Abnormal cardiovascular control during exercise: Role of insulin resistance in the brain

During exercise circulatory adjustments to meet oxygen demands are mediated by multiple autonomic mechanisms, the skeletal muscle exercise pressor reflex (EPR), the baroreflex (BR), and by feedforward signals from central command neurons in higher brain centers. Insulin resistance in peripheral tissues includes sensitization of skeletal muscle afferents by hyperinsulinemia which is in part responsible for the abnormally heightened EPR function observed in diabetic animal models and patients. However, the role of insulin signaling within the central nervous system (CNS) is receiving increased attention as a potential therapeutic intervention in diseases with underlying insulin resistance. This review will highlight recent advances in our understanding of how insulin resistance induces changes in central signaling. The alterations in central insulin signaling produce aberrant cardiovascular responses to exercise. In particular, we will discuss the role of insulin signaling within the medullary cardiovascular control nuclei. The nucleus tractus solitarius (NTS) and rostral ventrolateral medulla (RVLM) are key nuclei where insulin has been demonstrated to modulate cardiovascular reflexes. The first locus of integration for the EPR, BR and central command is the NTS which is high in neurons expressing insulin receptors (IRs). The IRs on these neurons are well positioned to modulate cardiovascular responses to exercise. Additionally, the differences in IR density and presence of receptor isoforms enable specificity and diversity of insulin actions within the CNS. Therefore, non-invasive delivery of insulin into the CNS may be an effective means of normalizing cardiovascular responses to exercise in patients with insulin resistance.