Furthermore, sVCAM-1 plasma concentrations were significantly more elevated in UT-SCT than that in UT-CON (P < 0.05; Fig. 1), whereas no significant difference was observed between T-SCT and T-CON subjects (P = 0.89). Plasma sICAM-1, sVCAM-1, and sE-selectin increased significantly in all groups at the end of the strenuous exercise as compared with resting levels (+8.2%, +12.0%, and +12.2%, respectively, P < 0.05) and returned to their baseline value 1 h after the end of exercise (T 1h). When plasma sE-selectin concentrations were expressed relative to the resting values, the return to the baseline was confirmed at T 1h in T subjects (Fig. 2), whereas it occurred only 2 h after the end of exercise (i.e., at T 2h) in UT subjects (Fig. 2).
Although incremental exercise did not statistically change sP-selectin concentrations in the T groups, a significant increase in these concentrations was measured in the UT groups between T rest and T ex (+167%, P < 0.005). These concentrations returned to basal values 1 h after the end of exercise (Table 3, Fig. 3).
The present study examined whether the training status of CON or SCT subjects affected the plasma concentrations of inflammatory and soluble adhesion molecules at rest and in response to an incremental maximal exercise. The main findings of our study were that 1) sVCAM-1 plasma concentrations were significantly lower in the T-SCT subgroups compared with the UT-SCT subgroups at rest, immediately (T ex), and 1 h (T 1h), 2 h (T 2h), and 24 h (T 24h) after the end of exercise; 2) sP-selectin significantly increased at the end of the exercise in UT, whereas no changes were observed among T group subjects; and 3) the postexercise decrease kinetics of plasma sE-selectin was faster in T than that in UT.
As shown in previous studies (27,28,40), MAP was not different among the SCT and the control subjects. However, MAP was significantly higher in the T group than that in the UT group (+23%, P < 0.001). In addition, UT-CON subjects were smaller and lighter by comparison with the three other groups. Therefore, as UT-CON and UT-SCT were matched for MAP and relative MAP; this difference should be of no consequence to the inflammatory and vascular adhesion findings.
No differences were found at rest between the four groups for hematological parameters, except for the mean corpuscular volume, which was significantly lower in the SCT carriers. This can be explained by the coexistence in half of SCT subjects (SCT-αT subjects) of α-thalassemia mutation (Table 1), which is known to diminish the RBC volume (38). Also and in accordance with Singh et al. (37), the T subjects exhibited a higher level of circulating platelets than the UT.
Effect of habitual physical activity on circulating inflammatory and adhesive molecules at rest.
The baseline levels of all the studied molecules were similar between the four groups except for sVCAM-1. Whereas sVCAM-1 concentrations were significantly more elevated in UT-SCT subjects than that in their CON counterparts (P < 0.05), those concentrations were similar between the two trained groups (T-SCT and T-CON). The present results are in accordance with those reported by Tripette et al. (40), who did not observe significant sVCAM-1 difference in trained SCT subjects compared with subjects with normal hemoglobin (HbAA). Conversely Monchanin et al. (27,28) observed significant higher sVCAM-1 concentrations in a trained SCT population compared with a healthy trained population at rest. Our results are difficult to compare with those of Monchanin et al. (27,28) and Tripette et al. (40) because i) our subjects were not experienced in cycle ergometer (that explains the poor measured MAP) and ii) several SCT carriers presented the double mutation α-thalassemia/SCT (63.6% for the T-SCT and 42.8% for the UT-SCT), which might be considered as beneficial for this population (26-28). Because plasma sVCAM-1 is reduced in SCT-αT subjects compared with SCT-only subjects (27,28), this might explain the variability of sVCAM-1 concentrations in our cohort and the discrepancies between our findings and those of Monchanin et al. (27,28) (Fig. 1).
Nevertheless, this study is the first to show a differential effect of habitual physical activity on sVCAM-1 concentrations in SCT carriers. Higher sVCAM-1 concentrations were observed in the UT-SCT compared with the T-SCT subjects (Fig. 1). The present results extend those of Adamopoulos et al. (2), who found, in healthy subjects and in patients with chronic heart failure, that baseline sVCAM-1 plasma concentrations were reduced after an exercise training program (30 min of cycling at 70%-80% V˙O2max per day, 5 d·wk−1 during 12 wk). Furthermore, Shiu et al. (34) showed that soluble adhesion molecules at rest reflect the degree of endothelium vascular activation; thus, the present results suggest that the vascular endothelium is less activated in the T group compared with the UT group. Because VCAM-1 is involved with very late antigen-4 (VLA-4) in the interactions between leukocytes (17) and reticulocytes/endothelial cells (37,34), increased endothelial expression of VCAM-1 should lead in SCT carriers to an increased risk of blood cell aggregation and thrombus formation, that is, vaso-occlusive events (15,31). We propose that the risk for such vaso-occlusive events might be more pronounced among the SCT subjects who do not engage in regular physical activity. Because high sVCAM-1 has been associated with pulmonary hypertension, organ failure, and mortality in SCD (19), further investigations will be required to determine the potential effects of physical activity in homozygous HbS carriers.
