h. Blood was sampled via a separate (noninfusion) iv catheter at the end of each energy expenditure measurement. Samples for urinary nitrogen, catecholamines, and creatinine determinations were collected over the 6-h infusion to determine protein oxidation, catecholamine spillover, the accuracy of the collection, and nonprotein respiratory quotient (RQ; fat vs. carbohydrate oxidation) from the metabolic cart measurements as described below. The protocol was started at the same time for each subject on each of the 3 test days. The subject voided 15 min before the primed-constant infusion. All urine was collected until the subject voided at the end of the test day (390 min).



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  		Figure 1. Protocol overview.

 
CRH

Ovine and human CRH were synthesized by Bachem (Torrance, CA) using GMP, and purity was confirmed by high pressure liquid chromatography. The peptides, as the acetate salts, were reconstituted to a final concentration of 0.5 mg/mL (free base) in either saline (oCRH) or saline acidified with acetic acid (hCRH). The peptides were coded, aliquoted, and frozen at -70 C until the day of use. An aliquot of each peptide was certified pyrogen free before use. The stock CRH was brought to a final concentration of 20 µg/mL (free base) in saline and passed through a 0.22-µm pore size filter before administration.

Monitoring

Heart rate and blood pressure were measured using an automated device (Dinamap, Tampa, FL). Cardiac rhythm was monitored using a Lifepak 9P monitor (Physio-Control Corp. Redmond, WA). Oral temperature was measured with a DiaTek digital thermometer.

Indirect calorimetry

Energy expenditure was measured by continuous indirect calorimetry using a ventilated hood system (Deltatrac Metabolic Monitor, DATEX, Helsinki, Finland) as previously described (21). In summary, calculations of oxygen consumption (VO2) and CO2 production (VCO2) were made from continuous measurements of oxygen and carbon dioxide concentrations in inspired and expired air diluted in a constant air flow generated by the analyzer. The accuracy of this system, as measured in vivo in our laboratory, is 99%, and the precision is 2%. We have found within-subject variation for RMR to be 3.2%. Before each 30-min measurement the calorimeter was calibrated with a reference gas containing 5% CO2 and 95% oxygen. Calculations of energy expenditure were made from VO2 and VCO2 recorded during the last 20 min for each 30-min period (22).

Laboratory analysis

Blood samples were collected via an indwelling catheter at the following time points: -30, -15, 30, 90, 150, 210, 270, 330, and 390 min. Samples were processed, frozen at -80 C, and analyzed in batch at the end of the trial. Glucose was analyzed on a Synchron Cx7 (Beckman Coulter, Inc., Brea, CA) using a glucose oxidase electrode. Cortisol, insulin, and ACTH were measured using immunoassays with chemiluminescent detection on an Immulite instrument (Diagnostic Products, Los Angeles, CA). Nonesterified fatty acid levels (NEFA) and glcyerol were analyzed on a Synchron CX5 (Beckman Coulter, Inc.). Both plasma and urinary catecholamines were analyzed using high pressure liquid chromatography with electrochemical detection (Bio-Rad Laboratories, Inc., Hercules, CA). Urinary creatinine was analyzed using a modified Jaffe reaction on the Synchron Cx7, and urinary nitrogen was measured on a pyrochemiluminescent analyzer (Antek Instruments, Houston, TX). Free levels of plasma hCRH were measured as previously described (23).

Statistical analysis

To provide an appropriate statistical model for the data resulting from the experiment, mixed linear models were implemented using the mixed procedure under releases 6.12 and 8.0 of SAS (SAS Institute, Inc., Cary, NC). For most variables, these models included treatment (hCRH, oCRH, and placebo) and dose as fixed effects in a factorial arrangement. Statements in the text that refer to treatment comparisons at the end of the infusion protocol are based on observations derived solely from the final 30 min of the infusion. The results cited throughout this report are the least squares means and the estimated SEM derived from the corresponding models. Multiple observations of any variable in a particular experimental subject at a given level of dose and treatment were treated as replicate measures.

