Disparity in osmolarity-induced vascular reactivity

Morphological, Mechanical and Hydrodynamic Aspects of Diaphragmatic Lymphatics

The diaphragmatic lymphatic vascular network has unique anatomical characteristics. Studying the morphology and distribution of the lymphatic network in the mouse diaphragm by fluorescence-immunohistochemistry using LYVE-1 (a lymphatic endothelial marker) revealed LYVE1⁺ structures on both sides of the diaphragm-both in its the muscular and tendinous portion, but with different vessel density and configurations. On the pleural side, most LYVE1⁺ configurations are vessel-like with scanty stomata, while the peritoneal side is characterized by abundant LYVE1⁺ flattened lacy-ladder shaped structures with several stomata-like pores, particularly in the muscular portion. Such a complex, three-dimensional organization is enriched, at the peripheral rim of the muscular diaphragm, with spontaneously contracting lymphatic vessel segments able to prompt contractile waves to adjacent collecting lymphatics. This review aims at describing how the external tissue forces developing in the diaphragm, along with cyclic cardiogenic and respiratory swings, interplay with the spontaneous contraction of lymphatic vessel segments at the peripheral diaphragmatic rim to simultaneously set and modulate lymph flow from the pleural and peritoneal cavities. These details may provide useful in understanding the role of diaphragmatic lymphatics not only in physiological but, more so, in pathophysiological circumstances such as in dialysis, metastasis or infection.

Water removal during automated peritoneal dialysis assessed by remote patient monitoring and modelling of peritoneal tissue hydration

Water removal which is a key treatment goal of automated peritoneal dialysis (APD) can be assessed cycle-by-cycle using remote patient monitoring (RPM). We analysed ultrafiltration patterns during night APD following a dry day (APD_DD; no daytime fluid exchange) or wet day (APD_WD; daytime exchange). Ultrafiltration for each APD exchange were recorded for 16 days using RPM in 14 patients. The distributed model of fluid and solute transport was applied to simulate APD and to explore the impact of changes in peritoneal tissue hydration on ultrafiltration. We found lower ultrafiltration (mL, median [first quartile, third quartile]) during first and second vs. consecutive exchanges in APD_DD (−61 [−148, 27], 170 [78, 228] vs. 213 [126, 275] mL; p < 0.001), but not in APD_WD (81 [−8, 176], 81 [−4, 192] vs. 115 [4, 219] mL; NS). Simulations in a virtual patient showed that lower ultrafiltration (by 114 mL) was related to increased peritoneal tissue hydration caused by inflow of 187 mL of water during the first APD_DD exchange. The observed phenomenon of lower ultrafiltration during initial exchanges of dialysis fluid in patients undergoing APD_DD appears to be due to water inflow into the peritoneal tissue, re-establishing a state of increased hydration typical for peritoneal dialysis.

Lymphatic Vessels and Their Surroundings: How Local Physical Factors Affect Lymph Flow

Simple Summary

Lymphatic vessels are responsible for the drainage of liquids, solutes, and cells from interstitial spaces and serosal cavities. Their task is fundamental in order to avoid fluid accumulation leading to tissue swelling and edema. The lymphatic system does not possess a central pump, instead lymph is propelled against an overall hydraulic pressure gradient from interstitial spaces to central veins thanks to two pumping mechanisms, which rely on extrinsic forces or the intrinsic rhythmic contractility of lymphatic muscle cells embedded in vessel walls. This latter mechanism can very rapidly adapt to subtle changes in the microenvironment due to hydraulic pressure, lymph flow-induced wall shear stress, liquid osmolarity, and local tissue temperature. Thus, endothelial and lymphatic muscle cells possess mechanosensors that sense these stimuli and promote a change in contraction frequency and amplitude to modulate lymph flow accordingly. In this review, we will focus on the known physical parameters that can modulate lymph flow and on their putative cellular and molecular mechanisms of transduction.

