Elevated Transmural Pressure Research Articles

Dilatation of the bridging cerebral cortical veins in childhood hydrocephalus suggests a malfunction of venous impedance pumping

Dogs with a naturally occurring form of hydrocephalus have an elevated transmural venous pressure leading to cortical vein dilatation. The purpose of this study is to discover if there is vein dilatation in childhood hydrocephalus and to estimate the pressure required to maintain any enlargement found. Children with hydrocephalus between the ages of 4 and 15 years were compared with a control group. Magnetic resonance venography (MRV) and flow quantification were performed. The arterial inflow, sagittal sinus and straight sinus venous outflow were measured and the outflow percentages compared to the inflow were calculated. The cross-sectional area of the veins were measured. There were a total of 18 children with hydrocephalus, compared to 72 age and sex matched control MRV’s and 22 control flow quantification studies. In hydrocephalus, the sagittal sinus venous return was reduced by 12.9%, but the straight sinus flow was not significantly different. The superficial territory veins were 22% larger than the controls but the vein of Galen was unchanged. There is evidence of a significant increase in the superficial vein transmural pressure in childhood hydrocephalus estimated to be approximately 4 mmHg. An impedance pump model is suggested to explain these findings.

Acidosis inhibits rhythmic contractions of human thoracic ducts.

Lymph vessels counteract edema by transporting interstitial fluid from peripheral tissues to the large veins and serve as conduits for immune cells, cancer cells, and pathogens. Because edema during inflammation and malignancies is frequently associated with acidosis, we tested the hypothesis that acid‐base disturbances affect human thoracic duct contractions. We studied, by isometric and isobaric myography, the contractile function of human thoracic duct segments harvested with written informed consent from patients undergoing esophageal cancer surgery. Human thoracic ducts produce complex contractile patterns consisting of tonic rises in tension (isometric myography) or decreases in diameter (isobaric myography) with superimposed phasic contractions. Active tone development decreases substantially (~90% at 30 vs. 7 mmHg) at elevated transmural pressure. Acidosis inhibits spontaneous as well as noradrenaline‐ and serotonin‐induced phasic contractions of human thoracic ducts by 70–90% at extracellular pH 6.8 compared to 7.4 with less pronounced effects observed at pH 7.1. Mean tension responses to noradrenaline and serotonin – averaged over the entire period of agonist exposure – decrease by ~50% at extracellular pH 6.8. Elevating extracellular [K+] from the normal resting level around 4 mmol/L increases overall tension development but reduces phasic activity to a level that is no different between human thoracic duct segments investigated at normal and low extracellular pH. In conclusion, we show that extracellular acidosis inhibits human thoracic duct contractions with more pronounced effects on phasic than tonic contractions. We propose that reduced phasic activity of lymph vessels at low pH attenuates lymph propulsion and increases the risk of edema formation.

