THE AUTODIGESTION HYPOTHESIS AND RECEPTOR CLEAVAGE IN DIABETES AND HYPERTENSION

One of the key features of cardiovascular complications, such as hypertension or diabetes, is that they often appear at the same time in the same individual together with other forms of co-morbidities. While clinically a recognized phenomenon, no molecular mechanism for such co-morbidities has received universal acceptance. We propose a new hypothesis that provides a molecular basis for co-morbidities in hypertension due to unchecked proteolytic activity and receptor destruction. Testing of the hypothesis in the spontaneously hypertensive rat reveals an unchecked matrix metalloproteinase and serine protease activity in plasma and on several cardiovascular and parenchymal cells. The elevated proteolytic activity causes extracellular cleavage of multiple receptor types, such that cleavage of one receptor type leads to loss of the function carried out by this receptor. Proteolytic cleavage of the extracellular domain of the β(2) adrenergic receptor in arteries and arterioles causes vasoconstriction and elevation of the central blood pressure while cleavage of the extracellular domain of the insulin receptor leads to insulin resistance and lack of transmembrane glucose transport. A diverse set of cell dysfunctions in the spontaneously hypertensive rat are accompanied by cleavage of the membrane receptors that are involved in these functions. Chronic inhibition of the unchecked protease activity in the spontaneously hypertensive rat serves to restore the extracellular receptor density and alleviates the corresponding cell dysfunctions. The mild unchecked proteolytic activity in the spontaneously hypertensive rat points towards a chronic autodigestion process as a contributor to the end organ injury encountered in this rat strain. The presence of various soluble receptors, which consist of extracellular fragments of membrane receptors, in the plasma of hypertensive and diabetic patients suggest that the autodigestion process may also be present in man.

Evidence for a second valve system in lymphatics: endothelial microvalves

The mechanism for interstitial fluid uptake into the lymphatics remains speculative and unresolved. A system of intralymphatic valves exists that prevents reflow along the length of the lymphatic channels. However, these valves are not sufficient to provide unidirectional flow at the level of the initial lymphatics. We investigate here the hypothesis that initial lymphatics have a second, separate valve system that permits fluid to enter from the interstitium into the initial lymph channels but prevents escape back out into the tissue. The transport of fluorescent microspheres (0.31 microm) across endothelium of initial lymphatics in rat cremaster muscle was investigated with micropipette manipulation techniques. The results indicate that microspheres can readily pass from the interstitium across the endothelium into the lumen of the initial lymphatics. Once inside the lymphatic lumen, the microspheres cannot be forced out of the lumen even after elevation of the lymphatic pressure by outflow obstruction. Reaspiration of the microspheres inside the lymphatic lumen with a micropipette is blocked by the lymphatic endothelium. This blockade exists whether the aspiration is carried out at the microsphere entry site or anywhere along the initial lymphatics. Nevertheless, puncture of the initial lymphatic endothelium with the micropipette leads to rapid aspiration of intralymphatic microspheres. Investigation of lymphatic endothelial sections fixed during lymph pumping shows open interendothelial junctions not found in resting initial lymphatics. These results suggest that initial lymphatics have a (primary) valve system at the level of the endothelium. In conjunction with the classical (secondary) intralymphatic valves, the primary valves provide the mechanism that facilitates the unidirectional flow during periodic compression and expansion of initial lymphatics.

Mechanisms of parenchymal cell death in-vivo after microvascular hemorrhage

OBJECTIVE

In vitro studies suggest that microhemorrhages with escape of red cells into the tissue may be cytotoxic to parenchymal cells due to oxygen free radical formation. We examined in the rat mesentery the impact of microhemorrhages on parenchymal cell death, as detected by propidium iodide staining, using an intravital approach.

METHODS AND RESULTS

Postcapillary venules were punctured with a closed-end micropipette, permitting escape of blood cells and plasma into the mesentery interstitium. Over a period of 2 h, no significant increase in parenchymal cell death was encountered in tissues with hemorrhagic sites compared with nonhemorrhagic control sites. Interstitial microinjections of plasma derived from whole blood incubated for several hours with and without a combination of sodium azide (2 mM) and hydrogen peroxide (1 mM) led to significantly increased levels of cell death compared to control experiments. Interventions against the hydroxyl radical with dimethylthiourea (DMTU, 2 mM) or 2,2'-dipyridyl (DPD, 2 mM), a lipid soluble iron chelator, provided no protective effect against the parenchymal cell death. DMTU slightly delayed tile cytotoxic reaction.

