Constant Flow Perfusion Research Articles

Cardioprotection by Preconditioning with Intralipid Is Sustained in a Model of Endothelial Dysfunction for Isolated-Perfused Hearts.

Endothelial dysfunction (ED) is closely associated with most cardiovascular diseases. Experimental models are needed to analyze the potential impact of ED on cardioprotection in constant pressure Langendorff systems (CPLS). One cardioprotective strategy against ischemia/reperfusion injury (I/RI) is conditioning with the lipid emulsion Intralipid (IL). Whether ED modulates the cardioprotective effect of IL remains unknown. The aim of the study was to transfer a protocol using a constant flow Langendorff system for the induction of ED into a CPLS, without the loss of smooth muscle cell functionality, and to analyze the cardioprotective effect of IL against I/RI under ED. In isolated hearts of male Wistar rats, ED was induced by 10 min perfusion of a Krebs-Henseleit buffer containing 60 mM KCl (K+), and the vasodilatory response to the vasodilators histamine (endothelial-dependent) and sodium-nitroprusside (SNP, endothelial-independent) was measured. A CPLS was employed to determine cardioprotection of pre- or postconditioning with 1% IL against I/RI. The constant flow perfusion of K+ reduced endothelial response to histamine but not to SNP, indicating reduced vasodilatory functionality of endothelial cells but not smooth muscle cells. Preconditioning with IL reduced infarct size and improved cardiac function while postconditioning with IL had no effect. The induction of ED neither influenced infarct size nor affected the cardioprotective effect by preconditioning with IL. This protocol allows for studies of cardioprotective strategies under ED in CLPS. The protection by preconditioning with IL seems to be mediated independently of a functional endothelium.

The Factors Affecting the Stability of IOP Homeostasis.

Shear-induced nitric oxide (NO) production by Schlemm's canal (SC) endothelial cells provides a fast, IOP-sensitive feedback signal that normally contributes to IOP homeostasis. Our goal was to analyze the response of this homeostatic system under constant flow perfusion (as occurs in vivo) vs. constant pressure perfusion (as typical for laboratory perfusions). A mathematical model of aqueous humor dynamics, including shear-mediated NO signaling, was formulated and analyzed for stability. The model includes Goldmann's equation, accounting for proximal and distal outflow resistance, and describes how elevated IOP causes narrowing of SC lumen that increases the shear stress on SC cells. Elevated shear stress stimulates NO production, which acts to reduce outflow resistance and relax trabecular meshwork cells to decrease trabecular meshwork stiffness, affecting the SC luminal caliber. During constant flow perfusion, the outflow system is typically stable, returning to baseline IOP after a perturbation. In contrast, during constant pressure perfusion, the outflow system can become unstable and exhibit a time-dependent change in outflow resistance that diverges from baseline. The stability of shear mediated IOP homeostasis is predicted to differ critically between constant flow vs. constant pressure perfusion. Because outflow facility is typically measured at a constant pressure in the laboratory, this instability may contribute to the characteristic time-dependent increase in outflow facility, known as washout, observed in many nonhuman species. Studies of IOP homeostasis should consider how the outflow system may respond differently under constant pressure vs. constant flow perfusion.

Sensitive quantitation of ultra-trace toxic aconitines in complex matrices by perfusion nano-electrospray ionization mass spectrometry combined with gas-liquid microextraction

The accurate analysis of ultra-trace (e.g. <10−4 ng/mL) substances in complex matrices is a burdensome but vital problem in pharmaceutical analysis, with important implications for precise quality control of drugs, discovery of innovative medicines and elucidation of pharmacological mechanisms. Herein, an innovative constant-flow perfusion nano-electrospray ionization (PnESI) technique was developed firstly features significant quantitative advantages in high-sensitivity ambient MS analysis of complex matrix sample. More importantly, double-labeled addition enrichment quantitation strategies of gas-liquid microextraction (GLME) were proposed for the first time, allowing highly selective extraction and enrichment of specific target analytes in a green and ultra-efficient (>1000-fold) manner. Using complex processed Aconitum herbs as example, PnESI-MS directly enabled the qualitative and absolute quantitative analysis of the processed Aconitum extracts and characterized the target toxic diester alkaloids with high sensitivity, high stability, wide linearity range, and strong resistance to matrix interference. Further, GLME device was applied to obtain the highly specific enrichment of the target diester alkaloids more than 1000-fold, and accurate absolute quantitation of trace aconitine, mesaconitine, and hypaconitine in the extracts of Heishunpian, Zhichuanwu and Zhicaowu was accomplished (e.g., 0.098 pg/mL and 0.143 pg/mL), with the quantitation results well below the LODs of aconitines from any analytical instruments available. This study built a systematic strategy for accurate quantitation of ultra-trace substances in complex matrix sample and expected to provide a technological revolution in many fields of pharmaceutical research.

