Microvascular Responses to Alterations in Oxygen Tension

The changes in microvascular diameter and perivascular oxygen tension resulting from alterations in suffusion solution Po 2 were investigated in a study of the participation of oxygen in the regulation of blood flow. Diffusion gradients for oxygen were altered by changing the Po 2 of a solution covering the surface of the hamster cheek pouch. As the solution Po 2 was raised from a low of 11 mm Hg, perivascular Po 2 of the large arterioles initially decreased to a minimum at approximately 40 mm Hg and then increased progressively as solution Po 2 was elevated further. Arterial capillary and tissue Po 2 remained relatively constant over a range of solution oxygen tensions between 11 and 40 mm Hg, suggesting that either the precapillary sphincters or the terminal arterioles were active in regulating tissue Po 2 as the input of O 2 from the solution was increased. The arterioles constricted as solution Po 2 was elevated. Average arteriolar diameter decreased by 13% as solution Po 2 increased from 11 to 47 mm Hg. A more pronounced constriction of 20% occurred when solution Po 2 was increased from 11 to 84 mm Hg. These experiments indicated that the response of large and small arterioles was not mediated by a direct effect of oxygen on the vascular smooth muscle, since decreases in perivascular oxygen tension were coincident with decreases in vascular diameter in these vessels over a range of solution Po 2 between 11 and 47 mm Hg. The data did not distinguish between a direct and an indirect effect of oxygen on the vascular smooth muscle of the terminal arterioles and precapillary sphincters. However, the oxygen tensions measured at these sites (18-30 mm Hg) required that the vascular smooth muscle cells respond to altered oxygen tension at levels higher than those which have been demonstrated experimentally.