Endothelin-1 focally constricts pulmonary veins in rats

The relationship between mixed venous blood oxygen saturation and pulmonary arterial and venous pressures in patients with heart failure

Recent discoveries have identified intrapulmonary bronchopulmonary anastomoses (IBAs) as a relatively common phenomenon forming intrapulmonary right-to-left shunts. This study hypothesizes that IBAs play a significant role in the pathophysiology of heart failure. We aim to investigate the impact of these intrapulmonary right-to-left shunts on pulmonary arterial and venous pressures in heart failure patients, utilizing mixed venous oxygen saturation (SvO₂) as a key measurement. This study included 237 patients with heart failure who underwent cardiac catheterization. The relationships between SvO₂ and pulmonary artery systolic pressure (sPAP), pulmonary artery wedge pressure (PAWP), and left ventricular end-diastolic pressure (LVEDP) were examined using various statistical methods (single regression analysis, partial correlation analysis, structural equation modeling, and Bayesian estimation). All statistical methods that we performed showed that SvO₂ was significantly and negatively correlated with both sPAP and PAWP (p < 0.01, respectively). However, SvO₂ did not significantly correlate with LVEDP. These results suggest that a decrease in SvO₂ leads to an increase in PAWP and sPAP, while LVEDP is only passively influenced by PAWP. This phenomenon likely reflects the impact of an intrapulmonary right-to-left shunt caused by IBAs. The decrease in SvO₂ causes an increase in sPAP and may also cause an increase in PAWP via IBAs.

Structural and functional definition of the pulmonary vein system in a chronic hypoxia-induced pulmonary hypertension rat model

Unlike the pulmonary artery (PA), the pathophysiological changes of the pulmonary vein (PV) in the development of pulmonary hypertension (PH) remain largely unknown. In this study, we comprehensively investigated the structural and functional changes in the PV isolated from the chronic hypoxia (CH; 10% O₂, 21 days)-induced PH rat model (CHPH). Results showed that CH caused an increase in right ventricular pressure but did not affect the mean pulmonary venous pressure and the left atrial pressure. Similar to the PA, vascular lumen stenosis and medial thickening were also observed in the intrapulmonary veins isolated from the CHPH rats. Notably, CH induced more severe loss in the endothelium of intrapulmonary veins than the arteries. Then, the contractile response to 5-HT and U46619 was significantly greater in the intrapulmonary small veins (ISPV) and arteries (ISPA) isolated from CHPH rats than those from normoxic rats but not in the extrapulmonary and intrapulmonary large veins. Treatment with nifedipine (Nif), SKF96365 (SKF), or ryanodine and caffeine either partially attenuated (Nif) or dramatically abolished (SKF or ryanodine and caffeine) 5-HT-induced maximal contraction in ISPV from both normoxic and CHPH rats. Because of the severe loss of endothelium in the PV of CHPH rats, the decrease in acetylcholine (ACh)-induced endothelium-dependent relaxation was significantly larger in ISPV than ISPA, whereas the sodium nitroprusside-induced endothelium-independent relaxation was not altered in both ISPA and ISPV. In conclusion, our results provide fundamental data to comprehensively define the PV system in CHPH rat model.

Acute hypoxia activates store-operated Ca(2+) entry and increases intracellular Ca(2+) concentration in rat distal pulmonary venous smooth muscle cells

RATIONALE

Exposure to acute hypoxia causes vasoconstriction in both pulmonary arteries (PA) and pulmonary veins (PV). The mechanisms on the arterial side have been studied extensively. However, bare attention has been paid to the venous side.

OBJECTIVES

To investigate if acute hypoxia caused the increase of intracellular Ca(2+) concentration ([Ca(2+)]i), and Ca(2+) influx through store-operated calcium channels (SOCC) in pulmonary venous smooth muscle cells (PVSMCs).

METHODS

Fluorescent microscopy and fura-2 were used to measure effects of 4% O2 on [Ca(2+)]i and store-operated Ca(2+) entry (SOCE) in isolated rat distal PVSMCs.

