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Original Studies |
Department of Pediatrics and the Childrens Clinical Research Center, Yale University School of Medicine, New Haven, Connecticut 06510
Address all correspondence and requests for reprints to: Rubina A. Heptulla, M.D., Department of Pediatrics, Division of Endocrinology, Yale University School of Medicine, 333 Cedar Street, New Haven, Connecticut 06510.
| Abstract |
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| Introduction |
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Studies from our laboratory have shown that insulin resistance and hypersecretion of insulin in healthy adolescents are correlated with elevated mean 24-h GH and IGF-I levels (1, 5), suggesting that the rise in circulating GH concentrations that occurs during the pubertal growth spurt may mediate changes in insulin sensitivity during adolescence. Moreover, there is evidence that the insulin resistance that occurs during puberty may be greater for glucose than for amino acid metabolism (5). We have hypothesized that selective insulin resistance and compensatory hyperinsulinemia induced by the pubertal rise in GH serve to amplify insulins effects on amino acid metabolism and to facilitate protein anabolism during the adolescent growth spurt. If this hypothesis is correct, a similar sequence of metabolic events should occur in children before or during puberty who are treated with exogenous GH for short stature. The present study was undertaken to examine this question.
| Subjects and Methods |
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The clinical characteristics of the study populations are shown
in Table 1
. The patients included four
males and four females (aged 12 ± 1 yr) who were about to begin
treatment with exogenous GH. Five patients had idiopathic GH
deficiency, two patients had nondeficient short stature, and one
patient had postsurgical GH deficiency. Five of the eight patients were
prepubertal. Bone age was delayed in all patients except the patient
with postsurgical GH deficiency.
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Hyperglycemic clamp procedure
Each patient had a hyperglycemic clamp study performed twice: before and approximately 6 months after the start of GH treatment (0.3 mg/kg·wk in three to six divided doses); controls were studied once. Hyperglycemic clamps were performed after the subjects had fasted for 1012 h overnight at the Childrens Clinical Research Center at Yale-New Haven Childrens Hospital. During GH treatment, each patient received the usual dose of GH on the evening before the study. Physical examination, including height, weight, blood pressure, and assessment of pubertal development, was performed on the morning of the study. Two iv catheters were then inserted: one in an antecubital vein for administration of exogenous glucose (20%) and the other in a vein of the dorsal part of the contralateral hand for blood sampling. The hand for blood sampling was kept in a heated box (6065 C) to "arterialize" venous blood samples (6). After a rest period of 3060 min, baseline fasting samples were obtained for the measurement of glucose, insulin, and IGF-I. The hyperglycemic clamp technique has been described in detail previously (7). With this procedure, plasma glucose was rapidly raised by 50 mg/dL above fasting values by infusing, in a decreasing logarithmic manner, a priming dose of exogenous glucose to achieve the desired hyperglycemic plateau quickly. Subsequently, plasma glucose (measured at 5-min intervals) was kept constant at this hyperglycemic level for 120 min by appropriate adjustment of the variable rate 20% glucose infusion. Blood samples were obtained 2, 4, 6, 8, and 10 min after the start of the glucose infusion and every 20 min thereafter for measurement of plasma insulin and C peptide. Branched chain amino acids were collected at 30 min before and 0, 60, 90, and 120 min after the start of the infusion. Urine was collected at the beginning and at the end of the procedure for determination of glucose content; all urine samples were free of glucose.
Analyses
Plasma glucose levels were measured by the glucose oxidase method with a Beckman glucose analyzer (Beckman Instruments, Brea, CA). Plasma insulin, C peptide, and GH levels were measured by commercially available, double antibody RIA kits (8, 9). Plasma IGF-I levels were measured by the Nichols Institute (San Juan Capistrano, CA). Glycosylated hemoglobin was measured chromatographically with a microcolumn method (Isolab, Akron, OH; nondiabetic range, 4.08.2%). Plasma branched chain amino acids were measured by ion exchange chromatography (10, 11, 12, 13).
Calculations
During the hyperglycemic clamp procedure, plasma insulin and C peptide responses were biphasic; first phase (010 min) and second phase (10120 min) responses were calculated as the mean hormone concentration during the respective time periods. The rate of glucose metabolism (M) during the hyperglycemic clamp procedure (expressed in milligrams per m2 body surface area/min) was calculated at 20-min intervals according to the equation M = INF - SC, where M is the glucose metabolism rate, INF is the glucose infusion rate, and SC is the correction for changes in the glucose space (7). For determination of the M/I ratio (an index of insulin sensitivity), the rate of glucose metabolism during the last 60 min of each clamp study (M) was divided by the mean plasma insulin level during this time period (I).
