Digital computer techniques have been employed to extract cortical evoked potentials to paired visual stimuli. Changes in the evoked potentials have been related to perceptual phenomena varying as a function of the interval between flashes. Evoked potentials to paired stimuli, which gave rise to perceptual interactions, could be approximated by algebraic summation of the responses to the stimuli when presented separately.
Summary: Studies have shown that in comparison to rapid occlusion of a vessel, gradual occlusion produces less severe tissue ischemia due to a more effective devel opment of collateral circulation. As other studies have shown that collateral circulation can be enhanced by stimulation of the endogenous renin-angiotensin II system, it was hypothesized that this system is involved in the mechanism of protection against ischemia that ob tains during gradual vascular occlusion. To test this hy pothesis, mortality rates were evaluated in gerbils sub jected to gradual vascular occlusion by means of progres sive carotid ligation while simultaneously infused with inhibitors of the renin-angiotensin II cascade-enala prilat or saralasin. Groups of animals with either abrupt or progressive carotid ligation infused with saline served as controls. Results showed that (1) in saline-infused anGradual occlusion of a vessel results in a less de leterious effect on the tissue elements of the distal territory in comparison to an abrupt occlusion. This difference is due to the presence of adaptive mech anisms that require time to be engaged and cannot be properly utilized when the occlusion is made quickly. As a result of these adaptive mechanisms, a more effective development of collateral circula tion occurs that reduces the severity of tissue isch emia. This effect can be seen not only in the arterial side of circulation, such as cerebral or coronary ar teries (Svarzbein et aI. , 1974; To moike et aI., 1981;Bache and Schwartz, 1983; Patterson et aI. , 1983), but also in the venous side such as the portal vein Received September 30, 1986; accepted October 21, 1987. Address correspondence and reprint requests to Dr. Leonardo A. Fernandez, at Section of Neurosurgery, Yale University School of Medicine, 333 Cedar Street, New Haven, CT 06510, U.S.A.
149imals, there was a significant decrease in the mortality rate of progressive-ligated animals when compared to abrupt-ligated animals, and (2) administration of either enalaprilat or saralasin to progressive-ligated animals re sulted in mortality rates that were indistinguishable from those of saline-infused abrupt-ligated animals. These re sults suggest that the endogenous renin-angiotensin system is indeed involved in an adaptive mechanism that occurs during progressive ligation of the carotid artery, and more specifically, that the relatively benign effect of progressive carotid ligation may be due to the action of angiotensin II to stimulate the development of collateral circulation and reduce the severity of focal brain isch emia.
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