A Case of Pleuro-esophageal Fistula

Introduction: Spontaneous esophageal rupture, or Boerhaave's syndrome, is caused by elevated transmural pressure from forceful emesis. Management includes intravenous antibiotics, chest tube drainage and parenteral nutrition. If this fails, surgery is an option but has a mortality rate of 16 to 33%. In poor surgical candidates, both endoscopic drainage and esophageal stent placement have been described, but are rarely done. We describe transesophageal endoscopic drainage of a mediastinal collection with esophageal stent placement for a case of spontaneous esophageal perforation.Figure: CT scan showing oral contrast within the left pleural space, arising from the GE junction region; concerning for pleuro-esophageal fistula.Figure: Lower third of the esophagus with fistula visualized.Figure: After intubating the fistula, the pleural cavity is visualized.Case Report: A 73 year old male with hypertension presented with abdominal pain, nausea and non-bloody emesis for two days. Initially, he was tachycardic, tachypneic and hypoxic. Physical examination revealed crackles at the left lung base and decreased breath sounds bilaterally. Epigastric tenderness was elicited without guarding. Laboratory values revealed no leukocytosis and an elevated lactate of 3.4 mmol/L. X-ray showed a left sided pneumothorax. A chest tube drained 1000ml of dark, foul-smelling feculent material. Computed tomography showed oral contrast in the left pleural space arising from the gastroesophageal (GE) junction, concerning for pleuroesophageal fistula. Pleural fluid cultures grew gram-negative rods and antibiotics were initiated. Esophagogastroduodenoscopy revealed a GE junction fistula which was intubated with the endoscope. An empyema was noted in the pleural space which was irrigated and drained endoscopically. A partially covered metal stent was placed across the defect. The patient improved and was discharged with follow up. Discussion: Conservative therapy fails in up to 65% of cases of Boerhaave's syndrome, necessitating surgical or endoscopic intervention. Surgical mortality is 16 to 33%, requiring less invasive techniques such as endoscopic mediastinal drainage. The first endoscopic treatment of a mediastinal collection was described in 1983. This technique has been an effective treatment of mediastinal collections.Wehrmann et al. described the use of a guidewire to transendoscopically enter an abscess cavity with successful extraction of necrotic material. Our case uses transluminal endoscopy for extraluminal mediastinal abscess drainage. It also shows the use of a partially covered stent to close a spontaneous perforation. Although there are no trials comparing this to surgery, our case, as well as limited case reports, supports this approach.

Low density lipoprotein transport through patient-specific thoracic arterial wall

Background and aimsThe distribution of Low density lipoprotein (LDL) within the arterial wall is helpful in understanding the onset and development of atherosclerosis. The objective of the study is to study the transport and LDL distribution within patient-specific arterial wall using computational analysis under normal and hypertensive conditions. MethodsPatient specific model of the thoracic aorta is computationally examined. The arterial wall is treated macroscopically as homogeneous (one layered) porous media of variable thickness. The interfacial lumen-arterial wall (endothelium) coupling is achieved by the Kedem-Katchalsky equation. ResultsHigh values of LDL are located at areas where WSS values range from 0.4 N/m2 to 1.5 N/m2 for normal conditions. In this case the Pearson correlation coefficient r between LDL values and WSS is equal to −0.655 denoting a strong negative linear correlation. In the case that hypertension takes place, high LDL values are located at areas where WSS values range from 0.59 N/m2 to 1.7 N/m2 and the corresponding Pearson correlation coefficient r is equal to −0.808 denoting a very strong negative linear correlation. For the same parabolic intake flow velocity profile, the luminal surface concentration of LDL is 0.2–2.1% higher than that of the bulk flow for the normal pressure and 0.4–3.4% higher than that of the bulk flow for the hypertensive pressure. For normal conditions, the concentration of LDL at the endothelium/media interface is considerably lower (almost 20 times) than the LDL concentration value at lumen/endothelium interface. For hypertensive conditions, the LDL concentration at the endothelium/media interface is only 4.5 times lower than the corresponding luminal (endothelium side) concentration. The lumen/endothelium side locations (mainly the concave parts) of low WSS - high LDL concentration values coincide with those of high wall-side LDL concentration. ConclusionsThe transport and LDL distribution is affected by elevated transmural pressure which causes higher LDL concentration. Thus, hypertensive conditions theoretically enhance atherosclerosis.

The Role of Sphingosine Kinase 1/Sphingosine-1-Phosphate Pathway in the Myogenic Tone of Posterior Cerebral Arteries

