Myogenic mechanisms in the cerebral circulation.

The autoregulatory responses of cerebral blood vessels during changes in perfusion pressure are mediated by several mechanisms. Metabolic mechanisms appear to predominate during decreases in perfusion pressure but myogenic responses can be detected under some conditions. There are major differences in the autoregulatory capacity of the cerebrum and brainstem and recent studies have suggested that the autoregulatory responses of cerebral vessels in vitro depend on the endothelium. To test this concept, we performed preliminary studies to produce selective endothelial injury of cerebral arterioles in rats, using a light-dye technique. The light-dye technique abolished the vasodilatory responses to acetylcholine and bradykinin, but the responses to adenosine and graded hypotension were preserved. These findings suggest that endothelial injury does not impair the autoregulatory responses of cerebral vessels in vivo. Calcium entry inhibitors greatly impair the autoregulatory responses of cerebral vessels, which suggests that entry of extracellular calcium is essential for cerebral autoregulation. Chronic hypertension alters cerebral autoregulation and may produce 'remodelling,' so that autoregulatory constriction is increased.