Redistribution of Cerebral Blood Flow during Severe Hypovolemia and Reperfusion in a Sheep Model: Critical Role of α1-Adrenergic Signaling

Abstract

Background: Maintenance of brain circulation during shock is sufficient to prevent subcortical injury but the cerebral cortex is not spared. This suggests area-specific regulation of cerebral blood flow (CBF) during hemorrhage.

Methods: Cortical and subcortical CBF were continuously measured during blood loss (≤50%) and subsequent reperfusion using laser Doppler flowmetry. Blood gases, mean arterial blood pressure (MABP), heart rate and renal blood flow were also monitored. Urapidil was used for α1A-adrenergic receptor blockade in dosages, which did not modify the MABP-response to blood loss. Western blot and quantitative reverse transcription polymerase chain reactions were used to determine adrenergic receptor expression in brain arterioles.

Results: During hypovolemia subcortical CBF was maintained at 81 ± 6% of baseline, whereas cortical CBF decreased to 40 ± 4% (p < 0.001). Reperfusion led to peak CBFs of about 70% above baseline in both brain regions. α1A-Adrenergic blockade massively reduced subcortical CBF during hemorrhage and reperfusion, and prevented hyperperfusion during reperfusion in the cortex. α1A-mRNA expression was significantly higher in the cortex, whereas α1D-mRNA expression was higher in the subcortex (p < 0.001).

Conclusions: α1-Adrenergic receptors are critical for perfusion redistribution: activity of the α1A-receptor subtype is a prerequisite for redistribution of CBF, whereas the α1D-receptor subtype may determine the magnitude of redistribution responses.

Keywords: adrenergic regulation; alpha-adrenergic; cerebral blood flow; cerebral hemodynamics; cerebrovasvular disease; head trauma; neurodegenerative disease; resuscitation.

Figure 1

Effects of 50% blood loss and reperfusion on arterial blood gases and lactate. Values are given for baseline (open bars), removal of 50% blood (filled bars) and complete reperfusion (hatched bars) in controls (blue) and after α1A-adrenergic blockade (red) for (A) pH, (B) partial pressure of carbon dioxide (pCO₂), (C) partial pressure of oxygen (pO₂), (D) oxygen saturation (sO₂), (E) base excess (BE) and (F) lactate. Mean ± SEM; * p < 0.05 compared to baseline.

Figure 2

Effects of 50% blood loss and reperfusion on vital parameters. (A) Mean arterial blood pressure (MABP), (B) heart rate (HR) and (C) renal blood flow (RBF). Controls in blue and α1A-adrenergic blockade in red. Mean ± SEM; * p < 0.05, ** p < 0.01 and *** p < 0.001 compared to baseline.

Figure 3

Effects of 50% blood loss and reperfusion on cortical and subcortical cerebral blood flow (CBF). Comparison of cortical and subcortical CBF in the control group (A) and under α1A-adrenergic blockade (B). Comparison of controls and α1A-adrenergic blockade for (C) cortex and (D) subcortex. Means ± SEM; * p < 0.05, ** p < 0.01 and *** p < 0.001 compared to baseline; ° p < 0.05, °° p < 0.01 and °°° p < 0.001 for comparison between two experimental groups.

Figure 4

Correlation of blood flow and MABP. Cortical or subcortical CBF or RBF in the control group (blue symbols) were plotted against MABP during blood withdrawal (A–C) and for the reperfusion phase (D–F), respectively. The effect of α1A-adrenergic blockade (red symbols) on the relationships of blood flow and MABP during blood withdrawal (G–I) or during reperfusion (J–L) is plotted analogously. Three-parameter logistic regression was calculated for each data set (solid lines). Correlation coefficients (r) are given in the respective panels, p < 0.00001 for all data sets.

Figure 5

Expression of α1-adrenergic receptors in cortical and subcortical brain arterioles. (A) Western blot analysis was performed for cortical and subcortical brain arterioles from 14 sheep, as described in the methods section. α1-adrenergic receptors (α1-AR) are detectable in cortex (Cx) and subcortex (Scx). 1–4, samples from different sheep; AU, arbitrary units; Ref., reference sample; (B) Quantification of receptor expression after normalization of band intensities to the housekeeping protein β-actin shows no differences of expression levels in cortex and subcortex, respectively; (C) Quantitative PCR-analysis was done using cDNA reverse transcribed from total RNA of 7 sheep. PCR-products of correct size were consistently detected for housekeeping genes GAPDH and ZO-1, and for the α1-adrenergic receptors A (ADRA1A) and D (ADRA1D), but not for the B-subtype receptor (ADRA1B). A, B, depict examples from two individual sheep; H₂O, no cDNA control; (D) Quantitation of receptor mRNA-expression in relation to the GAPDH mRNA and the ZO-1 mRNA, respectively, by the ΔΔC_t-method. As some data sets were not normally distributed all quantitations are presented as box plots, where boxes represent 25th and 75th percentiles, respectively. Medians are indicated by horizontal lines. Whiskers indicate 10th and 90th percentiles, respectively. p-values indicating significant differences are given in the panels.