AimsThe goal of the current study was to determine whether the sphingosine kinase 1 (SK1)/sphingosine-1-phosphate (S1P) pathway is involved in myogenic vasoconstriction under normal physiological conditions. In the present study, we assessed whether endogenous S1P generated by pressure participates in myogenic vasoconstriction and which signaling pathways are involved in SK1/S1P-induced myogenic response under normal physiological conditions.Methods and ResultsWe measured pressure-induced myogenic response, Ca2+ concentration, and 20 kDa myosin light chain phosphorylation (MLC20) in rabbit posterior cerebral arteries (PCAs). SK1 was expressed and activated by elevated transmural pressure in rabbit PCAs. Translocation of SK1 by pressure elevation was blocked in the absence of external Ca2+ and in the presence of mechanosensitive ion channel and voltage-sensitive Ca2+ channel blockers. Pressure-induced myogenic tone was inhibited in rabbit PCAs treated with sphingosine kinase inhibitor (SKI), but was augmented by treatment with NaF, which is an inhibitor of sphingosine-1-phosphate phosphohydrolase. Exogenous S1P further augmented pressure-induced myogenic responses. Pressure induced an increase in Ca2+ concentration leading to the development of myogenic tone, which was inhibited by SKI. Exogenous S1P further increased the pressure-induced increased Ca2+ concentration and myogenic tone, but SKI had no effect. Pressure- and exogenous S1P-induced myogenic tone was inhibited by pre-treatment with the Rho kinase inhibitor and NADPH oxidase inhibitors. Pressure- and exogenous S1P-induced myogenic tone were inhibited by pre-treatment with S1P receptor blockers, W146 (S1P1), JTE013 (S1P2), and CAY10444 (S1P3). MLC20 phosphorylation was increased when the transmural pressure was raised from 40 to 80 mmHg and exogenous S1P further increased MLC20 phosphorylation. The pressure-induced increase of MLC20 phosphorylation was inhibited by pre-treatment of arteries with SKI.ConclusionsOur results suggest that the SK1/S1P pathway may play an important role in pressure-induced myogenic responses in rabbit PCAs under normal physiological conditions.

Choroidal thickness following extrafoveal photodynamic treatment with verteporfin in patients with central serous chorioretinopathy

To evaluate the effect of verteporfin photodynamic treatment (PDT) on choroidal thickness in patients with central serous chorioretinopathy (CSC). Choroidal thickness was measured with enhanced depth imaging- optical coherence tomography (EDI-OCT) before and after verteporfin PDT (full-dose verteporfin, half-light dose) in 16 eyes in 16 patients with serous detachment of the fovea secondary to extrafoveal angiographic fluorescein leakage. Treatment was confined to the area of leakage, whereas choroidal thickness before and after treatment was assessed over a larger area of the fundus using OCT. Complete resolution of the serous detachment was seen in all 16 eyes within 1 month of extrafoveal PDT, while choroidal thickness in the area where PDT was applied decreased from 407 μm [mean; 95% confidence interval (CI(95) ) 356-458 μm] to 349 μm (mean; CI(95) 300-399 μm; p < 0.0001), and subfoveal choroidal thickness was reduced from 421 μm (mean; CI(95) 352-489 μm) to 346 μm (mean; CI(95) 278-414 μm; p = 0.0001). Initially, subfoveal choroidal thickness was significantly increased in the treated eye compared with the healthy fellow eye (mean 324 μm; CI(95) 273-376 μm; p = 0.0003), but after treatment, the difference was not significant. Photodynamic therapy of active CSC was followed by choroidal thickness reduction, not only locally but also at considerable distance from the treated area. Thus, the process that causes choroidal thickening in CSC appears to spread laterally within the choroid.

