Cerebral and pial microvessels: differential expression of gamma-glutamyl transpeptidase and alkaline phosphatase

Pial microvessels have several important blood-brain barrier (BBB) characte ristics in common with cerebral microvessels, despite lacking their astrocy tic ensheathment. We have therefore determined whether they have the same d istribution of two enzymes, gamma-glutamyl transpeptidase (GGTP) and alkali ne phosphatase, both of which are known to be astrocyte-dependent. GGTP was absent from all rat pial microvessels but strongly present in brain cortic al capillaries. Alkaline phosphatase was heterogeneously expressed in pial microvessels, including capillaries, but strongly positive in brain cortica l capillaries. Diffusible, inductive factors produced by astrocytes could a ccount for these differences in enzyme distribution between the two vessel types. Furthermore, differences in expression between the two markers may r eflect their differing sensitivities to the astrocytic factors. Caution is urged in the common usage of the pial microvessel as a model system in BBB studies.