Donald G. Welsh

Coupling red blood cell (RBC) supply to O 2 demand is an intricate process requiring O 2 sensing, generation of a stimulus, and signal transduction that alters upstream arteriolar tone. Although actively debated, this process has been theorized to be induced by hypoxia and to involve activation of endothelial inwardly rectifying K + channels (K IR ) 2.1 by elevated extracellular K + to trigger conducted hyperpolarization via connexin40 (Cx40) gap junctions to upstream resistors. This concept was tested in resting healthy skeletal muscle of Cx40 −/− and endothelial K IR 2.1 −/− mice using state-of-the-art live animal imaging where the local tissue O 2 environment was manipulated using a custom gas chamber. Second-by-second capillary RBC flow responses were recorded as O 2 was altered. A stepwise drop in PO 2 at the muscle surface increased RBC supply in capillaries of control animals while elevated O 2 elicited the opposite response; capillaries were confirmed to express Cx40. The RBC flow responses were rapid and tightly coupled to O 2 ; computer simulations did not support hypoxia as a driving factor. In contrast, RBC flow responses were significantly diminished in Cx40 −/− mice. Endothelial K IR 2.1 −/− mice, on the other hand, reacted normally to O 2 changes, even when the O 2 challenge was targeted to a smaller area of tissue with fewer capillaries. Conclusively, microvascular O 2 responses depend on coordinated electrical signaling via Cx40 gap junctions, and endothelial K IR 2.1 channels do not initiate the event. These findings reconceptualize the paradigm of blood flow regulation in skeletal muscle and how O 2 triggers this process in capillaries independent of extracellular K + .