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The Journal of Clinical Endocrinology & Metabolism Vol. 87, No. 5 2185-2193
Copyright © 2002 by The Endocrine Society

Other Original Articles

Body Composition and Circulating Levels of Insulin, Insulin-Like Growth Factor-Binding Protein-1 and Growth Hormone (GH)-Binding Protein Affect the Pharmacokinetics of GH in Adults Independently of Age

Troels Krarup Hansen, Jens Otto L. Jørgensen and Jens Sandahl Christiansen

Medical Department M (Endocrinology and Diabetes), Aarhus University Hospital, DK-8000 Aarhus C, Denmark

Address all correspondence and requests for reprints to: Troels Krarup Hansen, M.D., Medical Department M (Endocrinology and Diabetes), Aarhus University Hospital, Norrebrogade 42-44, DK-8000 Aarhus C, Denmark. E-mail: . tkh{at}dadlnet.dk

Abstract

The aim of our study was to scrutinize the association among age, body composition, and GH status in healthy adults. Using two-step, primed constant infusions of GH during suppression of endogenous GH secretion with octreotide in a group of 26 healthy nonobese men [mean age, 37.3 yr (range, 22–55 yr); body mass index, 24.6 ± 0.4 kg/m-2] we investigated the contributions of age, body composition, insulin, and binding proteins to the variability in the pharmacokinetics and acute actions of GH. All subjects were investigated twice, with the infusion rates of GH calculated according to either total body weight or intraabdominal fat mass. Body composition was determined using computed tomography and bioimpedance measurements. There was no correlation between age and body weight, yet strong positive correlations were observed between age and intraabdominal fat area (r = 0.78; P < 0.0001) and waist to hip ratio (r = 0.71; P < 0.0001) and to a lesser degree to sc fat area (r = 0.42; P < 0.03). The between-subject variability in steady state GH levels was significantly larger when GH was administered per cm2 intraabdominal fat area than per kg BW (P < 0.001). During primed constant infusions of GH at rates of 1.5 and 3.0 µg/kg·h, the corresponding MCRs of GH were 148.8 ± 5.4 and 89.8 ± 2.4 ml/min·m-2, respectively, and the MCRs were inversely related to the achieved steady state GH levels (P < 0.0001). The MCR was unrelated to age, but was negatively correlated to baseline concentrations of IGF-binding protein-1 (IGFBP-1; r = -0.53, P < 0.01) and positively correlated to basal levels of insulin (r = 0.46; P < 0.05), GH-binding protein (GHBP; r = 0.52; P < 0.01), IGFBP-3 (r = 0.47; P < 0.05), and total body fat (r = 0.44; P < 0.05). GH infusion caused significant changes in the concentrations of IGF-I, free fatty acids, GHBP, IGFBP-1, and insulin, but none of these effects was correlated to age.

Based on our results we conclude that 1) the clearance of GH is concentration dependent; 2) the pharmacokinetics and acute effects of GH are not affected by age per se; and 3) basal levels of insulin, IGFBP-1, and GHBP as well as age-related changes in body composition are important predictors of GH pharmacokinetics.

THE SENESCENT DECLINE in serum IGF-I concentration in combination with parallel changes in body composition have led to the suggestion that functional GH deficiency develops with increasing age even in the absence of hypothalamic-pituitary disease (1). The biological actions of endogenous GH depend on its secretion and clearance rates as well as sensitivity at the receptor level. Clinical experience with GH substitution in GH-deficient adults show that the dose requirements decline with age (2), which suggests that age-associated changes in GH clearance or sensitivity may exist. Whether these changes, in turn, are linked to age per se or are attributable to age-related changes in variables such as body composition, physical fitness, and circulating concentrations of other hormones and binding proteins remains unclarified. A decrease in both spontaneous and stimulated GH secretion with increasing age has been reported in several studies (3, 4, 5, 6, 7, 8). The effect of aging on the distribution and clearance of GH is less clear. Measurements of the half-life of endogenous GH using deconvolution analyses have suggested either no effect of age (7) or an age-related decline in t1/2 (5). A recent study employing a bolus of exogenous GH in a fixed dose in two age-separated groups of healthy subjects detected no significant differences in elimination half-life between the groups, but significantly higher values of MCR as well as distribution volume (Vd) in the older subjects (9). That study also indicated that MCR and Vd correlated positively with fat mass, which raised the hypothesis that fat mass is an important compartment for GH metabolism. In studies using constant infusion techniques, however, the MCR has been found to decrease with age (10, 11); a finding largely attributable to age-related differences in body weight (10). To what extent GH sensitivity at the target organ level changes with increasing age is not fully known.

