Adenosine and free-flow functional hyperemia in striated muscle

Striated muscle arteriolar responses to 1.5 min of 1-Hz contraction and/or increased tissue O2 partial pressure (PO2) were observed during exposure of the tissue interstitial space to adenosine deaminase (ADA) to evaluate the role of adenosine (ADO) as a regulator for blood flow. The microvasculature of the hamster cremaster muscle was continuously superfused with a bicarbonate buffer containing 11 micrograms ADA/ml and equilibrated with 5% CO2 and various O2 concentrations. Arterioles (resting diameter less than 30 micrometers) constricted a maximum of 55% when the superfusate gas tension was increased from 0 to 95% O2, but ADA had no effect on this behavior. Arterioles dilated during exercise, but the diameter change was decreased 20-25% during exercise with ADA treatment at both normal and elevated tissue PO2. As ADA had no effect on either the vasodilation to 2-chloroadenosine or resting arteriolar diameter, it was probably specific in its action. Assuming that all extracellular ADO was accessible to ADA and that ADA neutralized most newly formed ADO, we conclude that ADO is one component of a multifactor system mediating short periods of free-flow exercise hyperemia and that the release of ADO is not necessarily dependent on tissue hypoxia.