Abstract

Lymphatic vessels drain and propel lymph by exploiting external forces that surrounding tissues exert upon vessel walls (extrinsic mechanism) and by using active, rhythmic contractions of lymphatic muscle cells embedded in the vessel wall of collecting lymphatics (intrinsic mechanism). The latter mechanism is the major source of the hydraulic pressure gradient where scant extrinsic forces are generated in the microenvironment surrounding lymphatic vessels. It is mainly involved in generating pressure gradients between the interstitial spaces and the vessel lumen and between adjacent lymphatic vessels segments. Intrinsic pumping can very rapidly adapt to ambient physical stimuli such as hydraulic pressure, lymph flow-derived shear stress, fluid osmolarity, and temperature. This adaptation induces a variable lymph flow, which can precisely follow the local tissue state in terms of fluid and solutes removal. Several cellular systems are known to be sensitive to osmolarity, temperature, stretch, and shear stress, and some of them have been found either in lymphatic endothelial cells or lymphatic muscle. In this review, we will focus on how known physical stimuli affect intrinsic contractility and thus lymph flow and describe the most likely cellular mechanisms that mediate this phenomenon.

On the change of transport parameters with dwell time during peritoneal dialysis

The transitory change of fluid and solute transport parameters occurring during the initial phase of a peritoneal dialysis dwell is a well-documented phenomenon; however, its physiological interpretation is rather hypothetical and has been disputed. Two different explanations were proposed: (1) the prevailing view-supported by several experimental and clinical studies-is that a vasodilatory effect of dialysis fluid affects the capillary surface area available for dialysis, and (2) a recently presented alternative explanation is that the molecular radius of glucose increases due to the high glucose concentration in fresh dialysis fluid and that this change affects peritoneal transport parameters. The experimental bases for both phenomena are discussed as well as the problem of the accuracy necessary for a satisfactory description of clinical data when the three-pore model of peritoneal transport is applied. We show that the correction for the change of transport parameters with dwell time provides a better fit with clinical data when applying the three-pore model. Our conclusion is in favor of the traditional interpretation namely that the transitory change of transport parameters with dwell time during peritoneal dialysis is primarily due to the vasodilatory effect of dialysis fluids.

Vasoactive Components of Dialysis Solution

Background

Conventional peritoneal dialysis (PD) solutions elicit vasodilation, which is implicated in the variable rate of solute transport during the dwell. The components causing such vasoactivity are still controversial. This study was conducted to define the vasoactive components of conventional and new PD solutions.

Methods

Three visceral peritoneal microvascular levels were visualized by intravital video microscopy of the terminal ileum of anesthetized rats. Anesthesia-free decerebrate conscious rats served as control. Microvascular diameter and blood flow by Doppler measurements were conducted after topical peritoneal exposure to 4 clinical PD solutions and 6 prepared solutions designed to isolate potential vasoactive components of the PD solution.

Results

All clinically available PD solutions produced a rapid and generalized vasodilation at all intestinal microvascular levels, regardless of the osmotic solute. The pattern and magnitude of this dilation was not affected by anesthesia but was determined by arteriolar size, the osmotic solute, and the solution's buffer anion system. The greatest dilation occurred in the small precapillary arterioles and was elicited by conventional PD solution and heat re-sterilized solution containing low glucose degradation products (GDPs). Hypertonic mannitol solutions produced a dilation that was approximately 50% less than the dilation obtained with glucose solutions with identical osmolarity and buffer. Increasing a solution's osmolarity did not produce a parallel increase in the magnitude of dilation, suggesting a nonlinear relationship between the two variables. Lactate dissolved in an isotonic solution was completely non-vasoactive unless the solution's H+concentration was increased. At low pH, isotonic lactate produced a rapid but transient vasodilation. This vascular reactivity was similar in magnitude and pattern to that obtained with the isotonic 7.5% icodextrin solution (Extraneal; Baxter Healthcare, Deerfield, Illinois, USA).

