Periarteriolar and tissue PO2 during sympathetic escape in skeletal muscle.

The purpose of this study was to determine whether vascular escape from sympathetic nerve stimulation in skeletal muscle is caused by a fall in the tissue O2 level. O2 microelectrodes were used to measure PO2 at the wall of arterioles (periarteriolar PO2) and near the venous end of capillary networks (parenchymal tissue PO2) in the exteriorized cat sartorius muscle during sympathetic nerve stimulation. Measurements were made under a low O2 suffusate (equilibrated with 5% CO2-95% N2) and under a high O2 suffusate (10% O2-5% CO2-85% N2). During sympathetic stimulation under low O2, mean diameter of proximal (second-order) arterioles decreased from 55 to 32 micron before returning to 37 micron (sympathetic escape). Mean periarteriolar PO2 fell from 50 to 25 mmHg with no secondary increase. Distal (fifth-order) arterioles initially constricted from 7 to 4 micron before relaxing to 6 micron. Periarteriolar PO2 of these vessels fell from 40 to 13 mmHg with no secondary increase. During stimulation under high O2, periarteriolar PO2 levels of both proximal and distal arterioles were similar to those under low O2, yet escape was substantially reduced. The lack of relationship between periarteriolar PO2 and vascular escape argues against a role of vascular wall PO2 in this behavior. Parenchymal tissue PO2 fell to 9 mmHg during stimulation under low O2 but did not fall below 22 mmHg during stimulation under high O2. The attenuation of escape under conditions where tissue PO2 did not fall is consistent with the hypothesis that sympathetic escape in skeletal muscle is mediated through a fall in parenchymal cell PO2.