Conducted Capillary Signaling Enables Oxygen Responses in Skeletal Muscle Independent of Metabolite Production

Abstract

Red blood cell (RBC) flow through the microvasculature is closely matched to tissue O 2 requirements. At a fundamental level, O 2 demand‐supply coupling entails the sensing of PO 2 , and the generation of a stimulus that alters upstream arteriolar tone. However, where and how O 2 needs are sensed in the microcirculation is presently contested. One idea centers on hypoxia triggering the release of K + , activating endothelial K IR 2.1 channels and initiating a hyperpolarization that conducts upstream via gap junctions, comprised of connexins (Cx). We tested this idea in skeletal muscle where we controlled the local tissue O 2 environment using live animal imaging in Cx40 ‐/‐ and endothelial K IR 2.1 ‐/‐ mice. The extensor digitorum longus muscle positioned overtop a gas control chamber allowed second‐by‐second monitoring of capillary RBC flow responses as O 2 was altered around its physiological set point of 53 mmHg. A stepwise drop in PO 2 at the muscle surface (53 to 15 or 0 mmHg) increased RBC supply rate in control capillaries while elevated chamber O 2 elicited the opposite response; these capillaries robustly expressed Cx40. The RBC flow responses were rapid and tightly coupled to O 2 levels as readily observed when chamber O 2 was oscillated in a sinusoidal manner. In contrast, this blood flow response was significantly diminished in Cx40 ‐/‐ mice and translated into lower capillary RBC O 2 saturation. K IR 2.1 ‐/‐ mice, on the other hand, had normal resting RBC O 2 saturation and reacted normally to O 2 changes, albeit an oscillation or a sustained stepwise decrease. Furthermore, we confirmed that RBC flow responses are conserved in endothelial K IR 2.1 ‐/‐ mice even when the low O 2 challenge is applied to a restricted number of surface capillaries in the muscle; interestingly, these responses were dominated by capillary hematocrit changes in both control and endothelial K IR 2.1 ‐/‐ mice. Modelling this phenomenon supported the idea that the number of capillaries stimulated is indeed tied to conduction distance and tissue RBC distribution. In conclusion, we demonstrate that microvascular O 2 responses depend on coordinated electrical signaling via gap junctions comprised of Cx40 and that endothelial K IR 2.1channels do not drive the initiating electrical event. These findings reconceptualize our understanding of blood flow regulation and how O 2 initiates this process at the capillary level independent of metabolite production.