Postnatal development of γ-GT activity in rat brain microvessels corresponds to capillary growth and differentiation

It has been demonstrated histochemically that endothelial cells of the cerebral capillaries show a high activity in γ-glutamyl transpeptidase (γ-GT), an enzyme which takes part in the transfer of large neutral amino acids across the blood-brain barrier. Reports of the disappearance of enzyme activity in endothelial cells (EC) grown in culture¹² suggest that the presence of astroglial cells (AG) is required for the expression of γ-GT in these cells. The present study deals with the developmental changes in γ-GT activity in capillaries of rat cerebral cortex during ontogenesis (i.e. on days 2, 7, 11, 14, 21 and 60 after birth).

γ-GT activity is determined by measuring the enzyme kinetics of the histochemical reaction on isolated brain capillaries using a flying spot microscope densitometer. Enzyme activity is expressed as an increase in relative optical density (at 500 nm) in arbitrary units/min/μm² during the 2 min immediately after initiating incubation and corresponds to the γ-GT active area (in μm²) of the capillary segment. During early postnatal development, a biphasic change of γ-GT activity in capillaries of rat cerebral cortex is observed. The first phase (i.e. the postnatal period between the 2nd and 12th day) is characterized by a significant decrease in γ-GT activity, which coincides with the onset of the rapid mitotic proliferation of cortical endothelial cells. During the second phase (i.e. the postnatal period between the 12th and 21st day), a fast increase in the γ-GT activity can be measured. Then enzyme activity reaches the adult level between the 21st and 60th postnatal day.

With regard to changes in capillary length density and in astroglial proliferation the results correspond to the differentiation and maturation of the blood-brain barrier and confirm evidence stressing the important role of astroglial cells for the expression of γ-GT activity.