CHANGES OF THE NA K ATPASE ACTIVITY IN THE CEREBRAL CORTICAL MICROVESSELS OF RAT AFTER SINGLE INTRAPERITONEAL ADMINISTRATION OF MERCURIC-CHLORIDE - HISTOCHEMICAL-DEMONSTRATION WITH LIGHT AND ELECTRON-MICROSCOPY/

Since inorganic mercury salts only poorly penetrate the cerebral micro vascular endothelial cells comprising the blood-brain barrier (BBB), t heir neurotoxicity may be predicted to result from interference with B BB transport enzymes. In the present study, we tested the effect of me rcuric chloride (HgCl2) on Na+/K+ ATPase activity, a key enzyme involv ed in the ion transport in and out of the brain. Routine histochemical staining in conjunction with light and electron microscopy was used t o evaluate the changes in the Na+/K+ ATPase activity in cerebral corti cal microvessels of rats who received a single intraperitoneal injecti on of 6 mg/kg HgCl2. At 1 h after HgCl2 administration, light microsco py revealed uniform reduction of the Na+/K+ ATPase reaction in all cor tical layers. Electron microscopy confirmed the enzyme reaction to be very weak to completely absent in both the luminal and abluminal endot helial cell membranes, and the luminal plasmalemma showed invagination s and pinocytic vesicles indicative of changes in its transport functi ons. The enzyme inhibition coincided with, and was likely to contribut e to, profound perivascular swelling, involving mainly the astrocytic endfeet. The enzyme activity showed a partial recovery 18 h after HgCl 2 treatment, mainly in cortical layers II and III. After 5 days, the r ecovery of the enzyme activity appeared complete as observed by light and electron microscopy.The recovery of the microvascular Na+/K+ ATPas e coincided with the appearance of a strongly positive Na+/K+ ATPase r eaction in the adjacent astrocytic processes and with the diminution o f perivascular swelling.