To study more thoroughly the interaction between GH pharmacokinetics, body composition, and age we investigated a group of healthy adult males covering a wide age span. During octreotide-induced suppression of endogenous GH release, the subjects received primed constant infusion of GH at two different rates of infusion, each lasting 2 h. This protocol was performed twice in each subject, with the GH dose determined according to total body mass and amount of intraabdominal fat, respectively. By this approach we could test the possible impact of GH concentration, body composition, and age as well as baseline levels of insulin and pertinent GH-related binding proteins on GH pharmacokinetics.

Subjects and Methods

Subjects

Twenty-six healthy males (mean age, 37.3 ± 2.0 yr; range, 22–55 yr) were examined. The local ethical committee and the Danish National Board of Health approved the study, and informed consent was obtained from each subject before entering the study.

Study design

All subjects were examined on two occasions in random order separated by a minimum of 4 wk. After an overnight (>8-h) fast the subjects were admitted to the clinical research center at 0800 h and stayed in the supine position during the experiment. Intravenous cannulas were inserted in antecubital veins on each arm for blood sampling and infusions, respectively. A third iv cannula was inserted in a dorsal hand vein, and a continuous infusion of octreotide (2 µg/1.73 m-2·h; Sandostatin, Novartis, Basel, Switzerland) was given throughout the study to suppress endogenous GH secretion. After a baseline period of 60 min, continuous GH infusions were performed in two successive 2-h periods. Both infusion periods were primed by a bolus injection of 20 µg GH. On 1 study day, the infusion rate of GH was increased from 1.5 µg/kg·h during infusion period 1 to 3.0 µg/kg·h through infusion period 2. On the other study day the GH dose was determined according to the intraabdominal fat mass of each individual subject, as assessed by computed tomography scan, and infusion rates of 0.9 µg/cm-2 intraabdominal fat area·h in the first period and 1.8 µg/cm-2 intraabdominal fat area·h in the second period were used. These infusion rates were calculated based on an expected average intraabdominal fat area per kg BW of 1.67 cm-2/kg (8) and used to obtain comparable steady state concentrations of GH on either occasion. In five of the subjects the experiment was repeated after a washout period of a minimum of 4 wk using a constant infusion rate of 1.5 µg/kg·h for 4 h. A single batch of GH (Norditropin, Novo Nordisk A/S, Copenhagen, Denmark) was used for all experiments. Blood samples were drawn at 30-min intervals during the baseline period and every 15 min during the infusion periods. After termination of the GH infusions, blood samples were drawn every 5 min for the first hour and every 10 min for the next hour.

Analyses

Serum GH was determined by a double monoclonal immunofluorometric assay (DELFIA, Wallac, Inc., Turku, Finland). Intra- and interassay coefficients of variation ranged from 1.8–3.0% and 1.6–2.3% for 0.71–31.4 µg/liter GH, respectively, and the detection limit was 0.01 µg/liter. Serum IGF-I was measured with an in-house time-resolved immunofluorometric assay as previously described (12), and insulin was determined by a commercial ELISA (DAKO Corp., Glostrup, Denmark). Serum nonesterified fatty acids (NEFA) were determined by a colorimetric method employing a commercial kit (Wako Chemicals, Neuss, Germany), Serum IGF-binding protein-1 (IGFBP-1) was measured by ELISA (Medix Biochemica, Kainainen, Finland), IGFBP-3 by an immunoradiometric assay (Diagnostics Systems Laboratories, Inc., Webster, TX), and GH-binding protein (GHBP) with an in-house time-resolved immunofluorometric assay (13).