MEASUREMENTS AND MAIN RESULTS

In PVSMCs perfused with Ca(2+)-free Krebs Ringer bicarbonate solution (KRBS) containing cyclopiazonic acid to deplete Ca(2+) stores in the sarcoplasmic reticulum (SR) and nifedipine to prevent Ca(2+) entry through L-type voltage-depended Ca(2+) channels (VDCC), hypoxia markedly enhanced both the increase in [Ca(2+)]i caused by restoration of extracellular [Ca(2+)] and the rate at which extracellular Mn(2+) quenched fura-2 fluorescence. Moreover, the increased [Ca(2+)]i in PVSMCs perfused with normal salt solution was completely blocked by SOCC antagonists SKF-96365 and NiCl2 at concentrations that SOCE >85% was inhibited but [Ca(2+)]i responses to 60 mM KCl were not altered. On the contrary, L-type VDCC antagonist nifedipine inhibited increase in [Ca(2+)]i to hypoxia by only 50% at concentrations that completely blocked responses to KCl. The increased [Ca(2+)]i caused by hypoxia was completely abolished by perfusion with Ca(2+)-free KRBS.

CONCLUSIONS

These results suggest that acute hypoxia enhances SOCE via activating SOCCs, leading to increased [Ca(2+)]i in distal PVSMCs.

Expression of store-operated Ca2+ entry and transient receptor potential canonical and vanilloid-related proteins in rat distal pulmonary venous smooth muscle

Chronic hypoxia causes remodeling and alters contractile responses in both pulmonary arteries and pulmonary veins. Although pulmonary arteries have been studied extensively in these disorders, the mechanisms by which pulmonary veins respond to hypoxia and whether these responses contribute to chronic hypoxic pulmonary hypertension remain poorly understood. In pulmonary arterial smooth muscle, we have previously demonstrated that influx of Ca(2+) through store-operated calcium channels (SOCC) thought to be composed of transient receptor potential (TRP) proteins is likely to play an important role in development of chronic hypoxic pulmonary hypertension. To determine whether this mechanism could also be operative in pulmonary venous smooth muscle, we measured intracellular Ca(2+) concentration ([Ca(2+)](i)) by fura-2 fluorescence microscopy in primary cultures of pulmonary venous smooth muscle cells (PVSMC) isolated from rat distal pulmonary veins. In cells perfused with Ca(2+)-free media containing cyclopiazonic acid (10 μM) and nifedipine (5 μM) to deplete sarcoplasmic reticulum Ca(2+) stores and block voltage-dependent Ca(2+) channels, restoration of extracellular Ca(2+) (2.5 mM) caused marked increases in [Ca(2+)](i), whereas MnCl(2) (200 μM) quenched fura-2 fluorescence, indicating store-operated Ca(2+) entry (SOCE). SKF-96365 and NiCl(2), antagonists of SOCC, blocked SOCE at concentrations that did not alter Ca(2+) responses to 60 mM KCl. Of the seven known canonical TRP (TRPC1-7) and six vanilloid-related TRP channels (TRPV1-6), real-time PCR revealed mRNA expression of TRPC1 > TRPC6 > TRPC4 > TRPC2 ≈ TRPC5 > TRPC3, TRPV2 > TRPV4 > TRPV1 in distal PVSMC, and TRPC1 > TRPC6 > TRPC3 > TRPC4 ≈ TRPC5, TRPV2 ≈ TRPV4 > TRPV1 in rat distal pulmonary vein (PV) smooth muscle. Western blotting confirmed protein expression of TRPC1, TRPC6, TRPV2, and TRPV4 in both PVSMC and PV. Our results suggest that SOCE through Ca(2+) channels composed of TRP proteins may contribute to Ca(2+) signaling in rat distal PV smooth muscle.

Inflammatory response to pulmonary ischemia-reperfusion injury

Lung ischemia-reperfusion (IR) injury is one of the most important complications following lung transplant and cardiopulmonary bypass. The pulmonary dysfunction following lung IR has been well documented. Recent studies have shown that ischemia and reperfusion of the lung may each play significant yet differing roles in inducing lung injury. The mechanisms of injury involving neutrophil activation, and the release of numerous inflammatory mediators and oxygen radicals also contributes to lung cellular injury, pneumocyte necrosis, and apoptosis. We herein review the current understanding of the underlying mechanism involved in lung IR injury. The biomolecular mechanisms and interactions which lead to the inflammatory response, pneumocyte necrosis, and apoptosis following lung IR therefore warrant further investigation.