StatView for Windows version 5.0.1 (SAS Institute, Inc.) was used for all graphs. Comparisons between treatments with P < 0.05 were considered significant.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Both oCRH and hCRH administration caused transient tachypnea (rapid breathing) lasting 2–3 min. Both oCRH and hCRH resulted in flushing of the face and upper chest. The flushing occurred with the bolus injection of hCRH and oCRH and with the highest infusion level of both hCRH and oCRH (2 µg/kg·h). These effects have been described previously with oCRH and hCRH (24). Other than these changes and the metabolic responses described below, there were no serious adverse events.

There were no differences between men and women for any end point measured.

Energy expenditure is shown graphically in Fig. 2Go. hCRH increased energy expenditure at the end of the 2 µg/kg·h dose compared with placebo at the same time point (5.08 ± 0.35 vs. 4.46 ± 0.35 kJ/min; +13.9%; P < 0.05). oCRH had a small, but statistically significant, effect on EE at the end of the 2 µg/kg·h dose (4.71 ± 0.35 vs. 4.46 ± 0.35 kJ/min; +5.6%; P < 0.05) compared with placebo at the same time point.



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  		Figure 2. Energy expenditure measured by indirect calorimetry. Data are presented as the mean ± SEM (n = 8). The statistical model included treatment [hCRH, oCRH, and placebo (PL)] and dose as fixed effects in a factorial arrangement. *, P < 0.05 vs. placebo; #, P < 0.05, 2.0 µg/kg dose vs. placebo.

 
As expected during a 20-h fast, the RQ decreased over time during the placebo infusion. The RQ decreased during oCRH treatment to the same extent as with placebo. In contrast, infusion of hCRH caused a decrease in RQ (Fig. 3Go) and an increase in fat oxidation relative to placebo (Table 1Go). At the 1 µg/kg·h dose, fat oxidation increased 19.7% compared with placebo (P > 0.05), and at 2 µg/kg·h fat oxidation increased 46% (P < 0.05; Table 1Go). At the last time point, fat oxidation increased by 55% compared with placebo (P < 0.05; Table 1Go). Carbohydrate oxidation fell somewhat at the end of the hCRH treatment, but this change did not reach statistical significance compared with placebo (P = 0.17). Protein oxidation was not different among treatment groups (data not shown).



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  		Figure 3. RQ measured by indirect calorimetry. Data are presented as the mean ± SEM (n = 8). The statistical model included treatment [hCRH, oCRH, and placebo (PL)] and dose as fixed effects in a factorial arrangement. *, P = 0.06 vs. placebo; #, P < 0.05, 2.0 µg/kg dose vs. placebo.

 

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  		Table 1. Fat oxidation and carbohydrate oxidation

 
Heart rate increased from 57.7 ± 3.9 beats/min (bpm) at baseline to 71.4 ± 3.9 bpm with the 2.0 µg/kg·h hCRH infusion (Table 2Go). At the highest dose of hCRH, the heart rate was 10.7 bpm higher compared to that with placebo (P < 0.05 vs. placebo at 2 µg/kg). oCRH did not increase heart rate compared with placebo. Systolic blood pressure did not change during any of the treatments (data not shown). Diastolic blood pressure fell relative to the placebo value only at the highest dose of hCRH (-5.9 mm Hg; P < 0.05; data not shown). At the end of the infusion, the rate-pressure product (estimated mean arterial pressure x heart rate) was not different across treatments (data not shown). No arrhythmias were observed. Oral temperature was significantly increased compared with placebo at the 1.0 and 2.0 µg/kg doses (P < 0.05; Table 2Go).


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  		Table 2. Resting heart rate and oral temperature

 
The endocrine and metabolic effects of the CRH infusions were evaluated by serial blood sampling. Both oCRH and hCRH increased ACTH and cortisol as expected (Fig. 4Go). No differences were seen in cortisol levels between oCRH and hCRH, although both oCRH and hCRH were higher than placebo as expected. Glucose was higher during oCRH and hCRH treatments, but no differences were observed between oCRH and hCRH (Fig. 5Go). Although fat oxidation increased during hCRH infusion, neither glycerol nor NEFA was increased during hCRH infusion (Fig. 5Go).



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  		Figure 4. Plasma ACTH and cortisol. Data are presented as the mean ± SEM (n = 8). Cortisol and ACTH values were significantly higher in the oCRH and hCRH groups at all doses (P < 0.05).