CONCLUSIONS

These observations suggest that a newly formed microhemorrhage is not necessarily cytotoxic to parenchymal tissue cells. Interstitial microinjections of plasma, derived from whole blood after prolonged exposure to oxygen free radicals or just aging under in vitro conditions, may be cytotoxic to mesenteric parenchymal cells without effective blockade by interventions against the hydroxyl radical.

Inflammation in chronic venous insufficiency. Is the problem insurmountable?

One of the hallmarks of venous insufficiency is an elevated venous pressure. While a number of mechanisms have been proposed for vascular and parenchymal cell damage following venous pressure elevation, such as white cell infiltration, a key question remains as to what degree venous occlusion and flow interruption per se may constitute a risk factor in venous disease. To gain an insight into this mechanism, we examined the effect of venous occlusion followed by reperfusion. A draining venule (circa 50 micrometer) in the rat mesentery was occluded with a micropipette (1 h) followed by reperfusion (1 h). The procedure serves to raise the microvascular pressure to about 31 mm Hg during the occlusion while the flow is completely stopped in the local venous and capillary network. Parenchymal cell death in the mesentery was monitored by propidium iodide (PI) labeling. The number of PI-positive cells significantly increased predominantly during reperfusion. A 1-week treatment with a micronized purified flavonoid fraction (100 mg/kg/day) served to significantly reduce parenchymal cell death as well as leukocyte rolling, adhesion to postcapillary venule, and migration into the tissue both during occlusion and reperfusion. The results indicate, that even in an initially symptomless tissue, flow reduction combined with microvascular pressure elevation during venous occlusion results in tissue damage not only during reperfusion (as in arterial occlusion) but also during occlusion.

A mechanism of oxygen free radical production in the Dahl hypertensive rat

OBJECTIVE

To determine if oxygen free radicals derived from xanthine oxidase are involved in the development of salt-induced hypertension. Enhanced production of oxygen free radicals may play a role in hypertension by affecting vascular smooth muscle contraction and provide a mechanism for lesion formation.

METHODS

Dahl salt-sensitive (Dahl-S) and salt-resistant (Dahl-R) rats were fed either a low-salt, high-salt or high-salt + tungsten diet for 4 wk. In vivo production of superoxide (O2-) was detected by the reduction of a tetranitroblue tetrazolium (TNBT) dye in the rat mesentery, while plasma hydrogen peroxide (H2O2) production levels were determined using a modified electrochemical electrode technique.

RESULTS

The tungsten diet lowered the blood pressure of Dahl-S rats compared to high-salt-treated Dahl-S rats, but had no effect on blood pressure in Dahl-R rats. Light absorption of formazan deposits revealed that tungsten-treated Dahl-S rats had reduced TNBT staining along the endothelium of arterioles and venules compared to hypertensive, high-salt-treated Dahl-S rats. In addition, tungsten-treated Dahl-S rats had a lower plasma H2O2 concentration compared to hypertensive, high-salt-treated Dahl-S rats.

CONCLUSIONS

These findings indicate that xanthine oxidase-derived oxygen free radicals are involved in the pathogenesis of salt-induced hypertension.

Mast cell degranulation and parenchymal cell injury in the rat mesentery

OBJECTIVE

The objective of this study was to explore the degree of parenchymal cell injury after mast cell degranulation by application of compound 48/80 (CMP 48/80) in the absence of adherent leukocytes in the rat mesentery.

METHODS

Rats were rendered leukopenic by injection of an antibody against leukocytes, and the mesentery was superfused with CMP 48/80 during intravital microscopy. The extent of cell injury was determined using a fluorescent cell-viability indicator, propidium iodide (PI). In an additional group, mast cell degranulation with CMP 48/80 was prevented by using the mast cell stabilizer Ketotifen.

RESULTS

After a reduction in the number of circulating leukocytes, mast cell degranulation produced a mild increase in parenchymal cell injury. The injury levels significantly increased when individual regions of the mesentery were compared. Stabilization of the mast cells with Ketotifen reduced the injury to below baseline values.

CONCLUSIONS

In the absence of leukocyte adhesion to the endothelium, mast cell degranulation contributes to parenchymal cell injury in the mesentery.