Conclusions

( 1 ) Hyperosmolarity is the major vasoactive component of PD solution. ( 2 ) Hyperosmolarity and active intracellular glucose uptake account together for approximately 75% of PD solution-induced dilation, whereas GDPs contribute to approximately 25%. ( 3 ) Lactate is vasoactive only at low pH (high [H+]). ( 4 ) The magnitude of PD solution-mediated vasodilation is partially dependent on the nature of the osmotic solute, the GDP contents, and the [H+], which determine the vasoactivity of the lactate-buffer anion system. Studies are required to define the molecular mechanisms of PD-induced vasodilation and to determine the vasoactive properties of these solutions after chronic infusion.

Glycine improves peritoneal vasoreactivity to dialysis solutions in the elderly

Background: Peritoneal dialysis solution (PDS) dilates peritoneal microvessels predominantly by the activation of the endothelial nitric oxide (NO) pathway. We made an incidental observation of decreased PDS-induced, NO-dependent peritoneal microvascular vasoreactivity in elderly rats naïve to PDS exposure. We hypothesized that this subordinate NO-mediated peritoneal microvascular vasoreactivity is caused by increased oxidative stress in the aged endothelium, which compromises NO bioavailability in the elderly, and that peritoneal microvascular vasoreactivity can be improved by the supplementation of antioxidant glycine to PDS. Methods: We studied PDS-mediated vasoreactivity of four intestinal visceral arterioles of different orders by in vivo intravital microscopy in weaned, adult, and elderly rats to (i) confirm subordinate vasoreactivity to PDS in elderly rats; (ii) restore vasoreactivity by glycine supplementation; and (iii) establish age as an independent risk factor for endothelial cell dysfunction. Results: In a crossover series, peritoneal microvascular vasoreactivity to PDS exposure was remarkably decreased in elderly rats. This subordinate vasoreactivity was completely restored by the supplementation of glycine to PDS. In a separate series, we assessed in situ endothelial cell function in weaned and adult rats using the cumulative acetylcholine concentration–response curves. Unlike the adults, the weaned rats demonstrated remarkable sensitivity and reactivity to cumulative acetylcholine concentrations, suggesting the dependency of endothelial cell function on age. Conclusion: Aging is an independent risk factor for peritoneal microvascular endothelial cell dysfunction. Endothelial function in the elderly can be recovered by reinforcing the bioavailability of endothelial-derived NO through glycine. Dietary glycine supplementation is a potential therapeutic strategy to decrease the burden of oxidative stress on the aged endothelium.

Fluid Osmolarity Acutely and Differentially Modulates Lymphatic Vessels Intrinsic Contractions and Lymph Flow

Lymph formation and propulsion rely on an extrinsic mechanism based on the forces that surrounding tissues exert upon the vessel wall and lumen and an intrinsic mechanism based on spontaneous, rhythmic contractions of the lymphatic muscle layer of collecting vessels. The two spontaneous pacemakers described in literature involve chloride-dependent depolarizations (STDs) and I_f-like currents, both giving rise to a variable contraction frequency (f_c) of lymphatic vessels functional units (lymphangions). Several stimuli have been shown to modulate f_c, such as temperature, shear stress, and several tissue chemical modulators (prostaglandins, norepinephrine, acetylcholine, substance P, and others). However, no detailed description is present in literature on the acute modulation of f_c by means of osmolarity change of the surrounding interstitial space. Using a well-developed ex-vivo rat diaphragmatic preparation, in which osmolarity was changed by varying the concentration of D-mannitol in the perfusing solution and in later experiments the concentration of NaCl and then of Na⁺ and Cl⁻ ions separately by ionic substitution, we provide detailed experimental evidences that a stepwise increase in osmolarity from control value (308 mOsm) up to 324 mOsm caused a reduction of f_c down to ~-70% within the first 14 min, and that a stepwise decrease in osmolarity up to 290 mOsm induced an early f_c increase to ~+34% of control, followed by a decline to an f_c of ~-18% of control value. These variations were more dramatic when the same osmolarity changes were obtained by varying NaCl and/or Na⁺ or Cl⁻ ions concentration, which caused an almost complete arrest of spontaneous contractility within 14 min from the application. Diastolic and systolic diameters and stroke volume were not affected by osmolarity changes, so that modulation of lymph flow closely followed that of f_c. Modulation of lymph flow secondary to osmolarity changes is relevant if one considers that interstitial fluid balance is also dependent upon lymph drainage, and thus it is possible that, at least in the acute phase following variations of interstitial fluid osmolarity, its volume control might eventually be impaired due to the reduced or in the worst scenario null lymph drainage.

