Hormonal and metabolic responses to repeated cycling sprints under different hypoxic conditions

Highlights

• We examine hormonal and metabolic responses to exercise in different hypoxic levels.

• Severe hypoxia evokes a greater growth hormone (GH) response to sprint exercise.

• Greater increase in GH by severe hypoxic exercise accelerates post-exercise lipolysis.

• Exercise-induced GH release is related to post-exercise increase in circulating free fatty acid.

Abstract

Objective

Sprint exercise and hypoxic stimulus during exercise are potent factors affecting hormonal and metabolic responses. However, the effects of different hypoxic levels on hormonal and metabolic responses during sprint exercise are not known. Here, we examined the effect of different hypoxic conditions on hormonal and metabolic responses during sprint exercise.

Design

Seven male subjects participated in three experimental trials: 1) sprint exercise under normoxia (NSE); 2) sprint exercise under moderate normobaric hypoxia (16.4% oxygen) (HSE 16.4); and 3) sprint exercise under severe normobaric hypoxia (13.6% oxygen) (HSE 13.6). The sprint exercise consisted of four 30 s all-out cycling bouts with 4-min rest between bouts. Glucose, free fatty acids (FFA), blood lactate, growth hormone (GH), epinephrine (E), norepinephrine (NE), and insulin concentrations in the HSE trials were measured before exposure to hypoxia (pre 1), 15 min after exposure to hypoxia (pre 2), and at 0, 15, 30, 60, 120, and 180 min after the exercise performed in hypoxia. The blood samples in the NSE trial were obtained in normoxia at the same time points as the HSE trials.

Results

Circulating levels of glucose, FFA, lactate, GH, E, NE, and insulin significantly increased after all three exercise trials (P < 0.05). The area under the curve (AUC) for GH was significantly higher in the HSE 13.6 trial than in the NSE and HSE 16.4 trials (P < 0.05). A maximal increase in FFA concentration was observed at 180 min after exercise and was not different between trials.

Conclusion

These findings suggest that severe hypoxia may be an important factor for the enhancement of GH response to all-out sprint exercise.

Introduction

Exercise is a potent stimulus for growth hormone (GH) secretion. The GH response to acute exercise is related to exercise intensity with greater GH secretion observed at higher exercise intensities [1]. Although the precise physiological role of high intensity exercise-stimulated GH secretion has not been fully elucidated, it has been associated with a post-exercise rise in lipolysis [2]. Octreotide infusion blocks exercise-induced GH secretion and inhibits post-exercise adipose tissue lipolysis [3]. These results suggest that high intensity exercise-induced GH release enhances adipose tissue lipolysis during the post-exercise recovery period.

Exercise performed in acute severe systemic hypoxia also increases GH release [4], [5], [6]. We have recently reported that moderate- [4] and low- [5] intensity resistance exercise performed during acute severe hypoxia (13% oxygen) causes a larger increase in GH concentrations than when exercise is performed in normoxia. Additionally, submaximal endurance (cycle ergometer) exercise (750 kpm/min) performed under severe hypoxic conditions (4550 m above sea level; approx. 12% oxygen) increases levels of circulating GH and free fatty acids (FFA) to a greater extent than when exercise is performed under normoxic conditions [6]. In contrast, low-intensity (30% 1RM) resistance exercise performed under moderate hypoxia (15% oxygen) or normoxia caused similar levels of GH secretion [7]. Similarly, 30 min of submaximal endurance exercise (50% of peak oxygen uptake) performed under moderate hypoxia (2000 m above sea level; approx. 16.5% oxygen) or normoxia elicited similar GH and FFA responses [8]. These results suggest that the severity of hypoxia affects the exercise-induced GH secretion and adipose tissue lipolysis, and subsequent FFA mobilization.

All-out sprint cycling exercise is a powerful stimulus for GH release. Previous studies report a marked increase in circulating GH levels following sprint exercise [9], [10], [11], [12]. However, as far as we know, there are no data concerning the effect of different hypoxic levels on circulating GH and FFA responses during and post-sprint exercise. Improved repeated sprint ability performance in athletes occurs following sprint exercise training under hypoxia [13]. Indeed athletes, in particular international-level athletes, utilize hypoxic training rooms to gain a greater improvement in their physical performance. A reduction in body fat can further enhance athlete physical performance. Therefore, a better understanding of the relationship between hypoxia and circulating GH and FFA responses during and post-sprint exercise may assist in improving athlete performance. Here, we examined the effects of sprint exercise performed under both moderate and severe hypoxic conditions on hormonal and metabolic responses. We hypothesized that following sprint exercise performed in severe hypoxic conditions GH and circulating FFA levels would be elevated to a greater extent when compared to the sprint exercise performed in normoxic and moderate hypoxic conditions.