The Association of Growth Hormone Parameters with Skeletal Muscle Phosphocreatine Recovery in Adult Men

Peak stimulated GH and GH area-under-the-curve, on standard GH releasing hormone-arginine stimulation testing, and IGF-I are associated with skeletal muscle phosphocreatine recovery in adult men.

Abstract

Context:

Previous studies have suggested a relationship between GH and mitochondrial function. However, little is known about the relationship of specific GH indices and in vivo measures of mitochondrial function in humans.

Objective:

The objective of this study was to determine the association between GH, IGF-I, and phosphocreatine (PCr) recovery, a measure of mitochondrial function, in otherwise healthy adults.

Design:

Thirty-seven healthy men and women were studied at a single university medical center. Subjects underwent GH stimulation testing with GH releasing hormone-arginine and measurement of IGF-I. Mitochondrial function was determined by PCr recovery after submaximal exercise by ³¹Phosphorous magnetic resonance spectroscopy. Subjects underwent assessment of lean and fat mass with use of dual energy X-ray absorptiometry.

Results:

There were no differences in PCr recovery between men and women (men 20.7±1.5 vs. women 24.8±1.4 mm/min; P > 0.05). IGF-I (r = 0.33; P = 0.04) was associated with PCr recovery in all subjects. Among men, IGF-I (r = 0.69; P = 0.003), peak stimulated GH (r = 0.52; P = 0.04), and GH area under the curve (AUC) (r = 0.53; P = 0.04) were significantly associated with PCr recovery. However, neither IGF-I, peak stimulated GH, nor GH AUC (all P > 0.05) were associated with PCr recovery in women. After adjusting for age, race, and physical activity, IGF-I remained significantly associated with PCr recovery (β = 0.10; P = 0.02) among men.

Conclusions:

IGF-I, peak stimulated GH, and GH AUC are associated with skeletal muscle PCr recovery in men.

GH is a pituitary-derived anabolic hormone. Its actions are mediated by GH itself and by IGF-I derived from the liver. In vitro studies have demonstrated GH, or a synthetic activating GH receptor antibody, directly stimulates fatty acid oxidation in mitochondria of human fibroblasts while an antagonistic GH receptor antibody attenuates the effect of GH in a dose-dependent manner (1). Similarly, in vitro studies have demonstrated IGF-I stimulates mitochondrial activity in a prostate cancer cell line (2) and protects mitochondrial oxidative function in cardiac myocytes from hypoxia-reperfusion (3). Acromegaly, a condition of pathologic GH excess, results in increased serum IGF-I levels and is associated with morphologic changes in skeletal muscle mitochondria which resolve after surgical treatment (4). In addition, 14 h of continuous GH infusion in healthy volunteers increases IGF-I and mitochondrial oxidative capacity (5). These previous studies suggest a positive effect of either GH or IGF-I on mitochondrial function in humans. However, contrary to these findings, a microarray study in skeletal muscles of hypopituitary subjects treated with GH for 2 weeks demonstrated a surprising reduction of mitochondrial genes involved in β-oxidation and the respiratory chain (6). Therefore, the true relationship between GH and mitochondrial function in humans is not yet clear.

This study is the first, to our knowledge, to determine the relationship of critical parameters of the GH axis to in vivo assessment of mitochondrial function in otherwise healthy humans. We performed standard GHRH-arginine stimulation testing to determine peak stimulated GH and determined skeletal muscle mitochondrial function using ³¹Phosphorous magnetic resonance spectroscopy (MRS) to assess phosphocreatine (PCr) recovery rate after submaximal exercise in adult subjects. We related basal GH, peak stimulated GH, and GH area under the curve (AUC), as well as IGF-I to PCr recovery in vivo in men and women.