The evaluation of constant coronary artery flow versus constant coronary perfusion pressure during normothermic ex situ heart perfusion.

Evidence suggests that hearts that are perfused under ex-situ conditions lose normal coronary vasomotor tone and experience contractile failure over a few hours. We aimed to evaluate the effect of different coronary perfusion strategies during ex situ heart perfusion on cardiac function and coronary vascular tone. Porcine hearts (n=6 each group) were perfused in working mode for 6 hours with either constant aortic diastolic pressure (40 mmHg) or constant coronary flow rate (500 mL/min). Functional and metabolic parameters, cytokine profiles, cardiac and vascular injury, coronary artery function and oxidative stress were compared between groups. Constant coronary flow perfusion demonstrated better functional preservation and less edema formation (Cardiac index: flow control=8.33 vs pressure control=6.46 mL·min-1·g-1, p=0.016; edema formation: 7.92% vs 19.80%, p < 0.0001). Pro-inflammatory cytokines, platelet activation as well as endothelial activation were lower in the flow control group. Similarly, less cardiac and endothelial injury was observed in the constant coronary flow group. Evaluation of coronary artery function showed there was loss of coronary autoregulation in both groups. Oxidative stress was induced in the coronary arteries and was relatively lower in the flow control group. A strategy of controlled coronary flow during ex situ heart perfusion provides superior functional preservation and less edema formation, together with less myocardial damage, leukocyte, platelet, endothelial activation, and oxidative stress. There was loss of coronary autoregulation and decrease of coronary vascular resistance during ESHP irrespective of coronary flow control strategy. Inflammation and oxidative stress state in the coronary vasculature may play a role.

A novel bilayered expanded polytetrafluoroethylene glaucoma implant creates a permeable thin capsule independent of aqueous humor exposure.

The purpose of these studies was to evaluate clinical, functional, and histopathological features of glaucoma drainage implants (GDIs) fabricated from novel, custom‐tailored expanded polytetrafluoroethylene (ePTFE). Implants of matching footprints were fabricated from silicone (Control) and novel, bilayered ePTFE. ePTFE implants included: (a) one that inflated with aqueous humor (AH) (High), (b) one that inflated with a lower profile (Low), (c) an uninflated implant not connected to the anterior chamber (Flat), and (d) one filled with material that did not allow AH flow (Filled). All implants were placed in adult New Zealand White rabbits and followed over 1–3 months with clinical exams and intraocular pressure. The permeability of tissue capsules surrounding GDIs was assessed using constant‐flow perfusion with fluoresceinated saline at physiologic flow rates. After sacrifice, quantitative histopathological measures of capsule thickness were compared among devices, along with qualitative assessment of cellular infiltration and inflammation. Capsular thickness was significantly reduced in blebs over ePTFE (61.4 ± 53 μm) versus silicone implants (193.6 ± 53 μm, p = .0086). AH exposure did not significantly alter capsular thickness, as there was no significant difference between High and Filled (50.9 ± 29, p = .34) implants. Capsules around ePTFE implants demonstrated permeability with steady‐state pressure: flow relationships at physiologic flow rates and rapid pressure decay with flow cessation, while pressure in control blebs increased even at low flow rates and showed little decay. Perfused fluorescein dye appeared beyond the plate border only in ePTFE implants. ePTFE implants are associated with thinner, more permeable capsules compared to silicone implants simulating presently used devices.

Fluid flow-induced cell stimulation in bone tissue engineering changes due to interstitial tissue formation in vitro.

