DIMINISHED EFFECT OF INHIBITION OF NITRIC-OXIDE SYNTHASE ON REGIONAL CEREBRAL VASCULAR-RESISTANCE IN CONSCIOUS AND IN ISOFLURANE-ANESTHETIZED RATS DURING HEMORRHAGE

Effects of a nitric oxide (NO) synthase inhibitor on regional cerebral vascular resistance (rCVR) and regional cerebral blood flow (rCBF) we re studied during a severe hemorrhage in conscious and in isoflurane a nesthetized groups of rats. Half of each group was infused with N(G)-n itro-L-arginine-methyl ester (L-NAME), a NO synthase inhibitor, at a r ate of 2 mg . kg-1 . min-1 for 30 min. Half of the L-NAME infused and half of the normal saline infused rats were bled to reduce the mean ar terial blood pressure (MAP) to 44-49 mmHg. rCBF was measured using [C- 14]iodoantipyrine. rCVR was calculated as the ratio of MAP to rCBF. In the conscious non-hemorrhagic rats, L-NAME markedly increased rCVR in all the brain regions that we studied. In the conscious rats without L-NAME treatment, hemorrhage decreased rCVR in most of the brain regio ns. With L-NAME treatment in this group, hemorrhage increased rCVR onl y in the rostral part of the brain. Isoflurane decreased rCVR in most of the brain regions except the cortical area. L-NAME markedly increas ed rCVR in all the brain regions that we studied in the isoflurane ane sthetized rats. In the isoflurane anesthetized rats, hemorrhage did no t reduce rCVR in any of the brain regions. In the isoflurane anestheti zed hemorrhagic rats, L-NAME did not significantly affect rCVR in any of the brain regions that we studied, We found that L-NAME increased r CVR to a greater extent in the non-hemorrhagic rats than in the hemorr hagic rats in both the conscious and in the isoflurane anesthetized ra ts. The effect of L-NAME appeared to be similar both under conscious a nd under isoflurane anesthetized conditions.