EFFECTS OF NITRIC-OXIDE SYNTHESIS BLOCKADE AND ANGIOTENSIN-II ON BLOOD-FLOW AND SPONTANEOUS VASOMOTION IN THE RAT CEREBRAL MICROCIRCULATION

The effects of N(omega)-nitro-L-arginine methyl ester (L-NAME) an inhi bitor of NO synthesis, or angiotensin II on the frequency and amplitud e of rhythmic variations (vasomotion) in blood flow of the intact rat cerebral circulation were investigated using laser-Doppler flowmetry ( LDF). Experiments were performed on Sprague-Dawley rats anaesthetized with alpha-chloralose. The rat's head was fixed on a stereotaxic frame and the microvascular blood flow of the parietal cortex on the right or on both sides was measured via a small hole in the parietal bone, k eeping the dura and a thin bone layer intact. Following the intravenou s injection Of L-NAME, the mean arterial blood pressure (MABP) increas ed to 123 +/- 1 mmHg (1.25 mg kg-1) or to 144 +/- 3 mmHg (5.0 mg kg-1) but no significant changes in cerebral blood flow (CBF) or vasomotion could be detected. The observed increase in MABP was sustained until L-arginine was administered. In the presence Of L-NAME, during stepwis e reduction of MABP, CBF remained constant when MABP was kept between 60 and 130 mmHg, the vasomotion frequency was lower when MABP was abov e 80 mmHg but its amplitude was two times higher than in the control g roup. In another group of animals, angiotensin was infused to give com parable increments in blood pressure. In contrast to L-NAME, angiotens in II had no effect on either frequency or amplitude of vasomotion, co mpared to the control group, within the whole range of MABP studied. O ur study suggests that NO might be a significant modulator of the micr ovascular tonus in the brain and that it plays a significant role in c ontrolling vasomotion frequency and amplitude.