Materials and Methods

Study subjects

Thirty-seven adult participants, ages 18 to 55 yr, were recruited from the Boston metropolitan area. All participants were otherwise healthy without known endocrine dysfunction. Participants had no known diagnosis of diabetes mellitus or medical conditions known to affect weight or glucose metabolism. Subjects on estrogen, hormone replacement therapy, oral contraceptives, testosterone, glucocorticoids, anabolic steroids, GHRH, GH, or IGF-I within 3 months of enrollment were excluded from the study. The current study was approved by the Partners, Massachusetts General Hospital (MGH). Written informed consent was obtained from all participants. These subjects were part of a larger prior study relating mitochondrial function to aging, which did not include any data on GH (7).

³¹P-MRS protocol

Mitochondrial function was determined using ³¹P-MRS to assess PCr re-synthesis after submaximal exercise as previously reported (7). Briefly, subjects were placed in a 60-cm bore, Siemens 3.0T Tim Trio System with an operating frequency of 49.879 MHZ for ³¹Phosphorous-spectroscopy. Baseline resting spectra were obtained after which subjects were instructed to perform submaximal exercise for 3 min at a constant load [40% maximal voluntary contraction (MVC)] by performing bilateral quadriceps contractions at 0.5 Hz followed by a 5-min recovery period. An 8-cm diameter radiofrequency surface coil, tuned for ³¹Phosphorous, was fastened proximal to the superior aspect of the patella over the anteromedial aspect of the right thigh, keeping the coil's axis perpendicular to the z axis of the magnet. A hard pulse of 200 μsec was used for excitation of the ³¹Phosphorous signal. The amplitude of the RF pulse was optimized to obtain a maximal ³¹Phosphorous signal in single shot acquisitions. Spectral analysis was performed using integrated AUC. Mitochondrial function was determined by fitting the time course of PCr AUC during recovery with an exponential curve [β3 + β2 (1 − e ^−(t/τ))] to generate τPCr, which describes mitochondrial phosphorylation potential. Mitochondrial function was also described by the initial rate of PCr recovery (Vi_PCr). Vi_PCr represented as [(60/τPCr) × PCr depletion], is another kinetic parameter used to describe PCr recovery, and is also associated with mitochondrial oxidative capacity. Vi_PCr, is thought to be insensitive to end-of-exercise metabolic conditions, including end-of-exercise pH, and is favored by some investigators (8). Both τPCr and Vi_PCr were studied as end points.

Biochemical assessment

All hormone testing, including GHRH-arginine testing and measurement of free testosterone, was performed in the morning after an overnight fast. GHRH-arginine testing was performed with iv administration of sermorelin acetate (GHRH 1–29) (Geref, Serono Laboratories, Inc., Rockland, MA) at a dose of 1 μg/kg, followed by arginine hydrochloride (30 g/300 ml) at a dose of 0.5 g/kg (maximum 30 g). GH levels were assessed at 0, 30, 45, 60, 90, and 120 min after sermorelin administration. Serum GH was measured using the Beckman Access Ultrasensitive human GH assay, a paramagnetic particle, chemiluminescent immunoassay (Beckman Coulter, Chaska, MN). The analytical sensitivity of the assay is 0.002 μg/liter. The intraassay variation ranges from 1.90–2.78%, and the interassay variation ranges from 1.77–2.65%. IGF-I was measured using a commercially available EIA from ALPCO Diagnostics (Salem, NH) with an intraassay coefficient of variation (CV) of 6.6–9.7% and interassay CV of 11.3–13.7%.

Total testosterone from male subjects only was assayed at the Mayo Clinic Medical Laboratories using a high throughput liquid chromatography and tandem mass spectrometry (LC-MS/MS) (API 5000, Applied Biosystems-MDS Sciex, Foster City, CA) with an interassay variation of 2.2–11.6%. Free testosterone was measured by isotope dilution equilibrium dialysis also at the Mayo Clinic Medical Laboratories. Estradiol was assessed by a paramagnetic particle, chemiluminescent immunoassay from Beckman Coulter. The analytical sensitivity of the assay is 20 pg/ml with an estimated CV between 12–21%.

