Short- and Long-Term Effects of Growth Hormone (GH) Replacement on Protein Metabolism in GH-Deficient Adults

0021-972X/03/$15.00/0 Printed in U.S.A. The Journal of Clinical Endocrinology & Metabolism 88(12):5827–5833 Copyright © 2003 by The Endocrine Society doi: 10.1210/jc.2002-021943 Short- and Long-Term Effects of Growth Hormone (GH) Replacement on Protein Metabolism in GHDeficient Adults JIANJIAN SHI, RAJAGOPAL V. SEKHAR, ASHOK BALASUBRAMANYAM, KENNETH ELLIS, PETER J. REEDS, FAROOK JAHOOR, AND MORALI D. SHARMA 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 Reduced fat-free mass (FFM) in GH-deficient (GHD) adults is improved by GH replacement, but the protein metabolic changes are unclear. Using iv [2H3]leucine and oral l-[13C1]leucine infusions and dual emission x-ray absorptiometry, we compared leucine kinetics and body composition in eight GHD adults and eight healthy controls in the fasted and fed states, before and after 2 wk and 6 months of GH replacement. Leucine kinetics were not different between pretreatment GHD subjects and controls. After 2 wk of GH treatment, leucine oxidation decreased in the GHD subjects compared with baseline values [fasted, 41 ⴞ 6 vs. 30 ⴞ 5 ␮mol/kg FFM䡠h (P < 0.01); fed, 49 ⴞ 3 vs. 41 ⴞ 3.6 ␮mol/kg FFM䡠h (P < 0.05)], A DULTS WITH ACQUIRED GH deficiency (GHD) generally have reduced fat-free mass (FFM) and increased fat mass (FM) compared with body mass index (BMI)-matched healthy adults (1–5). Although it has been shown that GH replacement therapy results in an increase in FFM in GHD patients (1–3), the mechanism of this protein anabolic effect has not been clearly established. For example, whereas some studies have reported that GH replacement promotes protein anabolism by restraining catabolism and stimulating synthesis in both the fasted (6 – 8) and fed states (8), others have shown only a transient increase (2) or no increase (9) in protein synthesis. 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 Abbreviations: BMI, Body mass index; FFM, fat-free mass; FM, fat mass; GHD, GH deficient; KICA, ␣-ketoisocaproic acid. leucine balance improved [fasted, ⴚ14 ⴞ 4 vs. ⴚ3.5 ⴞ 3 ␮mol/kg FFM䡠h (P < 0.01); fed, 65 ⴞ 10 vs. 72 ⴞ 7 ␮mol/kg FFM䡠h (P ⴝ 0.07)], and protein synthesis increased [fasted, 116 ⴞ 5 vs. 131 ⴞ 6 ␮mol/kg FFM䡠h (P < 0.05); fed, 103 ⴞ 6 vs. 116 ⴞ 6 ␮mol/kg FFM䡠h (P < 0.05)]. After 6 months of GH treatment, these changes were not maintained in the fed state. The five GHD subjects with decreased FFM at baseline showed a significant increase after 6 months of GH treatment (P < 0.05). GH replacement in GHD acutely improves protein balance by stimulating synthesis and inhibiting catabolism. After 6 months, protein kinetics reached a new homeostasis to maintain the net gain in FFM. (J Clin Endocrinol Metab 88: 5827–5833, 2003) 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 5827 5828 J Clin Endocrinol Metab, December 2003, 88(12):5827–5833 ([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. Subjects and Methods The study was approved by the institutional review board of Baylor College of Medicine. Written informed consent was obtained from all subjects before their entry into the study. Eight adults (six women and two men) with panhypopituitarism, taking stable, physiological, replacement doses of glucocorticoid, thyroid hormone, and sex steroids, were recruited from the Neuroendocrine Clinic at Ben Taub General Hospital (Houston, TX). All patients had had panhypopituitarism for 3 yr or more. GH deficiency was defined as a peak plasma GH concentration below 2.5 ng/ml during an insulin tolerance test or an iv arginine (10% arginine hydrochloride) stimulation test. Of eight subjects with GHD, three were diagnosed by the insulin hypoglycemia test and five by the arginine stimulation test. The peak GH concentration in response to both stimulation tests was 1.1 ng/ml. The starting dose of GH was 2.5 ␮g/kg䡠d (0.0075 IU/kg䡠d) in women and 2 ␮g/kg䡠d (0.006 IU/kg䡠d) in men, administered as a single sc injection in the evening betwee (...truncated)