EFFECTS OF N-OMEGA-NITRO-L-ARGININE METHYL-ESTER ON THE CEREBRAL-CIRCULATION OF NEWBORN PIGLETS QUANTIFIED IN-VIVO BY NEAR-INFRARED SPECTROSCOPY

The effects of N-omega-nitro-L-arginine methyl ester (L-NAME) on basal cerebral vascular tone, the vasodilatory effects of acetylcholine (AC h), and the cerebrovascular response to alterations in arterial carbon dioxide tension (CBVR) were investigated using near-infrared spectros copy. Seven newborn piglets were anesthetized and mechanically ventila ted; mean arterial blood pressure (MAP) was monitored and near-infrare d spectroscopy used to measure changes in total cerebral Hb concentrat ion. At the beginning of the experiment, CBVR was measured and then 10 , 20, 30, and 100 mg . kg-1 L-NAME were administered sequentially; ACh (1, 2, 3, and 5 mug) was given before and after each injection of L-N AME. At the end of this sequence, CBVR was measured again and finally sodium nitroprusside (1.5 mg . kg-1) was administered. Ten and 20 mg . kg-1 L-NAME caused a significant decrease in total cerebral Hb concen tration of -0.59 (-3.21 to -0.02) and -1.46 (-6.50 to -0.15) mumol . L -1 (median and range), respectively (Wilcoxon p < 0.05), but subsequen t injections did not. Ten, 20, and 100 mg.kg-1 L-NAME caused an increa se in MAP (Wilcoxon p < 0.05). ACh caused an increase in total cerebra l Hb concentration and a decrease in MAP that was impaired but not abo lished by L-NAME (ANOVA p < 0.05). CBVR was not affected by L-NAME. So dium nitroprusside caused a reduction in mean (SD) MAP of 4.7 (1.6) kP a, and a slower rise in [tHb] of 13.44 (2.03) mumol . L-1. Postmortem examination of three animals revealed NADPH-diaphorase staining in neu rons, cerebral blood vessels, carotid artery, and jugular vein. These results demonstrate that nitric oxide modulates basal vasodilation and part of the vasodilatory action of ACh in the newborn piglet brain, b ut that these effects are quantitatively small. Nitric oxide was not s hown to be an important participant in CBVR. This is consistent with t he suggestion that the mechanism mediated by nitric oxide may coordina te perfusion rather than control bulk flow in the developing brain.