The effects of hexamethonium on cerebral blood flow and cerebral function during relative cerebral ischaemia in rats

Cerebral blood flow in either the cortex, thalamic region or the brain stem, as well as somatosensory evoked potentials were measured in a model of moderate cerebral ischaemia in three groups of anaesthetized spontaneously hypertensive rats. The rats were bled to reduce evoked potential amplitudes to approximately 50-60% of pre-haemorrhage control. The consequent blood pressure fall reduced blood flow to approximately 65, 80 and 85% of pre-haemorrhage control in the cortical, thalamic and brain stem regions, respectively, as measured with a laser Doppler flowmeter. Hexamethonium (10 mg kg-1 i.v.), an autonomic ganglion blocker, caused vasodilation and a slight (7-13 units of prebleeding control) increase in blood flow in all the three regions, and the somatosensory evoked potentials normalized. In addition, the latency of the first evoked potential component decreased toward prebleeding values. Heart rate decreased and a transient decrease was also observed in mean arterial pressure despite an attempt to keep it constant with a pressure regulating reservoir. It is possible that the slightly increased regional cerebral blood flow after hexamethonium injection can explain the improved cerebral function as indicated by the enhanced somatosensory evoked potentials. However, the results might also indicate an autonomic regulation of afferent sensory pathways.