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Aim Maintenance of the blood and extracellular volume requires tight control of endothelial macromolecule permeability, which is regulated by c AMP signalling. This study probes the role of the c AMP mediators rap guanine nucleotide exchange factor 3 and 4 (Epac1 and Epac2) for in vivo control of microvascular macromolecule permeability under basal conditions. Methods Epac1−/− and Epac2−/− C57 BL /6J mice were produced and compared with wild-type mice for transvascular flux of radio-labelled albumin in skin, adipose tissue, intestine, heart and skeletal muscle. The transvascular leakage was also studied by dynamic contrast-enhanced magnetic resonance imaging ( DCE- MRI) using the MRI contrast agent Gadomer-17 as probe. Results Epac1−/− mice had constitutively increased transvascular macromolecule transport, indicating Epac1-dependent restriction of baseline permeability. In addition, Epac1−/− mice showed little or no enhancement of vascular permeability in response to atrial natriuretic peptide ( ANP), whether probed with labelled albumin or Gadomer-17. Epac2−/− and wild-type mice had similar basal and ANP-stimulated clearances. Ultrastructure analysis revealed that Epac1−/− microvascular interendothelial junctions had constitutively less junctional complex. Conclusion Epac1 exerts a tonic inhibition of in vivo basal microvascular permeability. The loss of this tonic action increases baseline permeability, presumably by reducing the interendothelial permeability resistance. Part of the action of ANP to increase permeability in wild-type microvessels may involve inhibition of the basal Epac1-dependent activity. [ABSTRACT FROM AUTHOR]

Collagen XV and XVIII are ubiquitous constituents of basement membranes. We aimed to study the physiological roles of these two components of the permeability barrier non-invasively in striated muscle in mice deficient in collagen XV or XVIII by dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI). Structural information was obtained with transmission electron microscopy (TEM). MR data were analysed by two different analysis methods to quantify tissue perfusion and microcirculatory exchange parameters to rule out data analysis method-dependent results. Control mice (C57BL/6J Ola/Hsd strain) or mice lacking either collagen XV ( Col15a1−/−) or XVIII ( Col18a1−/−) were included in the study. MR images were acquired using a preclinical system using gadodiamide (Gd-DTPA-BMA, molecular weight 0.58 kDa) as a tracer. Exchange capacity (permeability ( P)-surface area ( S) product relative to blood flow ( FB)) was increased in test mice compared to controls, but the contributions from P, S, and FB were different in these two phenotypes. FB was significantly increased in Col18a1−/−, but slightly decreased in Col15a1−/−. PS was significantly increased only in Col18a1−/− even though P was increased in both phenotypes suggesting S might also be reduced in Col15a1−/− mice. Immunohistochemistry and electron microscopy demonstrated alterations in capillary density and morphology in both knockout mouse strains in comparison to the control mice. Both collagen XV and XVIII are important for maintaining normal capillary permeability in the striated muscle. DCE-MRI and the perfusion analyses successfully determined microvascular haemodynamic parameters of genetically modified mice and gave results consistent with more invasive methods. [ABSTRACT FROM AUTHOR]

Aim: Epac1 -/- mice, but not Epac2 -/- mice have elevated baseline permeability to albumin. This study extends the investigations of how Epac-dependent pathways modulate transvascular exchange in response to the classical inflammatory agent histamine. It also evaluates the limitations of models of blood-to-tissue exchange in transgenic mice in DCE-MRI measurements., Methods: We measured DCE-MRI signal intensity in masseter muscle of wt and Epac1 -/- mice with established approaches from capillary physiology to determine how changes in blood flow and vascular permeability contribute to overall changes of microvascular flux. We used two tracers, the high molecular weight tracer (Gadomer-17, MW 17 kDa, apparent MW 30-35 kDa) is expected to be primarily limited by diffusion and therefore less dependent on changes in blood flow and the low molecular weight tracer (Dotarem (MW 0.56 kDa) whose transvascular exchange is determined by both blood flow and permeability. Paired experiments in each animal combined with analytical methods provided an internally consistent description of microvascular transport., Results: Epac1 -/- mice had elevated baseline permeability relative to wt control mice for Dotarem and Gadomer-17. In contrast to wt mice, Epac1 -/- mice failed to increase transvascular permeability in response to histamine. Dotarem underestimated blood flow and vascular volume and Gadomer-17 has limited sensitivity in extravascular accumulation., Conclusion: The study suggests that the normal barrier loosening effect of histamine in venular microvessels do not function when the normal barrier tightening effect of Epac1 is already compromised. The study also demonstrated that the numerical analysis of DCE-MRI data with tracers of different molecular weight has significant limitations., (© 2018 Scandinavian Physiological Society. Published by John Wiley & Sons Ltd.)