Anthropometric assessment

Height and body weight were obtained after an overnight fast. Total body percent lean and fat mass was determined by dual-energy x-ray absorptiometry using a Hologic, Inc-4500 (Waltham, MA). Measurements of regional lean mass, specifically the right leg lean mass, using dual-energy x-ray absorptiometry were standardized (1995 User's Guide Hologic Inc). The technique has a precision error (1 sd) of 1.5% for lean mass (9).

Physical activity assessment

The physical activity level of the adults was evaluated by an accelerometer with daily step count averaged over a period of 7 d. Physical activity and training levels in our subjects were also assessed by self-reported regular structured exercise during direct interview and expressed as hours of activity per week.

Statistical analysis

Continuous variables were compared using the Student's t test. Nominal variables were compared using the χ² test. Univariate regression analysis was performed among all subjects and among men and women separately using the Pearson correlation coefficient. Multivariate regression analyses using standard least square modeling was performed to assess the contribution of age, race, physical activity, and gonadal hormones to the relationship between GH parameters and PCr recovery. Statistical analysis was performed using JMP Statistical Database Software (SAS Institute, Inc., Cary, NC). Statistical significance was determined as P < 0.05.

Results

Clinical characteristics of study subjects

The clinical characteristics of the subjects are presented in Table 1. Sixteen men and 21 women were enrolled in the study. Although the women were slightly younger (women: 35.4 ± 1.9 vs. men: 42.9 ± 2.6 yr old; P = 0.03), there were no differences in race or body mass index (BMI) between men and women (both P > 0.05). There were no differences in MVC (49.5 ± 4.0 vs. 43.9 ± 3.1 kg), end-of-exercise pH (6.71 ± 0.05 vs. 6.79 ± 0.03), PCr depletion (18.5 ± 0.7 vs. 19.6 ± 1.5 mm), τPCr (56.4 ± 3.5 vs. 49.3 ± 3.9 sec), or Vi_PCr (20.7 ± 1.5 vs. 24.8 ± 1.4 mm per minute) between men and women (all comparisons men and women respectively; all P > 0.05) (Table 1).

Table 1.

Baseline characteristics of study subjects stratified by gender

	All	Men	Women
No.	37	16	21
Age, yrs	38.6 ± 1.7	42.9 ± 2.6	35.4 ± 1.9*
Race, Caucasian (%)	21 (57%)	9 (56%)	12 (57%)
BMI, kg/m2	30.9 ± 1.1	29.9 ± 1.6	31.8 ± 1.6
Obese, BMI ≥30 kg/m2 (%)	26 (70%)	11 (69%)	15 (71%)
HbA1c, %	5.6 ± 0.1	5.7 ± 0.1	5.5 ± 0.1
Fasting glucose, mg/dl	89 ± 4	96 ± 9	84 ± 2
2-hour glucose, mg/dl	111 ± 7	118 ± 11	106 ± 8
Fasting insulin, μU/ml	6.8 ± 1.0	6.1 ± 1.8	7.3 ± 1.1
HOMA	2.3 ± 0.6	2.0 ± 1.0	2.6 ± 0.8
Lean right leg (kg)	9.9 ± 0.3	10.9 ± 0.4	9.1 ± 0.4*
% Total body fat mass	31.1 ± 1.8	23.3 ± 2.2	37.0 ± 1.6*
% Total body lean mass	65.8 ± 1.7	73.5 ± 2.1	60.1 ± 1.5*
Average daily step count by accelerometer, ×1,000 steps per day	11.4 ± 1.3	11.8 ± 2.5	10.9 ± 1.0
Self-reported exercise, hours per week	4.1 ± 0.8	5.5 ± 1.5	3.1 ± 0.8
Basal GH, μg/liter	0.5 ± 0.1	0.4 ± 0.2	0.5 ± 0.2
Peak GH, μg/liter	21.8 ± 3.7	17.5 ± 4.7	25.1 ± 5.5
GH AUC	1498.5 ± 251.3	1217.7 ± 328.5	1699.1 ± 361.4
IGF-I, μg/liter	95 ± 7	80 ± 7	106 ± 11
Total testosterone, ng/dl		539 ± 61	N/A
Free testosterone, ng/dl		13.8 ± 1.5	N/A
Estradiol, pg/ml	62.8 ± 10.0	30.3 ± 2.1	86.1 ± 15.2*
τPCr, sec	52.3 ± 2.7	56.4 ± 3.5	49.3 ± 3.9
ViPCr, mm/min	23.0 ± 1.1	20.7 ± 1.5	24.8 ± 1.4
PCr depletion, mm	19.1 ± 0.9	18.5 ± 0.7	19.6 ± 1.5
End-of-exercise pH	6.76 ± 0.03	6.71 ± 0.05	6.79 ± 0.03
MVC, kg	46.4 ± 2.5	49.5 ± 4.0	43.9 ± 3.1