Body composition

Body mass index (BMI) was defined as the weight of the subject in kilograms divided by the square of the height in meters. The waist to hip ratio was defined as the ratio between the circumference of the waist at the umbilical level and of the hips at the level of trochanter major. The same physician made all measurements. Body composition was measured using multifrequency bioelectrical impedance analysis (SEAC SFB3, SEAC, Brisbane, Australia), and the amounts of intraabdominal and sc fat were evaluated by computed tomography with a Somatom Plus-S scanner (Siemens, Erlangen, Holland). The subjects were studied in the supine position, and the areas scanned were 10-mm cross-sectional slices at the umbilical level. The same technician performed all scans, and all scans were evaluated blindly by the same two radiologists. Body fat percentage and lean body mass were calculated using the formula of Deurenberg (14) and data from multifrequency bioelectrical impedance analysis.

GH pharmacokinetics and metabolic response

The mean GH concentration of the last three serum samples in infusion periods 1 and 2 was used as an estimate of the steady state GH concentration (GHsteady state). For each infusion period, the MCR (MCR1 and MCR2) was calculated as: MCR = infusion rate/GHsteady-state. From the serum GH disappearance curve after termination of infusion period 2 the t1/2 was estimated according to a monoexponential model. The same approach was used after termination of the study during which GH was infused at the rate of 1.5 µg/kg·h for 4 h. The elimination constant, k, was determined as the slope of the log-linear regression of the GH disappearance curve, and the t1/2 was calculated as t1/2 = ln2/k. The distribution volume (Vd 1 and Vd 2) was calculated for each infusion period as MCR/k. Both MCR and Vd were corrected for surface area (15). The metabolic response to the GH infusions was estimated as the incremental area under the time-concentration curves (iAUC0–360 min) of IGF-I and NEFA using the linear trapezoidal rule.

Statistical methods

The Kolmogorov-Smirnov test was used to test for normality, and variables were log-transformed when appropriate. Within-subject differences in the pharmacokinetic variables with different GH infusion rates were assessed by the paired samples t test, and two-way ANOVA for repeated measurements was used to estimate the changes over time in hormones, metabolites, and binding proteins. The F test was used to test for differences in the variability of the GH concentration curves obtained during infusions adjusted to either body weight or intraabdominal fat area. Pearson’s product moment correlation or Spearman’s correlation coefficient with two-tailed probability values were used to measure the strength of association between the variables. As MCR and Vd were corrected for surface area, these variables were not independent of BMI, and hence no correlations were made between GH pharmacokinetics and BMI. Multiple linear regression and forward stepwise regression analyses were used to determine the strongest predictors among age, body composition estimates, and baseline values of hormones and binding proteins (independent variables) of pharmacokinetic parameters (dependent variables). Data are given as the mean ± SE, and statistical significance was assumed for P less than 0.05. All statistical calculations were performed with SPSS for Windows version 10.0 (SPSS, Inc., Chicago, IL).

Results

Descriptive data for the subjects are summarized in Table 1Go. Correlations between age and pertinent measures of body composition are depicted in Fig. 1Go. There was no correlation between age and body weight and only a weak positive correlation between age and BMI of the subjects, whereas age was strongly correlated with intraabdominal fat and waist to hip ratio. Age was also correlated with sc fat (r = 0.42; P = 0.03), but not with total body fat (r = 0.25; P = 0.21) or lean body mass (r = -0.02; P = 0.91). In Table 2Go correlations between age, body composition, and baseline concentrations of circulating hormones and binding proteins are shown. Only the IGF-I levels were correlated to age. The baseline levels of IGFBP-1 were negatively and the levels of insulin were positively correlated to total body fat, whereas GHBP levels were positively correlated to multiple indexes of adiposity.


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  		Table 1. Descriptive data of the subjects

 


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  		Figure 1. Correlations between age and various indexes of body composition.