A New PIXel in the Puzzle

Hypertension is a clinical situation associated with endothelial dysfunction and increased vascular reactive oxygen species (ROS) formation. The observations that antioxidants, such as Tempol or apocynin, lower the blood pressure in spontaneously hypertensive rats gave rise to the concept that ROS essentially contribute to hypertension. Recent work, however, has sharpened this view, and it is now clear that the underlying mechanism of ROS-induced hypertension may not be endothelial dysfunction but rather an ROS-dependent modulation of the sympathetic drive,1 the kidney, and potentially alterations in the immune system.2 Despite these findings, hypertension has been consistently linked to increased ROS formation in the vascular system. It is, therefore, plausible that ROS are not necessarily the cause but potentially the consequence of hypertension. This notion certainly does not exclude that ROS are involved in the pathogenesis of specific aspects of hypertension, such as the development of fibrosis. In keeping with the concept of hypertension-induced ROS formation, it was noted previously that distension of a vessel with an oversized balloon acutely increases ROS formation.3 Moreover, acute elevation of pressure from 80 to 160 mm Hg enhances the ROS formation and induces an ROS-dependent attenuation of the endothelium-dependent relaxation in isolated rat femoral arteries.4 Moreover, even in chronic models of isolated hypertension, such as aortic banding, the ROS formation was consistently higher in the hypertensive rather than in the normotensive part. There is, however, uncertainty regarding the enzymatic sources of ROS, and a contribution of mitochondria, endothelial NO …

Failure of vascular autoregulation in the upper limb with increased +Gz acceleration

Forearm pain occurring during high +Gz exposure has been linked with vascular distension from elevated transmural pressure of hydrostatic origin and is exacerbated by positive pressure breathing (PBG). We postulated that at high vascular transmural pressure vascular autoregulation might be overcome and be associated with worsened pain. Six volunteers were studied at +4, +5, +6, and +7 Gz on a human centrifuge. Forearm vascular resistance (FVR) was assessed by Doppler ultrasound resistive index (RI), and superficial forearm venous pressure (FVP) was measured via an indwelling catheter. Pain rating was assessed by numerical scale. The left arm was located at heart level (control position), or on the throttle (test position). Runs were completed with and without positive pressure breathing for G protection (PBG); subjects wore full coverage anti-G trousers and chest counter-pressure. In the test position, pain increased with increasing acceleration (P < 0.0001), and was more severe with PBG at +5 Gz and +7 Gz (P < 0.05). FVP rose substantially more in the test than control position (238 +/- 17 mmHg vs. 61 +/- 8 mmHg at +7 Gz, P < 0.0001) but the presence or absence of PBG had no effect on the FVP increase during acceleration in either position. In the test position, RI fell with increasing acceleration above +5 Gz (P < 0.0001), and the fall was greater with PBG (P < 0.05). Forearm pain was thus associated with a decrease in FVR and an increase in vascular transmural pressure. PBG exacerbated forearm pain and prompted a greater fall in RI, but had no effect on FVP response. These findings support FVR but not forearm venous distension in the aetiology of +Gz arm pain.

Sphingosine kinase functionally links elevated transmural pressure and increased reactive oxygen species formation in resistance arteries

Myogenic vasoconstriction, an intrinsic response to elevated transmural pressure (TMP), requires the activation of sphingosine kinase (Sk1) and the generation of reactive oxygen species (ROS). We hypothesized that pressure-induced Sk1 signaling and ROS generation are functionally linked. Using a model of cannulated resistance arteries isolated from the hamster gracilis muscle, we monitored vessel diameter and smooth muscle cell (SMC) Ca2+i (Fura-2) or ROS production (dichlorodihydrofluorescein). Elevation of TMP stimulated the translocation of a GFP-tagged Sk1 fusion protein from the cytosol to the plasma membrane, indicative of enzymatic activation. Concurrently, elevation of TMP initiated a rapid and transient production of ROS, which was enhanced by expression of wild-type Sk1 (hSk(wt)) and inhibited by its dominant-negative mutant (hSk(G82D)). Exogenous sphingosine-1-phosphate (S1P) also stimulated ROS generation is isolated vessels. Chemical (1 micromol/L DPI), peptide (gp91ds-tat/gp91ds), and genetic (N17Rac) inhibition strategies indicated that NADPH oxidase was the source of the pressure-induced ROS. NADPH oxidase inhibition attenuated myogenic vasoconstriction and reduced the apparent Ca2+ sensitivity of the SMC contractile apparatus, without affecting Ca2+-independent, RhoA-mediated vasoconstriction in response to exogenous S1P. Our results indicate a mandatory role for Sk1/S1P in mediating pressure-induced, NADPH oxidase-derived ROS formation. In turn, ROS generation appears to increase Ca2+ sensitivity, necessary for full myogenic vasoconstriction.

