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Division of Endocrinology, Department of Medicine, Children’s Nutrition Research Center (J.S., R.V.S., A.B., M.D.S.); Department of Pediatrics, Baylor College of Medicine (R.V.S., K.E., P.J.R., F.J.); and Ben Taub General Hospital (A.B., M.D.S.), Houston, Texas 77030
Address all correspondence and requests for reprints to: Morali D. Sharma, M.D., Division of Endocrinology, 6565 Fannin, Room B-250, Houston, Texas 77030. E-mail: msharma{at}bcm.tmc.edu.
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Abstract |
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Introduction |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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One possible explanation for these discrepant findings may be the fact that measurements of protein metabolism have been made at different times posttreatment, ranging from as early as 2 wk (7), when GH anabolic effect may have been maximum, to 6 months (2, 9), when FFM may have returned to normal and, hence, GH had no further anabolic effect. For example, a study by Beshyah et al. (9) reported no significant change in leucine kinetics after 6 months of GH replacement therapy in GHD adults. On the other hand, two different studies of fasted GHD patients reported increased nonoxidative leucine disposal, a measure of protein synthesis, and a decrease in leucine oxidation, an index of net protein catabolism, after 2 wk and 2 months, respectively, of GH treatment (6, 7).
Another factor that may confound the effects of GH replacement on protein metabolism is the prandial state of the subject during the measurement. It has been argued that the protein anabolic actions of GH require the presence of adequate insulin concentrations as well as normal insulin action (3). In the absence of insulin, the effects of GH on FFM are mainly catabolic, but in the presence of insulin the effects of GH on FFM are to restrain catabolism (3). To determine the physiological effects of GH treatment in GHD patients, it is therefore important to measure its effects on protein metabolism in both the postabsorptive state, when plasma insulin concentrations are low, and during feeding, when plasma insulin concentrations are increased. However, the majority of studies of the effects of GH replacement on protein metabolism in GHD patients have been performed in subjects in the fasted state (6, 7, 9). For these reasons it is important to determine the short- and long-term effects of GH therapy in the fed and fasted states.
There have been no previous studies of protein kinetics in GHD patients after short- and long-term treatment with GH in both the fasted and fed states. The goal of the present study was to quantify whole body and splanchnic protein metabolism in the fasted and fed states before and at two time points after GH replacement therapy. We hypothesized the following: 1) in the short term, GH treatment of GHD patients would lead to an increase in net protein synthesis in the fed state and a decrease in net protein loss in the fasted state to facilitate replenishment of FFM; and 2) after prolonged GH treatment and stabilization of body compositional changes, protein kinetics in GHD patients in the fed and fasted states would be indistinguishable from those in normal adults. To test these hypotheses, whole body leucine kinetics were measured using simultaneous infusions of an iv tracer ([2H3]leucine) and an oral tracer (l-[13C1]leucine) in eight GHD adult patients and in eight age-, sex-, and BMI-matched normal controls. The GHD patients were studied before starting GH replacement as well as after 2 wk and 6 months of physiological GH replacement therapy.
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Subjects and Methods |
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The starting dose of GH was 2.5 µg/kg·d (0.0075 IU/kg·d) in women and 2 µg/kg·d (0.006 IU/kg·d) in men, administered as a single sc injection in the evening between 2000–2100 h. The dose was doubled after 1 wk (the GH product used in the study was Humatrope, Eli Lilly & Co., Indianapolis, IN). Thereafter, the serum IGF-I concentration was measured every 6–8 wk, and the dose was titrated upward with the goal of maintaining the plasma concentration of IGF-I in the midnormal range for age and gender, up to a maximum daily dose of 4 µg/kg (0.012 IU/kg) in men and 5 µg/kg (0.015 IU/kg) in women. Female subjects receiving oral estrogen therapy were switched to transdermal estradiol (0.05 mg once a week) 4 wk before commencing GH therapy. GHD patients received oral glucocorticoid replacement according to the following regimen: 20 mg hydrocortisone at 0800 h and 10 mg at 1400 h throughout the period of the study, including the day of the metabolic studies in the General Clinical Research Center. Each metabolic study was performed 1 wk after a testosterone injection in GHD men receiving testosterone replacement and in midcycle in GHD women with normal menstrual cycles.
Controls were recruited by screening healthy volunteers for evidence of hypothalamic or pituitary disease by history and physical examination. All control subjects had the following normal laboratory tests: complete blood counts, liver and kidney function tests, free T4, TSH, LH, FSH, PRL, testosterone or estradiol, IGF-I, and morning cortisol.