Glucose dilates renal afferent arterioles via glucose transporter-1

Glomerular hyperfiltration occurs during the early stage of diabetes. An acute glucose infusion increases glomerular filtration rate. The involvement of tubuloglomerular feedback response and direct effect of glucose on the afferent arterioles (Af-Arts) have been suggested. However, the signaling pathways to trigger Af-Art dilatation have not been fully identified. Therefore, in the present study we tested our hypothesis that an increase in glucose concentration enhances endothelial nitric oxide synthesis activity and dilates the Af-Arts via glucose transporter-1 (GLUT1) using isolated mouse Af-Arts with perfusion. We isolated and microperfused the Af-Arts from nondiabetic C57BL/6 mice. The Af-Arts were preconstricted with norepinephrine (1 µM). When we switched the d-glucose concentration from low (5 mM) to high (30 mM) in the perfusate, the preconstricted Af-Arts significantly dilated by 37.8 ± 7.1%, but L-glucose did not trigger the dilation. GLUT1 mRNA was identified in microdisserted Af-Arts measured by RT-PCR. Changes in nitric oxide (NO) production in Af-Art were also measured using fluorescent probe when ambient glucose concentration was increased. When the d-glucose concentration was switched from 5 to 30 mM, NO generation in Af-Art was significantly increased by 19.2 ± 6.2% (84.7 ± 4.1 to 101.0 ± 9.3 U/min). l-Glucose had no effect on the NO generation. The GLUT1-selective antagonist 4-[({[4-(1,1-Dimethylethyl)phenyl]sulfonyl}amino)methyl]- N-3-pyridinylbenzamide and the nitric oxide synthase inhibitor N^G-nitro-l-arginine methyl ester blocked the high glucose-induced NO generation and vasodilation. In conclusion, we demonstrated that an increase in glucose concentration dilates the Af-Art by stimulation of the endothelium-derived NO production mediated by GLUT1.

EGb 761 promotes osteoblastogenesis, lowers bone marrow adipogenesis and atherosclerotic plaque formation

AIM OF THE STUDY

Our earlier study has demonstrated that EGb 761 (standardized extract of Ginkgo) has the bone sparing effect on the estrogen deficiency induced bone loss model. In the present study, we have addressed the question whether treatment of osteoporosis benefits arterial calcification or vice versa, because both adipocyte and osteoblast originate from the same mesenchymal cell of the bone marrow cell (BMC) population.

MATERIALS AND METHODS

Bone marrow cells were isolated to study the effect of EGb 761 on osteoblast and adipocytes. For in vivo effect hamsters were fed high fat diet and the effect of EGb 761 studied on atherosclerotic plaque formation and endothelial function.

RESULTS

BMC's undergoing induced osteogenic or adipogenic differentiations in the presence of EGb 761 show increase and decrease in mineralization and adipogenesis respectively. Osteogenic and adipogenic mRNAs, reveal lineage dependent expression patterns. Runx-2 (osteoblast transcription factor) showed a progressive increase, whereas PPAR-γ (adipogenic regulator) was attenuated, with same pattern of expression being for late osteogenic and adipogenic genes. EGb 761 led to increase in apoptotic cells and ROS, an important upstream signal. In vivo experiments in hamsters after induction with high cholesterol diet (HCD) show improvement in endothelial function by EGb 761 with lowering in total plasma cholesterol levels. EGb 761 led to vascular preservation of the aortic lumen with impairment of the endothelium dependent relaxation which was corroborated by micro-CT and histological sections of the thoracic region of the aorta.

