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 O2 requirements. At a fundamental level, O2 demand‐supply coupling entails the sensing of PO2, and the generation of a stimulus that alters upstream arteriolar tone. However, where and how O2 needs are sensed in the microcirculation is presently contested. One idea centers on hypoxia triggering the release of K+, activating endothelial KIR2.1 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 O2 environment using live animal imaging in Cx40‐/‐and endothelial KIR2.1‐/‐ mice. The extensor digitorum longus muscle positioned overtop a gas control chamber allowed second‐by‐second monitoring of capillary RBC flow responses as O2 was altered around its physiological set point of 53 mmHg. A stepwise drop in PO2 at the muscle surface (53 to 15 or 0 mmHg) increased RBC supply rate in control capillaries while elevated chamber O2 elicited the opposite response; these capillaries robustly expressed Cx40. The RBC flow responses were rapid and tightly coupled to O2 levels as readily observed when chamber O2 was oscillated in a sinusoidal manner. In contrast, this blood flow response was significantly diminished in Cx40‐/‐ mice and translated into lower capillary RBC O2 saturation. KIR2.1‐/‐mice, on the other hand, had normal resting RBC O2 saturation and reacted normally to O2 changes, albeit an oscillation or a sustained stepwise decrease. Furthermore, we confirmed that RBC flow responses are conserved in endothelial KIR2.1‐/‐mice even when the low O2 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 KIR2.1‐/‐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 O2 responses depend on coordinated electrical signaling via gap junctions comprised of Cx40 and that endothelial KIR2.1channels do not drive the initiating electrical event. These findings reconceptualize our understanding of blood flow regulation and how O2 initiates this process at the capillary level independent of metabolite production.