Capillary oxygen regulates demand-supply coupling by triggering connexin40-mediated conduction: Rethinking the...

Coupling red blood cell (RBC) supply to O₂ demand is an intricate process requiring O₂ 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_IR2.1^-/- mice using state-of-the-art live animal imaging where the local tissue O₂ environment was manipulated using a custom gas chamber. Second-by-second capillary RBC flow responses were recorded as O₂ was altered. A stepwise drop in PO₂ at the muscle surface increased RBC supply in capillaries of control animals while elevated O₂ elicited the opposite response; capillaries were confirmed to express Cx40. The RBC flow responses were rapid and tightly coupled to O₂; computer simulations did not support hypoxia as a driving factor. In contrast, RBC flow responses were significantly diminished in Cx40^-/- mice. Endothelial K_IR2.1^-/- mice, on the other hand, reacted normally to O₂ changes, even when the O₂ challenge was targeted to a smaller area of tissue with fewer capillaries. Conclusively, microvascular O₂ responses depend on coordinated electrical signaling via Cx40 gap junctions, and endothelial K_IR2.1 channels do not initiate the event. These findings reconceptualize the paradigm of blood flow regulation in skeletal muscle and how O₂ triggers this process in capillaries independent of extracellular K⁺.