Circulating levels of pegvisomant and endogenous growth hormone during prolonged pegvisomant therapy in patients with acromegaly.

OBJECTIVE: To investigate whether pegvisomant treatment in acromegaly induces gradual elevations in endogenous serum growth hormone (GH) levels and whether serum pegvisomant levels predict the therapeutic outcome. PATIENTS AND METHODS: Seventeen patients (6 women), mean age 46·3 years (range: 23·2-76·2), were studied. For each patient, four hospital visits were identified including 'active disease' (no treatment) and last follow-up. At each visit, 12 blood samples were drawn during 3 h including an oral glucose tolerance test (OGTT). Eight patients received a somatostatin analogue in addition to pegvisomant on the last visit. RESULTS: Median (range) pegvisomant doses (mg/day) were 10 (10-10), 15 (10-15) and 15 (10-15) at visits 2, 3 and 4, respectively, and the mean duration of pegvisomant treatment was 17·5 ± 3·2 (SEM) months. Serum IGF-I changed significantly during the treatment period with the highest level at baseline and lowest levels at visits 3 and 4. GH levels increased in a dose-dependent manner during pegvisomant treatment and decreased at visit 4. Changes in IGF-I levels correlated negatively with changes in serum pegvisomant levels between visits. Serum pegvisomant at each visit correlated with baseline growth hormone levels, whereas no associations between serum pegvisomant and either dose, gender, age or body weight were found. CONCLUSIONS: (1) Serum GH levels increased initially, but remained stable during prolonged pegvisomant treatment in patients with acromegaly, (2) serum pegvisomant levels predicted the reduction in serum IGF-I during treatment and (3) the interindividual variation in serum pegvisomant levels seems not predicted by either age, gender or body composition.

Sustained low-dose growth hormone therapy optimizes bioactive insulin-like growth factor-I level and may enhance CD4 T-cell number in HIV infection.

High-dose recombinant human growth hormone (rhGH) (2-6 mg/day) regimes may facilitate T-cell restoration in patients infected with human immunodeficiency virus (HIV) on highly active antiretroviral therapy (HAART). However, high-dose rhGH regimens increase insulin-like growth factor-I (IGF-I) to supra-physiological levels associated with severe side effects. The present study investigated whether lower doses of rhGH may improve T-cell restoration in patients infected with HIV following an expedient response of total and bioactive (i.e., free) IGF-I. A previous 16-week pilot-study included six HIV-infected patients on stable HAART to receive rhGH 0.7 mg/day, which increased total (+117%, P < 0.01) and free (+155%, P < 0.01) IGF-I levels. The study was extended to examine whether continuous use of low-dose rhGH (0.7 mg/day until week 60; 0.4 mg/day from week 60 to week 140) would maintain expedient IGF-I levels and improve CD4 T-cell response. Total and free IGF-I increased at week 36 (+97%, P < 0.01 and +125%, P < 0.01, respectively) and week 60 (+77%, P = 0.01 and +125%, P < 0.01) compared to baseline levels (161 +/- 15 and 0.75 +/- 0.11 microg/L). CD4 T-cell number increased at week 36 (+15%, P < 0.05) and week 60 (+31%, P = 0.01) compared to baseline levels (456 +/- 55 cells/microL). Following rhGH dose reduction, total IGF-I and CD4 T-cell number remained increased at week 88 (+44%, P = 0.01 and +33%, P < 0.01) and week 140 (+46%, P = 0.07 and +36%, P = 0.02) compared to baseline levels. These data support the notion that low-dose rhGH regimens may increase expediently total and bioactive IGF-I and improve T-cell restoration in patients infected with HIV on HAART.

The impact of pegvisomant treatment on substrate metabolism and insulin sensitivity in patients with acromegaly.

