Reply to Pancheva, Panchev, and Pancheva

Abstract

to the editor: We thank Pancheva and colleagues (4) for their interest in our study investigating the association between maximal aerobic capacity and cerebrovascular reactivity. In this study, we measured cerebral blood flow velocity during resting conditions through the middle cerebral artery (MCA), where little dilation is expected with a hypercapnic stimulus. In our group of older healthy adults, we found that greater maximal aerobic capacity was positively associated with the ability of the MCA to increase blood flow velocity in response to hypercapnia. Although we believe this finding has relevance to cognition, we are not able to confirm that aerobic exercise necessarily improves cognitive function (all subjects were screened for potential cognitive dysfunction using a standard battery of tests). We presume that the microvasculature is responding to CO2 and is responsible for increasing blood flow. Regular exercise may help protect the microvasculature so that it can respond to CO2 and increased neural activity. Pancheva and colleagues bring up an excellent point regarding the potential for capillary pumps in controlling blood flow to the brain, particularly in relation to exercise and exercise training. Our interpretation follows the conventional physiological data demonstrating the contribution of prostaglandins to blood flow regulation in both animals and humans, but we believe there are distinct mechanisms of CO2-mediated vasodilation and prostaglandin-mediated vasodilation. However, our study is unable to distinguish the exact mechanism underlying the association between aerobic capacity and cerebrovascular reactivity. The interaction between the capillaries and red blood cells (RBCs) is of great importance in the regulation of blood flow in any vascular bed. As Pancheva and colleagues have described, RBC rigidity and the functioning of capillary pumps may contribute to blood flow regulation (3). Aging likely increases RBC rigidity, yet it is unclear if RBC rigidity is affected by regular exercise, at least in middle-aged adults (2). RBC quality may be under-recognized in many human physiology studies. Additionally, it is unknown if regular exercise or endurance training in aging humans modifies capillary pump function. It is well accepted that exercise training induces angiogenesis and increases capillary density within the cortex (1). Such structural changes to the framework (capillary size or number), especially in aged animals with reduced capillarity, will certainly affect the regulation of cerebral blood flow. In our human physiology studies, investigating capillary pumps is not possible. Therefore, the role of capillary pumps in vivo, and how they fit into the currently accepted explanation for exercise-induced changes in the brain, needs to be elucidated.