Subregional topography of capillaries in the dorsal vagal complex of rats: II. Physiological properties

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Abstract
The differentiated cytoarchitecture, neurochemistry, and capillary organization of the rat dorsal vagal complex prompted this comprehensive investigation of micro vascular physiology in 11 subdivisions of area postrema, 5 subnuclei of nucleus tractus solitarii (NTS), the dorsal motor nucleus of the vagus nerve, and 4 other gray matter structures in the dorsal medulla oblongata. Microvascular exchangeable volume (residual plasma volume), capillary blood and plasma flow, and unidirectional transfer constants for a tracer amino acid, [14C]α-aminoisobutyric acid (AIB), varied considerably among the structures analyzed. Exchangeable volume, largest in area postrema medial zones (about 29 μl μl · g−1) and smallest in medullary gray matter (7–11 μl · g−1), correlated directly with subregional densities of capillaries and rates of tissue glucose metabolism. Capillary blood flow (range of 1,430–2,147 μl · g−1 · min−1), plasma flow, and tissue glucose metabolism (range of 0.48–0.71 μmol · g−1 · min−1) were linearly related in the dorsal vagal complex. The most striking quantitative difference among structures in this brain region were the rates of transcapillary influx and derived permeability X surface area (PS) products of [14C]AIB, which has physicochemical properties resembling those of hormones. PS products for AIB were negligible in most medullary gray matter regions (< 1 μl · g−1 · min−1, indicative of blood-brain barrier properties), but were 20–59 × and 99–402 × higher in NTS subnuclei and area postrema, respectively. An extraordinary feature of the microcirculation in area postrema was the long-duration transit of tracer sucrose and blood, a characteristic that would amplify the sensing ability of area postrema as it monitors the composition of the circulation.