Enhanced DNA fragmentation in the thymus of spontaneously hypertensive rats

The mechanisms contributing to organ injury in hypertension have been incompletely defined. The thymus gland of the spontaneously hypertensive rat (SHR) shows significant atrophy at the age of 15 wk compared with its normotensive control, the Wistar-Kyoto rat (WKY). The aim of the present study was to examine the thymus of SHR for evidence of DNA nicking as one of the mechanisms for thymic atrophy. SHR and WKY were subjected to adrenalectomy or sham surgery at 12 wk and studied at 15 wk. Adrenalectomy served to normalize the blood pressure in the SHR. DNA nicking was detected by in situ nick-end labeling (ISEL) of fixed tissue sections. Tissue sections were treated with proteolysis, and terminal deoxyribonucleotidyl transferase was used to incorporate biotinylated deoxynucleotides into DNA nick end in situ. Separately, DNA fragmentation was evaluated by measuring the level of released mono- and oligonucleosomes to the cytoplasm. A higher number of thymic ISEL-positive cells and a higher level of cytoplasmic mono- and oligonucleosomes were observed in SHR than in WKY. After adrenalectomy the enhanced level of ISEL and cytoplasmic mono- and oligonucleosomes in SHR was reduced to the level in WKY. Dexamethasone treatment (0.05 mg. kg-1. day-1) in WKY serves to decrease the thymus weight and significantly elevate the level of mono- and oligonucleosomes. Thus increased DNA fragmentation represents one of the mechanisms associated with thymic atrophy, a feature that reflects immune suppression in SHR.

Mechanisms of cell injury in rat mesentery and cremaster muscle

The events responsible for cell injury after a tissue stimulation are only incompletely understood. The purpose of this study was to examine mechanisms of cell injury in two tissues, rat mesentery and cremaster muscle, after tissue stimulation with N-formylmethionyl-leucyl-phenylalanine (FMLP) and platelet-activating factor (PAF). The response was studied in the same animal in random order using normal and leukopenic rats. The tissues were exteriorized after pentobarbital anesthesia. Five to six vascularized areas were chosen in each tissue, and cell injury and hydroperoxide production were assessed visually by continuous superfusion with 1 microM propidium iodide and 5 microM dichlorofluorescin diacetate (DCFH), respectively. FMLP (1 x 10(-8) M) and then PAF (1 x 10(-8) M) were added to the superfusate, and measurements were made at several time points. The second tissue was then examined using the same protocol. In the cremaster, there was little hydroperoxide production, and the tissue injury was eliminated after leukopenia. Leukopenia had no effect on tissue injury in the mesentery. Although hydroperoxide production was observed, there was no correlation between it and the tissue injury. The level of preactivation showed no correlation with either tissue injury or hydroperoxide production. In light of these results, mast cell degranulation may be an important mechanism of tissue injury in the mesentery.

Microvascular oxidative stress preceding leukocyte activation elicited by in vivo nitric oxide suppression

This study was aimed to determine the mechanism by which endogenous nitric oxide suppression promotes leukocyte adhesion in vivo. The rat mesenteric microcirculation was superfused with NG-nitro-L-arginine methyl ester (L-NAME; 100 microM), and intracellular oxidant formation in several microcirculatory cellular components such as arteriolar and venular endothelium and mast cells was visually monitored by digital microfluorography assisted by carboxydichlorofluorescein (CDCF), a hydroperoxide-sensitive fluorogenic probe. Adherent leukocyte density was measured simultaneously. L-NAME induced a significant time-dependent increase in CDCF fluorescence in arteriolar and venular endothelium and mast cells followed by firm adhesion of leukocytes. L-NAME-induced CDCF elevation showed a different spatial distribution compared with that evoked by N-formylmethionyl-leucyl-phenylalanine, in which only local venular segments with adhering leukocytes exhibited CDCF fluorescence enhancement. The level of hydroperoxide formation in arterioles and venules evoked by 60-min L-NAME superfusion was equivalent to that induced by the superfusion of approximately 880 microM tert-butyl hydroperoxide for 10 min. Pretreatment with anti-intracellular adhesion molecule-1, anti-P-selectin, or anti-CD18 monoclonal antibody attenuated L-NAME-elicited venular leukocyte adhesion without abolishing CDCF fluorescence in situ. Pretreatment with desferioxamine (50 mg/kg iv; 1 h before L-NAME superfusion) significantly diminished the iron-catalyzed hydroperoxide formation in arterioles and venules, but not in interstitial mast cells, as well as subsequent venular leukocyte adhesion. These findings indicate that endogenous nitric oxide may modulate oxidative stress in mast cells, arteriolar and venular microvascular endothelium and thereby can play a crucial role in leukocyte recruitment in venules.

