Further studies on reflectometric monitoring of cerebrocortical microcirculation. Importance of lactate anions in coupling between cerebral blood flow and metabolism.

In order to elucidate the possible role of lactate anions in the coupling between cerebral metabolism and blood flow (CBF), a circumscribed area of the brain cortex of chloralose anaesthetized cats was superfused with various concentrations (1-50 mM) of lithium-(1)-lactate. Cerebrocortical microcirculation (vascular volume, mean transit time, blood flow) and fluorescence of reduced nicotinamide adenine dinucleotide (NADH) were measured with a microscope fluoro-reflectometer. Before carrying out these experiments, the penetration depth of the 366 nm exciting light onto the brain cortex has been determined in vitro in cortical brain slices. In addition, in model experiments we studied the effect of deoxygenation of haemoglobin on intensity of the fluorescence and reflected lights measured at 450 nm and 366 nm, respectively. The in vitro experiments showed that approximately 95% of the exciting light may be absorbed by the 0.24 mm thick superficial layers of the brain cortex. A reasonably good linear relationship (correlation coefficient -0.96) was obtained between the blood content of the samples (blood content was varied between 1000 times and 10 times diluted arterial blood) and the reflectance signal (366 nm), and this relationship was not altered discernibly by 0.1 M sodium sulphite. These results indicate that the reflectance signal, measured at 366 nm in vivo in the intact brain cortex, is a good measure of the changes of cortical blood content, i.e. vascular volume. Topically applied lactate increased cerebrocortical NAD reduction in dependence on the concentration. The lactate-produced increase in NADH fluorescence was probably due to the conversion of lactate to pyruvate by the enzyme lactate dehydrogenase and, hence, the reduction of mitochondrial NAD (substrate effect). Because even 50 mM lactate increased CBF only slightly, by 9.4 +/- 2.5%, our results may rule out the involvement of lactate anions (without concomitant acidosis) in the coupling between cerebral metabolism and blood flow.