CONTEXT: Pegvisomant is a specific GH receptor antagonist that is able to normalize serum IGF-I concentrations in most patients with acromegaly. The impact of pegvisomant on insulin sensitivity and substrate metabolism is less well described. PATIENTS AND METHODS: We assessed basal and insulin-stimulated (euglycemic clamp) substrate metabolism in seven patients with active acromegaly before and after 4-wk pegvisomant treatment (15 mg/d) in an open design. RESULTS: After pegvisomant, IGF-I decreased, whereas GH increased (IGF-I, 621 +/- 82 vs. 247 +/- 33 microg/liter, P = 0.02; GH, 5.3 +/- 1.5 vs. 10.8 +/- 3.3 microg/liter, P = 0.02). Basal serum insulin and plasma glucose levels decreased after treatment (insulin, 54 +/- 5.9 vs. 42 +/- 5.3 pmol/liter, P = 0.001; glucose, 5.7 +/- 0.1 vs. 5.3 +/- 0.0 mmol/liter, not significant), whereas palmitate kinetics were unaltered. During the clamp, the glucose infusion rate increased after pegvisomant (3.1 +/- 0.5 vs. 4.4 +/- 0.6 mg/kg.min, P = 0.02), whereas the suppression of endogenous glucose production tended to increase (0.7 +/- 0.0 vs. 0.5 +/- 0.1 mg/kg.min, not significant). Total resting energy expenditure decreased after pegvisomant treatment (1703 +/- 109 vs. 1563 +/- 101 kcal/24 h, P = 0.03), but the rate of lipid oxidation did not change significantly. CONCLUSIONS: 1) Pegvisomant treatment for 4 wk improves peripheral and hepatic insulin sensitivity in acromegaly. 2) This is associated with a decrease in resting energy expenditure, whereas free fatty acid metabolism is unaltered. 3) The data support the important direct effects of GH on glucose metabolism and add additional benefits to pegvisomant treatment for acromegaly.

Continuous glucose monitoring in interstitial subcutaneous adipose tissue and skeletal muscle reflects excursions in cerebral cortex.

Continuous glucose monitoring (CGM) is being explored using several types of glucose sensors. Some are designed for subcutaneous adipose tissue. It is important to determine to which extent these glucose fluctuations in different tissues reflect changes taking place in the central nervous system, where glucose sensing is thought to occur. We studied the ability of subcutaneous adipose interstitial fluid measurements to parallel glucose propagations in blood, muscle, and central nervous system (CNS) during hyper- and hypoglycemia. A subcutaneous CGM system was applied in the CNS, subcutaneous adipose tissue, and skeletal muscle of nine Vietnamese potbellied pigs, and data were compared with frequent sampling in blood. Alterations in glucose levels were induced with intravenous glucose and insulin. During hyperglycemia, no difference was detected in delay between blood and interstitial glucose levels in subcutaneous adipose tissue (18.0 +/- 0.8 min), muscle (18.0 +/- 0.9 min), and CNS (20.3 +/- 1.2 min), respectively. During hypoglycemia, we found no time difference between interstitial parameters in the three tissues. However, the amplitude of glucose changes varied considerably, with a smaller magnitude of glucose change taking place in the brain. The timing of glucose excursions in subcutaneous adipose tissue and muscle reflect excursions in CNS. The reduced magnitude of glucose excursions in the brain suggests that different mechanisms of glucose transport are operative in CNS compared with subcutaneous adipose tissue and muscle.

Kinetics and secretion of placental growth hormone around parturition.

OBJECTIVE: During pregnancy, placental growth hormone (PGH) is secreted into the maternal circulation, replacing pituitary GH. It is controversial whether PGH levels decline during vaginal birth. After placental expulsion, PGH is eliminated from the maternal blood. GH binding protein (GHBP) and body mass index (BMI) influence GH kinetics, but their impact on PGH kinetics is unknown. The present study was undertaken to define the kinetics of PGH during vaginal delivery and Caesarian section and to relate these kinetics to GHBP and BMI. DESIGN: A short term, prospective cohort study. METHODS: Twelve women had repeated blood samples drawn during vaginal delivery. From 26 women undergoing planned Caesarian delivery (CS) repeated blood samples were withdrawn before, during and after the CS, allowing PGH half-life determination. RESULTS: During vaginal delivery, median PGH values did not change before expulsion of the placenta, although individual fluctuations were seen. Clearance of PGH from the maternal circulation was best described by a two-compartment model. The initial half-life of serum PGH was (mean +/- s.d.) 5.8 +/- 2.4 min, and the late half-life was (median) 87.0 min (range: 25.1-679.6 min). The late half-life was correlated to the pre-gestational BMI (r = 0.39, P = 0.047), but not to the serum GHBP concentration. CONCLUSIONS: Serum PGH did not decrease significantly during vaginal delivery. Elimination of PGH fitted a two-compartment model, with an estimated initial half-life of 5.8 min. The late phase serum half-life of PGH was related to BMI, suggesting a role for maternal fat mass in PGH metabolism.

Testosterone and estradiol regulate free insulin-like growth factor I (IGF-I), IGF binding protein 1 (IGFBP-1), and dimeric IGF-I/IGFBP-1 concentrations.

