The Effect of a Low pH Saline Perfusate upon the Integrity of the Energy-Depleted Rat Blood-Brain Barrier

The effect of a low pH perfusate upon the integrity of the rat blood-brain barrier was studied using an in situ supravital brain perfusion technique in which high-energy phosphates are depleted. Control animals were perfused for 10 min with a Ringer's salt solution containing the metabolic inhibitor 2,4-dinitrophenol (DNP) and adjusted to a pH of 7.4. In two separate experimental groups the perfusate, consisting of either the same medium as the controls or with additional buffering from Tris maleate, was switched after 5 min at a pH of 7.4, to a medium adjusted to pH 5.5 with lactic acid. Following a total perfusion time of 10 min, the integrity of the blood-brain barrier was assessed using the small molecular weight tracer [¹⁴C]mannitol. The cerebral perfusate flow rates (CPFR) after 10 min of perfusion were also determined in the three groups by perfusing for 40 s with [¹⁴C]iodoantipyrine. In each group, mannitol was excluded from the tissue of the brain to the same degree as has been previously reported in vivo, indicating an intact blood-brain barrier. There was also no significant pH-dependent change in CPFR. Ultrastructural examination of animals that had been perfusion fixed following in situ perfusion revealed no obvious differences between the cerebral endothelium of the control and low pH perfused animals. These results demonstrate that in the absence of energy-producing metabolism a perfusate pH of 5.5 is insufficient to disrupt the blood-brain barrier.