Exercise induces interleukin-8 expression in human skeletal muscle

Abstract

Skeletal muscle has been recognized as an endocrine organ, and muscle cell cultures express several cytokines with potential hormonal effects. Interleukin-8 (IL-8), a chemokine, which induces angiogenesis, is expressed in working muscles; however, the cell source of origin has not been identified. We aimed to elucidate if IL-8 protein is: (1) expressed in contracting muscle fibres and (2) whether there is a release of IL-8 from exercising muscle. Seventeen healthy male volunteers were included in two independent protocols: 3 h of ergometer bicycle exercise at 60% of VO2,max (n = 6) or rest (n = 5), and 3 h of two-legged knee-extensor exercise at 60% of maximal workload (n = 6). Repetitive muscle biopsy samples were obtained from the vastus lateralis in all experiments. A marked increase in IL-8 mRNA was found in muscle biopsy samples obtained after exercise. A marked IL-8 protein expression was demonstrated within the cytoplasm of muscle fibres in biopsy samples obtained in the recovery phase following 3 h of bicycle exercise, and the peak occurred 3-6 h postexercise. A small transient net release of IL-8 from working muscle was found at 1.5 h of knee-extensor exercise. However, the small release of IL-8 from muscle did not result in an increase in the systemic plasma concentration of IL-8, suggesting that muscle-derived IL-8 may play a local role, e.g. in angiogenesis.

Figure 1. Muscle IL-8 mRNA in rest and exercise groups

The exercise group: pre (0 h), immediately after 3 h of bicycle exercise (3 h), and 1.5 (4.5 h), 3 (6 h), 6 (9 h) and 21 h (24 h) postexercise. The rest group: pre (0 h), after 3, 4.5, 6 and 9 h of rest, and 24 h after the presample. Values are means ± s.e.m. (n = 6 subjects). *Significant (P < 0.05) difference from pre sample (0 h), and #significant (P < 0.05) difference from rest group.

Figure 2. IL-8 immunohistochemical expression in skeletal muscle tissue before and after 3 h of bicycle exercise

A, before the exercise began, IL-8 expression was almost absent in the muscle tissue. B, By 3 h, when the exercise had just ended, the muscle tissue showed a comparable IL-8 expression relative to the level seen before exercise. C, by 4.5 h after exercise, the IL-8 expression was significantly increased in the skeletal muscle tissue, which showed a high level of IL-8 both in general in the cytoplasm and related to the cell membranes, as well as in vessels in the muscle. D, ATPase-stained section. This is the neighbouring section to that seen in C. By comparing C and D, it is seen that both muscle fibre types express IL-8 after exercise. E and F, IL-8 expression was still very high, and peaked at 6 (E) and 9 h (F) following the bicycle exercise. G, after 24 h, the levels of IL-8 protein had decreased again, and the staining appeared homogeneous and mildly increased in the fibres. H and I, higher magnification of skeletal muscle tissue at 9 h following exercise. As shown, IL-8 protein is mainly expressed in the cytoplasm, and it is also expressed in the membranes, including the nuclei. We also detected intermittently vascular IL-8 expression in the vessel endothelium of the muscle tissue. Scale bars: A–G, 50 μm; H, 29 μm; I, 14 μm.

Figure 3. IL-8 venous plasma concentrations in rest and exercise groups

The exercise group: pre (0 h), immediately after 3 h of bicycle exercise (3 h), and 1.5 (4.5 h), 3 (6 h), 6 (9 h) and 21 h (24 h) postexercise. The rest group: pre (0 h), after 3, 4.5, 6, and 9 h of rest, and 24 h after the presample. Values are means ± s.e.m. (n = 6 subjects).

Figure 4. Muscle IL-8 mRNA before (0), during (0.5 and 1.5 h) and after 3 h of two-legged knee-extensor exercise, and 2 h after the end of exercise (5 h)

Values are means ± s.e.m. (n = 6). *Significant (P < 0.05) difference from before (0 h).

Figure 5

A, IL-8 arterial plasma concentration before (0 h), during (0.5 and 1.5 h) and after 3 h of two-legged knee-extensor exercise, and 2 h (5 h) after the end of exercise. Values are means ± s.e.m. (n = 6). B, leg plasma femoral venous–arterial (fv–a) IL-8 difference before (0 h), during (0.5 and 1.5 h) and after 3 h of two-legged knee-extensor exercise, and 2 h (5 h) post exercise. Values are means ± s.e.m. (n = 6). *Significant difference (P < 0.05) from before (0 h). C, leg blood flow before (0 h), during (0.5 and 1.5 h) and after 3 h of two-legged knee-extensor exercise, and 2 h (5 h) after the end of exercise. Values are means ± s.e.m. (n = 6). *Significant difference (P < 0.05) from before (0 h). D, leg net IL-8 balance (ng min⁻¹) values before (0 h), during (0.5 and 1.5 h) and after 3 h of two-legged knee-extensor exercise, and 2 h postexercise (5 h). Values are means ± s.e.m. (n = 6). *Significant difference (P < 0.05) from before (0 h).