CONCLUSION

From this data, it can be implied that EGb 761 controls bone loss, adiposity and lowers atherogenic risk factor after HCD induction.

The impact of dialysis solution biocompatibility on ultrafiltration and on free water transport in rats

This study compares different peritoneal dialysis fluids (PDF) in rats over a short contact time. For greater accuracy, net ultrafiltration (UF) and peritoneal transport indices, mass transfer area coefficient (MTAC) were scaled for the in vivo peritoneal surface area recruited (ivPSA) measured by microcomputerized tomography. Wistar rats underwent nephrectomy (5/6ths), were randomized into two groups and given 1.5% glucose PDF, either conventional acidic lactate (n = 14) or pH neutral bicarbonate (BicaVera) (n = 13); MTAC and UF were measured using a 90-min peritoneal equilibrium test (PET), fill volume (IPV) of 10 ml/100 g; small pore fluid transport was determined from sodium balance and used to calculate free water transport (FWT). Each ivPSA value was significantly correlated with the actual IPV, which varied from one rat to another. At 90 min of contact, there was no difference in recruited ivPSA in relation to PDFs. There was a difference (p < 0.01) in net UF/ivPSA 0.45 vs. 1.41 cm(2)/ml for bicarbonate versus lactate, as there was in the proportion of FWT with bicarbonate (42 ± 5% of net UF) compared to lactate (29 ± 4% of net UF). Net UF for individual values of ivPSA differs between conventional PDF and more biocompatible solutions, such as bicarbonate PDF. This observed change in UF cannot be fully explained by differences in glucose transport. The changes in FWT may be explained by the impact of the PDF biocompatibility on aquaporin function.

Changes in free water fraction and aquaporin function with dwell time during continuous ambulatory peritoneal dialysis

Diffusive (K(BD), A₀/Δx(t)) transport parameters and sieving coefficients (S) for small solutes and free water fraction (FWF), that is, the fraction of total water flow that is transported through aquaporins, were assessed as functions of dwell time t for 35 continuous ambulatory peritoneal dialysis patients using glucose 3.86% dialysis fluid.The individual values of the unrestricted pore area over diffusion distance, A₀/Δx(t), were estimated using the mixed effects nonlinear regression and applied for evaluation of S(t) for sodium and FWF(t). FWF decreased on average from the initial 51% of the total transcapillary water flow to 36% at 120 min, whereas the small pore water fraction and sodium sieving coefficient increased. Our results were consistent with the three-pore model if the contribution of the transcellular pores (α(TP)) at the beginning of dwell study was doubled and later decreased to the standard value of 0.02.We conclude that transport characteristics of fluid and small solutes should be considered as time-dependent variables during the peritoneal dialysis.

Vasoactive components of dialysis solution

BACKGROUND

Conventional peritoneal dialysis (PD) solutions elicit vasodilation, which is implicated in the variable rate of solute transport during the dwell. The components causing such vasoactivity are still controversial. This study was conducted to define the vasoactive components of conventional and new PD solutions.

METHODS

Three visceral peritoneal microvascular levels were visualized by intravital video microscopy of the terminal ileum of anesthetized rats. Anesthesia-free decerebrate conscious rats served as control. Microvascular diameter and blood flow by Doppler measurements were conducted after topical peritoneal exposure to 4 clinical PD solutions and 6 prepared solutions designed to isolate potential vasoactive components of the PD solution.

RESULTS

All clinically available PD solutions produced a rapid and generalized vasodilation at all intestinal microvascular levels, regardless of the osmotic solute. The pattern and magnitude of this dilation was not affected by anesthesia but was determined by arteriolar size, the osmotic solute, and the solution's buffer anion system. The greatest dilation occurred in the small precapillary arterioles and was elicited by conventional PD solution and heat re-sterilized solution containing low glucose degradation products (GDPs). Hypertonic mannitol solutions produced a dilation that was approximately 50% less than the dilation obtained with glucose solutions with identical osmolarity and buffer. Increasing a solution's osmolarity did not produce a parallel increase in the magnitude of dilation, suggesting a nonlinear relationship between the two variables. Lactate dissolved in an isotonic solution was completely non-vasoactive unless the solution's H(+) concentration was increased. At low pH, isotonic lactate produced a rapid but transient vasodilation. This vascular reactivity was similar in magnitude and pattern to that obtained with the isotonic 7.5% icodextrin solution (Extraneal; Baxter Healthcare, Deerfield, Illinois, USA).

