Hypothermia-induced reduction in the permeation of radiolabelled tracer substances across the blood-brain barrier.

The permeability of the rat blood-brain barrier during different levels of hypothermia was examined using a new and more sensitive quantitative radiotracer technique. In contrast to the findings of previous studies, tracer permeation as measured here by the calculated cerebrovascular permeability-area product (PA), did not increase during hypothermia. Rather, rats maintained at specific temperatures between 30-16 degrees C (1 h) by contact surface cooling, displayed lowered permeation (PA) of i.v. injected 14C-sucrose, 125I-bovine serum albumin and 3H-alpha-amino-isobutyric acid in all brain regions examined. This effect was temperature-dependent and reversible on rewarming to normothermic temperature. Elevated plasma tracer concentration vs. time was characteristic of hypothermic rats except those cooled to 30 degrees C. That no elevation in plasma tracer accompanied alterations in tracer permeation at 30 degrees C, indicates hypothermic induced reductions in PA are independent of altered plasma tracer levels. Furthermore, the nature of alpha-amino-isobutyric acid to rapidly distribute to intracellular vs. extracellular sites suggests the effects of hypothermia seen here are due primarily to the condition of cerebrovascular membranes.