Materials
Stable isotope studies of protein metabolism were performed on each subject at baseline before GH treatment and were repeated 2 wk and 6 months after the start of GH treatment. Control subjects underwent the same protocol only once. For each study the subject was admitted to the Adult General Clinical Research Center of Baylor College of Medicine after 2 d of stabilization on a standard diet (30 kcal and 1 g protein/kg·d). After a 12-h overnight fast, iv catheters were placed in the superficial veins of both arms. At 0800 h, a primed-constant iv infusion of [2H3]leucine (prime, 7.5 µmol/kg; infusion, 7.5 µmol/kg·h) was begun and continued for 8 h. Simultaneously, a primed-constant oral administration of l-[13C1]leucine (prime, 7.5 µmol/kg; infusion, 7.5 µmol/kg·h) was begun and continued for 8 h. The oral tracer was administered in boluses (2.5 ml) at 15-min intervals. During the first 4 h of the infusions, the subject remained fasting (fasted study). From 4–8 h, the subject consumed a mini-meal every 30 min (fed study). Each mini-meal consisted of 1/24th of the daily caloric and protein requirement. Serial blood and breath samples were collected at baseline and every 15 min during the last hour of the fasted (h 3–4) and fed (h 7–8) studies. Indirect calorimetry was performed for 30 min, once during the fasted study (h 2.5–3) and once during the fed study (h 6.5–7).
Laboratory analyses
The blood samples were drawn into prechilled tubes containing Na2-EDTA. They were centrifuged immediately at 4 C, and the plasma was removed and stored at -70 C for later analyses. Plasma leucine was isolated by ion exchange (Dowex 200X) chromatography and was converted to its heptafluorobutyramide derivative. The tracer/tracee ratio was measured by negative chemical ionization gas chromatography-mass spectrometric analysis by selectively monitoring ions at m/z ratios 349–352 as previously described (10). The plasma 
-ketoisocaproic acid (KICA) tracer/tracee ratio was measured by negative chemical ionization gas chromatography-mass spectrometry of its pentafluorobenzyl derivative (10). The breath samples were analyzed in duplicate for 13C abundance in carbon dioxide by gas isotope ratio mass spectrometry (Europa Scientific, Crewe, UK), monitoring ions at m/z ratios of 44 and 45.
FFM and FM were measured using dual emission x-ray absorptiometry in all subjects at baseline, and measurements were repeated in the GHD subjects after 6 months of GH treatment. Based on daily measurements over a 12-month period, the dual emission x-ray absorptiometry instrument at our institution has a precision of less than 2% for bone mineral density for a whole body phantom. For duplicate measurements in 10 subjects examined over a 5-d period, the precision was 3% for the soft tissue composition. Serum IGF-I assays were performed by chemiluminescent immunoassay at Arup Laboratories (Salt Lake City, UT).
Calculations
Carbon dioxide flux (Ra CO2) was calculated from the steady state equation:
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At isotopic steady state, the fraction of oral tracer that enters the systemic circulation (sampled compartment) is given by the plasma tracer/tracee ratio of the intragastric tracer to the plasma tracer/tracee ratio of the iv tracer normalized for their infusion rates. This fraction was calculated as:
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-KICA is the plateau enrichment of -KICA derived from the l-[13C1]leucine tracer.
Leucine oxidation (LEUox) was calculated as follows: LEUox (µmol/kg·h) = Ra 13CO2/Ep 13C--KICA, where Ra 13CO2 is the rate of production of labeled carbon dioxide (obtained from the product of Ra CO2 and the plateau isotopic enrichment of expired CO2 during the l-[13C1]leucine infusion).
Leucine used for protein synthesis (LEUsyn) was calculated as leucine flux minus leucine oxidation: LEUsyn (µmol/kg·h) = Q-LEUox.
Leucine derived from protein breakdown (LEUbrk) was calculated as the difference between leucine flux and all sources of leucine intake: LEUbrk (µmol/kg·h) = Q - (diet Leu + IG Leu + IV Leu).
Leucine balance (Leubal) was calculated as the difference between leucine intake and leucine oxidation: Leubal (µmol/kg·h) = (diet Leu + IG Leu + IV Leu) - LEUox.
The efficiency of utilization of leucine (LeuEU) was calculated as: LeuEU (%) = (Leu balance/diet Leu + IG Leu + IV Leu) x 100.