Role of veins in regulation of pulmonary circulation

Pulmonary veins have been seen primarily as conduit vessels; however, over the past two decades, a large amount of evidence has accumulated to indicate that pulmonary veins can exhibit substantial vasoactivity. In this review, the role of veins in regulation of the pulmonary circulation, particularly during the perinatal period and under certain pathophysiological conditions, is discussed. In the fetus, pulmonary veins contribute a significant fraction to total pulmonary vascular resistance. At birth, the veins as well as the arteries relax in response to endothelium-derived nitric oxide and dilator prostaglandins, thereby assisting in the fall in pulmonary vascular resistance. These effects are oxygen dependent and modulated by cGMP-dependent protein kinase. Under chronic hypoxic conditions, pulmonary veins undergo remodeling and demonstrate substantial constriction and hypertrophy. In a number of species, including the human, pulmonary veins are also the primary sites of action of certain vasoconstrictors such as endothelin and thromboxane. In various pathological conditions, there is an increased synthesis of these vasoactive agents that may lead to pulmonary venous constriction, increased microvascular pressures for fluid filtration, and formation of pulmonary edema. In conclusion, the significant role of veins in regulation of the pulmonary circulation needs to be appreciated to better prevent, diagnose, and treat lung disease.

Venous sphincters in the choroidea of spontaneously hypertensive rats

Ocular vasculopathy resulting from severe systemic hypertension affects retina, choroidea, and the optic nerve. While the pathologic changes of the arterial system, including luminal narrowing, are well documented, little is known about the ocular venous vessels in hypertension. Adult 18-week-old spontaneously hypertensive rats (SHRs) were untreated (control) or treated with lisinopril for 4 weeks; normotensive Wistar-Kyoto (WKY) rats were additional controls. The mean systemic blood pressure (MSP) was monitored chronically using telemetry. The ocular microvasculature was examined using scanning electron microscopy (SEM) of corrosion casts, histology of serial sections, and computer-based 3D reconstruction. The MSP in control SHRs (145 +/- 11.9 mmHg) was decreased to 68.1 +/- 4.9 mmHg (P < 0.001) following treatment, which was even below the baseline level of WKY (96.7 +/- 5.8) rats (P < 0.05). In addition to media thickening in arteries, the venous plexus of the choroidea in control SHRs revealed multiple tufts of smooth muscle cells (sphincters) that narrowed the lumen. Correlating to histology, SEM of casts and 3D reconstruction showed numerous constrictions and muscular tufts in veins of the choroidea, narrowing the vascular lumen up to 47%. Following antihypertensive treatment, the percentage of sphincter constriction was decreased to 6% (P < 0.001). The depth of venous sphincter contraction correlated significantly with MSP (r = 0.87). To our knowledge, these results indicate for the first time that venous sphincters occur in the choroidea of the SHRs, and that their muscularity correlates with MSP. Venous sphincters might be involved in the pathophysiology of hypertension-related ocular changes.

Clinical efficacy of sitaxsentan, an endothelin-A receptor antagonist, in patients with pulmonary arterial hypertension: open-label pilot study

STUDY OBJECTIVES

To evaluate the safety and efficacy of sitaxsentan, an endothelin-A receptor antagonist, in a 12-week, open-label trial of patients with pulmonary arterial hypertension (PAH).

PATIENTS

Six children and 14 adults with New York Heart Association (NYHA) functional class II, III, or IV primary pulmonary hypertension or PAH associated with either congenital systemic-to-pulmonary shunts or collagen vascular disease were enrolled.

MEASUREMENTS

Sitaxsentan was administered orally at 100 to 500 mg bid for 12 weeks. Cardiopulmonary hemodynamics via cardiac catheterization were obtained at baseline and week 12. Six-minute walk test distance was measured at baseline, week 6, and week 12.

RESULTS

Sitaxsentan treatment resulted in significant improvement in exercise capacity as assessed by the 6-min walk distance (baseline [mean +/- SD], 466 +/- 132 m; week 12, 515 +/- 141 m, n = 20, p = 0.006). Mean pulmonary artery pressure and pulmonary vascular resistance index also improved (63 +/- 20 to 52 +/- 22 mm Hg, n = 17, p = 0.0002; and 20 +/- 11 to 14 +/- 13 U x m(2), n = 17, p = 0.008, respectively). Serious adverse events included two cases of acute hepatitis (fatal in one patient).