The present study tests the clinical postulate that elevated testosterone (Te) and estradiol (E2) concentrations modulate the effects of constant iv infusion of saline vs. recombinant human IGF-I on free IGF-I, IGF binding protein (IGFBP)-1, and dimeric IGF-I/IGFBP-1 concentrations in healthy aging adults. To this end, comparisons were made after administration of placebo (Pl) vs. Te in eight older men (aged 61 +/- 4 yr) and after Pl vs. E2 in eight postmenopausal women (62 +/- 3 yr). In the saline session, E2 lowered and Te increased total IGF-I; E2 specifically elevated IGFBP-1 by 1.5-fold and suppressed free IGF-I by 34%; and E2 increased binary IGF-I/IGFBP-1 by 5-fold more than Te. During IGF-I infusion, the following were found: 1) total and free IGF-I rose 1.4- to 2.0-fold (Pl) and 2.1-2.5-fold (Te) more rapidly in men than women; 2) binary IGF-I/IGFBP-1 increased 3.4-fold more rapidly in men (Te) than women (E2); and 3) end-infusion free IGF-I was 1.6-fold higher in men than women. In summary, E2, compared with Te supplementation, lowers concentrations of total and ultrafiltratably free IGF-I and elevates those of IGFBP-1 and binary IGF-I/IGFBP-1, thus putatively limiting IGF-I bioavailability. If free IGF-I mediates certain biological actions, then exogenous Te and E2 may modulate the tissue effects of total IGF-I concentrations unequally.

Free rather than total circulating insulin-like growth factor-I determines the feedback on growth hormone release in normal subjects.

Pituitary GH secretion is feedback regulated by circulating IGF-I. However, it remains to be determined whether the feedback control is mediated through circulating free or total IGF-I. To study this, we compared the temporal changes in circulating levels of GH vs. free and total IGF-I during fasting. Seventeen healthy normal-weight subjects (body mass index 23.4 +/- 0.6 kg/m(2)) were studied during 80 h of fasting. Serum was assayed for GH every 3 h; total, free, and bioactive IGF-I, IGF binding protein (IGFBP)-1, -2, and -3 as well as IGFBP-1 bound IGF-I were assayed every morning. During fasting, mean 24-h GH levels increased from 1.41 +/- 0.20 to 3.01 +/- 0.46 and 2.09 +/- 0.30 microg/liter (d 1 vs. d 2 and 3; P < 0.03). After 24 h of fasting, free and bioactive IGF-I had decreased by 40 +/- 5 and 17 +/- 5%, respectively (P < 0.02), and both concentrations remained suppressed for the rest of the study. In contrast, total IGF-I remained unchanged until the end of d 3, at which levels were slightly reduced (P < 0.007). IGFBP-1 increased from 38 +/- 2 to 137 +/- 24, 212 +/- 32, and 214 +/- 22 microg/liter (d 1 vs. d 2, d 3, and end of d 3; P < 0.0001), and these changes closely paralleled those of IGFBP-1-bound IGF-I (P < 0.0001). IGFBP-2 increased only transiently at d 2 (P < 0.05), and IGFBP-3 remained unchanged. The increase in mean 24-h GH levels from d 1 to d 2 correlated inversely with the relative reduction in free IGF-I from d 1 to d 2 (r = -0.51; P = 0.04), i.e. the larger the reduction in free IGF-I, the larger the increase in GH. None of the other IGF-related parameters correlated with GH. In conclusion, the temporal relationship between the increase in GH and the reduction in free IGF-I supports the hypothesis that circulating free IGF-I mediates the feedback regulation of GH secretion.

Cotreatment of acromegaly with a somatostatin analog and a growth hormone receptor antagonist.

CONTEXT: Pegvisomant is a GH receptor antagonist that blocks the peripheral actions of GH in acromegaly. Pegvisomant, in contrast to somatostatin (SMS) analogs, does not suppress the activity of the GH-producing adenoma. OBJECTIVE: We assessed the effects of cotreatment with pegvisomant and SMS in acromegaly on GH secretion, IGF-I levels, and glucose tolerance. DESIGN, PATIENTS, AND INTERVENTIONS: Eleven patients with persistent disease despite previous therapy underwent the following fixed treatment algorithm: 1) on SMS therapy, 2) off therapy for 2 months, 3) 6-wk treatment with 10 mg/d pegvisomant, 4) 6-wk treatment with 15 mg/d pegvisomant, and 5) 3-month treatment with 15 mg pegvisomant plus SMS. Blood was sampled in the fasting state and during an oral glucose tolerance test. RESULTS: Total serum IGF-I levels (micrograms per liter) decreased after pegvisomant, but the lowest levels were obtained with cotreatment [458 +/- 67 (SMS), 562 +/- 78 (active), 376 +/- 51 (10 mg), 269 (15 mg), 195 +/- 24 (combined) (P < 0.0001)]. Free and bioactive IGF-I changed in a similar pattern. Steady-state pegvisomant levels (micrograms per liter) were obtained, but SMS cotreatment increased pegvisomant levels by 20% (P = 0.02) [2631 +/- 616 (10 mg), 6536 +/- 1413 (15 mg), 8030 +/- 1914 (combined)]. Pegvisomant increased endogenous GH levels (micrograms per liter), which was countered by SMS cotreatment [5.1 +/- 1.3 (SMS), 8.9 +/- 2.9 (active), 14.6 +/- 4.9 (10 mg), 19.7 +/- 6.5 (15 mg), 11.8 +/- 2.8 (combined) (P < 0.01)]. Plasma glucose levels (millimoles per liter) were highest during SMS and lowest during pegvisomant 15 mg [2-h oral glucose tolerance test: 10.3 +/- 0.7 (SMS), 8.9 +/- 0.7 (active), 7.2 +/- 0.7 (10 mg), 6.5 +/- 0.5 (15 mg), 8.0 +/- 0.8 (combined) (P = 0.02)]. CONCLUSIONS: Dual blockade of the GH axis with pegvisomant and a SMS analog is feasible in acromegaly.