Contribution of connexins to the function of the vascular wall.

Gap junction channels provide an enclosed conduit for direct exchanges of signalling molecules, including ions and small metabolites between cells. This system of communication allows cells to monitor the functional state of their neighbours, and is rapidly modulated to continuously adapt to the immediate needs of groups of coupled cells. In the major arteries, endothelial cells may express three connexins isotypes, namely Connexin 37 (Cx37), Cx40 and Cx43, whereas the underlying smooth muscle cells may express Cx37, Cx40, Cx43 and Cx45. Moreover, myoendothelial gap junctions have also been shown to be involved in the regulation of vascular tone. This review highlights the regulation of vessel connexins in response to injury, as observed during experimental hypertension or wound repair, as well as the consequences of loss of one connexin in different transgenic null mice. In view of the major endocrine role of the kidney in the control of blood pressure, we also discuss the distribution of connexins in the kidney vasculature. Cx40 is present between endothelial cells of vessels and glomeruli, as well as between renin-secreting cells, the modified smooth muscle cells which form the wall of the terminal part of afferent arterioles. Modulation of Cx40 expression in a model of renin-dependent hypertension suggests that this connexin may be implicated in the function of renin-secreting cells. Finally, to address the possible regulation of connexin expression by fluid pressure, we summarize the effects of elevated transmural urine pressure on bladder Cx43 expression.

A transmural pressure gradient induces mechanical and biological adaptive responses in endothelial cells.

A sudden increase in the transmural pressure gradient across endothelial monolayers reduces hydraulic conductivity (L(p)), a phenomenon known as the sealing effect. To further characterize this endothelial adaptive response, we measured bovine aortic endothelial cell (BAEC) permeability to albumin and 70-kDa dextran, L(p), and the solvent-drag reflection coefficients (sigma) during the sealing process. The diffusional permeability coefficients for albumin (1.33 +/- 0.18 x 10(-6) cm/s) and dextran (0.60 +/- 0.16 x 10(-6) cm/s) were measured before pressure application. The effective permeabilities (measured when solvent drag contributes to solute transport) of albumin and dextran (P(ealb) and P(edex)) were measured after the application of a 10 cmH(2)O pressure gradient; during the first 2 h of pressure application, P(ealb), P(edex), and L(p) were significantly reduced by 2.0 +/- 0.3-, 2.1 +/- 0.3-, and 3.7 +/- 0.3-fold, respectively. Immunostaining of the tight junction (TJ) protein zonula occludens-1 (ZO-1) was significantly increased at cell-cell contacts after the application of transmural pressure. Cytochalasin D treatment significantly elevated transport but did not inhibit the adaptive response, whereas colchicine treatment had no effect on diffusive permeability but inhibited the adaptive response. Neither cytoskeletal inhibitor altered sigma despite significantly elevating both L(p) and effective permeability. Our data suggest that BAECs actively adapt to elevated transmural pressure by mobilizing ZO-1 to intercellular junctions via microtubules. A mechanical (passive) component of the sealing effect appears to reduce the size of a small pore system that allows the transport of water but not dextran or albumin. Furthermore, the structures of the TJ determine transport rates but do not define the selectivity of the monolayer to solutes (sigma).