CONCLUSIONS

Patients with NYHA functional class II, III, or IV PAH showed a significant improvement in exercise capacity and cardiopulmonary hemodynamics over a 12-week period of treatment with sitaxsentan, an endothelin-A receptor antagonist. Further investigation is warranted to evaluate the safety and efficacy of sitaxsentan in patients with PAH.

Upregulation of ET-1 and its receptors and remodeling in small pulmonary veins under hypoxic conditions

Pulmonary veins show greater sensitivity to endothelin (ET)-1-induced vasoconstriction than pulmonary arteries, and remodeling was observed in pulmonary veins under hypoxic conditions. We examined, using an immunohistochemical method, the expression of Big ET-1, ET-converting enzyme (ECE), and ET(A) and ET(B) receptors in rat pulmonary veins under normoxic and hypoxic conditions. In control rats, Big ET-1 and ECE were coexpressed in the intima and media of the pulmonary veins, with an even distribution along the axial pathway. ET(A) and ET(B) receptors were expressed in the pulmonary veins, with a predominant distribution in the proximal segments. The expression of Big ET-1 was more abundant in the pulmonary veins than in the pulmonary arteries. After exposure to hypoxia for 7 or 14 days, the expression of Big ET-1, ECE, and ET receptors increased in small pulmonary veins. Increases in the medial thickness, wall thickness, and immunoreactivity for alpha-smooth muscle actin were also observed in the small pulmonary veins under hypoxic conditions. The upregulation of ET-1 and ET receptors in the small pulmonary veins is associated with vascular remodeling, which may lead to the development of hypoxic pulmonary hypertension.

Strategies to modulate the deleterious effects of endothelin in hepatic ischemia-reperfusion

BACKGROUND

This study evaluates whether bosentan (endothelin [ET] receptor antagonist) or preconditioning (mechanism that inhibits the postischemic ET release) could reduce the microvascular disorders and the injurious effects of tumor necrosis factor (TNF) associated with hepatic ischemia-reperfusion (I/R).

METHODS

Hepatic I/R was induced in rats and the effects of bosentan or preconditioning on the deleterious effects of ET in hepatic I/R were evaluated. Transaminase and TNF levels in plasma; edema, vascular permeability, lactate, ET, and TNF levels in liver; and edema and myeloperoxidase activity levels in lung were measured after hepatic reperfusion.

RESULTS

The administration of bosentan or the induction of preconditioning previous to I/R attenuated the increase in vascular permeability, edema and lactate levels observed in liver after I/R. However, the addition of ET before preconditioning abolished its benefits. Preconditioning prevented both the increase in hepatic TNF and its release from the liver into the systemic circulation. This resulted in an attenuation of liver and lung damage. Addition of ET or TNF to the preconditioned group abolished the benefits of preconditioning, whereas the previous inhibition of TNF release with GdCl3 in the preconditioned group pretreated with ET did not modify the effects of preconditioning. The inhibition of ET with bosentan prevented the increase of both hepatic and plasma TNF, thus attenuating the liver and lung injury, whereas TNF addition abolished the benefits of bosentan.

CONCLUSIONS

These findings suggest that both bosentan and preconditioning, by inhibition of ET could attenuate the microvascular disorders and the deleterious effect of TNF on the liver and lung elicited by hepatic I/R.

Combined treatment with endothelin- and PAF-antagonists reduces posttransplant lung ischemia/reperfusion injury

BACKGROUND

Pathophysiologic changes of posttransplant lung ischemia/reperfusion injury are mediated by redundant cellular and humoral mechanisms. We investigated the protective effect of combined administration of platelet activating factor (PAF) and endothelin (ET) antagonists after prolonged ischemia in a small animal lung transplantation model.