Fasting unmasks a strong inverse association between ghrelin and cortisol in serum: studies in obese and normal-weight subjects.

Administration of ghrelin, the endogenous ligand for the GH secretagogue receptor, stimulates not only GH secretion but also appetite and food intake in humans. Endogenous ghrelin levels display a distinct circadian rhythm, which is reciprocal to that of insulin and presumed to be meal dependent and not associated with GH secretion. We tested the hypothesis that food deprivation could impact circadian serum ghrelin levels and unmask meal-independent regulatory mechanisms. Thirty-three young adults, subdivided according to gender and level of obesity, were studied with blood sampling every 3 h from 12-84 h of fasting. Serum ghrelin levels showed a marked diurnal rhythm with a nadir in the morning (0800 h), peak levels in the afternoon, and a gradual decline during the night. This pattern was preserved during the entire fasting period and was independent of gender and obesity. Mean 24-h ghrelin levels exhibited a small but significant decline during the fast (P < 0.001). As expected, GH secretion increased with fasting in lean subjects, and a gradual decline in insulin concentrations was observed in all subjects. Neither GH nor insulin showed any significant relationship to ghrelin. In contrast, serum cortisol exhibited a strong inverse temporal association with ghrelin (r = -0.79; P < 0.0001). In conclusion, our study yields no evidence that ghrelin stimulates GH release during fasting. As a novel finding, ghrelin appears to be related to cortisol. However, further studies are needed to elucidate the physiological mechanisms behind this relationship.

Low IGF-I and elevated testosterone during puberty in subjects with type 1 diabetes developing microalbuminuria in comparison to normoalbuminuric control subjects: the Oxford Regional Prospective Study.

OBJECTIVE: To describe longitudinal variations in pubertal hormonal variables in subjects with and without microalbuminuria (MA). RESEARCH DESIGN AND METHODS: Blood samples collected annually from subjects recruited at diagnosis of type 1 diabetes and followed prospectively through puberty (median follow-up 9.3 years, range 4.7-12.8) were analyzed for total and free IGF-I, IGF binding protein-1, testosterone, sex hormone-binding globulin, and HbA(1c). A total of 55 subjects who developed MA (MA(+) group) were compared with 55 age-, sex-, and duration-matched control subjects who did not develop MA (MA(-) group). RESULTS: For female subjects, total IGF-I (MA(+) 1.2 mU/l vs. MA(-) 1.4 mU/l, P = 0.03) and free IGF-I levels (MA(+) 1,767 ng/l vs. MA(-) 2010 ng/l, P = 0.002) were lower, whereas the free androgen index (MA(+) 2.4 vs. MA(-) 2.0, P = 0.03) was higher in those with MA. These changes were less pronounced in male subjects. For both sexes, in a Cox model after adjusting for puberty, the presence of MA was associated with lower free IGF-I levels, higher testosterone standard deviation score (SDS), and poor glycemic control. We found that 22 of 55 case subjects (40%) developed persistent MA, whereas 33 (60%) had transient MA. In the persistent MA group compared with the transient and control groups, total IGF-I levels were lower (1.1 vs. 1.3 vs. 1.4 mU/l, P = 0.002) as were free IGF-I levels (1,370.9 vs. 1,907.3 vs. 1,886.7 ng/l, P < 0.001), whereas HbA(1c) levels were higher (11.8 vs. 10.3 vs. 9.9%, P < 0.001). CONCLUSIONS: Poor glycemic control and differences in IGF-I levels and androgens, particularly in female subjects, accompany development of MA at puberty. These differences may in part account for the sexual dimorphism in MA risk during puberty and could relate to disease progression.