Connexins 43 and 26 Are Differentially Increased after Rat Bladder Outlet Obstruction

To evaluate the regulation of connexin expression by fluid pressure, we have studied the effects of elevated transmural urine pressure on Connexin43 (Cx43) and Cx26. We chose to focus on these two proteins out of the five connexins (Cx26, 43, 40, 37, and 45) which we found by RT-PCR to be expressed in the rat bladder, since in situ hybridization and immunofluorescence showed that Cx43 is the predominant connexin expressed by smooth muscle cells (SMC), whereas Cx26 is abundantly expressed only in the latter cell type. To evaluate whether these connexins are affected by changes in transmural urine pressure, we used a rat model of bladder outlet obstruction, in which a ligature is placed around the urethra. Under conditions of increased fluid pressure due to urine retention, we observed that the expression of both Cx43 and Cx26 increased at both transcript and protein levels, reaching a maximum 7–9 h after the ligature. Further analysis revealed that these changes were accounted for by a fourfold increase in Cx43 mRNA of SMC but not urothelial cell and by a fivefold increase in Cx26 mRNA of urothelium. Scrape-loading of propidium iodide showed that the latter change was paralleled by a twofold increase in coupling between urothelial cells. The data show that Cx43 and Cx26 are differentially regulated during bladder outlet obstruction and contribute to the response of the bladder wall to increased voiding pressure, possibly to control its elasticity.

20-HETE Contributes to Myogenic Activation of Skeletal Muscle Resistance Arteries in Brown Norway and Sprague–Dawley Rats

To evaluate the role of 20-hydroxyeicosatetraenoic acid (20-HETE), a product of arachidonic acid omega-hydroxylation via cytochrome P450 (CP450) 4A enzymes, in regulating myogenic activation of skeletal muscle resistance arteries from normotensive Brown Norway (BN) and Sprague-Dawley (SD) rats. Gracilis arteries (GA) were isolated from each animal, viewed via television microscopy, and vessel diameter responses to elevated transmural pressure were measured with a video micrometer under control conditions and following pharmacological inhibition of the CP450 4A enzyme system. Under control conditions, GA from both rat groups exhibited strong, endothelium-independent myogenic activation, which was impaired following treatment with either 17-octadecynoic acid (17-ODYA) or dibromo-dodecenylmethylsulfimide (DDMS), two mechanistically different inhibitors of 20-HETE production. The addition of tetraethylammonium (KCa channel inhibitor) to 17-ODYA-treated GA restored myogenic reactivity to levels comparable to those under control conditions. Treatment of GA from BN and SD rats with 6(Z),15(Z)-20-HEDE, a selective antagonist for 20-HETE receptors, mimicked the effects of 17-ODYA and DDMS treatment on myogenic reactivity. These results suggest that the production of 20-HETE via CP450 4A enzymes contributes to the myogenic activation of skeletal muscle resistance arteries from normotensive BN and SD rats. 20-HETE may act through a receptor-mediated process to block vascular smooth muscle KCa channels in response to the elevated transmural pressure.

20‐HETE Contributes to Myogenic Activation of Skeletal Muscle Resistance Arteries in Brown Norway and Sprague‐Dawley Rats

ABSTRACTObjective: To evaluate the role of 20‐hydroxyeicosatetraenoic acid (20‐HETE), a product of arachidonic acid ω‐hydroxylation via cytochrome P450 (CP450) 4A enzymes, in regulating myogenic activation of skeletal muscle resistance arteries from normotensive Brown Norway (BN) and Sprague‐Dawley (SD) rats.Methods: Gracilis arteries (GA) were isolated from each animal, viewed via television microscopy, and vessel diameter responses to elevated transmural pressure were measured with a video micrometer under control conditions and following pharmacological inhibition of the CP450 4A enzyme system.Results: Under control conditions, GA from both rat groups exhibited strong, endothelium‐independent myogenic activation, which was impaired following treatment with either 17‐octadecynoic acid (17‐ODYA) or dibromo‐dodecenylmethylsulfimide (DDMS), two mechanistically different inhibitors of 20‐HETE production. The addition of tetraethylammonium (KCa channel inhibitor) to 17‐ODYA‐treated GA restored myogenic reactivity to levels comparable to those under control conditions. Treatment of GA from BN and SD rats with 6(Z), 15(Z)‐20‐HEDE, a selective antagonist for 20‐HETE receptors, mimicked the effects of 17‐ODYA and DDMS treatment on myogenic reactivity.Conclusions: These results suggest that the production of 20‐HETE via CP450 4A enzymes contributes to the myogenic activation of skeletal muscle resistance arteries from normotensive BN and SD rats. 20‐HETE may act through a receptor‐mediated process to block vascular smooth muscle KCa channels in response to the elevated transmural pressure.

