Metabolic factors involved in regulation of muscle blood flow.

Muscles with various compositions of muscle fibers show a different degree of functional hyperemia. This can be caused by either a different pattern of vascular recruitment, a release of various metabolites, or a heterogeneous sensitivity of vessels in different muscles to individual metabolites. The possible involvement of hypoxia, adenosine, ATP, lactate, pH, hyperosmolarity, potassium, and inorganic phosphate in functional hyperemia is discussed. It is concluded, on the basis of the data from the literature as well as on that of our own experiments, that potassium and inorganic phosphate are equally involved in functional hyperemia in mixed muscles during short-lasting isotonic or isometric rhythmic or tetanic contractions. Potassium seems to be more important in long-lasting contractions, whereas inorganic phosphate seems to be more important in muscles with a high proportion of oxidative fibers. Other factors, such as low pH, might enhance the vasodilating potency of phosphate, and adenosine seems to be involved only to a small extent.