Capillary Oxygen Regulates Demand-supply Coupling by Triggering Connexin40-mediated Conduction: Rethinking the Metabolic Hypothesis

Coupling red blood cell (RBC) supply to O demand is an intricate process requiring O 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) 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 and endothelial mice using state-of-the-art live animal imaging where the local tissue O environment was manipulated using a custom gas chamber. Second-by-second capillary RBC flow responses were recorded as O was altered. A stepwise drop in PO at the muscle surface increased RBC supply in capillaries of control animals while elevated O elicited the opposite response; capillaries were confirmed to express Cx40. The RBC flow responses were rapid and tightly coupled to O; computer simulations did not support hypoxia as a driving factor. In contrast, RBC flow responses were significantly diminished in mice. Endothelial mice, on the other hand, reacted normally to O changes, even when the O challenge was targeted to a smaller area of tissue with fewer capillaries. Conclusively, microvascular O responses depend on coordinated electrical signaling via Cx40 gap junctions, and endothelial K2.1 channels do not initiate the event. These findings reconceptualize the paradigm of blood flow regulation in skeletal muscle and how O triggers this process in capillaries independent of extracellular K.