Repeated, short-term ischemia augments bradykinin-mediated opening of the blood–tumor barrier in rats with RG2 glioma

Abstract

The objective of this study was to investigate the effects of repeated, short-term ischemia on bradykininmediated permeability of the blood-brain barrier (BBB) and the blood-tumor barrier (BTB). The mechanism by which bradykinin transiently opens the BTB, involves B2 receptors, Ca2+ flux, nitric oxide (NO) and cyclic GMP (cGMP). Since global and focal cerebral ischemia are known to increase levels of brain nitric oxide synthase (bNOS) and endothelial nitric oxide synthase (eNOS) we tested the hypothesis that bradykinin may increase the BTB permeability to a greater extent under ischemic rather than nonischemic conditions. The vertebral arteries in female Wistar rats were coagulated immediately after intracerebral implantation of RG2 glioma. Short-term ischemia was produced in some rats by a modification of the fourvessel occlusion procedure for incomplete forebrain ischemia, in which the common carotid arteries were clamped daily for 15 min on days 7, 8 and 9 after tumor implantation, after which reperfusion was allowed. On day 10 after tumor implantation, bradykinin (10 µg kg-1 min-1) or phosphate-buffered saline (PBS) was infused for 15 min into the right carotid artery of anesthetized, sham-operated (nonischemic controls) and ischemic rats, followed by an intravenous bolus (100 µCi kg-1) each of [14C]-iodo-antipyrine (IAP), [14C]dextran or [14C]-aminoisobutyric acid (AIB) to measure regional cerebral blood flow (rCBF), blood volume, or unidirectional transfer constant Ki, respectively, by quantitative autoradiography. A single 15-min ischemic episode significantly decreased rCBF in the tumor center (158.9 ± 17.33 in control vs. 58.78 ± 24.45 ml 100 g-1 min-1 in ischemic group; p < 0.01) and in the tumor periphery (106.82 ± 7.34 in control vs. 70.55 ± 26.66 ml 100 g-1 min-1 in ischemic group; p < 0.05). Respective mean blood volume in tumors (11.7 ± 13.3, 12.7 ± 14.0, and 13.3 ± 14.5 µlg-1) from ischemic-PBS, nonischemic-bradykinin, and ischemic-bradykinin groups, respectively, was not significantly different; mean blood volume in normal brain (3.7, 3.1 and 3.8 µlg-1) was not significantly different among these groups either. Intracarotid infusion of bradykinin following repeated ischemia significantly increased mean Ki, as compared to bradykinin infusion in nonischemic controls, in both the tumor center (36.60 ± 8.4 vs. 22.90 ± 4.61 µlg-1 min-1, p < 0.05) and in tumor periphery (17.70 ± 5.93 vs. 8.50 ± 4.42 µlg-1 min-1, p < 0.05). Mean Ki values for tumor center and tumor periphery of ischemic rats receiving intracarotid bradykinin were 3-fold greater than those of nonischemic rats infused with PBS. Immunohistochemical and Western blot analyses showed that repeated, short-term ischemia significantly increased the levels of bNOS in tumor cells and eNOS in tumor capillaries, but neither induced iNOS nor affected B2 receptor levels in tumor cells in vivo, as compared with nonischemic controls. Taken together, these results demonstrate for the first time that repeated, short-term ischemia augments bradykinin-mediated opening of the BTB. We conclude that the elevated intratumoral levels of bNOS and eNOS may 'prime' the NO generating capacity of tumor cells. Consequently, increased de novo synthesis and a correspondingly elevated concentration of NO within the tumor, therefore, may be one mechanistic explanation for the significantly increased, bradykinin-mediated BTB opening under ischemic conditions, reported here. [Neurol Res 2001; 23: 631-640]