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The Journal of Clinical Endocrinology & Metabolism Vol. 89, No. 8 3956-3964
Copyright © 2004 by The Endocrine Society

Changes in Free Rather Than Total Insulin-Like Growth Factor-I Enhance Insulin Sensitivity and Suppress Endogenous Peak Growth Hormone (GH) Release following Short-Term Low-Dose GH Administration in Young Healthy Adults

Kevin Yuen, Jan Frystyk, Margot Umpleby, Linda Fryklund and David Dunger

Department of Paediatrics (K.Y., D.D.), University of Cambridge, Cambridge CB2 2QQ, United Kingdom; Medical Research Laboratories (J.F.), Aarhus University Hospital, Aarhus, Denmark DK-8000; Department of Diabetes and Endocrinology (M.U.), Guy’s King’s and St. Thomas’ School of Medicine, King’s College, London SE1 7EH, United Kingdom; and Pfizer Health AB (L.F.), Stockholm SE-11287, Sweden

Address all correspondence and requests for reprints to: Professor David B. Dunger, University Department of Paediatrics, Level 8, Box 116, Addenbrooke’s Hospital, Hills Road, Cambridge CB2 2QQ, United Kingdom. E-mail: dbd25{at}cam.ac.uk.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
High-dose GH administration is commonly associated with impaired insulin sensitivity (SI) in humans. Paradoxically we have shown that low-dose GH (1.7 µg/kg·d) administration enhances ß-cell function in young healthy adults. In the present double-blind, placebo-controlled, cross-over study, we explored the physiological effects of this low GH dose on glucose metabolism in 12 young healthy adults (seven males, 19–29 yr). At pretreatment and after each 14-d treatment block, overnight metabolic profiles were assessed followed by a hyperinsulinemic euglycemic clamp, whereas fasting blood samples were collected weekly.

In subjects treated with GH first (group A, n = 6), GH treatment increased total IGF-I (P < 0.05) and IGF binding protein-3 (P < 0.01) after 7 d, but these levels subsequently returned to pretreatment levels after 14 d. In contrast, free IGF-I increased (P < 0.05), and overnight GH pulse peak amplitude decreased (P < 0.01) after 14 d. In subjects treated with placebo first (group B, n = 6), all biochemical parameters were unchanged after placebo treatment, whereas the changes in free and total IGF-I were similar to those of group A after GH treatment. Combined clamp data from both groups A and B (n = 12) showed that 14-d GH treatment decreased overnight plasma insulin levels (P < 0.02) and hepatic glucose appearance (P < 0.05) and increased SI (P < 0.01). Of note, the GH-induced changes in SI positively correlated with the changes in free IGF-I (r = 0.72, P < 0.01).

In conclusion, low-dose GH administration enhanced SI and suppressed endogenous peak GH release, and we hypothesize that these effects are the direct result of increased serum levels of free IGF-I.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
THE INSULIN ANTAGONISTIC actions of GH are well described in conditions of GH excess such as acromegaly (1), after GH administration in GH-deficient adults (2, 3), and in healthy adults (4, 5). Whereas most studies demonstrating these effects used supraphysiological GH doses (4, 5, 6), there is also evidence that short-term GH administration exerts temporary acute insulin-like effects (7, 8, 9) by inhibiting endogenous glucose production (8, 10, 11).

We recently demonstrated that 7-d low-dose GH (1.7 µg/kg·d) administration, a dose much lower than those used in previous studies and closely resembling the daily physiological GH production rate in adults (12), enhanced ß-cell function and decreased fasting blood glucose levels without modifying insulin sensitivity (SI) in young healthy adults (9). In that study, SI was estimated using the homeostasis model assessment (HOMA) (13), which is derived from fasting glucose and fasting insulin levels. Fasting glucose levels are largely determined by basal hepatic glucose production (14, 15); thus, the HOMA may provide a better estimate of hepatic rather than peripheral SI. However, the HOMA methodology for assessing SI may be too imprecise for studies of small sample sizes.