Splanchnic release of ghrelin in humans.

Ghrelin is a novel polypeptide identified in rat stomach, and it increases GH release, gastric motility, and appetite and modulates many other organ functions. It has been reported that ghrelin may be released from a variety of other tissues, but the absolute contribution of the splanchnic bed remains to be defined. We quantified the splanchnic output of ghrelin in 22 healthy people after an overnight fast, with indwelling catheters in the femoral artery and hepatic vein. Splanchnic ghrelin output was calculated by multiplying veno-arterial difference in ghrelin concentration with splanchnic plasma flow (measured by indicator dye dilution technique). Plasma ghrelin was measured using (125)I-labeled ghrelin and rabbit polyclonal antibody raised against octanoylated human ghrelin. Ghrelin concentrations in the artery and in the hepatic veins were 960 +/- 82 pg/ml and 1102 +/- 90 pg/ml (P < 0.001), respectively. The veno-arterial concentration difference was 143 +/- 38 pg/ml, amounting to 15% of the arterial concentration. The splanchnic output of ghrelin was 141 +/- 43 ng/min (P < 0.003). Assuming that the half-life of ghrelin is less than 60 min, the splanchnic output would explain the entire amount of circulating ghrelin in the postabsorptive state. We conclude that a substantial amount of ghrelin is being released from the splanchnic bed in the postabsorptive state in healthy human subjects and that splanchnic bed is the major source of circulating ghrelin in humans.

Development and clinical evaluation of a novel immunoassay for the binary complex of IGF-I and IGF-binding protein-1 in human serum.

Correlation studies have suggested that IGF-binding protein (IGFBP)-1 is a dynamic regulator of free IGF-I. To further study this, we developed a monoclonal immunofluorometric assay specific for the binary complex of IGF-I and IGFBP-1 in human serum. An IGFBP-1 antibody, which recognizes all phospho-forms of IGFBP-1, was used for coating. An europium-labeled IGF-I antibody served as tracer. Assay incubation was performed at conditions approaching those in vivo (i.e. pH 7.4, 37 C). The assay was highly specific: no signal was obtained unless both IGF-I and IGFBP-1 were present and neither IGFBP-2, -3, -4, nor IGF-II caused any cross-reaction. The linear standard curve covered 3 orders of magnitude, and within and in-between assay coefficients of variation were less than 5 and 15%, respectively. To study the dynamic relationship between free IGF-I and binary complex formation, seven healthy subjects were fasted for 72 h. Samples were collected every 3 h. During fasting, free IGF-I was reduced by two thirds (P < 0.0001). IGFBP-1 and the binary complex increased in parallel (P < 0.0001), and levels correlated positively in all subjects (0.89 < or = r < or = 0.98; P < 0.0001). Free IGF-I correlated inversely with IGFBP-1 (-0.81 < or = r < or = -0.48; 0.0001 < or = P < or = 0.05) and the binary complex (-0.79 < or = r < or = -0.41; 0.0001 < or = P < or = 0.05). To study overnight fasting levels, we compared healthy controls and patients with type 1 diabetes and chronic renal failure (n = 10), because these patients show profound alterations in their IGF-system. In both groups, the binary complex was increased about 2.5-fold (P < 0.0001), whereas IGFBP-1 was increased by 5- to 6-fold (P < 0.0001). Accordingly, free IGF-I was severely reduced (P < 0.0001). In conclusion, the assay enables us to study the role of IGFBP-1 as a dynamic regulator of free IGF-I. Our results clearly show that IGFBP-1 and free IGF-I are tightly associated peptides. Furthermore, it has now become possible to compare levels of IGF-I carried within the binary complex IGFBP-1:IGF-I in different (patho-) physiological conditions.

Residual beta-cell function more than glycemic control determines abnormalities of the insulin-like growth factor system in type 1 diabetes.