 

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  		Table 2. Baseline serum concentrations of hormones and binding proteins, correlations with age, and age-adjusted correlation coefficients between the baseline values and indexes of body composition

 
GH pharmacokinetics

The mean GH concentrations obtained during infusions at doses determined according to intraabdominal fat area were somewhat lower than those obtained when using body weight-adjusted infusion rates and showed a significantly higher degree of variability (Fig. 2Go; steady state level 1, 4.5 ± 0.7 vs. 7.0 ± 0.3 µg/liter, steady state level 2, 14.7 ± 2.5 vs. 22.7 ± 0.6 µg/liter; F test for equal variances: P < 0.001 and P < 0.001, respectively). Subsequently, only data obtained during the body weight-adjusted infusions were used. In Fig. 3Go the changes in serum GH and various hormones, binding proteins, and metabolites during study d 1 are depicted. The use of body weight-adjusted infusion rates resulted in very similar GH concentration curves in all subjects, but at the end of each infusion period no clear-cut steady state was evident despite the use of a priming GH bolus and an infusion period of 7 times the expected t1/2 of GH. The degree of nonequilibrium, assessed as the slope from a log-linear regression of the last three measurements in each infusion period (slope 90–120 and slope 210–240), was not correlated to the mean GH concentrations of the three samples (r = 0.33; P = 0.10 and r = -0.12; P = 0.57 respectively), and hence these mean values were used as estimates of the steady state GH levels in the calculations of MCR. In the experiment involving a continuous 240-min infusion of GH at the lower dose, a robust constant GH level was recorded after 180 min of infusion (Fig. 4Go). The pharmacokinetics of GH are given in Table 3Go. The apparent GH steady state concentrations were more than tripled when the GH infusion rate was doubled from 1.5 to 3.0 µg/kg·h, resulting in a significant decrease in the MCR with increasing infusion rate. The t1/2 calculated from the GH disappearance curve after infusion period 2 was not correlated to the preceding serum GH concentration (r = 0.22; P = 0.28) and was not significantly different from the t1/2 observed when the GH infusion rate was kept constant at 1.5 µg/kg·h for 4 h (Fig. 4Go and Table 3Go). The observation that the t1/2 is concentration independent indirectly implies that the Vd changes in parallel with the MCR within the range of GH concentrations achieved in this study. None of the pharmacokinetic variables was correlated to age in a crude correlation. However, when the analysis was adjusted for intraabdominal fat area, there was a positive correlation between age and t1/2 (Table 4Go). Table 5Go shows correlations of GH pharmacokinetics with body composition and baseline levels of various hormones. IGFBP-1 was negatively correlated to MCR1, Vd 1, and MCR2, whereas insulin, GHBP, IGFBP-3, and total body fat all were positively related to MCR1. Multiple linear and forward stepwise regression analyses revealed IGFBP-1 to be the most important predictor of both MCR1 and MCR2, whereas total body fat was the strongest predictor of t1/2.



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  		Figure 2. Serum GH concentration profiles in 26 subjects during two-step GH infusion and concomitant continuous iv infusion of octreotide (2 µg/1.73 m-2·h). Each infusion period was preceded by a bolus injection of 20 µg GH (indicated by arrows). A, On study d 1 the infusion rates were calculated according to body weight (infusion period 1, 1.5 µg/kg·h; infusion period 2, 3.0 µg/kg·h). B, On study d 2 the infusion rates were calculated according to intraabdominal fat area (infusion period 1, 0.9 µg/cm-2 intraabdominal fat·h; infusion period 2, 1.8 µg/cm-2 intraabdominal fat·h).

 


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  		Figure 3. Illustration of study protocol and the resulting serum concentration profiles of GH, IGF-I, GHBP, IGFBP-1, insulin, and NEFA. Endogenous GH secretion was suppressed by iv infusion of octreotide (2 µg/1.73 m-2·h) throughout the experiment. GH infusions were performed in two successive 2-h periods, followed by a 2-h washout period. Both infusion periods were preceded by a bolus injection of 20 µg GH (indicated by arrows), and the infusion rate of GH was increased from 1.5 µg/kg·h during infusion period 1 to 3.0 µg/kg·h through infusion period 2. Data are the mean ± SE (n = 26).

