EFFECT OF HEMORRHAGIC HYPOTENSION ON CORTICAL OXYGEN-PRESSURE AND STRIATAL EXTRACELLULAR DOPAMINE IN CAT BRAIN

This study investigated the relationships between blood pressure, cort ical oxygen pressure, and extracellular striatal dopamine in the brain of adult cats during hemorrhagic hypotension and retransfusion. Oxyge n pressure in the blood of the cortex was measured by the oxygen depen dent quenching of phosphorescence and extracellular dopamine, dihydrox yphenylacetic acid (DOPAC) and homovanillic acid (HVA) by in vivo micr odialysis. Following a 2 h stabilization period after implantation of the microdialysis probe in the striatum, the mean arterial blood press ure (MAP) was decreased in a stepwise manner from 132 +/- 2 Ton (contr ol) to 90 Ton, 70 Ton and 50 Ton, holding the pressure at each level f or 15 min. The whole blood was then retransfused and measurements were continued for 90 min. As the MAP was lowered there was a decrease in arterial pH, from a control value of 7.37 +/- 0.05 to 7.26 +/- 0.06. T he PaCO2 decreased during bleeding from 32.3 +/- 4.8 Ton to 19.6 +/- 3 .6 Ton and returned to 30.9 +/- 3.9 Ton after retransfusion. The PaO2 was 125.9 +/- 15 Ton: during control conditions and did not significan tly change during bleeding. Cortical oxygen pressure decreased with de crease in MAP, from 50 +/- 2 Ton (control) to 42 +/- 1 Ton, 31 +/- 2 T on and 22 +/- 2 Ton, respectively. A statistically significant increas e in striatal extracellular dopamine, to 2,580 +/- 714% of control was observed when MAP decreased to below 70 Ton and cortical oxygen press ure decreased to below 31 Ton. When the MAP reached 50 Ton, the concen tration of extracellular dopamine increased to 18,359 +/- 2,764% of th e control value. A statistically significant decrease in DOPAC and HVA were observed during the last step of bleeding. The data show that de creases in systemic blood pressure result in decrease in oxygen pressu re in the microvasculature of the cortex, suggesting vascular dilation is not sufficient to result in a full compensation for the decreased MAP. The decrease in cortical oxygen pressure to below 32 Ton is accom panied by a marked increase in extracellular dopamine in the striatum, indicating that even such mild hypoxia can induce significant disturb ance in brain metabolism.