In tissue engineering experiments in vitro, bioreactors have been used for applying wall shear stress (WSS) on cells to regulate cellular activities. To determine the loading conditions within bioreactors and to design tissue engineering products, in silico models are used. Previous in silico studies in bone tissue engineering (BTE) focused on quantifying the WSS on cells and the influence on appositional tissue growth. However, many BTE experiments also show interstitial tissue formation (i.e., tissue infiltrated in the pores rather than growing on the struts ‐ appositional growth), which has not been considered in previous in silico studies. We hereby used a multiscale fluid‐solid interaction model to quantify the WSS and mechanical strain on cells with interstitial tissue formation, taken from a reported BTE experiment. The WSS showed a high variation among different interstitial tissue morphologies. This is different to the situation under appositional tissue growth. It is found that a 35% filling of the pores results (by mineralised bone tissue) when the average WSS increases from 1.530 (day 0) to 5.735 mPa (day 28). Furthermore, the mechanical strain on cells caused by the fluid flow was extremely low (at the level of 10−14‐10−15), comparing to the threshold in a previous mechanobiological theory of osteogenesis (eg, 10−2). The output from this study offers a significant insight of the WSS changes during interstitial tissue growth under a constant perfusion flow rate in a BTE experiment. It has paved the way for optimising the local micro‐fluidic environment for interstitial tissue mineralisation.

Study of Hepatic Vascular Dynamics Based on Symmetrical Pulsating Perfusion.

BackgroundThe traditionally used perfusion method is constant flow. This study proposes a novel method called Symmetric Pulsating Flow (SPF) and verified that this method is applicable.Material/MethodsThe fluid dynamic behavior of perfusate in the vessel, the shear stress, and the vascular deformation were simulated based on the bi-directional fluid-structure interaction. The differences of the fluid dynamic behaviors and the mechanical characteristics of vascular wall were studied and compared between the 2 methods during the process of hepatic perfusion. The simulations and comparisons were carried out on 3 different vascular models.ResultsUtilizing the constant flow perfusion, a double vortex clearly appeared at the rear end of the foreign matter and reflux retention can be caused by the double vortex. The reflux retention caused lower shear stress against the vascular wall and thus brought new accumulation of foreign matter. The SPF perfusion, however, prevented the double vortex, and avoided such reflux retention during the vascular perfusion. In addition, the SPF can clean the vascular wall better with a slower speed, which causes less injury to the vessel, and the pulsating effect can reduce the accumulation of new foreign matter.ConclusionsThe SPF perfusion can clean the vascular wall more thoroughly with less injury.

Effects of Constant Flow vs. Constant Pressure Perfusion on Fluid Filtration in Severe Hypothermic Isolated Blood-Perfused Rat Lungs.

Victims of severe accidental hypothermia are prone to fluid extravasation but rarely develop lung edema. We hypothesize that combined hypothermia-induced increase in pulmonary vascular resistance (PVR) and a concomitant fall in cardiac output protect the lungs against edema development. Our aim was to explore in hypothermic-isolated blood-perfused rat lungs whether perfusion at constant pressure influences fluid filtration differently from perfusion at constant flow. Isolated blood-perfused rat lungs were hanging freely in a weight transducer for measuring weight changes (ΔW). Fluid filtration coefficient (Kfc), was determined by transiently elevating left atrial pressure (Pla) by 5.8 mmHg two times each during normothermia (37°C) and during hypothermia (15°C). The lung preparations were randomized to two groups. One group was perfused with constant flow (Constant flow group) and the other group with constant pulmonary artery pressure (Constant PPA group). Microvascular pressure (Pmv) was determined before and during elevation of Pla (ΔPmv) by means of the double occlusion technique. Kfc was calculated with the formula Kfc = ΔW/ΔPmv/min. All Kfc values were normalized to predicted lung weight (PLW), which was based on body weight (BW) according to the formula: PLW = 0.0053 BW - 0.48 and presented as KfcPLW in mg/min/mmHg/g. At cessation, bronchoalveolar lavage (BAL) fluid/perfusate protein concentration (B/P) ratio was determined photometrically. Data were analyzed with parametric or non-parametric tests as appropriate. p < 0.05 considered as significant. Perfusate flow remained constant in the Constant flow group, but was more than halved during hypothermia in the Constant PPA group concomitant with a more fold increase in PVR. In the Constant flow group, KfcPLW and B/P ratio increased significantly by more than 10-fold during hypothermia concerted by visible signs of edema in the trachea. Hemoglobin and hematocrit increased within the Constant flow group and between the groups at cessation of the experiments. In hypothermic rat lungs perfused at constant flow, fluid filtration coefficient per gram PLW and B/P ratio increased more than 10-fold concerted by increased hemoconcentration, but the changes were less in hypothermic lungs perfused at constant PPA.