The data are presented as mean±sem.

^*, P < 0.05 comparing men vs. women.

Of the 37 total subjects, 11 were considered normal weight (BMI < 25 kg/m²) and 26 were obese (BMI ≥ 30 kg/m²). The normal weight subjects had a mean BMI of 22.2 ± 0.4 kg/m² compared with our obese subjects who had a mean BMI of 34.6 ± 0.8 kg/m² (P < 0.0001). There were no differences in age, gender, race, τPCr, Vi_PCr, PCr depletion, pH at end-of-exercise, or MVC, between the normal weight and obese subjects (all P > 0.05).

Among all subjects, neither τPCr nor Vi_PCr were associated with body composition parameters (lean mass, fat mass, BMI), measures of insulin or glucose, or physical activity. Vi_PCr (r = −0.39; P = 0.02) but not τPCr (r = 0.25; P = 0.14) was significantly associated with age.

Relationship of phosphocreatine recovery to the GH axis

Vi_PCr (r = 0.33; P = 0.04), but not τPCr (P = 0.31), was associated with serum IGF-I in all subjects. Neither Vi_PCr nor τPCr were significantly associated with peak stimulated GH, fasting GH, or GH AUC among all subjects (all P > 0.05).

Both Vi_PCr (r = 0.69; P = 0.003) (Fig. 1A) and τPCr (r = −0.54; P = 0.03) were significantly associated with serum IGF-I in men. Vi_PCr was also associated with peak stimulated GH (r = 0.52; P = 0.04) (Fig. 1B) and GH AUC (r = 0.53; P = 0.04) but not basal GH (P = 0.54) in men. τPCr was not significantly associated with peak stimulated GH, GH AUC, or basal GH in men. Neither τPCr nor Vi_PCr were associated with total testosterone, free testosterone, or estradiol among men (all P > 0.05).

Fig. 1.

Univariate analyses demonstrating association of (A) IGF-I and (B) peak stimulated GH with Vi_PCr in men and (C) IGF-I and (D) peak stimulated GH with Vi_PCr in women.

Neither Vi_PCr nor τPCr were significantly associated with serum IGF-I (Fig. 1C), peak stimulated GH (Fig. 1D), GH AUC, or basal GH in women. Neither τPCr nor Vi_PCr were associated with estradiol in women (all P > 0.05).

Effects of physical activity on phosphocreatine recovery

There were no differences between men and women in total daily step counts determined by accelerometer or the amount of time spent in regular structured exercise as determined by direct interview (both P > 0.05). Neither total daily step counts by accelerometer nor reported physical activity levels were associated with PCr recovery parameters of τPCr or Vi_PCr in all subjects. In men only, self-reported physical activity was positively associated with Vi_PCr (r = 0.71; P = 0.002) and trended to a negative association with τPCr (r = −0.44; P = 0.09). Self-reported physical activity or daily step count by accelerometer were not associated with either parameter of PCr recovery in women.

