EFFECT OF HEMORRHAGIC HYPOTENSION ON EXTRACELLULAR LEVEL OF DOPAMINE,CORTICAL OXYGEN-PRESSURE AND BLOOD-FLOW IN BRAIN OF NEWBORN PIGLETS

The present study describes the relationships between extracellular st riatal dopamine, cortical oxygen pressure and blood flow in brain of n ewborn piglets during hemorrhagic hypotension. Cerebral oxygen pressur e was measured optically by the oxygen dependent quenching of phosphor escence; extracellular dopamine by in vivo microdialysis; striatal blo od flow was monitored by a laser Doppler. Following a 2 h stabilizatio n period after implanting the microdialysis and laser Doppler probes i n the striatum, the mean arterial blood pressure (MABP) was decreased in stepwise manner from 87 +/- 4 Torr (control) to 35 +/- 5 Torr, duri ng 63 min. The whole blood was then reinfused and measurements were co ntinued for 45 min. Statistically significant decrease in blood flow, 10%, was observed when arterial blood pressure decreased to about 53 T orr. With further decrease blood pressure to 35 Torr, blood flow decre ased to about 35% of control (P < 0.01). Cortical oxygen pressure decr eased almost proportional to decrease in blood pressure. The progressi ve decrease in MABP from 87 +/- 4 Torr to 65 +/- 6, 52 +/- 7, and 35 /- 5 Torr resulted in cortical oxygen pressure decreasing from 45 +/- 4 Torr to 33 +/- 3 Torr (P < 0.05), 24 +/- 4 Torr (P < 0.01) and 13 +/ - 3 Torr (P < 0.01). The levels of extracellular dopamine in the stria tum increased with decreasing cortical oxygen pressure. As cortical ox ygen decreased, the extracellular dopamine increased to 230%, 420% and 3200% of control, respectively. Our results show that in mild hypoten sion total blood flow is well maintained but oxygen pressure in the mi crovasculature decreases, possibly due to heterogeneity in the regulat ory mechanism. The decrease in cortical oxygen pressure in striatum is parallel to increase in striatal dopamine which then can be use as ea rly indicator of brain hypoxia.