Introduction

The development of techniques for isolation of capillaries from either laboratory animal brain or human autopsy brain was an important event in the evolution of blood–brain barrier (BBB) methodologies. It was the availability of isolated capillaries that allowed BBB research to evolve from organ physiology to the cellular and molecular level. Bovine and human brain capillaries were initially isolated by Siakatos et al. (1969) and by several groups in the early and mid-1970s (Joo and Karnushina, 1973; Brendel et al., 1974; Goldstein et al., 1975). These methodologies uniformly used mechanical homogenization techniques and subsequent studies showed that capillaries isolated with mechanical homogenization methods did not exclude Trypan Blue and were metabolically leaky (Williams et al., 1980). Although microvessels isolated with an enzymatic homogenization technique and which are not mechanically disturbed do generally exclude Trypan Blue, these microvessels are also metabolically impaired. The adenosine triphosphate (ATP) concentration in capillaries isolated from brain with either a mechanical or an enzymatic homogenization technique is decreased more than 90% (Lasbennes and Gayet, 1984). The serious depletion of ATP in brain microvessels isolated with an enzymatic homogenization technique is a puzzling phenomenon since, in general, cells that are isolated with an enzymatic homogenization technique have normal ATP supplies (Kilberg, 1982).