The present study was therefore undertaken to analyze the effects of 14-d GH administration, using an identical low GH dose (1.7 µg/kg·d), on SI using the gold standard hyperinsulinemic euglycemic clamp technique as well as ß-cell function, circulating levels of IGF-I, and IGF binding protein (IGFBP)-1 and -3 in young healthy adults. Additionally, the effects of the low GH dose on hepatic and peripheral SI, and their relationship to whole-body SI, and overnight metabolic profiles were also examined.


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

Twelve healthy nondiabetic subjects (seven males, age range 19–29 yr, body mass index [mean ± SE] 22.9 ± 3.8 kg/m2) participated in the study. Body weight was stable for at least 3 months before participation and during the 5-wk study period. The subjects had a mean waist circumference of 81.4 ± 7.6 cm, total fat mass of 12.9 ± 7.0 kg, and total lean body mass of 57.8 ± 10.1 kg. Ethical permission for the study was granted by the Cambridge Local Research Ethics Committee, and each subject gave written informed consent before participation. The subjects were asked to maintain their normal physical activity and diet for at least 3 d before the investigations, and all subjects were free from any illnesses at the time of the investigations.

Anthropometric assessment

Height, weight, waist circumference, body fat mass, and fat-free mass were measured at each visit. Total body fat mass and fat-free mass were obtained using a bioelectrical impedence monitor (Bodystat 1500, Isle of Man, UK).

Study design

The study was a placebo-controlled, double-blinded, cross-over study (Fig. 1Go). At study entry (visit 1) and at the end of each treatment phase (visits 3 and 6), the subjects were admitted to the ward at 1900 h for an overnight stay. The subjects were given a standardized carbohydrate meal in the evening before commencing fasting until midday of the following day. After the evening meal, overnight GH (15-min sampling from 2000 to 0800 h), nonesterified fatty acid (NEFA) (60-min sampling from 0000 to 0800 h), ß-hydroxybutyrate (60-min sampling from 0000 to 0800 h), glucose (15-min sampling from 0000 to 0800 h), and insulin profiles (60-min sampling from 0000 to 0800 h) were performed, followed by fasting blood sample collection and the hyperinsulinemic euglycemic clamp the following morning.



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  		FIG. 1. Study design.

 
Fasting blood samples

After a 10- to 12-h fast, blood samples were collected at 0800 h the following morning at each visit (visits 1–6) for the analysis of glucose, insulin, C-peptide, IGF-I, IGFBP-3, IGFBP-1, and NEFA concentrations. Fasting blood glucose was analyzed immediately, and the remaining samples were placed on ice and centrifuged on-site in a cooled centrifuge. Aliquoted samples were then stored at –80 C within 4 h until assayed.

Hyperinsulinemic euglycemic clamp

At 0500 h, the basilic vein in the antecubital fossa of one arm was cannulated, and a primed continuous infusion of [6,6 2H2] glucose (170 mg iv bolus followed by continuous infusion of 1.7 mg/min) was commenced and continued until 1100 h; the basilic vein of the other arm was cannulated and the arm was put into a heated sleeve for blood sampling collection. Following a 180-min basal and tracer equilibration period to allow tracer to achieve steady-state enrichment, a hyperinsulinemic euglycemic clamp [insulin bolus (Actrapid; Novo Nordisk, Bagsvaerd, Denmark), 2.3 mU/kg, followed by insulin infusion (Actrapid; Novo Nordisk), 0.5 mU/kg·min] was performed between 0800 h and 1100 h. During the clamp, blood glucose was determined every 5 min and maintained at a level 0.5 mmol/liter lower than the mean fasting blood glucose level from 0730 to 0800 h using a variable rate of 20% dextrose infusion. The 20% dextrose was enriched with 7 mg [6,6 2H2] glucose to prevent marked decreases in blood tracer enrichment and consequent calculation errors. Plasma enrichment of [6,6 2H2] glucose was determined every 5 min during each steady-state period (basal: 0730 to 0800 h and clamp: 1030 to 1100 h) and every 30 min between the steady-state periods. Throughout the clamp, blood samples were also collected for the analysis of insulin, NEFA, and ß-hydroxybutyrate.

