Tissue Water Content and the permeability-surface area product (PS) for [51Cr]-EDTA
Change in microvascular permeability and surface area for transvascular exchange following the various treatments was investigated by measurement of the permeability-surface area product (PS) for [51Cr]-EDTA, as described earlier [30, 31]. For this purpose the animals received a bolus infusion of about 370 kBq of the tracer [51Cr]-EDTA, (0.5 mL) (GE Health Care, Stockholm, Sweden). The bolus infusion was followed by a continuous infusion of the tracer at a rate of 0.33 mL/h (3.7 MBq/mL). Arterial blood samples (10 μL) for analyses of plasma [51Cr]-EDTA concentration were collected at 2.5, 5, 10, 15, 25, 35 and 40 min post start of the bolus injection. After 37 min, a bolus dose of about 25 kBq of [125I]-albumin dissolved in 0.1 mL isotonic saline was given in the femoral vein for calculation of plasma volume in the brain. Three min later, the experiment was finished by an arterial blood sample and the animal was sacrificed by decapitation. Before each experiment [125I]-albumin was purified from free iodine, using centrifugal filtration. The brain and the TGs were removed and put on a chilled support. In order to analyze PS in the cortex, periaqueductal gray (PAG), trigeminal nuclei caudalis (TNC) and cerebellum, tissue structures were extracted and immediately weighed. Tissue and blood tracer activities of 51 Cr-EDTA and 125I-albumin were determined in a gamma counter. Tissue samples were then dried in an oven for 24 h at 100 °C and brain water content was calculated as [(wet weight-dry weight)/wet weight)] × 100. Arterial hematocrit was measured before and during tracer infusion in order to convert blood concentrations into plasma concentrations. The blood to brain transfer constant (Ki) for [51Cr]-EDTA was then calculated according to the following equation Ki = B/ 0∫T Ca (t) d t [32], where B is the amount of tracer that has moved from blood to brain (tissue uptake of tracer minus regional tracer concentration in plasma), Ca is concentration of the tracer in arterial plasma as a function of time, and T is the duration of the experiment. Ki is a function of capillary plasma flow per unit mass of tissue (FV) and the permeability-surface area product (PS), the latter reflecting microvascular permeability and surface area available for diffusional exchange. The mathematical expression for this relationship is Ki = FV [1-e-PS/FV] which can be rewritten as PS = -FV ln (1-Ki/FV) [33]. From this expression it can be deduced that with a Ki/FV ratio of less than 0.1 the Ki value approximates PS with an error of less than 6 % [34].