Pre-existing level of tone is an important determinant of cerebral artery autoregulatory responsiveness.

The objective of this study was to test the hypothesis that cerebral artery autoregulatory responses to increasing transmural pressure depend on the pre-existing level of vascular tone. Pial arteries were obtained from adult normotensive Wistar-Kyoto (WKY) rats, and were cannulated and studied under pressurized conditions using a video-assisted perfusion system. Myogenic tone developed spontaneously during equilibration, reducing the lumen diameter by 31% (n = 6). Each artery was then subjected to a series of step increases in transmural pressure of between 50 and 100 mmHg, a manoeuvre that produces autoregulatory constriction. Afterwards, serotonin was added to the perfusate to decrease lumen diameter by another 27% and the transmural pressure changes were repeated once more. The autoregulatory effectiveness in response to a change of 50----100 mmHg in transmural pressure was expressed as both the percentage change in diameter and the myogenic 'gain'. During the increased activation induced by serotonin, the autoregulatory responses to a change in transmural pressure were completely abolished; lumen diameter, which had previously decreased by 4.9 +/- 1.5%, increased by 10.5 +/- 4.5% (P less than 0.05). The nature of this change is further reflected in the myogenic gain, which was 0.39 +/- 0.14 in vessels with myogenic tone alone, and -1.14 +/- 0.56 for the same arteries at the higher level of activation (P less than 0.05). In additional experiments (n = 6), a comparable degree of constriction was induced with 0.5% ethanol, which increases calcium influx through receptor-independent mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)