 


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  		Figure 4. Serum GH during a 4-h constant infusion of 1.5 µg/kg·h GH, followed by a 2-h washout period. The infusion was preceded by a bolus injection of 20 µg GH (indicated by an arrow), and endogenous GH secretion was suppressed by iv infusion of octreotide (2 µg/1.73 m-2·h) throughout the experiment. Data are the mean ± SE (n = 5).

 

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  		Table 3. GH pharmacokinetics under steady state conditions during GH infusions at two different infusion rates

 

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  		Table 4. Correlations between age and GH pharmacokinetics before and after adjustment for various indexes of body composition

 

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  		Table 5. Correlations between GH pharmacokinetics and baseline hormone concentrations and indexes of body composition

 
Metabolic response to GH infusion

During the infusion of GH and octreotide, significant changes over time, as assessed by two-way ANOVA for repeated measurements, were observed in IGF-I (P < 0.001), NEFA (P < 0.001), GHBP (P < 0.001), insulin (P < 0.05), and IGFBP-1 (P < 0.001; Fig. 3Go). During the GH infusions, a small, but significant, increase in IGF-I was observed (iAUCIGF-I = 6.36 ± 2.42 µg/liter·min). There was no correlation between age and iAUCIGF-I (r = 0.34; P = 0.10), but there were positive correlations between iAUCIGF-I and Vd 1 (r = 0.52; P < 0.01) and sc fat area (r = 0.47; P < 0.05). The lipolytic response to the GH infusions, as indicated by iAUCNEFA, was positively correlated to iAUCGHBP (r = 0.43; P < 0.05), but showed no correlation to age, GH pharmacokinetics, baseline hormonal levels, or body composition. The increase in GHBP during the GH infusions (iAUCGHBP, 0.12 ± 0.01 nM/min) was positively correlated to sc fat area (r = 0.55; P < 0.01) and intraabdominal fat area (r = 0.42; P < 0.05; Fig. 5Go, A and B) and negatively correlated to baseline IGFBP-1 levels (r = -0.43; P < 0.05). The infusion of octreotide caused a significant decrease in insulin levels. However, during the GH infusions a minor increase in insulin concentrations was observed despite continuous octreotide administration. IGFBP-1 expressed a biphasic serum curve during the GH infusions, which most likely reflects the integrated response to the concomitant GH and octreotide administration as well as changes in insulin concentrations.



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  		Figure 5. Relations between incremental area under the GHBP serum curve (iAUCGHBP) during GH infusions and sc (A) and intraabdominal (B) fat areas in 26 subjects.

 
Discussion

In the present study we employed primed, two-step, incremental infusions of GH during pharmacological suppression of endogenous GH secretion to investigate the impact of GH concentration, age, body composition, binding proteins, and other hormones on the pharmacokinetics and acute metabolic effects of GH. Our data suggest that the effects of age on the pharmacokinetics and acute pharmacodynamics of GH are minimal. On the other hand, our results indicate that the clearance of GH is concentration dependent and also considerably influenced by insulin, GHBP, IGFBP, and body composition.

Evidence from a number of clinical experiments points to a complex control of the GH elimination kinetics, including inputs from age, sex, body composition, renal function, other hormones, binding proteins, and GH itself (9, 10, 11, 16, 17, 18, 19, 20, 21). To focus on the impact of age, we reduced the number of potential variables by studying healthy, normal weight men covering a continuous age range. There was no correlation between age and either body weight or total body fat in the subjects, but we did observe a strong correlation between age and intraabdominal fat. This increase in the relative amount of intraabdominal fat in clinically nonobese males with age is well known (22). There was no correlation between age and any of the pharmacokinetic variables, but as the t1/2 was negatively correlated to the intraabdominal fat area, adjustment for this revealed a positive correlation between age and t1/2. Apparently even in nonobese subjects the effect of age by itself on t1/2 is counterbalanced by the more or less inevitable increase in intraabdominal fat with increasing age. During GH infusion significant alterations in the concentrations of IGF-I, NEFA GHBP, IGFBP-1, and insulin were observed, but none of these changes was correlated to age. This finding is in line with recent experimental data indicating that the hepatic sensitivity to exogenous GH is unaltered with increasing age (23). If both the clearance and the effects of GH are independent of age, it thus appears that the well established negative correlation between baseline IGF-I values and age is primarily attributable to an age-related decline in the spontaneous GH secretion.