All kinetic data are expressed per kilogram of FFM.
Statistical analysis
A sample size of eight was selected based on Dr. Jahoor’s previous data on leucine kinetics (11): leucine flux, 167 ± 10 µmol/kg·h (coefficient of variation, 6%); leucine oxidation, 37 ± 4 µmol/kg·h (coefficient of variation, 11%). Based on these data, a sample size of eight should detect a SD difference of 1.5 with a power of 0.80, assuming a correlation of at least 0.5 between repeated measures.
The nonpaired t test was employed for statistical comparison of baseline data between GHD patients and controls, and repeated measures one-way ANOVA was used to compare the data obtained at the three time points in the GHD patients: baseline, 2 wk, and 6 months. If the ANOVA was significant, pairwise comparisons were made with the Bonferroni post hoc test. Results are presented as the mean ± SEM, with significance at P < 0.05.
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Results |
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). GHD patients were matched with control subjects for age, sex, and BMI. Both groups had normal liver and kidney function, and fasting glucose and hemoglobin A1c values (Table 2). Subjects 1 and 4 had normal menstrual periods and therefore did not require sex steroid replacement. Subjects 2 and 3 had mildly elevated PRL levels; subject 2 had a small residual prolactinoma, and subject 3 had a small nonfunctioning pituitary tumor; both tumors had been stable in terms of size and hormone production for more than 3 yr. The mean GH dosage was 0.41 ± 0.15 mg/d. The mean IGF-I concentration before starting GH was 71 ± 16 ng/ml (9.2 ± 2 nmol/ml), and it increased to 222 ± 46 ng/ml (28.9 ± 6 nmol/ml) after 6 months of GH therapy (P < 0.01; Fig. 1). Six of the eight patients tolerated the GH treatment protocol without any side-effects. One patient developed mild edema and arthralgia requiring temporary dosage reduction. Another patient developed fasting hyperglycemia; hence, her GH dose was reduced, and she was maintained on 2.5 µg/kg·d throughout the remainder of the study, without recurrence of hyperglycemia. Serum cortisol and free T4 concentrations measured at 1000 h at each study visit were within normal limits in all GHD patients. There was no significant change in mean fasting plasma glucose [83 ± 0.7 vs. 88 ± 4 mg/dl (4.6 ± 0.04 vs. 4.8 ± 0.2 mmol/liter)], insulin [7.6 ± 2.5 vs. 8.3 ± 1.9 µU/ml (54 ± 18 vs. 59 ± 14 pmol/liter)], or hemoglobin A1c (5.5 ± 0.1% vs. 5.6 ± 0.3%) values after 6 months of GH therapy.
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There were no significant differences in the rates of leucine oxidation and leucine balance between GHD patients and controls in the fasted and fed states (Table 3). After 2 wk of GH therapy, while GH dosage and plasma IGF-I levels were still subphysiological, there were significant decreases in the rates of leucine oxidation in GHD patients in both the fasted and fed states (P < 0.01 and P < 0.05, respectively; Table 3). As a result of the reduction in leucine oxidation, leucine balance, representing the leucine used for protein synthesis, improved in both the fasted and fed states (P < 0.01 and P = 0.07, respectively; Table 3).
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There were no significant differences in the rates of leucine released from protein breakdown or leucine used for protein synthesis between GHD patients and controls in the fasted and fed states (Table 4). After 2 wk of GH therapy, there was an increase in leucine used for protein synthesis in both the fed and fasted states (P < 0.05 for both; Table 4). After 6 months of GH treatment, whereas leucine used for protein synthesis remained higher compared with the pretreatment value (P < 0.05), there was no difference in the fed state. GH therapy had no significant effect on leucine flux from protein breakdown (Table 4). The percentage of enteral leucine taken up by the splanchnic bed was almost identical in controls and GHD subjects, and GH treatment had no significant effect on splanchnic leucine kinetics (Table 5).
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Body composition was measured in controls once and in GHD subjects before and after 6 months of GH therapy. Both FFM and FM were modestly lower in GHD patients than controls, but the difference was not significant (Table 6). The ratio of FFM to body weight (0.59 ± 0.4) in the GHD subjects was almost identical to that in the controls (0.61 ± 0.04). After 6 months of GH replacement, FFM increased in five of the eight GHD subjects, showed no change in one, and decreased in two. Hence, although there was an overall mean increase in FFM of 1 kg, this change was not significantly different from pretreatment values. A closer examination of the data of individual GHD subjects, however, showed that five of the eight GHD subjects had a lower pretreatment FFM/body weight ratio (0.54 ± 0.4) compared with the other three GHD subjects (0.66 ± 0.06) and to the controls (0.61 ± 0.04). For this subset of five subjects, the gain in FFM was 3.5 kg, which was significantly greater than the pretreatment value (P < 0.05), and their FFM/body weight ratio increased significantly to 0.57 ± 5 (P < 0.05).