Randomization

On completion of the overnight visit at study entry, the subjects were randomized into two groups: group A to receive GH followed by placebo treatment (GH-placebo) and group B to receive placebo followed by GH treatment (placebo-GH) for 14 d separated by a 7-d wash-out period (Fig. 1Go). The subjects self-injected 1.7 µg/kg·d GH (Genotropin; Pharmacia Ltd., Milton Keynes, UK) or placebo sc at 2200 h with a 0.3-ml insulin syringe, thus allowing the GH and placebo doses to be administered to the nearest 0.053 mg (i.e. minimal increments of 0.01 ml from the syringe is equivalent to 0.053 mg GH).

Assays

Blood glucose concentrations were measured using a YSI (Yellow Springs Instrument, Yellow Springs, OH) model 2300 stat plus analyzer (Farnborough, Hants, UK). The intraassay coefficient of variation (CV) was 1.5% at 4.1 mmol/liter, and inter-assay CVs were 2.8 and 1.7% at 4.1 and 14.1 mmol/liter, respectively. Plasma insulin, C-peptide, total IGF-I, IGFBP-3, IGFBP-1, and GH concentrations were measured using a Diagnostic Systems Laboratories (Webster, TX) ELISA (Oxford Bio-Innovations, Upper Heyford, Oxon, UK) according to the manufacturer’s instructions. For insulin, intraassay CV was 4.4% at 62 pmol/liter and 5.1% at 215 pmol/liter, and equivalent interassay CVs were 4.3 and 2.9%, respectively. For C-peptide, intraassay CV was 4.8% at 0.7 ng/ml and 3.2% at 2.2 ng/ml, and equivalent interassay CV was 15.8 and 8.1%, respectively. For IGF-I, sensitivity was 0.080 ng/ml, intraassay CVs were 3.4 and 1.5% at 9.4 and 263.6 ng/ml, and equivalent interassay CVs were 8.2 and 3.7%. For IGFBP-3, sensitivity was 0.5 ng/ml, intraassay CVs were 3.9, 3.2, and 1.8% at 7.4, 27.5, and 82.7 ng/ml, and equivalent interassay CVs were 0.6, 0.5, and 1.9%, respectively. For IGFBP-1, sensitivity was 0.25 ng/ml, intraassay CVs were 5.3 and 6.1% at 48.4 and 7.0 ng/ml, and equivalent interassay CVs were 5.1 and 10.4, respectively. For GH, intraassay CVs were 9.7% at 0.7 ng/ml and 6.5% at 6.4 ng/ml, and equivalent interassay CVs were 10.4 and 5.5%, respectively. Serum-free IGF-I concentrations were determined using ultrafiltration by centrifugation at conditions approaching those in vivo (16). All samples were analyzed in triplicate and within the same run, with the intraassay CV of 13.9% and interassay CV of 16.5%. NEFA concentrations were measured using a peroxidase technique with a commercial kit (Half Micro test, Roche Molecular Biochemicals, Lewes, UK). ß-Hydroxybutyrate was measured using a standard enzymatic technique (ß-hydroxybutyric acid kit, Sigma Diagnostics, Poole, Dorset, UK). Glucose enrichment was determined from deproteinized plasma using the methoxime-trimethylsilyl ether derivative (17) and was measured by gas chromatography mass spectrometry using a Hewlett Packard 5971A MSD (Agilent Technologies, Berks, UK).

Analysis of GH pulsatility

The Pulsar detection program (18) was used to analyze the various characteristics of spontaneous overnight GH secretion. The program detects GH pulses from 49 consecutive 15-min samples as deviations based on both height and duration from a smoothed, detrended baseline, using the assay SD as a scale factor. The intraassay SD coefficients were calculated from large single pool assays and in the parabolic equation, SD x 100 = ax (2) + bx + c, for our ELISA (and based on GH concentrations measured in nanograms per milliliter), the values of a, b, and c were 0.13, 11.70, and 17.00, respectively. The peak GH height is the highest GH level, whereas the GH amplitude is defined as the peak GH height minus baseline GH levels.