 


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  		Figure 5. Blood glucose, insulin, glycerol, and NEFA. Data are presented as the mean ± SEM (n = 8). Glucose values were significantly higher during the oCRH and hCRH treatment than during placebo. No differences were observed between the effects of oCRH and hCRH on glucose (P > 0.05). No differences were observed in the effects of treatments on insulin, glycerol, or NEFA (P > 0.05).

 
Urinary catecholamines did not differ among treatment groups (data not shown). Plasma catecholamine levels are presented in Fig. 6Go. Concentrations of plasma norepinephrine differed between hCRH and oCRH groups at the highest dose (P < 0.05). There were no significant differences for norepinephrine and epinephrine between hCRH and placebo treatments at any time point. Plasma hCRH levels increased to approximately 9 ng/mL during hCRH infusion (Fig. 7).



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  		Figure 6. Venous plasma catecholamines. Data are presented as the mean ± SEM (n = 7). No differences were observed between the effects of oCRH and hCRH, compared with placebo, on epinephrine (P > 0.05). No differences were observed between the effects of oCRH or hCRH, compared with placebo, on norepinephrine (P > 0.05). *, Norepinephrine values were significantly different during hCRH and oCRH treatments at 2.0 µg/kg·min (P < 0.05).

 

   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
To test the hypothesis that peripheral administration of CRH would be thermogenic in humans, we compared oCRH or hCRH vs. placebo in a counterbalanced cross-over design using eight normal healthy volunteers infused over 6.5 h. Thermogenesis increased by 13.9% after infusion of hCRH.

Compared with placebo, oral temperature increased during hCRH, but not oCRH, infusion. Fat oxidation was increased 55% at the end of the infusion compared with that during placebo without evidence of increased adipose tissue lipolysis or activation of the sympathetic nervous system (SNS). These results demonstrate for the first time that peripheral infusions of CRH increase fat oxidation and energy expenditure in man.

CRH is a potent peptide that is involved in the regulation of a variety of processes, such as ACTH secretion, anxiety disorders, depression, memory and learning, and body weight regulation (18). Central administration of CRH is also thought to increase thermogenesis in rodents. CRH has been demonstrated to increase brown fat thermogenesis (1), plasma catecholamines (25) and heart rate levels (25) when administered into the brain of rodents. These observations are compatible with central activation of sympathetic outflow by CRH. In contrast to rodents, humans have a circulating CRHBP, which binds circulating CRH. CRH receptors are distributed not only in the brain (9, 26), but also in multiple peripheral organs.

Resting energy expenditure (REE) accounts for the majority of total daily energy expenditure. Low REE predicts weight gain. Several drugs used to treat obesity have been examined to determine their effects on REE. Catecholamines increase REE by approximately 20% (27, 28). After acute administration, ephedrine given with caffeine increases REE by 13% (29). Sibutramine increases REE 3–5% (30). Our results demonstrate that hCRH increases REE by 13.9%. This thermogenic response to hCRH is greater than that with any of the currently available antiobesity drugs.

Cortisol can increase REE, but is unlikely to account for the increase in REE that we observed, because cortisol increased during both oCRH and hCRH infusions. For example, Brillon et al. demonstrated that a 12-h infusion of hydrocortisone increased REE by 8–15% (31). The increase in REE in our study is not due to an increase in cortisol, because cortisol increased after oCRH treatment, but REE did not. Stated another way, cortisol elevation occurs during ovine CRH treatment, and oCRH does not increase REE, so therefore cortisol alone cannot logically be responsible for the increase in REE in the hCRH group. Alternate explanations for this result include an interaction between hCRH and cortisol to increase REE or oCRH blockade of the cortisol-induced increase in REE. We believe that the latter two explanations are unlikely. Brillon et al. reported an increase in REE after 12 h of cortisol infusion. CRH should be completely suppressed via feedback inhibition at that time. Suppression of cortisol-mediated increases in REE by ovine, but not human, CRH seems unlikely. These alternate hypotheses could be accepted or rejected if glucocorticoid receptor (GR) antagonists were infused along with hCRH. Unfortunately, potent GR antagonists are not available for human use.