It is now well established that physical exercise disturbs homeostasis and that one of the main results of physical training is to attenuate these perturbations. Thus, among SCT carriers, known factors that promote sickling, such as increased pH, could be less influential and contribute to limiting the polymerization process after training compared with before training. Consequently, endothelial activation and sVCAM-1 expression (33,34) as suggested in the present study would be accordingly attenuated. A putative alternative explanation would be that because training is known to restrict the inflammatory response induced by an acute exercise bout (by limiting pro-inflammatory responses and increasing the anti-inflammatory cytokine release) (32), exercise training might limit vascular activation by its anti-inflammatory pathway (32,39). This has been suggested by Tripette et al. (40), who showed increased postexercise IL-6 concentrations in trained SCT subjects. This cytokine has been demonstrated to exert anti-inflammatory effects through an inhibition of the production of TNFα and IL-1 and a stimulation of the production of IL-1ra and IL-10 (32). Further studies will have to confirm the possibility that an elevated physical activity may limit the adhesive potential in HbS carriers.
Effects of training on incremental exercise and recovery of circulating inflammatory and adhesion molecules.
A significant increase in sVCAM-1, sE-selectin, sICAM-1, and sCD40L plasma concentrations was observed in the four groups in response to incremental exercise (Tables 2 and 3). Although these findings are in accordance with several previous studies in healthy subjects or in patients with peripheral arterial disease or SCT carriers (5,27,28,35,36), they contrast with other findings obtained in healthy subjects or in SCT carriers (27,40,42). The use of the endothelial circulating molecules to observe the effects of an acute exercise on the endothelial activation is because at rest, soluble molecules reflect the level of underlying endothelial activation (34). A postexercise increase in plasma adhesion molecules is thought to result from the shedding of these molecules from the cell surface, possibly as a result of inflammation or increase in shear stress induced by exercise (24). As shown in Figures 1 and 2, plasma sVCAM-1 and sE-selectin concentrations were raised in response to incremental exercise. These molecules are known to reflect endothelial dysfunction and mediate the interaction between leukocytes and platelets/endothelium (5) and between leukocytes and reticulocytes/endothelium (34), respectively. Similarly, the marked increase in sCD40L could reflect increased CD40L expression, primarily on lymphocytes' surface (8) but also on platelets, endothelial cells, smooth muscle cells, macrophages, dendritic cells, fibroblasts, and mast cells (42). The rise of sCD40L has been associated to the pathogenic processes of chronic inflammatory disease (42) and to the presence of increased prothrombotic activity (22). The present findings thus suggest that incremental exercise might be responsible for endothelial activation in HbAA as in HbAS subjects, independent of their habitual physical activity levels. Of note, the rise of the soluble form of ICAM-1 is difficult to interpret because ICAM-1 is expressed in many types of cells, including endothelial cells, fibroblasts, epithelial cells, and multiple cells of hematopoietic lineage (36), such that it is impossible to ascertain whether the present increase of sICAM-1 indeed results from an increase in endothelial ICAM-1. The latter possibility is unlikely because no significant increases in sICAM-1 have been reported after maximal exercise in SCT subjects (27) or in HbAA subjects (4). The sICAM-1 increase reported by Akimoto et al. (4) after endurance or downhill exercise may be attributed to muscular damage and inflammation induced by these types of exercise.
Although widely used, assessment of soluble molecules to indirectly evaluate the endothelium expression immediately after an acute exercise is controversial. Tripette et al. (40) recently argued that the postexercise increase in adhesion molecules observed in several studies may be accounted for by an exercise-induced hemoconcentration. Although water loss during a 15-min incremental exercise should be rather minor in our population (30), the "hemoconcentration hypothesis" should not be dismissed altogether. Thus, the postexercise changes described heretofore should be viewed as the result of both exercise-induced physiological responses (endothelial activation) and hemoconcentration.
No significant changes in plasma interleukins (TNF-α, IL-4, IL-5, IL-8, and IL-10) were observed in response to exercise. These results were unexpected because IL-4, IL-8, and TNF-α are pro-inflammatory cytokines that are responsible for endothelial activation and adhesion molecule expression (23,34). It is thus possible that alternative pathways such as oxidative stress (18) may be recruited.
Differential effects of habitual physical activity on exercise-induced variations of soluble P- and E-selectin.