Calculations

Insulin sensitivity was determined during the final 30 min of the clamp from the mean infusion rate of 20% dextrose required to maintain euglycemia per kilogram body weight per minute (M-value: milligram per kilogram per minute).

The rates of basal glucose turnover, insulin-stimulated glucose appearance (endoRa), and disappearance (Rd) were calculated from the final 30 min before and during the insulin clamp using the non-steady-state Steele equations (19, 20, 21) modified for use with stable isotopes (22, 23). Glucose enrichments were expressed as the tracer/tracee ratio, and the effective volume of distribution of glucose was assumed to be 143 ml/kg (21, 24, 25).

The HOMA (13, 26), previously validated against independent measures of ß-cell function (27, 28), was used to estimate ß-cell function (percent) from fasting glucose and C-peptide levels. These values are expressed in relation to values in a standard individual in which these parameters are each assigned the value 100%. The HOMA-CIGMA computer program (13, 26) calculated the HOMA values, and with such a method, high HOMA values denote increased ß-cell function.

Order, carryover, and rebound effect

Order and carryover effects were sought because the study was of a cross-over design. To determine whether the sequence in which GH or placebo treatment was administered had influenced the results of the study, we examined the difference between pre- and posttreatment levels of plasma total IGF-I and IGFBP-3 for the two treatment orders (i.e. groups A and B). However, no order effect was observed because the treatment differences in total IGF-I and IGFBP-3 at pre-treatment for the two treatment orders were similar, regardless of the order in which the subjects received GH or placebo injections (data not shown).

For the analysis of a carryover effect, subjects were analyzed separately according to the sequence in which they had received placebo or GH treatment. In the group that received GH first, we found a carryover effect because the effects of GH on total IGF-I and IGFBP-3 persisted during the wash-out and the initial placebo treatment phase, whereas no changes in these parameters were noted when placebo treatment was administered first. Data are therefore presented separately as groups A and B. However, the carry-over effect was observed at d 21, whereas overnight profiles and clamp data were collected at d 1, 14, and 35 and therefore were not considered to be affected by the carryover effect. Accordingly, these data were combined in the analysis.

Power calculations

The sample size and power calculations for the present study were based on our previous study (9), in which 12 subjects would enable the detection of 10% difference in SI with a 90% power at the 1% level.

Statistical analyses

Statistical analyses were performed using SPSS for Windows (version 10.0, SPSS Inc., Chicago, IL). Data are expressed as means ± SE. Skewed variables (M-value and free IGF-I levels) were logarithmically transformed to allow parametric testing. The order effect was analyzed by comparing the treatment difference in plasma total IGF-I and IGFBP-3 from pretreatment between the two study groups (i.e. groups A and B) using the unpaired Student’s t test. The carryover and rebound effect was analyzed using the two-tailed paired Student’s t test to evaluate statistical differences. Comparisons between the means of the three study nights were made using paired Student’s t test, with each treatment phase being compared with pretreatment (i.e. visit 1). Repeated-measures ANOVA was used to analyze overnight and clamp data measured at multiple time points, with the Dunnett’s test used as a post hoc test. Two-tailed P values < 0.05 were considered statistically significant.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Anthropometric characteristics of study subjects

No significant changes in body weight, waist circumference, total fat mass, and total lean body mass were observed before and after GH or placebo injections during the study.

