Collagen and glycosaminoglycans (GAGs) constituting the extracellular matrix may limit the space available and thus exclude macromolecules from a fraction of the interstitial fluid phase. This exclusion phenomenon is of importance for transcapillary fluid and solute exchange. The purpose of the study was to examine the range of interstitial exclusion in rat skin by using probes within a span of molecular weights and electrical charge, and also to test if a change in interstitial composition occurring as a consequence of aging affected exclusion. To this end we used a novel approach, involving the exact determination of albumin concentration and mass in interstitial fluid and tissue eluate by high performance liquid chromatography and thereafter expressing the corresponding numbers relative to albumin for a set of probe proteins assessed by quantitative proteomics. Albumin was excluded from 55 ± 4% (n=8) of the extracellular fluid phase. There was a highly significant positive correlation between probe Stokes-Einstein (SE) radius and fractional excluded volume (VEF) described by VEF = 0.078*SE radius + 0.269 (p<0.001), and oppositely, a negative correlation between probe isoelectric point (pI) and exclusion for proteins with comparable size, VEF = -0.036 * pI +0.719 (p=0.04). Aging resulted in a significant reduction in skin hydration and sulphated GAGs and a moderate increase in hyaluronan, and a corresponding reduced VEF for albumin and the other macromolecular probes. Our findings suggest that the changes in the extracellular matrix in aged skin may result in delayed adjustments of fluid perturbations and reduced ability for salt storage.
In peritoneal dialysis (PD) patients, the frequent exposure to "unphysiological" dialysis fluids elicits a chronic state of a low-grade peritoneal inflammation leading to interstitial matrix remodeling and angiogenesis. Proinflammatory cytokines are important regulators involved in this inflammatory process that ultimately leads to dysfunction of the peritoneum as a dialysis membrane. We aimed to measure the local concentrations of proinflammatory cytokines in the peritoneal interstitial fluid (IF). Furthermore, we wanted to assess how the driving forces for fluid and solute exchanges are affected in a remodeled interstitial matrix and thus measured the colloid osmotic pressure (COP) gradient in rats that were exposed to chronic PD. After 8 wk of peritoneal dialysis, IF from peritoneum was isolated using a centrifugation method, and was analyzed for cytokine content and COP along with plasma. For several of the proinflammatory cytokines there were gradients from IF to plasma, showing local production. For some cytokines, the concentration in IF was increased severalfold, whereas IL-18 was increased systemically due to PD. Furthermore, the presence of the catheter per se seemed to increase cytokine levels. COP in IF was significantly decreased in the PD group, while collagen and hyaluronan content was increased. Collectively, our data suggest that the increased levels of proinflammatory cytokines after PD may be an integral component of the development of fibrosis and angiogenesis commonly seen in PD patients, and the decreased COP in IF after chronic PD may shift the Starling equilibrium across peritoneal capillaries to an absorptive state.
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