Changes in cytochrome oxidation in outer and inner stripes of outer medulla

To examine regional cytochrome oxidation in the outer medulla, we developed fiber optic probes that allowed us to obtain localized reflectance measurements from the outer and inner stripes of the outer medulla. We measured directional changes in cytochrome oxidation in these two regions. In the outer stripe furosemide surprisingly caused a significant decrease in cytochrome oxidation. The decrease occurred concomitantly with a fall in outer medullary blood flow as measured by laser-Doppler flowmetry. Saralasin, an antagonist of angiotensin II caused a significant increase in cytochrome oxidation in the outer stripe. In the inner stripe furosemide tended to increase cytochrome oxidation, and saralasin had no effect. These results indicate that the two regions of the outer medulla may be affected differently by the same agent. They suggest that cytochrome oxidation in the outer stripe is predominantly influenced by outer medullary blood flow, whereas the inner stripe is predominantly influenced by the rate of oxygen consumption. The advantages and limitations of this methodology are discussed.