Introduction

Cerebral microvessels possess properties that are typical for peripheral blood vessels, and in addition have properties of epithelial tissues (for review see Joó, 1996). In contrast to the peripheral vasculature, the endothelial cells of brain microvessels are connected by tight junctions and thus provide a permeability barrier between blood and brain interstitial fluid. This so-called blood–brain barrier (BBB) has properties in common with tight epithelia, i.e. an input resistance of 1–2 k φ/cm2, and different transport mechanisms in the luminal and antiluminal (brain-facing) membrane (Betz and Goldstein, 1986). The Na+/K+-ATPase is present in the antiluminal membrane and transports Na+ from inside the cell into the brain interstitial fluid in exchange for K+ ions (Betz et al., 1980). It has been demonstrated that an amiloride-sensitive Na+ influx as well as a Na+/Cl- co-transport, inhibited by furosemide, is present on the luminal side of the capillaries (Betz, 1983). This enables the transport of salt and water from the luminal to the brain side. However, K+ transport is mainly directed from the brain to the blood side (Hansen et al., 1977), and it has been concluded that the blood–brain barrier is involved in homeostasis of brain K+ concentration (Bradbury and Stulcová, 1970; Goldstein, 1979).

The development of the patch-clamp technique by Neher and Sakmann (1976) provided new possibilities to study ion transport across cell membranes. The cell culture technique has primarily been applied to study electrogenic transport systems in endothelial cells.