In older studies, the MCR of pituitary-derived GH was found to be independent of the serum GH concentrations (24, 25, 26), but our observation of an inverse correlation between MCR and the steady state serum GH levels is in accordance with more recent data (11, 17, 20). The GH concentrations never reached a definite steady state even after 2 h of constant GH infusion. This may cause a minor overestimation of the clearance rates, but as it was observed in all subjects independently of the GH concentrations reached, it is unlikely that the small degree of nonequilibrium otherwise biases our findings. The disappearance of GH from the vascular pool (t1/2) was independent of the preceding serum GH concentrations within the range of concentrations obtained in our study, and consequently the apparent distribution volume (Vd) also showed a negative correlation to the steady state level of GH. The decrease in MCR with increasing GH concentrations signifies that saturation of one or more elimination mechanism occurs, and our data indicate that the saturation occurs even at physiological GH concentrations. This is in accordance with previous data indicating that extrarenal clearance of GH reaches half-maximal saturation at a plasma GH concentration of 12 µg/liter (20). The renal clearance of GH, which accounts for approximately 50% of the total MCR (27), has been shown to be independent of serum GH levels over a wide range of concentrations (20), and thus, the saturation most likely occurs in the extrarenal clearance pathway. GH receptors (GHRs) are widely expressed in human tissues, with the highest concentration in the liver (28), and experimental studies indicate that receptor-mediated uptake of GH in the liver constitutes the major extrarenal clearance mechanism (29, 30). Circulating GHBP is derived from GHRs by proteolytic cleavage (31, 32, 33), and is generally believed to reflect tissue GHR status. Tiong and Herington (34) found the liver to be the primary source of GHBP, but observations of a strong correlation between GHBP levels and visceral adipose tissue (35, 36, 37) indirectly suggest that adipose tissues also play a significant role in the generation of GHBP. This is supported by our observation of strong positive correlations between the increments in GHBP during GH administration and sc and intraabdominal fat mass. The MCR at low steady state GH concentrations (MCR1) showed a strong positive correlation to baseline GHBP levels and total body fat, whereas the reduced MCR at high steady state GH levels (MCR2) did not. This could indicate that the saturation of GH clearance is connected to GHBP. However, saturation of circulating GHBP, resulting in an increase in the unbound fraction of GH, has been reported to significantly increase the elimination of GH (16). A plausible explanation is that saturation occurs at the level of the GHRs, and that the GHBP concentration in the present context predominantly reflects whole body GHR status. Furthermore, it is possible that interference from GHBP in the GH assay employed (38, 39) to some extent may influence the results.

As the MCR of GH is positively correlated with various indexes of adiposity, especially intraabdominal fat mass (9, 11, 17, 18), we tested whether adjustment of the GH infusion rates according to the amount of intraabdominal fat would reduce the interindividual differences in steady state GH levels compared with determination of dose by total body weight. Although the apparent steady state GH concentrations obtained during infusion of GH per cm2 intraabdominal fat area were somewhat lower than those during infusion according to body weight, the variability was significantly larger, and evidently more predictable GH serum concentrations were obtained when GH was administered per kg BW than per cm2 intraabdominal fat area.

