Histamine decreases selectivity of sheep lung blood-lymph barrier

Histamine increases lymph flow and protein flux from the sheep lung. This increase in water and protein movement has been attributed to an increase in lung microvascular permeability-surface area product caused by histamine. Whether it was permeability that was changed or whether it was surface area cannot be discerned from past work. Histamine might increase either of these determinants of transport in the sheep lung. Previous measurements of several plasma protein fractions in plasma and lymph showed no change in the sieving behavior of the microcirculation with intravenous histamine, suggesting that only surface area was changed. In the present study, even larger test molecules were used to probe the limits of transport through the lung's blood-lymph barrier. Fluorescein isothiocyanate-labeled dextrans (FITC-dextran) that can normally permeate lung lymph do so more readily after intravenous histamine infusion (3 micrograms X kg-1 X min-1). Formerly impermeable FITC-dextran fractions also appear after histamine administration. No definitive size limit to the transport of dextran was found in histamine-stimulated sheep lungs. Intra-arterial histamine does not increase lung lymph, protein, or FITC-dextran flow. These results suggest that histamine changes the permeability or sieving characteristics of the lung microvascular barrier making it less size selective to large molecules. Surface area for transport may also increase.