The microcirculation is responsible for orchestrating adjustments in vascular tone to match local tissue perfusion with oxygen demand. Beyond this metabolic dilation, the microvasculature plays a critical role in modulating vascular tone by endothelial release of an unusually diverse family of compounds including nitric oxide, other reactive oxygen species, and arachidonic acid metabolites. Animal models have provided excellent insight into mechanisms of vasoregulation in health and disease. However there are unique aspects of the human microcirculation that serve as the focus of this review. The concept is put forth that vasculo-parenchymal communication is multimodal, with vascular release of nitric oxide eliciting dilation and preserving normal parenchymal function by inhibiting inflammation and proliferation. Likewise, in disease or stress, endothelial release of ROS both mediates dilation and parenchymal inflammation leading to cellular dysfunction, thrombosis, and fibrosis. Some pathways responsible for this stress-induced shift in mediator of vasodilation are proposed. This paradigm may help explain why microvascular dysfunction is such a powerful predictor of cardiovascular events, and help identify new approaches to treatment and prevention.

Not all of the effects of telomerase can be easily explained by its nuclear actions. Growing evidence suggests that TERT, the catalytic subunit of telomerase, can reversibly translocate from the nucleus to organelles (including the mitochondria), in a dose-and time-dependent manner, in response to stressors, Objective: We examined the hypothesis that telomerase activity modulates microvascular flow-mediated dilation, and loss of telomerase activity contributes to the change of mediator from nitric oxide to mitochondrial hydrogen peroxide in patients with coronary artery disease (CAD). Clinical Track Methods and Results:Human coronary and adipose arterioles were isolated for videomicroscopy. Flow-mediated dilation was measured in vessels pretreated with the telomerase inhibitor BIBR-1532 or vehicle. Statistical differences between groups were determined using a 2-way analysis of variance repeated measure (n≥4; P<0.05). L-NAME (N ω -nitro-L-arginine methyl ester; nitric oxide synthase inhibitor) abolished flow-mediated dilation in arterioles from subjects without CAD, whereas polyethylene glycol-catalase (PEG-catalase; hydrogen peroxide scavenger) had no effect. After exposure to BIBR-1532, arterioles from non-CAD subjects maintained the magnitude of dilation but changed the mediator from nitric oxide to mitochondrial hydrogen peroxide (% max diameter at 100 cm H 2 O: vehicle 74.6±4.1, L-NAME 37.0±2.0*, PEG-catalase 82.1±2.8; BIBR-1532 69.9±4.0, L-NAME 84.7±2.2, PEG-catalase 36.5±6.9*). Conversely, treatment of microvessels from CAD patients with the telomerase activator AGS 499 converted the PEG-catalase-inhibitable dilation to one mediated by nitric oxide (% max diameter at 100 cm H 2 O: adipose, AGS 499 78.5±3.9; L-NAME 10.9±17.5*; PEG-catalase 79.2±4.9). Endothelial-independent dilation was not altered with either treatment. Conclusions:

Small arterioles (40–150 μm) contribute to the majority of vascular resistance within organs and tissues. Under resting conditions, the basal tone of these vessels is determined by a delicate balance between vasodilator and vasoconstrictor influences. Cardiovascular homeostasis and regional tissue perfusion is largely a function of the ability of these small blood vessels to constrict or dilate in response to the changing metabolic demands of specific tissues. The endothelial cell layer of these microvessels is a key modulator of vasodilation through the synthesis and release of vasoactive substances. Beyond their vasomotor properties, these compounds importantly modulate vascular cell proliferation, inflammation, and thrombosis. Thus the balance between local regulation of vascular tone and vascular pathophysiology can vary depending upon which factors are released from the endothelium. This review will focus on the dynamic nature of the endothelial released dilator factors depending on species, anatomic site, and presence of disease, with a focus on the human coronary microcirculation. Knowledge how endothelial signaling changes with disease may provide insights into the early stages of developing vascular inflammation and atherosclerosis, or related vascular pathologies.

Brachial artery flow mediated vasodilation in exercise trained (ET) individuals is maintained after a single bout of heavy resistance exercise compared to sedentary (SED) individuals. The purpose of this study was to determine if vasodilation is also maintained in the microcirculation of ET individuals. A total of 51 SED and ET individuals underwent gluteal subcutaneous fat biopsy before and after performing a single bout of leg press exercise. Adipose arterioles were cannulated in an organ bath and vasodilation to acetylcholine was assessed ± the endothelial nitric oxide inhibitor L-NAME, the cyclooxygenase inhibitor indomethacin, or the hydrogen peroxide scavenger PEG-catalase. Separate vessels (isolated from the same groups) were exposed to an intraluminal pressure of 150 mmHg for 30 minutes to mimic the pressor response which occurs with isometric exercise. Vasodilation to acetylcholine was reduced in microvessels from SED subjects after either a single weight lifting session or exposure to increased intraluminal pressure whereas microvessels from ET individuals maintained acetylcholine-mediated vasodilation. Prior to weight lifting, vasodilation of microvessels from ET individuals was reduced in the presence of L-NAME and indomethacin. After weight lifting or exposure to increased intraluminal pressure, PEG-catalase significantly reduced vasodilation while L-NAME and indomethacin had no effect. These results indicate that 1) endothelium-dependent vasodilation in the microvasculature is maintained following heavy resistance exercise in ET individuals but not in SED subjects, and that 2) high pressure alone or during weight lifting may induce a mechanistic switch in the microvasculature to favor hydrogen peroxide as the vasoactive mediator of dilation.