Insulin, but not IGF-I, acutely increases the availability of surface GHR in liver cells (40). In contrast, there are observations suggesting that both insulin and, to a lesser degree, IGF-I cause down-regulation of GHRs in peripheral tissues (41, 42). The net effect of insulin on total body GHR status remains unclarified, but the observation in the present study of a positive correlation between baseline insulin concentration and MCR1 suggests that insulin increases the extrarenal clearance of GH, which could be linked to an overall increase in GHR expression. Recently, fasting insulin levels have been reported to be negatively correlated with 24-h integrated GH concentrations independently of age, gender, and other physiological factors (43). The researchers concluded that insulin negatively influences GH secretion, but it could be argued that a positive relation between insulin levels and GH clearance would equally affect the 24-h integrated GH concentration. Insulin by itself stimulates IGF-I synthesis in hepatocytes, but a synergistic effect is seen when insulin is administered in combination with GH, and even pretreatment with insulin increases the sensitivity of hepatocytes to GH (44). Likewise, administration of GH alone and administration of ip, but not sc, insulin partly restore the reduced hepatic IGF-I mRNA expression in diabetic rats, whereas only coadministration of GH and insulin restores IGF-I mRNA levels to those in control animals (45). It is well known that patients with insulin-dependent diabetes have elevated GH levels, but inappropriately low levels of total and particularly free IGF-I (46, 47), suggesting a state of GH resistance. It seems plausible that this hepatic GH resistance could be a consequence of low portal insulin concentrations with subsequent down-regulation of liver GHR expression.

We found that the baseline level of IGFBP-1 was a major negative predictor of GH clearance. IGFBP-1 is down-regulated by insulin (48, 49, 50) and GH (51, 52), and recent data suggest that serum IGFBP-1 reflects insulin sensitivity in humans (53, 54, 55, 56). It could be hypothesized that the association between IGFBP-1 and GH clearance is secondary to the correlation between IGFBP-1 and insulin, with changes in portal delivery of and hepatic sensitivity to insulin being reflected in both changed IGFBP-1 levels and altered expression of liver GHRs. However, in GH-deficient adults serum levels of both IGFBP-1 and insulin are elevated, but still closely correlated (57), which suggests that GH contributes to the regulation of IGFBP-1 independently of the effect of insulin, and it seems possible that IGFBP-1 levels can be used as an independent predictor of GH clearance and sensitivity. Interestingly, IGFBP-1 has been reported to show strong positive correlations to the basal GH secretion rate and to the mass of GH secreted per burst, as assessed by deconvolution analysis of 24-h serum GH concentration profiles (6). One could speculate about whether the close negative correlation between IGFBP-1 and clearance influences the observed positive correlation between IGFBP-1 and deconvoluted GH secretion.

The lipolytic effect of the GH infusions, as assessed by changes in serum concentrations of NEFA, was positively correlated to the changes in GHBP, but otherwise showed no correlation to any of the measured variables. This might be attributable to saturation of the GH-induced lipolysis with the infusion rates employed. In a previous study bolus injections of GH, with subsequent peak GH concentrations of approximately 10, 15, and 30 µg/ml, caused similar increments in serum NEFA levels (58), which indicates that the maximal lipolytic effect of GH is reached even at the lowest of these GH concentrations. To reconcile our pharmacokinetic and metabolic data one could speculate that the accumulation of fat mass occurs with age as a result of external factors, such as changes in dietary habits and physical activity. This increase in fat mass elevates circulating NEFA levels, resulting in feedback suppression of pituitary GH release. As a compensatory response, GHR expression in adipose tissue is increased, which, in turn, increases the clearance of GH through receptor internalization. To some extent this elicits GH signal transduction and subsequent stimulation of lipolysis, which, however, only partially counterbalances the accumulation of fat.

In conclusion, our results indicate that GH pharmacokinetics and actions are not determined by age per se, but rather by interactions between age-associated changes in body composition and GHR expression and functioning.

Acknowledgments

We are indebted to Joan Hansen for skillful technical assistance. Novo Nordisk A/S generously supplied the GH.

Footnotes

This work was supported by Grant 960082 from the Danish Research Council (Novo Nordisk Center for Research in Growth and Regeneration, Århus University).

Abbreviations: BMI, Body mass index; GHBP, GH-binding protein; GHR, GH receptor; iAUC0–360 min, incremental area under the time-concentration curves; IGFBP-1, IGF-binding protein-1; NEFA, nonesterified fatty acids; Vd, distribution volume.

Received November 12, 2001.

Accepted January 28, 2002.

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