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Discussion |
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Our observation of a decrease in leucine oxidation and an increase in the rate of utilization of leucine for protein synthesis in GHD subjects after 2 wk of GH therapy suggest that GH replacement improves protein balance by inhibiting amino acid oxidation, thereby making more amino acids available for the synthesis of protein. This finding corroborates the earlier findings of Lucidi et al. (7) in fasted GHD subjects studied at 12 and 36 h after 2 wk of GH therapy on alternating days. They reported a 30% decrease in the leucine oxidation rate and a 13% increase in nonoxidative leucine disposal (that is, leucine used for protein synthesis), results almost identical to our present findings in the fasted state. Our results also corroborate the findings of Russell-Jones et al. (6, 8) in fasted GHD subjects after 2 months of GH therapy. They reported a decrease in leucine oxidation and an increase in nonoxidative leucine disposal (an index of protein synthesis) in a GH-treated group compared with a placebo-treated group of GHD subjects. Together, our present findings and those of previous studies suggest that in the fasted state, GH replacement therapy exerts a protein anabolic effect in GHD patients primarily by inhibiting oxidation of amino acids and stimulating protein synthesis.
Whereas most studies of the protein anabolic effect of GH have been conducted in the fasted state, very few studies have investigated the response in the fed state, especially in GHD patients. The effect of feeding is important because of the possible synergistic effects of meal-induced hyperinsulinemia, known to restrain protein breakdown rate (12), and GH replacement therapy, known to stimulate protein synthesis (6, 7, 8). In one of their studies, Russell-Jones et al. (8) determined the effect of 2 months of GH therapy on leucine kinetics in GHD subjects immediately after a meal. Using nonsteady state formulas to calculate leucine kinetics, they reported increases in both leucine oxidation and nonoxidative disposal rates in GH-treated compared with placebo-treated subjects. However, the increase in the nonoxidative leucine disposal rate was greater than the increase in the oxidation rate, indicating that GH also promotes protein balance in the fed state by stimulating protein synthesis. In the present study we mimicked a steady state by feeding mini-meals every 30 min for 4 h and made the same observations reported by Russell-Jones et al. (8). That is, although meal ingestion resulted in a marked increase (37%) in the leucine oxidation rate compared with the fasted value, leucine balance and the rate of leucine utilization for protein synthesis were increased relative to pretreatment rates. This was due to the fact that GH treatment attenuated leucine oxidation, that is, protein catabolism, despite the known stimulatory effects of a meal on amino acid oxidation (12). In another experiment using an almost identical feeding schedule (hourly feeds) as that in the present study, but with a different tracer method to measure protein kinetics after 1 month of GH therapy, Binnerts et al. (2) reported essentially the same finding, that is, increases in protein balance and synthesis compared with pretreatment values. Together the combined findings of these studies suggest that in the short-term, GH replacement therapy promotes protein anabolism in GHD subjects by inhibiting amino acid catabolism and stimulating protein synthesis in both the fed and fasted states.
After 6 months of treatment, the improvements in leucine balance and protein synthesis observed after 2 wk of GH therapy were maintained in the fasted state, but not in the fed state. This was due to the fact that leucine oxidation in the fed state returned to the pretreatment value, suggesting an inability of GH to restrain the meal-induced increase in amino acid oxidation. The same finding was reported for fed subjects after 6 months of GH therapy in the study by Binnerts et al. (2). Both protein balance and synthesis reached a new steady state while maintaining an increase in FFM. In another study, Beshyah et al. (9) looked at the effect of 6 months of GH therapy on leucine kinetics in GHD subjects in the fasted state. In agreement with our present results in the fasted state, they reported a modest, although not significant, improvement in protein synthesis. However, unlike our present finding in fasted GHD subjects, they did not observe a decrease in the leucine oxidation rate. Another variant finding reported by Beshyah et al. (9) was their observation of lower rates of protein synthesis and breakdown in patients with hypopituitarism compared with healthy controls, whereas our present data show no significant differences in leucine kinetics in GHD patients and controls. Nevertheless, our overall findings suggest that after 6 months of GH therapy, protein metabolism reaches a new steady state to maintain the net gain in FFM.