20-HETE Contributes to Myogenic Activation of Skeletal Muscle Resistance Arteries in Brown Norway and Sprague-Dawley Rats

Objective: To evaluate the role of 20-hydroxyeicosatetraenoic acid (20-HETE), a product of arachidonic acid ω-hydroxylation via cytochrome P450 (CP450) 4A enzymes, in regulating myogenic activation of skeletal muscle resistance arteries from normotensive Brown Norway (BN) and Sprague-Dawley (SD) rats. Methods: Gracilis arteries (GA) were isolated from each animal, viewed via television microscopy, and vessel diameter responses to elevated transmural pressure were measured with a video micrometer under control conditions and following pharmacological inhibition of the CP450 4A enzyme system. Results: Under control conditions, GA from both rat groups exhibited strong, endothelium-independent myogenic activation, which was impaired following treatment with either 17-octadecynoic acid (17-ODYA) or dibromo-dodecenylmethylsulfimide (DDMS), two mechanistically different inhibitors of 20-HETE production. The addition of tetraethylammonium (KCa channel inhibitor) to 17-ODYA-treated GA restored myogenic reactivity to levels comparable to those under control conditions. Treatment of GA from BN and SD rats with 6(Z), 15(Z)-20-HEDE, a selective antagonist for 20-HETE receptors, mimicked the effects of 17-ODYA and DDMS treatment on myogenic reactivity. Conclusions: These results suggest that the production of 20-HETE via CP450 4A enzymes contributes to the myogenic activation of skeletal muscle resistance arteries from normotensive BN and SD rats. 20-HETE may act through a receptor-mediated process to block vascular smooth muscle KCa channels in response to the elevated transmural pressure.

Chronic increases in transmural pressure reduce NO-mediated dilations in isolated resistance arteries of the hamster.

It is unclear whether the impairment of NO-mediated dilation in hypertension is the cause or the consequence of high blood pressure. We therefore studied in isolated resistance arteries whether elevated transmural pressure affects NO-mediated dilation. Arteries (n=5-7) were perfused at hydrostatic pressures of either 45, 120 or 160 mmHg for 48 h. Subsequently, diameter and calcium responses (fura 2) were studied at a transmural pressure of 45 mmHg. Pre-exposure to 120 and 160 mmHg reduced resting diameters and minimal diameters after stimulation with noradrenaline and significantly increased corresponding intracellular free calcium levels in vascular smooth muscle. Moreover, the NO-mediated dilation in response to acetylcholine was significantly reduced although the increase in endothelial calcium was not altered. Dilations induced by the NO donor SNP were not affected. It is concluded that chronically elevated pressure per se impairs endothelial NO production by a mechanism distal to receptor-dependent calcium increases.

Coarctation induces alterations in basement membranes in the cardiovascular system.

A coarctation hypertensive rat model was used to examine the effects of elevated blood pressure on basement membrane component synthesis by cardiac myocytes and aorta using immunohistochemistry and Northern blot analysis. Carotid arterial pressure increased immediately on coarctation, and left ventricular hypertrophy was maximal within 5 days. In immunohistochemical studies, fibronectin and laminin were increased and the basement membrane chondroitin sulfate proteoglycan decreased in both the subendothelial space and smooth muscle cell basement membranes of the aorta above the clip compared with controls, whereas only fibronectin was elevated in the aorta below the clip. No change in basement membrane staining intensity for the cardiac myocytes was observed. Alterations in steady-state mRNA levels for fibronectin and laminin in the aorta paralleled those observed by immunohistochemical analysis with regard to protein and tissue type affected as well as intensity of the changes. However, changes in mRNA levels (but not protein deposition) for perlecan and type IV collagen were also observed in aortas from hypertensive rats compared with controls. Increases in steady-state mRNA levels for all basement membrane components in the heart and vasculature peaked before maximal cardiac hypertrophy (5 days). These studies indicate that alterations in basement membrane component deposition in the hypertrophied vasculature occur at both transcriptional and translational levels and suggest that the cell attachment glycoproteins fibronectin and laminin may be important factors in the vascular response to elevated transmural pressure.