Multivariate regression analyses

Multivariate regression analyses for Vi_PCr were performed in all subjects. Controlling for the effects of self-reported physical activity or daily step count by accelerometer, IGF-I remained significantly associated with Vi_PCr in all subjects (P < 0.05 for both methods of assessing physical activity).

Multivariate regression analyses for Vi_PCr were subsequently performed in men only. In a separate model controlling for the effects of age, race, and physical activity determined either by self-reported activity or step count in men only, IGF-I remained significantly associated with Vi_PCr (for self-reported activity: β=0.10; P = 0.02; overall R² for the model = 0.75; overall P for the model = 0.003). Similarly, peak stimulated GH and GH AUC also remained significantly associated with Vi_PCr after controlling for age, race, and self-reported physical activity in men (P < 0.05 for both peak GH and GH AUC).

Using IGF-I, peak stimulated GH, and GH AUC as independent variables, analysis demonstrates IGF-I is the most significant GH parameter associated with Vi_PCr (β = 0.13; P = 0.05).

Mitochondrial function by GH status

Using a peak stimulated GH of ≤4.2 μg/liter as a cut-off, subjects were stratified into GH sufficiency and relative GH deficiency. The cut-off of 4.2 μg/liter has previously been validated in obese men and women with hypothalamic pituitary disease as having the most discriminative power to diagnose GH deficiency in obesity (10) and has previously been used by our group to demonstrate an increased cardiovascular disease risk among obese subjects with reduced GH levels (11). Normal-weight subjects had higher peak stimulated GH compared with obese subjects as expected (normal weight: 47.5 ± 7.7 vs. obese: 11.0 ± 1.7 μg/liter; P < 0.0001). While none of the normal weight subjects had peak stimulated GH ≤ 4.2 μg/liter, four of the 26 obese subjects had peak stimulated GH ≤ 4.2 μg/liter. Subjects with peak stimulated GH ≤ 4.2 μg/liter had higher τPCr and lower Vi_PCr compared with subjects with peak stimulated GH > 4.2 μg/liter, indicative of delayed PCr recovery, however the results did not reach statistical significance (τPCr: 62.1 ± 7.3 vs. 51.1 ± 2.9 sec; P = 0.22; Vi_PCr: 18.6 ± 1.6 vs. 23.6 ± 1.2 mm/min; P = 0.16) (peak GH ≤4.2 μg/liter vs. peak GH > 4.2 μg/liter, respectively) given the small number of subjects with peak GH ≤ 4.2 μg/liter.

Discussion

This is the first study, to our knowledge, to correlate discrete parameters of the GH axis with an in vivo assessment of mitochondrial function in healthy men and women. For the first time, we demonstrate a significant association between serum IGF-I, peak stimulated GH, and GH AUC and PCr recovery, in adult men. We also demonstrate gender-specific differences in the relationship between GH parameters and PCr recovery.

Our results, demonstrating an association between PCr recovery and parameters of the GH axis, are consistent with the one prior study in otherwise healthy human subjects without hypopituitarism, where continuous infusion of GH increased serum IGF-I, and muscle mitochondrial ATP production rate and citrate synthase activity. In addition, GH treatment also led to an increase in expression of mitochondrial genes COX3, COX4, and mitochondrial transcription factor A in skeletal muscle (5). Although a prior study in hypopituitary subjects treated with GH for two weeks demonstrated a reduction, rather than an increase, in mitochondrial genes involved in β-oxidation and the respiratory chain in skeletal muscles by microarray, only six of the genes were confirmed by quantitative real-time RT-PCR, and a detailed description of the hypopituitary patients and their history (duration of hypopituitarism, loss of other pituitary axes, etc.) were not included, making a clear interpretation difficult. Furthermore, in the prior study, the genetic expression data were also not consistent with the physiological data which demonstrated increased whole body resting energy expenditure and fat oxidation, which would suggest increased mitochondrial oxidative function (6).