The increase in energy expenditure observed with hCRH was entirely due to an increase in fat oxidation. There was a fall in carbohydrate oxidation during the hCRH infusion; however, due to a large degree of variability, this result is not statistically significant. Carbohydrate oxidation decreased in the hCRH-treated group. Protein oxidation did not change. Regulation of fat oxidation is poorly understood (32), but is known to involve carbohydrate availability, the mitochondrial fatty acid transport protein CPT-I (33), activation of phosphodehydrogenase kinase (34), and fatty acid availability. Catecholamine infusions increase fat oxidation (27) (28), but other pharmacological interventions for obesity do not increase fat oxidation (29, 30). As such, activation of CRH pathways provides a novel pharmacological strategy to increase fat oxidation.

At the highest dose of hCRH, heart rate increased by 10.7 bpm relative to that after placebo treatment. This effect was modest and may be due to direct effects of CRH on the heart, to activation of the SNS by hCRH, or, alternately, to a reflex withdrawal of the vagus in response to the fall in diastolic blood pressure. CRH receptors have been described in the myocardium and throughout the vasculature (14, 35), and hypotension has been described in response to bolus administration of CRH (24). Although heart rate increases, we do not believe that this accounts for any significant portion of the increase in energy expenditure. Myocardial oxygen consumption is closely related to the product of mean blood pressure and heart rate, the rate pressure product (36). Mean blood pressure fell and heart rate increased during hCRH infusion. As such, the rate pressure product did not change. Accordingly, we estimate that the overall effect of hCRH on myocardial oxygen consumption is negligible. Further study of the contribution of the effects on heart rate to overall thermogenesis is warranted.

hCRH increased fat oxidation, as evidenced by a larger fall in the respiratory quotient compared with placebo. This effect was not seen during the infusion of oCRH and therefore cannot be secondary to the increase in cortisol, as hCRH and oCRH produced similar increases in cortisol, and RQ fell during placebo infusion as would be expected during prolonged fasting. If hCRH caused adipose tissue lipolysis, we would expect that glycerol and/or NEFA concentrations would have increased. However, hCRH produced no significant changes in either glycerol or NEFA, arguing against an effect on adipose tissue lipolysis. This suggests that hCRH infusion may act to increase fat oxidation by mobilization and oxidation of intracellular stores of triglyceride or alternately by increasing very low density triglyceride hydrolysis and oxidation. Intracellular skeletal muscle triglyceride stores are linked to insulin resistance (37) and appear to be mobilized by exercise (38). The effect of hCRH to increase fat oxidation in the absence of an increase in glycerol or NEFA is in contrast to catecholamine infusions that increase glycerol and NEFA (28). Again, this argues against a catecholamine-mediated process.

CRH receptors are present on sympathetic ganglia (13), and hCRH increases sympathetic tone (1) and plasma catecholamines (25) when administered into the brain of animals. This suggests that hCRH might stimulate the release of catecholamines. This could occur via activation of the SNS or alternately through release of epinephrine from the adrenal. However, urinary and plasma catecholamines did not change during hCRH infusion compared with levels during placebo infusion. This argues against the involvement of the SNS in the actions of hCRH to increase REE and fat oxidation. Plasma norepinephrine was significantly different between oCRH and hCRH at the 2 µg/kg dose. The explanation for this observation is unclear; however, it is possible that the rise in cortisol produced by oCRH decreases sympathetic tone, whereas the infusion of hCRH does not produce this effect. Regardless, plasma catecholamines do not differ between hCRH and placebo infusions and therefore do not account for the thermogenic effects and increase in fat oxidation seen with hCRH infusion. It is important to note that these measures are not sensitive indicators of sympathetic outflow and we cannot satisfactorily exclude the possibility that the SNS is involved in the thermogenic response to hCRH. Further studies using blockade of the SNS are necessary to conclusively exclude activation of the SNS as the primary mechanism of action for hCRH.

In contrast to our expectations, only hCRH increased REE and fat oxidation, whereas both oCRH and hCRH increased ACTH and cortisol (Table 3Go). Figure 5Go and Table 3Go also highlight the similarities between the effects of hCRH and oCRH on glucose metabolism; insulin levels probably fall due to prolonged fasting, and glucose probably rises due to increased gluconeogenesis (a known effect of cortisol).


