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Rucker, M., O. Strobel, B. Vollmar, F. Roesken, and M. D. Menger. Vasomotion in critically perfused muscle protects adjacent tissues from capillary perfusion failure. Am J Physiol Heart Circ Physiol 279: H550-H558, 2000.--We analyzed the incidence and interaction of arteriolar vasomotion and capillary flow motion during critical perfusion conditions in neighboring peripheral tissues using intravital fluorescence microscopy. The gracilis and semitendinosus muscles and adjacent periosteum, subcutis, and skin of the left hindlimb of Sprague-Dawley rats were isolated at the femoral vessels. Critical perfusion conditions, achieved by stepwise reduction of femoral artery blood flow, induced capillary flow motion in muscle, but not in the periosteum, subcutis, and skin. Strikingly, blood flow within individual capillaries was decreased (P [is less than] 0.05) in muscle but was not affected in the periosteum, subcutis, and skin. However, despite the flow motion-induced reduction of muscle capillary blood flow during the critical perfusion conditions, functional capillary density remained preserved in all tissues analyzed, including the skeletal muscle. Abrogation of vasomotion in the muscle arterioles by the calcium channel blocker felodipine resulted in a redistribution of blood flow within individual capillaries from cutaneous, subcutaneous, and periosteal tissues toward skeletal muscle. As a consequence, shutdown of perfusion of individual capillaries was observed that resulted in a significant reduction (P [is less than] 0.05) of capillary density not only in the neighboring tissues but also in the muscle itself. We conclude that during critical perfusion conditions, vasomotion and flow motion in skeletal muscle preserve nutritive perfusion (functional capillary density) not only in the muscle itself but also in the neighboring tissues, which are not capable of developing this protective regulatory mechanism by themselves. critical perfusion; intravital fluorescence microscopy; microcirculation; skin; subcutis; periosteum; felodipine; redox state; reduced nicotinamide adenine dinucleotide