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POINT-COUNTERPOINT COMMENTS
Department of Physiology
Akdeniz University Faculty of Medicine
Antalya, Turkey
The following letters are in response to the Point:Counterpoint: "Sickle cell trait should/should not be considered asymptomatic and as a benign condition during physical activity" that appears in this issue.
To the Editor: There is ample evidence for hemorheological abnormalities in individuals with sickle cell trait (SCT). These abnormalities include impaired red blood cell (RBC) deformability (2) and increased RBC aggregation (5). Point and Counterpoint articles by Le Gallais et al. (4) and Connes et al. (3) both indicate that a variety of circulatory disorders have been reported in these individuals. Such disorders are sporadic and many SCT subjects can be asymptomatic, although a careful hemorheological analysis may reveal abnormalities of RBC mechanical properties (2). It is thus important to recognize that vascular autoregulatory mechanisms of the cardiovascular system can easily compensate for certain degrees of RBC mechanical alterations, but only if sufficient vasodilatory reserve exists (1). For example, in the isolated perfused rat hind limb, it has been demonstrated that the hemodynamic effects of experimentally impaired RBC deformability (i.e., 
20% decrease) are eliminated if vasomotor activity was intact (1). Thus, under non-stressed conditions in subjects with normal vascular antiregulatory reserve, SCT might be totally nonsymptomatic despite impaired RBC mechanical properties (2). However, exercise poses several challenges: 1) it is the most well-known physiological condition that consumes autoregulatory reserve to maintain homeostasis despite greatly increased demands for blood flow, and 2) it is known to induce acute adverse hemorheological effects (6). Therefore, even slight, nonsymptomatic vascular abnormalities may become manifest under such conditions, with the mechanical impairments of RBC in SCT subjects likely to promote these manifestations.
REFERENCES
Environmental Physiology Laboratory
Medical College of Georgia
Augusta, Georgia
To the Editor: As Le Gallais et al. (3) and Connes et al. (2) would likely agree, athletes and others with sickle cell trait (SCT) routinely participate in physical exercise, training, and competition without apparent signs and symptoms of unique metabolic responses or distress. However, as we have shown (1), with sufficient heat-exercise strain, measurable sickling and neutrophil activation can occur. Monchanin et al. (4) further demonstrated greater soluble vascular cell adhesion molecule-1 levels during and following incremental exercise in those with SCT. Collectively, these and other identified humoral changes, hemorheological parameters, and indicators of potential vascular dysfunction associated with SCT at least support the plausibility of hemoglobin S percent-dependent intravascular sickling, acute vaso-occlusion, and related clinical risk during strenuous exercise. Field reports on incidents of collapse and death further advocate this scenario (5, 6); intravascular sickling, however, has not yet been established as the precipitating cause of exertional rhabdomyolysis or endothelial damage in the microvasculature in such cases. Additional research should be diligently pursued to more definitively defend or dismiss the notion of greater clinical risk during intense physical activity and especially heat strain for those with SCT. However, for now, empirical evidence to date warrants implementing appropriate activity-related precautions and guidelines, so that those with SCT can still participate competitively but safely.
REFERENCES
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