Evidence for role of capillaries in regulation of skeletal muscle oxygen supply.

How oxygen (O2 ) supply to capillaries is regulated to match the tissue's demand is unknown. Erythrocytes have been proposed as sensors in this regulatory mechanism since they release ATP, a vasodilator, in an oxygen saturation (SO2 )-dependent manner. ATP causes hyperpolarization of endothelial cells resulting in conducted vasodilation to arterioles. OBJECTIVE: We propose individual capillary units can regulate their own O2 supply by direct communication to upstream arterioles via electrically coupled endothelium. METHODS: To test this hypothesis, we developed a transparent micro-exchange device for localized O2 exchange with surface capillaries of intact tissue. The device was fabricated with an O2 permeable micro-outlet 0.2 × 1.0 mm. Experiments were performed on rat extensor digitorum longus (EDL) muscle using dual wavelength video microscopy to measure capillary hemodynamics and erythrocyte SO2 . Responses to local O2 perturbations were measured with only capillaries positioned over the micro-outlet. RESULTS: Step changes in the gas mixture %O2 caused physiological changes in erythrocyte SO2 , and appropriate changes in flow to offset the O2 challenge if at least 3-4 capillaries were stimulated. CONCLUSION: These results support our hypothesis that individual capillary units play a role in regulating their erythrocyte supply in response to a changing O2 environment.

A micro-delivery approach for studying microvascular responses to localized oxygen delivery.

BACKGROUND: In vivo video microscopy has been used to study blood flow regulation as a function of varying oxygen concentration in microcirculatory networks. However, previous studies have measured the collective response of stimulating large areas of the microvascular network at the tissue surface. OBJECTIVE: We aimed to limit the area being stimulated by controlling oxygen availability to highly localized regions of the microvascular bed within intact muscle. DESIGN AND METHOD: Gas of varying O(2) levels was delivered to specific locations on the surface of the Extensor Digitorum Longus muscle of rat through a set of micro-outlets (100 µm diameter) patterned in ultrathin glass using state-of-the-art microfabrication techniques. O(2) levels were oscillated and digitized video sequences were processed for changes in capillary hemodynamics and erythrocyte O(2) saturation. RESULTS AND CONCLUSIONS: Oxygen saturations in capillaries positioned directly above the micro-outlets were closely associated with the controlled local O(2) oscillations. Radial diffusion from the micro-outlet is limited to ~75 µm from the center as predicted by computational modeling and as measured in vivo. These results delineate a key step in the design of a novel micro-delivery device for controlled oxygen delivery to the microvasculature to understand the fundamental mechanisms of microvascular regulation of O(2) supply.

Modeling steady state SO2-dependent changes in capillary ATP concentration using novel O2 micro-delivery methods.

Adenosine triphosphate (ATP) is known to be released from the erythrocyte in an oxygen (O2) dependent manner. Since ATP is a potent vasodilator, it is proposed to be a key regulator in the pathway that mediates micro-vascular response to varying tissue O2 demand. We propose that ATP signaling mainly originates in the capillaries due to the relatively long erythrocyte transit times in the capillary and the short ATP diffusion distance to the electrically coupled endothelium. We have developed a computational model to investigate the effect of delivering or removing O2 to limited areas at the surface of a tissue with an idealized parallel capillary array on total ATP concentration. Simulations were conducted when exposing full surface to perturbations in tissue O2 tension (PO2) or locally using a circular micro-outlet (~100 µm in diameter), a square micro-slit (200 × 200 µm), or a rectangular micro-slit (1000 µm wide × 200 µm long). Results indicated the rectangular micro-slit has the optimal dimensions for altering hemoglobin saturations (SO2) in sufficient number capillaries to generate effective changes in total [ATP]. This suggests a threshold for the minimum number of capillaries that need to be stimulated in vivo by imposed tissue hypoxia to induce a conducted micro-vascular response. SO2 and corresponding [ATP] changes were also modeled in a terminal arteriole (9 µm in diameter) that replaces 4 surface capillaries in the idealized network geometry. Based on the results, the contribution of terminal arterioles to the net change in [ATP] in the micro-vascular network is minimal although they would participate as O2 sources thus influencing the O2 distribution. The modeling data presented here provide important insights into designing a novel micro-delivery device for studying micro-vascular O2 regulation in the capillaries in vivo.