ObjectivesWe hypothesized that the loss of cerebral blood flow (CBF) auto-regulation under hypertensive conditions could promote cerebrovascular over-perfusion and haemorrhage formation. The possibility that CBF auto-regulation becomes defective prior to haemorrhagic stroke development was assessed in Wistar–Kyoto stroke-prone spontaneously hypertensive rats (SHRsp) and related to the myogenic responsiveness of the cerebrovasculature to pressure.MethodsLaser Doppler techniques were used to measure relative CBF in relation to mean arterial pressure (MAP 130-260 mmHg) within the perfusion domains of the middle (MCA) and posterior (PCA) cerebral arteries. The ability of isolated MCAs and PCAs to constrict to a 120 mmHg pressure step (pressure-dependent constriction) was measured using a pressure myograph.ResultsTwo weeks prior to stroke, 10-week-old pre-stroke SHRsp exhibited near-constant CBF regulation to a 200 mmHg MAP. Thirteen-week-old pre-stroke SHRsp and age-matched post-stroke SHRsp lost their ability to auto- regulate CBF in the MCA and PCA perfusion domains. CBF increased at a high rate and in a linear manner with MAP. A distinct upper limit to CBF auto-regulation was absent. Pressure-dependent constriction was attenuated prior to stroke, and lost after stroke in isolated MCAs, but not the PCAs, of SHRsp.ConclusionsThe loss of CBF auto-regulation prior to stroke in SHRsp could enhance cerebral perfusion and facilitate the initiation of haemorrhage. Such dysfunction after stroke could produce secondary haemorrhages. Defects in pressure-dependent constriction cannot fully account for the pattern of CBF auto-regulation loss observed in post-stroke SHRsp.