CONCLUSIONS

(1) Hyperosmolarity is the major vasoactive component of PD solution. (2) Hyperosmolarity and active intracellular glucose uptake account together for approximately 75% of PD solution-induced dilation, whereas GDPs contribute to approximately 25%. (3) Lactate is vasoactive only at low pH (high [H(+)]). (4) The magnitude of PD solution-mediated vasodilation is partially dependent on the nature of the osmotic solute, the GDP contents, and the [H(+)], which determine the vasoactivity of the lactate-buffer anion system. Studies are required to define the molecular mechanisms of PD-induced vasodilation and to determine the vasoactive properties of these solutions after chronic infusion.

Chronic infusion of sterile peritoneal dialysis solution abrogates enhanced peritoneal gene expression responses to chronic peritoneal catheter presence

Chronic exposure to sterile peritoneal dialysis (PD) solutions is associated with microvascular and interstitial changes within the blood-peritoneal barrier (peritoneum). These changes are commonly linked to loss of peritoneal function over time, presumably because of angiogenesis-related increased vascular area. However, the effects on peritoneal microvascular function of chronic peritoneal exposure to PD solutions are unknown. The present study examined peritoneal microvascular function after chronic exposure to sterile PD solution. Six rats underwent permanent catheter insertion under anesthesia. Three rats were treated with approximately 16 mL conventional PD solution daily for 6 weeks; catheter insertion controls received 1 mL saline daily. At 6 weeks, visceral peritoneal microvascular function was assessed in vivo using intravital microscopy. Endothelial cell functions were assessed using messenger RNA (mRNA) gene microarray analysis. In both groups, significant angiogenesis was seen, predominantly in the base of the mesentery. Sensitivity and reactivity of the intestinal visceral peritoneal pre-capillary arterioles (A3 arterioles, 8 - 15 microm in diameter) were decreased in the catheter controls, but not in the chronic PD infusion rats. Chronic catheter presence increased the expression of 18 genes in the controls as compared with 12 genes in the chronic infusion rats. In both groups, expression of fibronectin, integrin-beta, integrin-alpha5, collagen type XVIII-alpha1, and matrix metalloproteinase was enhanced. Endothelial expression of proinflammatory genes (interleukin-1beta, tissue pathway inhibitor, chemokine ligand 2) was enhanced by chronic catheter insertion, but not after chronic PD fluid infusion. Increased expression of genes encoding proteins involved in inflammation and tissue remodeling results from peritoneal catheter-related endothelial cell activation. Chronic exposure of the nonuremic peritoneum to sterile PD solutions overrides the catheter-related endothelial cell proinflammatory phenotype to restore peritoneal microvascular function.

Cellular edema regulates tissue capillary perfusion after hemorrhage resuscitation

BACKGROUND

Hemorrhage-induced activation of endothelial cell Na+/H+ -exchanger results in cellular swelling, which physically impedes capillary filling and compromises gut perfusion. We hypothesized that correction of the vascular volume deficit by conventional resuscitation does not improve capillary filling unless cellular swelling is prevented. Also, we hypothesized that adjunctive direct peritoneal resuscitation (DPR) with topical peritoneal dialysis solution (Delflex; Fresenius USA, Inc., Ogden, Ut) enhances capillary filling and gut perfusion by mechanisms that are independent of the Na+/H+ function.