In vivo visualization of oxidative changes in microvessels during neutrophil activation

This study was designed to characterize temporal changes of oxidative metabolism in microvascular endothelium in vivo associated with neutrophil adhesion and diapedesis. The rat mesentery was loaded with dichlorofluorescein (DCFH) diacetate, a hydroperoxide-sensitive fluorescent probe that is trapped within viable cells as a nonfluorescent form and is converted to fluorescent dichlorofluorescein (DCF) by hydroperoxides. The fluorescent light emission was recorded with digital microscopy and its camera response was calibrated by superfusion of the mesentery with known concentrations of hydroperoxides. After rinsing with a precursor-free solution, it was possible to observe both the neutrophil adhesion and fluorescence changes in venular endothelium during superfusion with platelet-activating factor (PAF, 100 nM) by alternately changing light sources. In the PAF-treated animals, DCF fluorescence in the venular wall was markedly elevated concurrent with an increase in the number of adherent neutrophils. An onset of a significant DCF fluorescence was observed as early as 10 min after PAF application. The venular oxidative changes were co-localized with neutrophils adhering to the endothelium or migrating into the interstitium. The oxidative impact in the venular wall superfused with 100 nM PAF for 40 min was equivalent to that induced by the 10-min superfusion of 0.8 mM of tert-butyl hydroperoxide. Pretreatment with dipyridyl, an iron chelator, or dimethylthiourea, a hydroxyl radical scavenger, significantly attenuated the PAF-induced oxidative changes in the venule but had little effect on neutrophil adhesion. These findings suggest that an endothelial cell-dependent mechanism involving an iron-catalyzed reaction may have a role in enhancement of neutrophil-mediated oxidative stress in venular endothelium.

Wall structure of arteries and arterioles feeding the spinotrapezius muscle of normotensive and spontaneously hypertensive rats

Vessel wall structure in feed arteries and their hierarchical ramifications were investigated in a rat skeletal muscle. Emphasis was placed on the transition of wall structure elements from the axillary and thoracodorsal arteries to the feeder arteries and the arcade and transverse arterioles forming the microcirculation of the spinotrapezius muscle. Semi-thin sections, prepared for light microscopy, and ultra-thin sections for electron microscopy were used to compare mature (15-20 week old) normotensive and hypertensive animals. The results demonstrate a gradual transition of the elastica intima and the smooth muscle from the central arteries towards the arterioles, accompanied by an attenuation of the adventitia. The elastica intima in 60-40 microns arcade arterioles is reduced from a continuous sheet in the central arteries to single fibers oriented parallel to the vessel axis. The multiple layers of smooth muscle cells at the level of the feeder arteries are reduced in the majority of arcade arterioles and in transverse arterioles to a single smooth muscle coat. Comparisons of the smooth muscle mass per unit length of vessel at selected sites of normotensive and spontaneously hypertensive rats show evidence for some degree of wall hypertrophy in the axillary artery, but no evidence for wall hypertrophy in arterioles of the microcirculation. There is no evidence of a pressure-mediated structural medial wall hypertrophy along this peripheral vascular pathway.

Microvascular adjustments during irreversible hemorrhagic shock in rat skeletal muscle

A study was made of microvessel response to hemorrhagic hypotension and the subsequent restoration of blood volume. The experiments were conducted on anesthetized rats in which the cremaster muscle was exteriorized for intravital microscopy. Variables measured during hypotension (40 mm Hg for 60 min) and after blood restoration (120 min observation) included systemic blood pressure, heart rate, arteriolar and venular diameter, sensitivity to epinephrine, velocity and volumetric flow rate. These findings were correlated with 24-hr survival statistics. The response to hemorrhagic hypotension is a reflection of two separate adjustments, cardiac output and peripheral vascular behavior. In survivors, the microvascular sequelae following blood replacement was one of continuous improvement of muscle perfusion, whereas in nonsurvivors the picture was one of progressive deterioration. The consistent hallmark of irreversibility, arteriolar hyposensitivity, was associated with a continuous falling off in mean arterial pressure despite restoration of normal blood volume.