Nitric oxide and blood-brain barrier integrity.

The blood-brain barrier (BBB) is comprised of the endothelial cells that line the capillaries of the brain. The unique characteristics of this barrier include tight intercellular junctions, a complex glycocalyx, a paucity of pinocytic vesicles, and an absence of fenestra. These properties allow for the selective exchange of substances between the systemic circulation and the extracellular fluid compartment of the brain. It is well established that there are many conditions, including those mediated by nitric oxide (NO), that can lead to an opening of the BBB, eventually leading to vasogenic edema and secondary brain damage. The precise molecular mechanisms mediating NO-induced tissue injury and the breakdown of the BBB are complex and not completely understood. NO is a soluble, easily diffusible gas that is generated by NO synthase. Two of the isoforms of NO synthase are constitutive, calcium-dependent enzymes that modulate many physiological functions, including the regulation of smooth muscle contraction and blood flow. The third isoform is calcium-independent and inducible and can be stimulated by stress, inflammation, and infection. Under these conditions, NO can be generated in large quantities and has detrimental effects on the CNS. NO has been shown to increase permeability of the BBB, allowing substances to enter into the brain passively. This review considers the role of NO and BBB integrity.

[14C]7-ethoxycoumarin metabolism by precision-cut rat hepatic slices.

The metabolism of [14C]7-ethoxycoumarin ([14C]7-EC) has been studied in rat liver slice cultures in vitro by using a direct radiometric high-pressure liquid chromatography method. [14C]7-EC was extensively biotransformed in these incubations to 7-hydroxycoumarin (7-OHC), 7-hydroxycoumarin glucuronide, and 7-hydroxycoumarin sulfate, as well as to a large number of previously unrecognized metabolites, the majority of which are sulfate conjugates. The liver slice [14C]7-EC metabolite profile was also very complex and seemed to be qualitatively similar to the medium metabolite profile. Quantitative comparisons, however, demonstrated that there was approximately two to five times more 7-OHC in the liver slice than in the medium, whereas 7-hydroxycoumarin sulfate, the most abundant metabolite in the medium, was present only at low levels in the liver slice. These data demonstrate that 7-OHC levels are considerably underestimated when only levels in the medium are considered. Total metabolite levels were approximately equal in the medium and liver slice after a 2-hr incubation, with considerably higher total metabolite levels present in the medium at the end of the incubation period (8 hr). Additional studies are needed to identify the structures of the previously unrecognized metabolites observed in this study and the enzymes responsible for their formation, as well as studies to define the metabolism of [14C]7-EC in other in vitro models by using tissue from humans and other animal species.