The GH-IGF-I axis is disturbed in patients with type 1 diabetes. Our aim was to investigate whether abnormalities are found in patients in very good glycemic control and, if so, to estimate the role of residual beta-cell function. Patients with hemoglobin A1c (HbA1c) less than 6% (reference range, 3.6-5.4%) were selected for the study. Twenty-two men and 24 women, aged 41.3 +/- 13.8 yr (mean +/- SD), with a diabetes duration of 17.8 +/- 14.6 yr participated. Healthy controls (15 women and nine men), aged 41.3 +/- 13.0 yr, were also studied. Overnight fasting serum samples were analyzed for HbA1c, C peptide, free and total IGFs, IGF-binding proteins (IGFBPs), GH-binding protein, and IGFBP-3 proteolysis. HbA1c was 5.6 +/- 0.5% in patients and 4.4 +/- 0.3% in controls. Total IGF-I was 148 +/- 7 microg/liter in patients and 178 +/- 9 microg/liter in controls (P < 0.001). Free IGF-I, total IGF-II, IGFBP-3, and GH-binding protein were lower, whereas IGFBP-1, IGFBP-1-bound IGF-I, and IGFBP-2 were elevated compared with control values. Patients with detectable C peptide (> or =100 pmol/liter) had higher levels of total IGF-I, free IGF-I, and total IGF-II and lower levels of IGFBP-1 and IGFBP-2 than those with an undetectable C peptide level despite having identical average HbA1c. IGFBP-3 proteolysis did not differ between patients and controls. Despite very good glycemic control, patients with type 1 diabetes and no endogenous insulin production have low free and total IGF-I. Residual beta-cell function, therefore, seems more important for the disturbances in the IGF system than good metabolic control per se, suggesting that portal insulin delivery is needed to normalize the IGF system.

Estradiol supplementation modulates growth hormone (GH) secretory-burst waveform and recombinant human insulin-like growth factor-I-enforced suppression of endogenously driven GH release in postmenopausal women.

The present study tests the mechanistic postulate that estrogen confers resistance to negative feedback by systemic IGF-I. To this end, eight postmenopausal women received a constant iv infusion of recombinant human (rh)IGF-I (10 micro g/kg.h x 6 h) and saline in randomized order on the 10th day of supplementation with oral estradiol (E(2)) and placebo (Pl). GH secretion was quantitated by 10-min blood sampling, immunochemiluminometry assay, and deconvolution analysis. Administration of E(2) compared with Pl followed by saline infusion: 1) stimulated pulsatile GH secretion ( micro g/liter.6 h), viz., 12 +/- 3.3 (Pl) and 18 +/- 4.6 (E(2)) (mean +/- SEM, paired comparison, P < 0.05); 2) halved the time latency (min) to achieve peak GH secretion after GHRH injection, 24 +/- 2.2 (Pl) and 12 +/- 2.1 (E(2)) (P < 0.01); and 3) did not alter the mass of GH secreted ( micro g/liter) in response to a maximally effective dose of GHRH, 30 +/- 7.2 (Pl) and 37 +/- 11 (E(2)). Exposure to E(2) compared with Pl followed by rhIGF-I infusion: 1) accelerated the rate of decline of GH concentrations by 3.3-fold, viz., absolute slope ( micro g/liter.1000 min), 3.8 (range, 2.5-5.0) (Pl) and 12 (range, 10-14) (E(2)) (P < 0.001); 2) augmented the algebraic decrement in GH concentrations ( micro g/liter) enforced by rhIGF-I infusion, 0.73 +/- 0.21 (Pl) and 1.6 +/- 0.25 (E(2)) (P < 0.01); 3) halved the time delay (min) to peak GHRH-induced GH secretion, 20 +/- 1.2 (Pl) vs. 10 +/- 1.3 (E(2)) min (P < 0.01). In contradistinction, E(2) did not alter: 1) the capability of rhIGF-I to suppress GHRH-stimulated GH secretory burst mass significantly, viz., by 50 +/- 8% (Pl) and 52 +/- 14% (E(2)) (P < 0.05 each vs. saline); 2) the hourly rate of rise of infused (total) IGF-I concentrations; and 3) total and ultrafiltratably free IGF-I concentrations ( micro g/liter) attained at the end of the two rhIGF-I infusions. In summary, compared with Pl, E(2) supplementation in postmenopausal women: 1) amplifies endogenously driven GH secretory-burst mass; 2) initiates rapid onset of GHRH-stimulated GH release; and 3) potentiates IGF-I-dependent suppression of unstimulated GH concentrations. Based upon companion modeling data, we postulate that E(2) facilitates the upstroke and IGF-I enforces the downstroke of high-amplitude GH secretory bursts in estrogen-replete individuals.

The effect of growth hormone on the insulin-like growth factor system during fasting.

The present study investigates the possible stimulatory effect of endogenous GH on IGF and IGF-binding protein (IGFBP) levels during fasting. Eight normal subjects were examined on four occasions: 1) in the basal postabsorptive state; 2) after 40 h of fasting; 3) after 40 h of fasting with somatostatin suppression of GH; and 4) after 40 h of fasting with suppression of GH and exogenous GH replacement. The two somatostatin experiments were identical in terms of hormone replacement (except for GH). Short-term fasting led to a 50% reduction in free IGF-I. The reduction in free IGF-I was paralleled by an increase in IGFBP-1, an increase in the complex formation of IGFBP-1 and IGF-I, and a modest reduction in IGFBP-3 proteolysis. GH deprivation during fasting led to a 35% reduction in total IGF-I and a 70% reduction in free IGF-I. GH replacement increased free and total IGF-I to levels similar to those observed during plain fasting and decreased IGFBP-1, however, without affecting IGFBP-1-bound IGF-I. Finally, IGFBP-3 proteolysis was slightly increased by GH replacement. In conclusion, the major new finding of the present study is that the GH hypersecretion seen during short-term fasting is not merely secondary to a reduction in IGF bioactivity.