Transport of Low-Density Lipoprotein Into the Blood Vessel Wall During Atherogenic Diet in the Isolated Rabbit Carotid Artery.

Atherosclerosis is a chronic fibroproliferative disease that includes accumulation of cholesterol-rich lipids in the arterial wall. Though numerous studies have investigated atherosclerosis, not enough is known about the exact mechanisms of low-density lipoprotein (LDL) transport into the blood vessel wall. Therefore, we explored the (125)I-LDL transport into the arterial wall under constant perfusion flow and pressure as well as the influence of duration of atherogenic diet on (125)I-LDL transport and biomechanical properties of carotid artery. The isolated segment of rabbit carotid artery was used under constant perfusion flow and pressure-induced (0 mmHg and 140 mmHg) blood vessel distension, with the possibility to change and precisely calculate shear stress during the experiment. Obtained results indicate the influence of atherogenic diet duration and consequent variation of shear stress on (125)I-LDL transport into the blood vessel wall. (125)I-LDL transport into the blood vessel wall at low pressure-induced blood vessel distension decreases by the increase of the shear stress and in relation to the atherogenic diet duration. At high pressure-induced blood vessel distension, (125)I-LDL transport increases in relation to the atherogenic diet duration and the increase of shear stress. The influence of shear stress is a more dominant parameter on LDL uptake at low pressure-induced blood vessel distension; however, the atherogenic diet duration has more of a dominant influence on LDL uptake at high pressure-induced vessel distension.

A Novel Vascular Homing Peptide Strategy to Selectively Enhance Pulmonary Drug Efficacy in Pulmonary Arterial Hypertension

A major limitation in the pharmacological treatment of pulmonary arterial hypertension (PAH) is the lack of pulmonary vascular selectivity. Recent studies have identified a tissue-penetrating homing peptide, CARSKNKDC (CAR), which specifically homes to hypertensive pulmonary arteries but not to normal pulmonary vessels or other tissues. Some tissue-penetrating vascular homing peptides have a unique ability to facilitate transport of co-administered drugs into the targeted cells/tissues without requiring physical conjugation of the drug to the peptide (bystander effect). We tested the hypothesis that co-administered CAR would selectively enhance the pulmonary vascular effects of i.v. vasodilators in Sugen5416/hypoxia/normoxia-exposed PAH rats. Systemically administered CAR was predominantly detected in cells of remodeled pulmonary arteries. Intravenously co-administered CAR enhanced pulmonary, but not systemic, effects of the vasodilators, fasudil and imatinib, in PAH rats. CAR increased lung tissue imatinib concentration in isolated PAH lungs without increasing pulmonary vascular permeability. Sublingual CAR was also effective in selectively enhancing the pulmonary vasodilation by imatinib and sildenafil. Our results suggest a new paradigm in the treatment of PAH, using an i.v./sublingual tissue-penetrating homing peptide to selectively augment pulmonary vascular effects of nonselective drugs without the potentially problematic conjugation process. CAR may be particularly useful as an add-on therapy to selectively enhance the pulmonary vascular efficacy of any ongoing drug treatment in patients with PAH.