Separated data

Group A (GH-Placebo group). GH increased total IGF-I (P < 0.05) and IGFBP-3 (P < 0.01) levels at d 7, but these parameters returned to pretreatment levels at d 14 (Table 1Go). In contrast, GH increased free IGF-I (P < 0.05) and decreased C-peptide (P < 0.05) levels at d 14, compared with pretreatment levels (Table 1Go). After wash-out and before placebo administration (d 21), total IGF-I (P < 0.05), IGFBP-3 (P < 0.05), and C-peptide (P < 0.05) levels were again raised and were significantly higher than pre-GH treatment levels (d 1), indicating a carryover rebound effect (Tables 1Go and 2Go). C-peptide levels tended to rise at d 7 during GH treatment but decreased at d 14 (P < 0.05) (Table 2Go). On stopping GH treatment, C-peptide levels rose again by d 21 (P < 0.01), before returning to pre-GH treatment levels at d 35 (Table 2Go). Similar rebound changes in total IGF-I and IGFBP-3 levels were also observed after wash-out and during the placebo treatment phase before these levels returned to pre-GH treatment levels at d 35 (Table 1Go). In contrast, fasting glucose, insulin, and IGFBP-1 levels were unchanged throughout the GH treatment phase. The full extent of the carryover effect is shown in Tables 1Go and 2Go and Fig. 2Go.


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  		TABLE 1. Changes in IGF-I and IGFBP-3 levels separated by order of GH/placebo administration

 

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  		TABLE 2. Changes in fasting glucose, insulin, and C-peptide levels separated by order of GH/placebo administration

 


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  		FIG. 2. Schematic representation of the time-dependent changes in IGF-I, IGFBP-3, and C-peptide levels during GH and placebo treatment phases for both groups A and B.

 
Group B (placebo-GH group). In this study group, all parameters remained unchanged during the 14 d of placebo injections and the 7-d wash-out (i.e. d 1–21). During GH treatment, identical changes to those observed in group A were noticed for total IGF-I, free IGF-I, and C-peptide (Tables 1Go and 2Go). Similarly, fasting glucose, insulin, and IGFBP-1 remained unchanged. The only difference between the two GH treatment periods were related to IGFBP-3, which increased transiently after 7 d of GH treatment in group A, but not in group B (Tables 1Go and 2Go).

Data in combined study groups

Overnight profiles and clamp studies after 14 d of GH vs. placebo. Overnight blood glucose levels (0000–0800 h) remained unchanged after 14 d of GH administration, despite a significant reduction in overnight plasma insulin levels (P < 0.02). No nocturnal changes were observed for NEFA and ß-hydroxybutyrate concentrations (Table 3Go). GH pulse analyses from 2000 to 0800 h demonstrated increased smoothed basal GH levels (P < 0.005), decreased pulse peak GH amplitude (P < 0.01), and peak GH height (P < 0.05), whereas the number of GH peaks remained unchanged after GH administration (Table 4Go).


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  		TABLE 3. Overnight, fasting, and hyperinsulinemic euglycemic clamp data at pretreatment and after placebo and GH treatment (n = 12)

 

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  		TABLE 4. Overnight (2000–0800 h) GH pulse characteristics at pretreatment and after placebo and GH treatment (n = 12)

 
IGF-I, IGFBP-1 and -3, fasting C-peptide levels, and ß-cell function. Fourteen-day GH administration increased free IGF-I (P < 0.02) and decreased fasting C-peptide levels (P < 0.01) and ß-cell function (P < 0.05), whereas total IGF-I and IGFBPs-1 and -3 levels were unchanged (Table 3Go).

Clamp data. After 14 d of GH administration, the glucose clamp technique demonstrated an enhancement in SI (M-value) (P < 0.005) and a reduction in endoRa (P < 0.05) when compared with pretreatment, whereas Rd was unchanged (Table 3Go). The changes in SI were found to positively correlate with the increase in free IGF-I levels (r = 0.72; P < 0.01) (Fig. 3Go). However, blood glucose, plasma insulin, NEFA, and ß-hydroxybutyrate levels were unchanged after placebo and GH administration (Table 3Go).



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  		FIG. 3. Correlation between changes in insulin sensitivity (M-value) and free IGF-I levels from baseline.

