Spatial variation of aortic wall oxygen diffusion coefficient from transient polarographic measurements

Polarographic current transients following a voltage step (turn-on transient) were measured with bare cathodes (25 microns diameter) and shallowly recessed oxygen microelectrodes (< 5 microns diameter). Except for the initial part of the current transient, the experimental measurements were in excellent agreement with simple models in the literature, which predict an inverse relationship with square root of t. Turn-on transients were measured in aqueous solutions with known physical properties, and in aortic wall tissue from three different species (n = 6 rabbits, n = 3 dogs, and n = 1 miniature pig). Oxygen diffusion coefficients (D) were determined in vitro by comparing time constants measured by the same microelectrode in saline and in strips of aortic wall tissue at 37 degrees C. On the inner side (endothelium and intima) of the aorta, D averaged (+/- S.E.) 7.0 (+/- 0.8) x 10(-6) cm2/s in 6 rabbits, 6.4 (+/- 1.0) x 10(-6) cm2/s in 3 dogs, and was 4.6 x 10(-6) cm2/s in the pig. On the adventitial side, D was 9.5 x 10(-6) cm2/s in 1 rabbit, 11.4 (+/- 1.2) x 10(-6) cm2/s in 3 dogs, and 8.1 x 10(-6) cm2/s in the pig. For every aortic strip on which D was measured from both sides, D for the inner wall was always lower, overall by a little more than one third (p < 0.001). The lower D on the endothelial side may limit oxygen transport to the vascular wall and play a role in atherogenesis.