METHODS

Orthotopic left lung transplantation was performed after 20 hours cold ischemia in male Fischer (F344) rats weighing 200-250 g. Group I served as control. In Group II, donors received 1 mg/kg body weight of the endothelin antagonist TAK-044, and recipients 2 mg/kg. Group III was treated with the PAF antagonist TCV-309 (donor: 50 microg/kg; recipient: 100 microg/kg) (Takeda Chemicals Ltd.). Group IV received a combined treatment with both substances at the same dosage. Twenty-four hours after reperfusion, the native contralateral lung was occluded to assess gas exchange of the graft only, and 5 minutes later the thoracic aorta was punctured for arterial blood gas analysis (n = 5). In other animals (n = 5), lung tissue was frozen 24 hours after reperfusion and assessed for myeloperoxidase activity (MPO) and thiobarbituric acid reactive substances.

RESULTS

Combined inhibition of PAF and ET-1 at the receptor level resulted in significantly improved graft function as compared to controls (Group I), and to groups treated with either TAK-044 or TCV-309. This was determined by a higher arterial oxygen content (112 +/- 9 mmHg, p = .00061 vs control, 48 +/- 5 mmHg), reduced MPO activity (0.35 +/- 0.02 deltaOD/mg/min, p = .000002 vs control, 1.1 +/- 0.1 deltaOD/mg/min) and reduced lipid peroxidation (59.5 +/- 2.5 pmol/g, p = .011 vs control, 78.5 +/- 4.1 pmol/g). The improvement of arterial oxygen (Group II 77 +/- 10 mmHg, p = .027 vs control; Group III 84 +/- 8 mmHg, p = .0081 vs control) and reduction of MPO activity (Group II 0.85 +/- 0.061 deltaOD/mg/min, p = .017; Group III 0.92 +/- 0.079 deltaOD/mg/min, p = .058) in groups treated with either a PAF antagonist or an ET antagonist was significantly less than in Group IV.

CONCLUSIONS

Combined donor and recipient treatment with an ET antagonist and a PAF antagonist results in superior posttransplant graft function 24 hours after reperfusion, suggesting a synergistic role of ET-1 and PAF in the mediation of reperfusion injury in this model. Single treatment with either of the antagonists revealed only a slight improvement compared to untreated controls.

Differentiation of the peptidergic vasoregulatory response to standardized splanchnic hypoperfusion by acute hypovolaemia or sepsis in anaesthetized pigs

This study was performed to integratively investigate the vasoregulatory response during standardized splanchnic hypoperfusion in pigs. Splanchnic perfusion was reduced to 50% of baseline by: haemorrhage by 20 and 40% of the estimated total blood volume; femoral venous infusion of live E. coli to establish sepsis of systemic origin; portal venous infusion of live E. coli to establish sepsis of splanchnic origin. Invasive haemodynamic monitoring and radioimmunoassay analyses of arterial plasma concentrations of angiotensin II, endothelin-1 and atrial natriuretic peptide were carried out. Acute hypovolaemia reduced systemic and splanchnic vascular resistances following transient increases and increased angiotensin II levels (+587%), whereas endothelin-1 and atrial natriuretic peptide levels did not change significantly. Systemic sepsis following femoral venous infusion of E. coli resulted in increased splanchnic vascular resistance and increased levels of angiotensin II (+274%), endothelin-1 (+134%) and atrial natriuretic peptide (+185%). Infusion of E. coli via the portal venous route induced an increase in splanchnic vascular resistance associated with particularly elevated levels of angiotensin II (+1770%) as well as increased endothelin-1 (+201%) and atrial natriuretic peptide (+229%) concentrations. Hypovolaemia and sepsis, although standardized with a predefined level of splanchnic hypoperfusion, elicited differentiated cardiovascular and vasopeptidergic responses. Sepsis, particularly of portal origin, notably increased splanchnic vascular resistance related to increased production of the vasoconstrictors angiotensin II and endothelin-1. The role of atrial natriuretic peptide as a vasodilator seems to be of subordinate importance in hypovolaemia and sepsis.

Morphometry of human coronary arterial trees

BACKGROUND

Some groups, including ours, have been generating arterial tree models using constrained constructive optimization (CCO). Arterial trees have been grown to arbitrary resolution without input of anatomical data. We performed this study to learn about the shortcomings that might have resulted from neglecting the anatomical data in CCO models.