 

   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
The insulin antagonistic effects of high-dose GH administration in humans are well documented (4, 5). However, this appears to be a dose-dependent phenomenon because the use of a much lower and physiologically approximated GH dose appears to enhance ß-cell function and lowers fasting blood glucose levels in young healthy adults (9). The present study was undertaken to further elucidate the insulin-sensitizing effects of this low dose GH treatment regimen. In this study, similar changes in total IGF-I levels were noted after 7 d to our previous observations (9); however, by prolonging low dose GH administration for a further 7 d, free IGF-I increased but paradoxically total IGF-I returned to pre-GH treatment levels. We hypothesize that the initial changes in total IGF-I levels at d 7 could have resulted from the additive effects of endogenous and exogenous GH on IGF-I generation at the liver, whereas the increase in free IGF-I levels at d 14 may have led to the suppression of endogenous peak GH secretion and the reversion of total IGF-I back to pre-GH treatment levels.

Our findings accord well with those reported by Chapman et al. (29) in which they described an increased sensitivity of the suppressive effects of free compared with total IGF-I on endogenous GH secretion in healthy adults and later demonstrated that cessation of free IGF-I feedback inhibition was associated with the rapid recovery of endogenous GH release (30). In another study, Veldhuis et al. (31) demonstrated that short-term reduction in total IGF-I levels augmented the GH secretory burst amplitude and increased basal GH release in healthy adults. Therefore, the increased serum-free IGF-I levels generated by the low-dose GH administration in our study may account for the reduction in GH secretory burst amplitude from the pituitary gland and therefore explain the normalization of total IGF-I and IGFBP-3 levels observed at d 14. In contrast to the recovery of endogenous GH secretion observed by Dall et al. (32) 14 d after stopping GH therapy, we observed a rebound effect in that plasma total IGF-I and IGFBP-3 levels were noted to increase after only 7 d of wash-out, suggesting that the shorter time taken for the recovery of endogenous peak GH secretion may be related to the lower GH dose used.

However, the mechanism responsible for the increase in free IGF-I but return of total IGF-I to pretreatment levels at d 14 is unclear, although similar observations have been previously reported in human obesity (33) during an oral glucose tolerance test and fasting (16). GH has been previously shown to affect several of the IGFBPs, including IGFBP-1 (34) and IGFBP-3 (35), whereas IGFBP-2 has also been proposed to reflect changes in GH status (35). Thus, minor changes in the concentration of these IGFBPs may be responsible for the observed increase in free IGF-I levels. In addition, it is possible that low-dose GH administration to healthy adults increases free IGF-I to a greater magnitude than total IGF-I. Support for this hypothesis is derived from a study by Skjaerbaek et al. (36), where they demonstrated that GH administration to healthy adults increased free IGF-I relatively more than total IGF-I. However, although Skjaerbaek et al. (36) postulated that the observed decrease in IGFBP-1 levels may be partly responsible for this, we found no evidence for this in the present study.

Many hormones are bound to circulating carrier proteins, whereas the unbound free component is considered to be the dominant biologically active entity (37). In accordance with this hypothesis, the positive correlation of changes in free IGF-I levels with the changes in M-value would suggest that free rather than total IGF-I directly improved SI. Our results also accord well with the findings by Simpson et al. (38), who recently demonstrated the increase in SI after recombinant IGF-I administration to type 1 diabetic subjects was the result of a direct effect of IGF-I, independent of its suppressive effects on endogenous GH secretion.