Our present finding that GH treatment had no effect on the protein breakdown rate corroborates the previous findings of Russell-Jones et al. and Lucidi et al. (6, 7, 8) and suggests that the protein anabolic action of GH may not be mediated exclusively through the action of either insulin or IGF-I, both of which have been shown to promote protein anabolism primarily through their inhibitory effects on proteolysis (13, 14). Furthermore, in the present study the hyperinsulinemia that accompanies feeding did not cause a further improvement in leucine balance after 2 wk of GH therapy. Actually, the GH-induced improvement in leucine balance of 10.5 µmol/kg FFM·h in the fasted state was better than the 7 µmol/kg FFM·h in the fed state. Finally, the maximum protein anabolic effect of GH was elicited after 2 wk of GH therapy, when the plasma IGF-I concentration was still in the subphysiological range.
Previous studies have reported decreased FFM and increased FM in GHD adults compared with controls (1, 2, 3, 4, 5), and that GH therapy results in a gain in FFM (1, 2, 3, 4). Hence, it was rather unexpected that 6 months of GH therapy did not cause a significant gain in FFM in the GHD group as a whole. Actually, the mean ratio of FFM to body weight (0.59 ± 0.4) for the entire GHD group was almost identical to that of the control group (0.61 ± 0.04); hence, it is not surprising that 6 months of GH therapy did not cause any significant overall gain in FFM. The loss of FFM in GHD patients is known to correlate positively with the duration of GHD, and the mean duration of GHD in our subjects was 6.6 yr, with a range of 3–15 yr. Furthermore, our GHD patients had multiple pituitary hormone deficiencies, and despite clinically and biochemically adequate replacement with glucocorticoids, sex hormones, and T4, it is possible that variations in the dosage of or responses to these hormones might have caused variability in FFM among individuals in the GHD group. Hence, some individuals in the GHD group did not have significantly lower FFM than the controls before starting GH replacement therapy. A closer examination of the data of individual GHD subjects revealed that five of the eight GHD subjects had a lower ratio of FFM to body weight (0.54 ± 0.4) compared with the other three GHD subjects (0.66 ± 0.06) and to the controls. The three subjects who did not have low FFM to body weight ratio compared with their matched controls included the two men receiving testosterone replacement therapy, suggesting that the anabolic effect of testosterone might have played a role in maintaining their FFM at a higher level. In the subset of five GHD subjects with low baseline FFM, the mean gain in FFM was 3.5 kg, which was significantly greater than pretreatment values (P < 0.05). Moreover, their posttreatment FFM/body weight ratio increased to 0.57 ± 5, a value very similar to that in the control group. This gain of 3.5 kg FFM after 6 months of GH replacement in this subgroup of five GHD patients is comparable to the 4.0 kg reported by Binnerts et al. (2) after 6 months of treatment. The attainment of a normal FFM in all GHD subjects after 6 months of GH treatment may explain, in a teleological fashion, why GH lost its protein anabolic effect, as it was physiologically unnecessary to promote protein anabolism any further. The lack of further FFM accretion despite continued GH administration suggests the presence of a negative feedback mechanism from FFM regulating the anabolic effects of GH.
We are not aware of any other study of the effect of GH treatment on splanchnic leucine metabolism. The percentage of enteral leucine extracted by the splanchnic bed (
30%) was remarkably similar in the two groups of subjects and to previously reported values for adult humans (14). Although there was a trend toward a decrease in both the absolute and the percent uptake of leucine during GH treatment, these changes were not significant.
In summary, we have demonstrated that short-term GH replacement therapy increases the efficiency of amino acid utilization, decreases protein catabolism, and enhances net protein synthesis in GHD patients in both the fasted and fed states. After 6 months of GH replacement, FFM normalizes and protein kinetics reach a new homeostasis. Finally, the anabolic actions of GH do not seem to be mediated exclusively through the actions of either insulin or IGF-I.
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Acknowledgments |
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Footnotes |
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J.S. and R.V.S. contributed equally to this work.
Abbreviations: BMI, Body mass index; FFM, fat-free mass; FM, fat mass; GHD, GH deficient; KICA, -ketoisocaproic acid.
Received December 11, 2002.
Accepted September 10, 2003.
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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