METHODS

In vivo intravital videomicroscopy and Doppler velocimeter were used by us to measure microvascular diameter and flow, capillary filling (index of functional capillary density, FCD), and endothelial cell function in the terminal ileum of anesthetized rats. Rats were bled to 50% mean arterial pressure for 60 min and resuscitated with the shed blood plus 2 volumes of saline (conventional resuscitation). Prevention of endothelial cell swelling was achieved with topical amiloride (specific Na+/H+ inhibitor) in the tissue bath before hemorrhage or simultaneously with conventional resuscitation. DPR was simulated by instillation of Delflex in the tissue bath as adjunctive to conventional resuscitation. Sham no hemorrhage group and a simulated DPR group that received topical amiloride treatment served as controls.

RESULTS

Conventional resuscitation from hemorrhagic shock restored and maintained central hemodynamics but caused progressive and persistent intestinal vasoconstriction and hypoperfusion associated with low FCD and endothelial cell dysfunction. Prevention of endothelial cell swelling when combined with conventional resuscitation, preserved endothelial cell function, and restored local intestinal microvascular variables to near-prehemorrhage levels. Simulated adjunctive DPR produced rapid, sustained, and generalized vasodilation associated with restoration of endothelial cell function, and maximum recruitment of FCD independent of the Na+/H+ -exchanger function.

CONCLUSIONS

Paradoxical endothelial cell swelling occurs early during hemorrhagic shock because of activation of the Na+/H+ exchanger. This cellular edema, which is not resolved by correction of the vascular volume deficit, explains the persistent postresuscitation endothelial cell dysfunction and gut hypoperfusion. Simulated adjunctive DPR in this study reversed endothelial cell swelling and enhanced gut perfusion by mechanisms that are independent of the Na+/H+ exchanger activity.

Mechanisms of direct peritoneal resuscitation-mediated splanchnic hyperperfusion following hemorrhagic shock

Conventional resuscitation (CR) from hemorrhagic shock causes a persistent and progressive splanchnic vasoconstriction and hypoperfusion despite hemodynamic restoration with intravenous fluid therapy. Adjunctive direct peritoneal resuscitation (DPR) with a clinical peritoneal dialysis solution instilled into the peritoneal cavity has been shown to restore splanchnic tissue perfusion, down-regulate the gut-derived exaggerated systemic inflammatory response, promote early fluid mobilization, and improve overall outcome. This study was conducted to define the molecular mechanisms of DPR-induced gut hyperperfusion after hemorrhagic shock. Male rats were bled to 50% baseline mean arterial pressure and resuscitated with the shed blood plus two volumes of saline (CR). In vivo videomicroscopy and Doppler velocimetry were used to assess terminal ileal microvascular diameters and blood flow. Direct peritoneal resuscitation animals received CR and topical application of a clinical glucose-based peritoneal dialysis solution (Delflex). Inhibitors, glibenclamide (K(+)ATP channels), N-monomethyl-L-arginine (L-NMMA) (nitric oxide synthase), 8-cyclopentyl-1,3-diprophylxanthine (DPCPX) (A1 adenosine receptor), tetrabutylammonium (K(+)Ca2+ channels), and mefenamic acid (cyclooxygenase) were topically applied (individually or in combination) with DPR according to protocol; BQ-123 (endothelin A receptor antagonist) and BQ-788 (endothelin B receptor antagonist) were used topically with CR to define the mechanism of post-CR vasoconstriction and hypoperfusion. Conventional resuscitation caused a persistent progressive intestinal vasoconstriction and hypoperfusion that can be abolished with endothelin antagonists. In contrast, adjunctive DPR caused an instantaneous sustained vasodilation and hyperperfusion. Glibenclamide or L-NMMA partially attenuated DPR-induced vasodilation, whereas the addition of DPCPX to the two inhibitors eliminated the dilation. Cyclooxygenase and K(+)Ca2+channels were not active in DPR-mediated microvascular effects. In conclusion, DPR improves splanchnic tissue perfusion by endothelium-dependent mechanisms mediated by activations of glibenclamide-sensitive K(+) channels (KATP), adenosine A1 receptor subtype activation, and nitric oxide release. Direct peritoneal resuscitation preserves endothelial dilatory functions, thereby overriding any endothelium-derived constrictor response triggered by hemorrhagic shock and CR.