The present study describes the relationships between extracellular striatal dopamine, cortical oxygen pressure and blood flow in brain of newborn piglets during hemorrhagic hypotension. Cerebral oxygen pressure was measured optically by the oxygen dependent quenching of phosphorescence; extracellular dopamine by in vivo microdialysis; striatal blood flow was monitored by a laser Doppler. Following a 2 h stabilization period after implanting the microdialysis and laser Doppler probes in the striatum, the mean arterial blood pressure (MABP) was decreased in stepwise manner from 87 ± 4 Torr (control) to 35 ± 5 Torr, during 63 min. The whole blood was then reinfused and measurements were continued for 45 min. Statistically significant decrease in blood flow, 10%, was observed when arterial blood pressure decreased to about 53 Torr. With further decrease blood pressure to 35 Torr, blood flow decreased to about 35% of control (P < 0.01). Cortical oxygen pressure decreased almost proportional to decrease in blood pressure. The progressive 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 striatum increased with decreasing cortical oxygen pressure. As cortical oxygen decreased, the extracellular dopamine increased to 230%, 420% and 3200% of control, respectively. Our results show that in mild hypotension total blood flow is well maintained but oxygen pressure in the microvasculature decreases, possibly due to heterogeneity in the regulatory mechanism. The decrease in cortical oxygen pressure in striatum is parallel to increase in striatal dopamine which then can be use as early indicator of brain hypoxia.
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