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  		Table 3. Summary of effects of CRH (compared to placebo)

 
There are several important points regarding the divergence of the thermogenic response. First, we expected oCRH to have greater bioavailability because it does not bind to CRHBP, which is inhibitory for CRH binding in vitro (15). As such, we can conclude that CRHBP does not prevent hCRH thermogenesis and fat oxidation at the highest doses of hCRH administered. CRHBP may modulate CRH action in the periphery. In pregnancy, for example, up-regulation of CRHBP is thought to limit CRH action early in pregnancy, whereas later in pregnancy CRHBP levels fall, raising free CRH levels (39) (Fig. 7Go). An alternate explanation is that CRHBP enhances hCRH action. This explanation is at odds with in vitro data demonstrating inhibitory effects of CRHBP (15) and is therefore unlikely.



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  		Figure 7. Plasma free hCRH. Data are presented as the mean ± SEM (n = 8). Only the data for hCRH and placebo are presented, as oCRH was not assayed.

 
A second explanation for these observations is that CRHBP acts to transport bound CRH to an active site. CRHBP is cleared from the serum in response to infusions of hCRH, but not oCRH (20). CRHBP has a 200-fold lower affinity for oCRH than for hCRH (20). It is possible that CRHBP acts to transport hCRH to active sites in the brain or to increase hCRH concentrations at active sites in the periphery. Reverse transport of CRH into the brain of rodents is not observed (40), arguing against a role for reverse transport into the brain.

A third explanation for the divergence between the effects of oCRH and hCRH on thermogenesis and fat oxidation is that hCRH binds and activates a receptor not activated by oCRH. Alternately, hCRH and oCRH might act through the same receptor, but activate alternate signaling pathways. Adenylate cyclase is the classic signaling pathway activated by CRH, but other signaling pathways have also been demonstrated (41, 42, 43, 44). We are not aware of a direct in vitro comparison of adenylate cyclase activation by the different hCRH receptors and splice forms in response to oCRH and hCRH. Receptor binding studies suggest that hCRH has a higher affinity for CRH receptor type 2 (CRH-R2) than oCRH (18). Conversely, CRH-R1 affinity for hCRH is similar to its affinity for oCRH. Receptor binding studies with recombinant human CRH-R1 demonstrate a Ki for oCRH of 1.0 nmol/L vs. 2.0 nmol/L for hCRH (18). The Ki of the CRH-R2 for oCRH is 184 nmol/L vs. 30.7 nmol/L for hCRH (18). Combined with equal activation of ACTH release, a CRH-R1-mediated event, this suggests that CRH-R2 may be more involved in the thermogenic responses than CRH-R1. Alternatively, CRH receptor splice variations might account for the observed differences between oCRH and hCRH (7, 8). Skeletal muscle is a major component of lean mass and accounts for the majority of REE (45). Consistent with the idea that hCRH acts directly in skeletal muscle to increase REE, CRH-R2 is present in skeletal muscle (7). Unfortunately, agonists/antagonists specific for the CRH receptor subtypes are not available to directly test the hypothesis that CRH-R2 mediates the observed thermogenic responses.

Obviously, the rise in cortisol prevents the use of hCRH as a therapeutic maneuver to increase REE in humans. However, if the effects of hCRH to increase REE are mediated by the CRH receptor in thermogenic tissues, it may be possible to engineer compounds that activate thermogenesis without stimulating an increase in cortisol. As such, this thermogenic system provides a novel method to increase energy expenditure and fat oxidation in man, i.e. a therapeutic target.

In conclusion, these results demonstrate for the first time that peripheral infusions of hCRH increase fat oxidation and REE in vivo. Thermogenesis increased by 13.9% after the infusion of hCRH. Fat oxidation increased 55% without evidence of increased adipose tissue lipolysis or activation of the SNS. oCRH and hCRH increased ACTH and cortisol to an equal extent; however, only hCRH increased fat oxidation and REE. This suggests that either binding of hCRH to CRHBP facilitates transport of hCRH to active sites, or CRH-R2 may be more involved in the thermogenic and fat oxidation responses of hCRH than CRH-R1. Peripheral CRH administration offers a novel strategy to increase energy expenditure.


   Acknowledgments
 
We gratefully thank Laura Manderfield and Connie Murla for expert technical and database assistance, respectively.


   Footnotes
 
1 This work was supported by NIH Grant R44-DK-51983. Back

Received September 28, 2000.

Revised January 22, 2001.

Accepted January 30, 2001.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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