NITRIC-OXIDE SCAVENGING BY HEMOGLOBIN OR NITRIC-OXIDE SYNTHASE INHIBITION BY N-NITRO-L-ARGININE INDUCES CORTICAL SPREADING ISCHEMIA WHEN K+IS INCREASED IN THE SUBARACHNOID SPACE

We investigated the combined effect of increased brain topical K+ conc entration and reduction of the nitric oxide (NO.) level caused by nitr ic oxide scavenging or nitric oxide synthase (NOS) inhibition on regio nal cerebral blood flow and subarachnoid direct current (DC) potential . Using thiopental-anesthetized male Wistar rats with a closed cranial window preparation, brain topical superfusion of a combination of the NO. scavenger hemoglobin (Hb; 2 mmol/L) and increased Kf concentratio n in the artificial cerebrospinal fluid ([K+](ACSF)) at 35 mmol/L led to sudden spontaneous transient ischemic events with a decrease of CBF to 14 +/- 7% (n = 4) compared with the baseline (100%). The ischemic events lasted for 53 +/- 17 minutes and were associated with a negativ e subarachnoid DC shift of -7.3 +/- 0.6 mV of 49 +/- 12 minutes' durat ion. The combination of the NOS inhibitor N-nitro-L-arginine (L-NA, 1 mmol/L) with [K+](ACSF) at 35 mmol/L caused similar spontaneous transi ent ischemic events in 13 rats. When cortical spreading depression was induced by KCl at a 5-mm distance, a typical cortical spreading hyper emia (CSH) and negative DC shift were measured at the closed cranial w indow during brain topical superfusion with either physiologic artific ial CSF(n = 5), or artificial CSF containing increased [K+](ACSF) at 2 0 mmol/L (n = 4), [K+](ACSF) at 3 mmol/L combined with L-NA (n = 10), [K+](ACSF) at 10 mmol/L combined with L-NA (five of six animals) or [K +](ACSF) at 3 mmol/L combined with Hb (three of four animals). Cortica l spreading depression induced long-lasting transient ischemia instead of CSH, when brain was superfused with either [K+](ACSF) at 20 mmol/L combined with Hb (CBF decrease to 20 +/- 20% duration 25 +/- 21 minut es, n = 4), or [K+](ACSF) at 20 mmol/L combined with L-NA (n = 19). Tr ansient ischemia induced by NOS inhibition and [K+](ACSF) at 20 mmol/L propagated at a speed of 3.4 +/- 0.6 mm/min, indicating cortical spre ading ischemia (CSI). Although CSH did not change oxygen free radical production, as measured on-line by in vivo lucigenin-enhanced chemilum inescence, CSI resulted in the typical radical production pattern of i schemia and reperfusion suggestive of brain damage (n = 4). Nimodipine (2 mu g/kg body weight/min intravenously) transformed CSI back to CSH (n = 4). Vehicle had no effect on CSI (n = 4). Our data suggest that the combination of decreased NO. levels and increased subarachnoid KC levels induces spreading depression with acute ischemic CBF response. Thus, a disturbed coupling of metabolism and CBF can cause ischemia. W e speculate that CSI may be related to delayed ischemic deficits after subarachnoid hemorrhage, a clinical condition in which the release of Hb and K+ from erythrocytes creates a microenvironment similar to the one investigated here.