METHODS

In a total of 450 segments obtained from 4 human cast hearts, the ratio ofbifurcating daughter segment radii (O < Sbif = r(2)/r(1) < 1) was examined, which corresponds to the split of the total flow of the mother segment. For any complete bifurcation, where the radii of the parent segments and the radii of daughters were known, the area expansion ratio was computed (Aexp = [r(1)2 + r(2)2]/r(parent)2).

RESULTS

The bifurcating ratio was found to be distributed in a nonnormal fashion, with a median of 0.76. The average area expansion ratio Aexp, characterizing the change of cross-sectional area of the vasculature from proximal to distal, was 0.93+/-0.26. The 'rate of branching' (d(i)/(d(0)) was defined by the segment diameter relative to the diameter of the root segment. Averaging the rate of branching over segments within each bifurcation level resulted in a decreasing function of bifurcation level.

CONCLUSIONS

This article provides new experimental data on branching geometry of coronary arteries (i.e., the trees evaluated in this study are purely delivering rather than conveying). Based on these facts, we suggest that the analytical bifurcation law in CCO might be replaced by the bifurcation rule obeyed on a stochastic basis only.

Endothelin reactivity and receptor profile of pulmonary vessels in postobstructive pulmonary vasculopathy

Chronic ligation of one pulmonary artery results in pulmonary vascular remodeling and bronchial angiogenesis, collectively known as postobstructive pulmonary vasculopathy (POPV). To determine whether the reactivity of pulmonary vessels to endothelins (ET) was altered in POPV and to explore potential mechanisms, we ligated the left main pulmonary artery of 18 rats. Four weeks later, using a lung explant technique, we compared POPV lungs with controls for contractile responses of intrapulmonary vessels to ET-1 and ET-3 and for relaxant responses to ET-1 and sodium nitroprusside (SNP) after precontraction with U-46619. Morphometric measurements were made on vessels studied pharmacologically. Competition receptor binding studies with 125I-labeled ET-1 and unlabeled ET-1 and BQ-123 were performed using membrane proteins of pulmonary vessels. We found, in arteries, that contractile responses to ET-1 and ET-3 were significantly increased and that relaxant responses to ET-1 but not to SNP were reduced; in veins, only relaxation to SNP was increased. Morphometry showed that arteries and veins in POPV had reduced diameters without altered muscle thickness. Receptor binding studies showed that the proportion of ETA receptors in arteries was significantly increased in POPV (66%) vs. controls (54%). We conclude that, in POPV, the increase in reactivity to ET-1 and ET-3 is primarily related to an augmented proportion of ETA receptors.

Endothelin-1 causes luminal constrictions in rat cochlear veins

Serum levels of the vasoconstrictor endothelin-1 (ET-1) increase in ischemia and systemic hypertension. We examined the effects of ET-1 on the cochlear microvasculature. Blood vessels were cast with methacrylate in adult male Wistar Kyoto rats, 10 min after intravenous injection of ET-1 (1.0 microg/kg); control animals received saline. Systemic blood pressure was recorded continuously. ET-1 increased the average systolic pressure by 18% and average diastolic pressure by 22% (P < 0.01). Scanning electron microscopy of cast vessels showed multiple circumscribed luminal constrictions on: (1) postcapillary venules; (2) collecting veins; (3) where collecting veins merged with the spiral modiolar vein; (4) on the spiral modiolar vein itself. Circumscribed constrictions in arteries were not observed. In ET-1 injected animals focal contractions of collecting veins reduced luminal width by 13.4% +/- 2.9 (P < 0.01). In control rats, constrictions on venous casts were minimal and constrictions on arteries were not observed. The present study shows that ET-1 is involved in local control of cochlear blood flow in that it focally contracts cochlear veins. It is suggested that this might be due to the high affinity of ET-1 receptors and/or the large number of ET-1 receptors on contractile cells in venous walls.

Inflammatory response to cardiopulmonary bypass: mechanisms involved and possible therapeutic strategies

BACKGROUND

Recent study of the inflammatory reactions occurring during and after cardiopulmonary bypass (CPB) has improved our understanding of the involvement of the inflammatory cascade in perioperative injury. However, the exact mechanisms of this complex response remain to be fully determined.

METHODS

Literature on the inflammatory response to CPB was reviewed to define current knowledge on the possible pathways and mediators involved, and to discuss recent developments of therapeutic interventions aimed at attenuating the inflammatory response to CPB.