The reduction in endoRa may be related to effects of free IGF-I and the suppression of endogenous peak GH secretion in enhancing hepatic SI. Free IGF-I may act on the liver by cross-reacting with the insulin receptor, hybrid IGF-I/insulin receptors, or IGF-I receptors in the liver, although in one study by Caro et al. (39), few IGF-I receptors in the human liver were expressed. However, this study was performed in two obese adults, one of whom had type 2 diabetes, which may not give an accurate reflection of the true number of hepatic IGF-I receptors in humans. Conversely, the improvement in hepatic SI by free IGF-I has been demonstrated recently by Cusi and DeFronzo (40) after recombinant IGF-I administration in poorly controlled type 2 diabetic subjects. It is unlikely that IGF-I suppressed hepatic glucose production via the insulin receptor because in vitro studies have shown that IGF-I binds to the insulin receptor with an affinity of only 5% to that of insulin (41, 42). By contrast, IGF-I may exert its hepatic effect via binding to hybrid IGF-I/insulin receptors, which behave more like IGF-I receptors (43) and are more widely distributed in human tissues (43, 44). In vitro studies by Sakai and colleagues (45, 46) supported this notion by demonstrating that the biological actions of free IGF-I are mediated predominantly through the hybrid IGF-I/insulin receptor. More recently, studies of tissue-selective animal knockout models have demonstrated that low circulating IGF-I levels and increased GH levels led to the impairment of SI (47) and that by blocking the effect of GH predominantly enhanced hepatic SI (48, 49), thus highlighting the role of GH antagonism of insulin action at the level of the liver.

Because circulating plasma free fatty acid and excessive rates of free fatty acid/lipid oxidation may influence hepatic glucose production (50), the preservation of NEFA and ßhydroxybutyrate levels suggests that the low GH dose did not induce lipolysis. Curiously we did not observe any changes in fasting blood glucose levels. However, because the subjects were fasted, it is possible that the fasted state may have opposed the reduction in IGFBP-1 to act as a protective mechanism against the insulin-like activity of further increases of free IGF-I in inducing hypoglycemia. Indeed, levels of free IGF-I have been noted by Katz et al. (51) to steadily decline and inversely correlate to IGFBP-1 levels during fasting.

A limitation to our study was the short wash-out period and the absence of overnight serum GH profile before starting placebo treatment in group A subjects. However, although serum GH levels were not measured, a rebound increase in endogenous GH secretion remains the most plausible explanation for the observed increase in total IGF-I and IGFBP-3 levels before placebo treatment was commenced in this group. It is unlikely that the rebound increase in endogenous GH secretion could influence the overnight data analysis performed at the end of placebo treatment because the GH-dependent parameters had returned to pre-GH treatment values by then. However, because of this rebound GH effect, the effects of GH treatment on overnight metabolic profiles and glucose turnover during the clamp were not directly compared with placebo treatment.

In conclusion, short-term low-dose GH administration in young healthy adults improved SI, an effect likely to be the result of the direct effect of free IGF-I in enhancing hepatic SI. The mechanism remains unclear, and whether IGF-I acts directly via hepatic IGF-I or hepatic IGF-I/insulin hybrid receptors requires further clarification. Additionally, short-term low-dose GH administration induced time-dependent changes in total IGF-I and IGFBP-3 levels, suggesting a readjustment in the feedback inhibition in the pituitary GH/IGF-I axis corresponding to the changes in serum free IGF-I levels. However, further clinical studies are required to clarify whether the effects of low-dose GH on SI in healthy subjects are sustainable and whether these effects are applicable in disease states.


   Acknowledgments
 
We thank Pfizer Ltd. for providing the recombinant human GH used in this study. The authors are grateful to Angela Watts, Karen Whitehead, Martin White, Rachel Williams, Fariba Shojaee-Moradie, and Premila Croos for their technical support; Kirsten Nyborg and Susanne Sorensen for the measurements of free IGF-I; and the nursing staff at the Wellcome Trust Clinical Research Facility (Addenbrooke’s Hospital, Cambridge, UK). Finally, we also thank the subjects who kindly volunteered to participate in this study.


   Footnotes
 
K.Y. is supported by a research grant from Pfizer Ltd., and J.F. is supported by a grant from the Danish Health Research Council.

Abbreviations: CV, Coefficient of variation; endoRa, glucose appearance; HOMA, homeostasis model assessment; IGFBP, IGF binding protein; NEFA, nonesterified fatty acid; Rd, glucose disappearance; SI, insulin sensitivity.

Received February 17, 2004.

Accepted May 12, 2004.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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