Dose dependency of the pharmacokinetics and acute lipolytic actions of growth hormone.

The sensitivity to GH is subject to substantial interindividual variations, which has been attributed to differences in age, sex, and body composition. We investigated 18 healthy nonobese men (aged 24-56 yr) on four occasions. The pharmacokinetics and acute lipolytic effects of GH were evaluated using iv bolus injections of either placebo or GH (1, 3, and 6 micro g/kg(-1)). Body composition was determined by computed tomography and bioimpedance measurements, and the lipolytic response was assessed through measurements of circulating lipid intermediates and adipose tissue microdialysis. The metabolic clearance rate was dose dependently reduced with increasing GH doses (57.2 +/- 5.1, 45.2 +/- 3.8, and 39.2 +/- 2.4 ml/min(-1) per meter(-2) following injection of 1, 3, and 6 micro g/kg(-1) GH, respectively, P < 0.001), and half-time was increased (14.2 +/- 0.6, 16.2 +/- 0.4, and 18.0 +/- 0.5 min, respectively, P < 0.0001). The pharmacokinetic variables were not correlated to age or body composition at any GH dose, but GH-binding protein was the major predictor of metabolic clearance rate following the two highest GH doses as indicated by multivariate regression analysis (r(2) = 0.55, P < 0.001 and r(2) = 0.35, P = 0.012, respectively). There was a significant dose-response relationship between injected GH and the subsequent increments in lipid intermediates, but the integrated lipolytic response was not correlated to GH pharmacokinetics, age, or body composition at any GH dose. Taken together, our findings suggest that differences in GH-binding protein concentrations, which possibly reflect GHR expression, determine GH pharmacokinetics rather than age or body composition per se.

Acute growth hormone administration causes exaggerated increases in plasma lactate and glycerol during moderate to high intensity bicycling in trained young men.

We studied the acute effects of a single, sc GH dose on exercise performance and metabolism during bicycling. Seven highly trained men [age, 26 +/- 1 yr (mean +/- SEM); weight, 77 +/- 3 kg; maximal oxygen uptake, 65 +/- 1 ml O(2).min(-1).kg(-1)] performed 90 min of bicycling 4 h after receiving 7.5 IU (2.5 mg) GH or placebo in a randomized, double-blinded, cross-over design trial. A standardized pre-exercise meal was given 2 h before exercise. Blood was sampled at rest and during exercise and analyzed for GH, IGF-I, glucose, lactate, insulin, glycerol, and nonesterified fatty acids (NEFA). In the placebo trial, all subjects completed the exercise protocol without any difficulties. In contrast, two subjects were not able to complete the exercise protocol in the GH trial, and one subject barely managed to complete the protocol. In addition, GH administration resulted in exaggerated increases in plasma lactate concentrations during exercise (P < 0.0001). The combined lipolytic effect of GH and exercise, evidenced by increased plasma glycerol and serum NEFA concentrations, was 3-fold greater than the effect of exercise alone (P < 0.0001), but this increased substrate availability did not result in increased whole body fat oxidation (indirect calorimetry). Plasma glucose was, on average, 9% higher during exercise after GH administration compared with placebo (P < 0.0001). We conclude that a single, relevant GH dose causes exaggerated increases in plasma lactate and glycerol as well as serum NEFA during 90 min of subsequent bicycling at moderate to high intensity. The exaggerated increase in plasma lactate may be associated with substantially decreased exercise performance.

Gene expression of the GH receptor in subcutaneous and intraabdominal fat in healthy females: relationship to GH-binding protein.