Enhanced norepinephrine sensitivity in renal arteries at elevated transmural pressure

The electrical and mechanical responses of vascular smooth muscle to norepinephrine were compared at three different levels of transmural pressure (60, 100, and 140 mmHg) in cannulated small arteries that were isolated from dog kidneys. Elevation of transmural pressure caused depolarization of the vascular smooth muscle and a significant increase in vessel sensitivity to norepinephrine. Norepinephrine produced less depolarization as transmural pressure was elevated. Comparison of the relationship between transmembrane potential and norepinephrine concentration at these different transmural pressures suggests that membrane depolarization contributes to, but is not the sole explanation of, the increased norepinephrine sensitivity at higher transmural pressures. The results of this study suggest that transmural pressure in a blood vessel may be an important determinant of its sensitivity to vasoactive agonists.

A rate-sensitive component to the myogenic response is absent from bat wing arterioles.

The responses of three sequential branching orders of arterioles in the bat wing to rapid and slow changes in transmural pressure were studied. Arterial and venous pressures to the wing were elevated simultaneously by pressurizing a box containing the body of the animal, while the wing was exposed to atmospheric pressure. Box pressure was elevated from 0 to +48 mmHg at two rates: 24 and 0.6 mmHg/s. During this time, continuous recordings of hydrostatic pressure and diameter were made in single arterioles using intravital microscopic techniques. Second-order arterioles and arcuate arterioles from skin and skeletal muscle constricted in response to elevated transmural pressure but did not show an enhanced response to rapid pressure changes. There was a trend for terminal arterioles to show a transient peak constriction during rapid stretch, but this response was always associated with a biphasic change in arteriolar pressure. These results suggest that the transient arteriolar resistance changes associated with rapid transmural pressure increases in previous experiments may be primarily a result of transient pressure changes in small arterioles. We find no evidence that arterioles in this preparation exhibit a rate-sensitive component to their myogenic response.

Effect of reduced oxygen availability upon myogenic depolarization and contraction of cat middle cerebral artery.

The goal of this study was to determine whether electrophysiological mechanisms contribute to the relaxation of cat middle cerebral artery in response to decreased ambient Po2 and whether decreased Po2 alters the myogenic depolarization and contraction of this vessel in response to elevations in transmural pressure. In one series of experiments, arterial segments (200-500 micron outer diameter) were isolated and mounted in an in vitro tension transducer to allow continuous measurement of active tension as bath Po2 was reduced. In these experiments, vessel relaxation occurred primarily between 150 mm Hg Po2 and 40 mm Hg Po2, suggesting that cerebral arteries are sensitive to alterations of Po2 in the physiological range. Relaxation did not result from the activation of dilator nerves in the vessel wall, since it was unaffected by tetrodotoxin. Arterial segments were also cannulated with micropipettes and subjected to elevations in transmural pressure during 300 mm Hg Po2 and 50 mm Hg Po2 superfusion. During 300 mm Hg Po2 superfusion, cannulated vessels exhibited myogenic depolarization and maintained their diameter as transmural pressure was increased; 50 mm Hg Po2 superfusion inhibited spontaneous spike activity, decreased the slope of the myogenic depolarization, and partially inhibited vessel contraction in response to elevated transmural pressure. These effects are independent of the parenchymal cell environment and appear to be mediated, at least in part, by electrophysiological mechanisms.