LATE EFFECTS OF X-IRRADIATION ON REGULATION OF CEREBRAL BLOOD-FLOW AFTER WHOLE-BRAIN EXPOSURE IN RATS

Hemodynamic parameters such as total cerebral blood volume (total CBV) , cerebral parenchymal blood volume (CBV), cerebral blood flow (CBF) a nd cerebral blood velocity index were measured in rats 6, 12 and 18 mo nths after single exposures of brain to 5, 10, 15 and 20 Gy X rays for total CBV, CBF and blood velocity index, and only 20 Gy for CBV. Tota l CBV and blood velocity index were determined by a noninvasive blood dilution method using [Tc-99m]pertechnetate and CBF by [I-131]iodoanti pyrine brain extraction. The CBV was obtained from both parenchymal pl asma and erythrocyte volumes measured in isolated brain by I-125-label ed serum albumin and Cr-51-labeled erythrocytes, respectively. Neither the dose nor the time after irradiation influenced total CBV. Neverth eless, CBV decreased slightly while CBF decreased strongly at 12 and 1 8 months after 20 Gy. In contrast, the blood velocity index increased progressively at 12 and 18 months after 15 Gy and at all times after 2 0 Gy. According to the coexistence in irradiated brains of a remodelin g with microvascular occlusions and dilated abnormal vessels, this low ered CBF can be explained by the smaller number of open capillaries an d a ''steal phenomenon'' through low-resistance channels developed in the parenchymal and extra-parenchymal vasculatures. Such a ''steal phe nomenon'' is also supported by the response of the blood velocity inde x, which appears to be the earliest sensitive index for the detection of hemodynamic changes with respect to time (6 months) and dose of rad iation (15 Gy). (C) 1997 by Radiation Research Society.