The effects of increased intracranial pressure (ICP) on pial vessel diameters were investigated in rats using a cranial window and fluorescence microscopy. The brain surface was superfused with mock cerebro-spinal fluid (CSF) at a constant rate (5 ml/h), and the ICP was raised up to 20 mmHg by elevating a CSF-reservoir connected to the CSF-outlet of the window. Arterioles dilated as the ICP increased (+ 12% dilation at 20 mmHg ICP). Following a rapid reduction of the raised ICP to normal, arteriolar diameters did not return to control values (+ 7% dilatation), while venules dilated (+ 3%), indicating reactive hyperaemia. At this time, CO2 inhalation induced a low response in the arterioles (+ 0.4%/mmHg PaCO2 increase) and an over-response in the venules (+ 0.3%/mmHg). The CO2 response decreased in smaller arterioles (< or = 30-40 microns phi). In addition, a few animals showed extravasation of Na(+)-fluorescein administered intravenously. Our results indicate that reactive hyperaemia can take place following a rapid return from an increased ICP to an normal level, even in cases of mild intracranial hypertension; a disruption of the blood-brain barrier may follow.
Animals Arterioles