A persistent increase in lung endothelial permeability to protein is a hallmark of the Adult Respiratory Distress Syndrome. Moreover, inflammatory mediator-initiated increase in the Area/Unit pathlength (Arho/Ax) of the Endothelial Junctional Space is the foundation of elevated solute permeability of the microvascular wall. The long term objectives of this research are to better understand the endothelial mechanisms that modulate pulmonary microvascular permeability to macromolecules. Intracellular level of cyclic nucleotide, adenosine 3',5' cyclic monophosphate (cAMP), or the activities of protein kinase C and A, and intracellular free Ca2+ concentration ([Ca2+]) will be specifically manipulated in vitro. The hypotheses to be tested are: 1) Agonist-induced changes of intracellular second messengers modulate endothelial morphology leading to the discrete Solute Permeability Mechanism: Restricted Diffusion or Convection and 2) Different mediator induced distinct cellular responses by discrete spatiotemporal alterations in [Ca2+] of cytoskeleton protein will be assessed in specific aims 1-3. Specific aim 4 will test the Crone Hypothesis. These aims will be tested using bovine lung endothelial cells from two distinct locations: large vessel (pulmonary artery endothelial cells, AEC) and macrovessel (MEC). Monolayers of AEC and MEC will be used as cellular models of the lung microvascular barrier to solutes. The following assays will be used: 1) The solute permeability mechanisms that define the barrier properties of endothelial monolayer will be measured for each experimental condition, 2) Endothelial surface area will be measured from differential interference contrast (DIC) digital images using Image-1 software (universal Imaging Corp), 3) Agonist-induced spatiotemporal alterations of intracellular [Ca2+] will be measured by fura-2 fluorescence digital images. 4) Agent-induced rearrangement of endothelial F-actin/myosin II will be measure by fluorescence digital images in living cells, 5) Electron microscopic analysis by rotary shadowing of the endothelial cytoskeleton will be performed, 6) Intracellular [cAMP] and [cGMP] will be measured by radioimmunoassay, 7) Protein kinase activities will be measured by P-labelled immunoaffinity protein SDS polyacrylamide gel electrophoresis.