Descriptive and inferential statistics were performed using Systat version 5.1 for Windows (SPSS, Chicago, IL). All data are presented as the mean ± SEM. Patients with and without GH insufficiency responded similarly to treatment with respect to clinical and metabolic changes. Consequently, data for all eight patients were combined for analysis. Multiple group comparisons were made using ANOVA and repeated measures ANOVA. Dunnetts procedure for multiple comparisons was used post-hoc to localize effects. Differences were considered significant at the 0.05 level. The two-tailed paired t test was applied for paired comparisons in the patients before and after treatment.
| Results |
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2- to 3-fold) and the decrease in
M/I ratio (
75%) were similar in prepubertal and pubertal subjects
even though estrogen and testosterone levels remained in the
prepubertal range in the Tanner I patients (Table 1
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| Discussion |
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The effects of GH on glucose metabolism have been examined in a large number of in vitro and in vivo studies (15, 16, 17). However, the metabolic consequences of GH therapy in currently recommended doses for treatment of children with GH deficiency and other causes of growth failure has not been extensively studied. Standard parameters have included changes in fasting plasma glucose and glycosylated hemoglobin levels, which, as in our patients, have generally not been affected by therapy (18, 19), although a rare patient has developed diabetes while receiving treatment (20). Using oral glucose tolerance testing and iv glucose administration, Walker and colleagues (21) found a similar increase in fasting and glucose-stimulated insulin levels after 1 yr of GH therapy in children with non-GH-deficient short stature. Surprisingly, using the euglycemic clamp procedure, they were unable to demonstrate a change in the rate of peripheral glucose disposal after GH treatment, suggesting that peripheral insulin sensitivity was not altered by GH therapy. It should be noted, however, that the steady state plasma insulin levels achieved in those subjects during the euglycemic clamp were close to maximally stimulatory values and much higher than those observed in our subjects during the hyperglycemic clamp. Thus, the hyperglycemic clamp procedure used here may have been able to uncover more subtle defects in peripheral insulin action induced by GH than the euglycemic hyperinsulinemic clamp procedure involving high doses of insulin that were used by Walker et al. (21).
The observation that first and second phase plasma C peptide as well as plasma insulin levels increased after GH therapy suggests that the secretion of insulin was increased with therapy. It is noteworthy, however, that the changes in plasma C peptide concentrations posttreatment were more modest than those of plasma insulin and did not achieve statistical significance vs. pretreatment values. These discrepancies may be due to the small sample size and greater variability in the C peptide responses. Alternatively, GH therapy may have reduced hepatic clearance of insulin leading to greater peripheral hyperinsulinemia or enhanced renal clearance of C peptide due to increased glomerular filtration rate (22, 23), leading to lower circulating C peptide concentrations. As urinary excretion of C peptide was not determined, we are unable to evaluate this issue.
Despite the marked reduction in insulin sensitivity caused by GH therapy, the overall rate of glucose metabolism at the end of the clamp study was similar in the patients before and after treatment and in comparison to control values. Thus, the patients were able to maintain normal rates of glucose metabolism during therapy by increasing peripheral plasma insulin concentrations. The compensatory hyperinsulinemia did not have adverse effects on blood pressure, findings consistent with those in a large sample of healthy Finnish adolescents in whom basal hyperinsulinemia correlated weakly, if at all, with systolic and diastolic blood pressure (24). On the other hand, the compensatory hyperinsulinemia may have contributed to the increase in growth velocity seen with therapy. We have previously hypothesized that compensatory hyperinsulinemia might serve a beneficial role if the metabolic defects induced by GH were restricted to glucose metabolism and spared other insulin-sensitive metabolic fuels (5). Therefore, in this study we were interested in determining the effects of GH treatment on changes in branched chain amino acid levels during the clamp procedure. Branched chain amino acids are essential amino acids that are primarily metabolized in muscle and under these fasting conditions, the fall in circulating branched chain amino acids is due to an inhibition of protein breakdown (25). The finding that the greater insulin response during the clamp in the patients during treatment was associated with greater suppression of branched chain amino acid levels compared to pretreatment values is particularly noteworthy. Thus, increases in plasma insulin as well as IGF-I levels may play a role in the somatotropic effects of GH therapy.
| Footnotes |
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2 Recipient of a postdoctoral fellowship award from the Juvenile
Diabetes Foundation International. ![]()
Received April 4, 1997.
Revised June 16, 1997.
Accepted June 26, 1997.
| References |
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