Increased microvascular permeability in mice lacking Epac1 (Rapgef3)

Aim

Maintenance of the blood and extracellular volume requires tight control of endothelial macromolecule permeability, which is regulated by cAMP signalling. This study probes the role of the cAMP 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^−/− C57BL/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.

Role of Hyperplasia of Gingival Lymphatics in Periodontal Inflammation

Lymphatic vessels are important for maintenance of tissue fluid homeostasis and afferent antigen transport. In chronic inflammation, lymphangiogenesis takes place and is characterized by lymphatic endothelial cell proliferation and lymphatic hyperplasia. Vascular endothelial growth factor C (VEGFC) is the main known lymphangiogenic growth factor, and its expression is increased in periodontitis, a common chronic infectious disease that results in tissue destruction and alveolar bone loss. The role of lymphangiogenesis during development of periodontitis is unknown. Here, we test if transgenic overexpression of epithelial VEGFC in a murine model is followed by hyperplasia of lymphatic vessels in oral mucosa and if the lymphatic drainage capacity is altered. We also test if lymphatic hyperplasia protects against periodontal disease development. Transgenic keratin 14 (K14)-VEGFC mice had significant hyperplasia of lymphatics in oral mucosa, including gingiva, without changes in blood vessel vasculature. The basal lymph flow was normal but slightly lower than in wild-type mice when oral mucosa was challenged with lipopolysaccharide from Porphyromonas gingivalis. Under normal conditions, K14-VEGFC mice exhibited an increased number of neutrophils in gingiva, demonstrated enhanced phagocyte recruitment in the cervical lymph nodes, and had more alveolar bone when compared with their wild-type littermates. After induction of periodontitis, no strain differences were observed in the periodontal tissues with respect to granulocyte recruitment, bone resorption, angiogenesis, cytokines, and bone-related protein expressions or in draining lymph node immune cell proportions and vascularization. We conclude that overexpression of VEGFC results in hyperplastic lymphatics, which do not enhance lymphatic drainage capacity but facilitate phagocyte transport to draining lymph nodes. Hyperplasia of lymphatics does not protect against development of ligature-induced periodontitis.

High-salt diet increases hormonal sensitivity in skin pre-capillary resistance vessels

AIMS

Recent data indicate that the skin of rats on a high-salt diet is able to accumulate Na(+) without commensurate water. This extrarenal mechanism of Na(+) homoeostasis could affect skin vasoregulation. We hypothesized that the major resistance vessel of rat skin, the pre-capillary arterioles, has increased vasoreactivity within the physiological range of circulating ANG II, a hormone relevant to salt-sensitive hypertension.

METHODS AND RESULTS

Skin arterioles from skin and muscle were isolated using the agar-infusion technique. Vessels from rats fed high-salt and low-salt diet had similar lumen diameter and media area/lumen area ratio. Contractile sensitivity to ANG II was increased in skin vessels from high-salt vessels at all doses tested starting at 10(-10) m (P < 0.01). Pre-capillary arterioles from muscle displayed similar contractions to ANG II, independent of the diet. As ANG II and the renin-angiotensin system are strongly involved in salt conservation, we explored whether vasoreactivity for noradrenaline was increased as well, because this is a functionally unrelated hormone. At low doses, contractions were similar, but at 10(-5) and 10(-4) m, noradrenaline produced stronger contractions in skin vessels from high-salt compared with low-salt rats (P < 0.01).

CONCLUSIONS

Our data demonstrate significantly increased hormonal vasoreactivity of skin vessels from rats on a high-salt diet, which could increase peripheral resistance in many situations and contribute to higher pressure in salt-sensitive hypertension. As vessels from adjacent muscle were unaffected, we raise the interesting possibility that increased vasoreactivity in the skin could be linked to osmotically inactive Na(+) accumulation.

Lowering of interstitial fluid pressure in rat trachea after substance P alone and in combination with calcitonin gene-related peptide

Neurogenic inflammation is mediated following a release of sensory neuropeptides including calcitonin gene-related peptide (CGRP) and substance P (SP). The release of peptides can be mediated chemically with capsaicin, or electrically by stimulation of the vagal nerve, both inducing vasodilation, plasma protein extravasation and lowering of interstitial fluid pressure (Pif) which will contribute to the enhancement of oedema formation.

AIM

Lowering of Pif has previously been demonstrated following intravenous (i.v.) treatment with CGRP, but it was not possible to demonstrate that SP had this effect under the same condition.

METHODS

Micropuncture measurements of Pif in the submucosa, without opening of the trachea, was conducted on rats anaesthetized with pentobarbital sodium (50 mg kg-1) and cardiac arrest was induced with i.v. KCl.

RESULTS

Pif in vehicle-treated animal averaged -1.7 +/- 0.4 (SD) mmHg (n = 9). Intravenous injection of SP induced significant lowering of Pif compared with control, both at low dose (0.47 nmol kg-1 body weight) with 1 min distribution time (P < 0.007, -4.2 +/- 2.3 mmHg) and at high dose with seconds of distribution time (P < 0.03, -4.2 +/- 1.6 mmHg). The same response was observed after treatment with SP co-injected with CGRP.

CONCLUSIONS

Substance P alone or in combination with CGRP is able to induce a rapid lowering of Pif showing that this peptide is a potent agent in increasing the hydrostatic driving pressure initially transporting fluid into the tissue during an acute inflammatory reaction.