The different temporal trajectories among T and UT groups in sE-selectin and sP-selectin after an acute exercise are of remarkable interest. First, in accordance with some authors (10,35), a significant rise in sP-selectin was observed at the end of the incremental exercise in the UT subjects only (+167%). However, despite the present statistically significant finding concerning the sP-selectin increase, when we observe the individual results, two UT populations are distinguished. Six of the 16 UT subjects exhibit more than 50% postexercise increase (high responders). The 10 other subjects are characterized by 13%-50% postexercise sP-selectin increase (low responders). As sP-selectin is an important marker of platelet activation (42,43), our results suggest that platelet activation in response to an acute exercise is characterized by an important interindividual variability and is more pronounced in subjects with low physical activity than that in their trained counterparts. This is in agreement with the observation of Kestin et al. (21), who showed in a healthy population that strenuous exercise resulted in both platelet activation and platelet hyperactivity in sedentary subjects but not in physically active subjects. These authors further strengthened the link between platelet activation/aggregation and ischemia and emphasized the fact that sedentary subjects are more vulnerable to death during exercise (21). Chronic exercise is also known to increase fibrinolysis and to reduce platelets aggregation and activation (24), all of which may contribute to limit the thrombus formation in response to an acute exercise in T-SCT subjects. However, our hypothesis contrasts with those of Connes et al. (14), who did not find differences in the coagulation markers (prothrombin time, activated partial thromboplastin time, and antithrombin III activity) between SCT carriers and sedentary subjects in response to a maximal exercise. These authors hypothesized that the clinical complications observed in some SCT carriers during exercise are not the consequence of increased blood coagulation activity (14). In addition, no difference between T and UT was observed concerning sCD40L plasma concentrations, which are considered as prothrombotic activity marker (22). To resolve these discrepancies, the effects of habitual physical activity on platelet metabolism need to be further examined, particularly in SCT carriers.
A significant increase in sE-selectin was also observed in response to graded exercise in all groups. However, when sE-selectin is expressed relative to its resting values, slower recovery kinetics was observed in the UT group (2 h) as compared with the T subjects (1 h). This finding is in accordance with Boos et al. (6), who found that sE-selectin increased immediately after exercise in subjects with cardiovascular disease, without any decrease 30 min later (6). We hypothesize that the differences in the recovery process between the T and the UT groups might be accounted for either by better clearance of sE-selectin in the T subjects or by faster recovery of the basal plasma volume in the T group (i.e., auto-hemodilution or better hydration postexercise for the trained subject-not measured). The latter might be particularly relevant for SCT carriers because a higher hemodilution should restrict the occurrence of two factors involved in vaso-occlusive events, namely, blood viscosity (which increases shear stress and consequently the vascular adhesion activation) (27) and RBC dehydration, which would lead to an impaired RBC deformability (12).
Taken together, our results might appear contradictory compared with those of the literature presenting case reports of accidents in SCT carriers after exercise (16,25). In these earlier studies, vaso-occlusive crises sometimes leading to sudden death were reported in young athletes and military recruits, and both of these populations can be viewed as "trained." These studies suggested that these populations appear to be "at high risk" for vaso-occlusive events. However, our subjects performed an acute exercise paradigm in standardized conditions of temperature, duration, and hydration. This contrasts with the multiple "extreme" and adverse environmental conditions (e.g., altitude, temperature, dehydration, exhausting long-lasting exercises) that were present and described in the case-report studies. These differences in environmental conditions might explain the aforementioned apparent contradiction. Nevertheless, such explanations should not minimize our awareness and need for precautions, considering the many unanswered questions around SCT and exercise that need to be resolved in the future.
In conclusion, a physically active lifestyle is associated with low baseline and postexercise levels of sVCAM-1 in the SCT carrier group, with a limited plasma sP-selectin increase in response to an incremental exercise and with a rapid recovery in plasma sE-selectin. Thus, increased levels of chronic physical activity may confer beneficial effects on endothelial activation and possibly reduce the likelihood of vascular occlusions in SCT carriers. As endothelial activation and sVCAM-1 concentrations are associated with pulmonary hypertension, organ failure, and mortality in SCD (19), these results should be of interest and support further investigations in SCD patients.
This study was supported by grants from l'Ambassade de France au Cameroon in Yaoundé.
The authors thank Mrs. Gaëlle Lepape, Pr. Christophe Nouedoui, and Mrs. Philippe Stofft and François-Xavier Owona† for their helpful assistance and the general direction of the Central Hospital of Yaoundé for its hospitality.
The authors also thank the financial and logistic support of the Cameroonian Ministries of Public Health and of Higher Education.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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Keywords:©2010The American College of Sports Medicine
VASCULAR CELL ADHESION MOLECULES; CYTOKINES; SELECTINS; MAXIMAL EXERCISE; HEMOGLOBINOPATHY