The presence of both GH receptors (12) and IGF-I receptors (13) have been demonstrated in skeletal muscle. Furthermore, GH has been shown to stimulate peroxisome proliferator-activated receptor γ coactivator 1-α (PGC-1α), a nuclear encoded mitochondrial gene involved in mitochondrial biogenesis, in rat skeletal muscle to increase mitochondrial function (14), and in vitro can directly stimulate mitochondrial function in fibroblasts (1). Although IGF-I has yet to be demonstrated to be directly involved in mitochondrial function in skeletal muscle, IGF-I has been associated with improved mitochondrial function in other cell types. For example, treatment of DU145 prostate cancer cells with IGF-I in vitro results in increase in mitochondrial activity as measured by high-resolution respirometry, potentially through activation of phosphoinositide 3-kinase (PI3K)/AKT signaling pathway (2). Isolated hepatic mitochondria from Wistar rats with carbon tetracholoride-induced cirrhosis treated in vivo with IGF-I for four weeks demonstrate significant improvements in mitochondrial membrane potential, free radical production, and ATP synthase activity (15). Furthermore, treatment of cardiomyocytes isolated from adult male Sprague–Dawley rats, with IGF-I in vitro, also prevents the collapse of the mitochondrial membrane potential and maintains mitochondrial oxidative activity and citrate synthase activity in the face of hypoxia-reperfusion (3). The association of IGF-I, peak stimulated GH, and GH AUC with PCr recovery in men is consistent with these data in animal/experimental models.

We demonstrate, for the first time, a gender-specific relationship between parameters of the GH axis and mitochondrial function, with a significant association between IGF-I, peak stimulated GH, and GH AUC and PCr recovery evident in men but not in women. Previous studies between men and women, including our own, have not revealed any significant differences in PCr recovery by gender (16), but such studies did not evaluate the relationship of specific parameters of the GH axis to PCr recovery within genders. Testosterone or estradiol does not appear to play a role in in vivo mitochondrial function per se based on our data. Women have higher fat mass and lower fat free mass compared with men (17, 18). However, the relationship between IGF-I and PCr recovery that we show is unlikely to be secondary to differences in fat mass or muscle mass given the lack of association between these parameters and PCr recovery in men and women. Women are known to have higher 24-h pulsatile GH secretion compared with men (19) but serum IGF-I levels generally do not differ significantly between men and women (20), as seen in the current study. Further studies are needed to assess gender differences and possible effects of gonadal steroids in the relationship of PCr recovery and mitochondrial function to specific indices of the GH axis.

In a prior study, we assessed the relationship between age and PCr recovery in a larger cohort (n = 121) ranging in age from 8 to 55 yr and demonstrated a statistically significant relationship with τPCr (7). In this current study, we now assess the relationship of GH parameters to mitochondrial function for the first time, using available data from a group of 37 subjects with a more limited age range. In the current study, there was a trend toward an association between τPCr and age, similar to our prior observation, but this did not reach statistical significance. Differences in sample size and age ranges between the studies may explain relative differences in the strength of the statistical association. Another measure of PCr recovery, Vi_PCr, which is thought to be unrelated to exercise frequency and insensitive to end-of-exercise metabolic conditions such as PCr depletion and pH at end-of-exercise (8), was significantly associated with age in our current study, consistent with our prior observation. With regards to the association between GH and IGF-I and measures of PCr recovery in the current study, we controlled for the effects of age in a multivariate model and continued to demonstrate a significant association between IGF-I and PCr recovery, suggesting this relationship is most likely not mediated by age per se.