OBJECTIVE: Circulating GH-binding protein (GHBP) is produced by proteolytical cleavage of the extracellular part of the GH receptor (GHR) and is positively correlated to the amount of body fat. To test the hypothesis that adipose tissue may contribute to the production of circulating GHBP, we compared gene expression of two GHR isoforms in adipose tissue with serum GHBP concentrations in healthy females. DESIGN: Twenty-two healthy females undergoing surgery for benign gynecological conditions were included in the study. METHODS: During surgery, s.c. and intraabdominal fat biopsy samples were taken. Gene expression of the full-length GHR and a truncated GHR (GHRtr) was assessed by RT-PCR relative to the expression of beta-actin. RESULTS: The full-length GHR was expressed to a much higher level than GHRtr in both tissues. The levels of both GHR and GHRtr mRNA were similar in intraabdominal and s.c. adipose tissues. Surprisingly, concentrations of circulating GHBP were negatively correlated to the levels of mRNA transcripts of both the full-length GHR and GHRtr in intraabdominal fat. Whole body resistance (as a measure of lean body mass) was positively correlated to mRNA levels for both GHRs in intraabdominal fat. CONCLUSIONS: (i) The full-length GHR is expressed to a much higher level than GHRtr in s.c. as well as visceral abdominal fat; (ii) the observation of a significant correlation between GHR expression and GHBP levels further emphasizes the link between adipose tissue and GHBP; (iii) it remains, however, to be demonstrated whether circulating GHBP is produced to a significant degree by adipose tissue.

Insulin-like growth factors, insulin-like growth factor-binding proteins, insulin-like growth factor-binding protein-3 protease, and growth hormone-binding protein in lipodystrophic human immunodeficiency virus-infected patients.

Human immunodeficiency virus (HIV)-lipodystrophy is associated with impaired growth hormone (GH) secretion. It remains to be elucidated whether insulin-like growth factors (IGFs), IGF-binding proteins (IGFBPs), IGFBP-3 protease, and GH-binding protein (GHBP) are abnormal in HIV-lipodystrophy. These parameters were measured in overnight fasting serum samples from 16 Caucasian males with HIV-lipodystrophy (LIPO) and 15 Caucasian HIV-infected males without lipodystrophy (NONLIPO) matched for age, weight, duration of HIV infection, and antiretroviral therapy. In LIPO, abdominal fat mass and insulin concentration were increased (>90%, P < .01) and insulin sensitivity (Log10ISI(composite)) was decreased (-50%, P < .001). Total and free IGF-I, IGF-II, IGFBP-3, and IGFBP-3 protease were similar between groups (all P > .5), whereas, in LIPO, IGFBP-1 and IGFBP-2 were reduced (-36%, P < .05 and -50%, P < .01). In pooled groups, total IGF-I, free IGF-I, total IGF-II, and IGFBP-3, respectively, correlated inversely with age (all P < .01). In pooled groups, IGFBP-1 and IGFBP-2 correlated positively with insulin sensitivity (age-adjusted all P < .05). IGFBP-3 protease correlated with free IGF-I in pooled groups (r(p) = 0.47, P < .02), and in LIPO (r(p) = 0.71, P < .007) controlling for age, total IGF-I, and IGFBP-3. GHBP was increased, whereas GH was decreased in LIPO (all P < .05). GH correlated inversely with GHBP in pooled groups (P < .05). Taken together the similar IGFs and IGFBP-3 concentrations between study groups, including suppressed GH, and increased GHBP in LIPO, argue against GH resistance of GH-sensitive tissues in LIPO compared with NONLIPO; however, this notion awaits examination in dose-response studies. Furthermore, our data suggest that IGFBP-3 protease is a significant regulator of bioactive IGF-I in HIV-lipodystrophy.

Physical capacity influences the response of insulin-like growth factor and its binding proteins to training.

The influence of initial training status on the response of circulating insulin-like growth factor (IGF) and its binding proteins (IGFBP) to prolonged physical training was studied in young men. It was hypothesized that highly standardized training would result in more extensive changes in the circulating IGF system in untrained subjects because of lower fitness level. Seven untrained (UT) and 12 well-trained (WT) individuals performed 11 wk of intense physical training (2-4 h daily). Fasting serum samples were analyzed for total and free IGF-I and -II, for IGFBP-1 to -4, as well as for IGFBP-3 proteolysis. Eleven weeks of physical training resulted in decreased levels of total IGF-I, free IGF-I, and IGFBP-4 in both the UT and WT groups. In the UT group, IGFBP-2 increased, IGFBP-3 decreased [from 4,255 +/- 410 (baseline) to 3,896 +/- 465 (SD) microg/l (week 4); P < 0.05], and IGFBP-3 proteolysis increased [from 28 +/- 8% (baseline) to 37 +/- 7% (week 4) and 39 +/- 12% (week 11); P < 0.05], whereas no significant changes were found in the WT group. In conclusion, intense physical training results in a marked influence on the IGF system and its binding proteins with generally more extensive changes seen in the untrained individuals. Also, prolonged physical training resulted in increased IGFBP-3 proteolysis in previously untrained individuals only, indicating that intense physical training affects trained and untrained individuals differently.