RESULTS

CPB has been shown to induce complement activation, endotoxin release, leukocyte activation, the expression of adhesion molecules, and the release of many inflammatory mediators including oxygen-free radicals, arachidonic acid metabolites, cytokines, platelet-activating factor, nitric oxide, and endothelins. Therapies aimed at interfering with the inflammatory response include the administration of pharmacologic agents such as corticosteroids, aprotinin, and antioxidants, as well as modification of techniques and equipment by the use of heparin-coated CPB circuits, intraoperative leukocyte depletion, and ultrafiltration.

CONCLUSIONS

Improved understanding of the inflammatory reactions to CPB can lead to improved patient outcome by enabling the development of novel therapies aimed at limiting this response.

Morphologic sites for regulating blood flow in the exocrine pancreas

The exocrine pancreas has a lobular structure and an intricate capillary network supplies the lobules. Casts of these capillaries are either straight and of constant width, provided with many shallow crests, or undulating and of varying diameter, provided with bulges and deeper constrictions. The mean capillary cast diameter is 6.32 microns (SD 0.53) and 3.91 microns (SD 0.84) at constriction sites. The first type corresponds to non-fenestrated capillaries, makes 24% of capillaries and is more frequently provided with pericytes (2.7 +/- 0.9 pericytes per capillary profile). The second type corresponds to fenestrated capillaries, comprises 76% of the capillaries and is less frequently provided with pericytes (1.5 +/- 0.6 pericytes per capillary profile). The endothelial cells of capillaries regularly form intermediate junctions and microvilli and contain microtubuli and cytoplasmic filaments. Intravital observations show that capillaries are capable of contracting and narrowing the capillary lumen. This contractility is accomplished by endothelial cells both at and apart from their nuclear regions while pericytes never contracted spontaneously during our in vivo observations. The capillary diameters estimated by intravital measurements, 3.53 microns (SD 1.05), are similar to cast measurements but differ at constricted segments from cast measurements. Flow reduction shows more variability in smaller capillaries and the flow is more reduced in capillaries of 5 microns diameter to about 40% of open capillaries vs. 68% in capillaries with 7.5 microns diameter. Veins are either provided with smooth muscle sphincters or with valves. These results indicate that corrosion casting accurately shows the geometry of capillaries. However, where the capillaries are drastically constricted, they might not be filled and therefore may be underestimated during measurements. Since none of the intravital luminal constrictions are small enough to reduce flow (smaller than 1 micron luminal diameter) and because many constrictions are effective to reduce flow, we conclude that capillaries of the exocrine pancreas are always capable of maintaining continuous blood flow yet can influence blood perfusion. The presence of venous valves in association with venous sphincters constitutes a new situation concerning blood drainage regulation in the exocrine pancreas.

Pulmonary venous sphincters in cattle

BACKGROUND

The pulmonary veins of rats have regular focal narrowing by tufts of smooth muscle (sphincters) that can contract in response to a variety of stimuli, but these structures are not well studied in other species, and there is little information about their innervation and control.

METHODS

The pulmonary veins of 21 cattle were cast with methacrylate, and the casts were studied by scanning electron microscopy, or the fixed tissue was studied by light microscopy with immunocytochemistry and transmission electron microscopy.

RESULTS

Constrictions occurred in series along the course of veins (9.6/500 microns), giving the cast veins a string-of-pearl look, with narrowing of 33-81% of the outer diameter. No resin appeared beyond the most narrowed veins. The percentage of contraction did not correlate with the diameter of the veins. With immunohistochemistry using antibodies to S-100, protein gene peptide 9.5, neuron-specific enolase, neurofilament 200, and glial fibrillary acidic protein and with transmission electron microscopy, we could identify no neuronal elements associated with the venous smooth muscle tufts. Bronchial smooth muscle bundles in the same sections stained positively.

CONCLUSIONS

The veins of cattle are unlike the rat because the focal venous smooth muscle protrudes deeply into the venous lumen and may completely obstruct perfusion. If the focal venous muscle has no innervation (this study) and can constrict without blood flow (as shown previously), then the venous constriction and, hence, local blood flow regulation must be controlled by local mediators.