Although previous studies using ³¹P-MRS have relied on the saturation transfer method to assess mitochondrial function, we have favored the use of the dynamic PCr recovery method after submaximal exercise to determine mitochondrial function. The saturation transfer method assesses mitochondrial ATP synthesis at rest, which is predominantly a demand driven process. In the absence of a perturbation to the system, lower basal ATP synthesis may therefore only reflect lower ATP demand. However, alterations in PCr recovery in the face of increased demand would suggest true mitochondrial dysfunction. PCr recovery after submaximal exercise to increase demand occurs oxidatively (21). The rate of PCr recovery after submaximal exercise therefore reflects mitochondrial oxidative function (22) and prolonged PCr recovery is indicative of decreased mitochondrial oxidative function. In previous studies, we have demonstrated a strong association between our method of assessing PCr recovery and mitochondrial DNA content in skeletal muscle in healthy subjects after treatment with the mitochondrial toxin stavudine (23). Phielix et al. (24) also found the PCr recovery rate of the vastus lateralis after submaximal exercise was significantly correlated with state three respiration of skeletal muscle tissue ex vivo in a study including subjects with diabetes, first degree relatives of diabetics, and normal controls. An animal study in rats demonstrated the PCr recovery method to be a more sensitive indicator of mitochondrial dysfunction compared with the saturation transfer method after treatment with the mitochondrial complex I inhibitor diphenyleneiodonium (DPI). DPI treatment for two weeks produced a significant decrease in ADP-stimulated mitochondrial oxygen consumption rate in isolated muscles ex vivo and a 46% decrease in in vivo mitochondrial function as determined by PCr recovery after electrical stimulation of the muscle. ATP synthesis rates determined by saturation transfer, however, was not different between control and DPI-treated rats (25). We therefore believe the use of PCr recovery after submaximal exercise is a sensitive indicator of mitochondrial dysfunction.

We chose to study the PCr recovery after submaximal exercise in the vastus lateralis muscle as it appears to provide the most robust change in metabolically challenged subjects. In a recent study comparing the deltoid vs. the vastus lateralis muscle of diabetic vs. control subjects, ex vivo analyses of mitochondrial function by O₂ flux via state III respiration demonstrated dysfunction in diabetic subjects only in the vastus lateralis but not the deltoid muscle. This difference between muscle groups was present despite matching for age, gender, and BMI and demonstrating similarities in fiber types between the muscle groups assayed (26). This suggests measurement of the muscles in the lower extremity, specifically the vastus lateralis, as opposed to upper extremity, is more likely to discern differences among metabolically challenged subjects. While other investigators have also measured other muscle groups in the lower extremity including the tibialis anterior muscles, the vastus lateralis muscle appears to produce the most robust response. In a study comparing the vastus lateralis and tibialis anterior muscle in healthy trained vs. untrained individuals, Larsen et al. (16) demonstrated the vastus lateralis muscle had a higher rate of phosphocreatine recovery after maximal voluntary isometric contraction (26% higher in untrained and 22% higher in trained subjects). Similarly, the maximum rate of oxidative phosphorylation of the vastus lateralis muscle was higher compared with the tibialis anterior muscle regardless of training status (16). Given these intrinsic differences in the oxidative capacity and mitochondrial function of differing muscle groups, it appears most reasonable to measure PCr recovery from the vastus lateralis, to optimize our sensitivity to detect differences in metabolically challenged subjects.

This was a cross-sectional study, and causality cannot be determined. While we hypothesize that low GH activity may contribute to some degree of relative mitochondrial dysfunction, especially in men, the converse may be true. Further interventional studies administering IGF-I vs. GH and determining their effects on mitochondrial function are necessary. In addition, further studies in both men and women to identify the source of this gender dimorphic effect will be necessary.

In summary, this is the first study to demonstrate an association between PCr recovery after submaximal exercise on ³¹P-MRS, and IGF-I, peak stimulated GH, and GH AUC in otherwise healthy adult men. In addition, we also demonstrate a lack of an association between GH parameters and PCr recovery in women suggesting a gender-specific effect in this relationship. Further studies will be needed to confirm and expand on this novel finding.