Experimental and mathematical model for evaluation of LDL uptake by the isolated blood vessels

Objective. The aim of this study is to present the experimental model which can be used to determine LDL transport into the blood vessel wall. Method. We used isolated rabbit carotid arteries under physiologically relevant constant pressure and perfusion flow in conditions more similar to in vivo. We used Rapid dual-isotope dilution method, Steady - state method, testing of transport of LDL-before and after removal of the endothelium, in conditions with intact endothelium and after its removal. Results. First we used Rapid dual-isotope dilution method and the data obtained showed that the transport of LDL could not be precisely determined in this way because it was in the range of standard error. Then we used Steady - state method and uptake (Us) was 3.52 ± 1.07% at higher pressure (140±10 mmHg) than at lower pressure (p<0.05). Also, LDL uptake was evaluated before and after the endothelium removal. The results after endothelial removal showed that Us of LDL was almost 3 times higher (9.2%) than with intact endothelium (p<0.05) and that the accumulation of LDL in the blood vessel wall was 0.1% (0.06% in intact endothelium) indicating that intact endothelium was a strong barrier. Conclusion. Our experimental model and applied mathematical procedures can provide a precise description of the LDL uptake by the blood vessel wall.

The action of vasoactive drugs on longitudinal and circular muscle of dog mesenteric vein.

Abstract The dog anterior mesenteric vein has outer longitudinal and inner circular muscle layers. The response of the vein to drugs was recorded without disrupting the muscle arrangements by combining constant-flow perfusion of a segment with isometric recording of longitudinal tension. Noradrenaline and acetylcholine were potent stimulants. Tachyphylaxis developed to histamine, 5-hydroxytryptamine and angiotensin. Cocaine and guanethidine potentiated the responses to noradrenaline and fluorescent microscopy confirmed the presence of adrenergic nerves in both muscle layers. Isoprenaline partially relaxed the stimulated vein and this action was blocked by propranolol. Evidence was obtained which suggests the existence in the vein of specific receptors for several of the agonists investigated.

Effects of phenylephrine on the sublingual microcirculation during cardiopulmonary bypass

The objective of the present study was to investigate sublingual microvascular blood flow and microcirculatory haemoglobin oxygen saturation (Smc(O(2))) during cardiopulmonary bypass (CPB) using constant systemic blood flow but different perfusion pressures achieved by phenylephrine administration. Fifteen patients undergoing coronary artery bypass grafting were enrolled in this pilot study. Systemic haemodynamics, oxygen transport variables, arterial and mixed venous blood gas analysis, and microcirculatory variables were determined after initiation of general anaesthesia, during CPB (systemic blood flow=2.4 litre m(-2)), after increasing perfusion pressure by 20 mm Hg with a continuous infusion of phenylephrine, and after termination of phenylephrine infusion. CPB immediately resulted in a significant (P<0.05) decrease in systemic oxygen transport without alterations in sublingual microcirculatory blood flow and Smc(O(2)). Increasing perfusion pressure from 47 (SD 9) to 68 (7) mm Hg using phenylephrine=1.4 (1.0) microg kg(-1) min(-1) resulted in a significant decrease in sublingual small vessel blood flow (from median 2.5 to 1.8 arbitrary units) representing mostly capillary blood flow, but not in medium-sized vessels (median 3 to 2.8 arbitrary units). Concurrently, global tissue blood flow from 110 (54) to 197 (100) perfusion units and Smc(O(2)) increased from 72 (11)% to 84 (7)%, suggesting significant microcirculatory blood flow shunting in vessels with diameters >25 microm. Our data demonstrate that an increased perfusion pressure produced by phenylephrine at constant CPB flow may decrease microcirculatory blood flow in the sublingual mucosal microcirculation due to microvascular blood flow shunting.

Comparisons of the release of vasodilator substances from left and right cardiac chambers of the isolated perfused rabbit heart: Implications for intraventricular thrombus formation

Prostacyclin (PgI 2) and endothelium-derived nitric oxide (EDNO) are produced by the arterial and venous endothelium. In addition to their vasodilator action on vascular smooth muscle, both act together to inhibit platelet aggregation and promote platelet disaggregation. EDNO also inhibits platelet adhesion to the endothelium. EDNO and PgI 2 have been shown to be released from the cultured endocardial cells. In this study, we examined the release of vasoactive substances from the intact endocardium by using isolated rabbit hearts perfused with physiological salt solution (95% O 2/5% CO 2, T = 37 °C). The right and left cardiac chambers were perfused through separate constant-flow perfusion loops (physiological salt solution, 8 ml min −1). Effluent from left and right cardiac, separately, was bioassayed on canine coronary artery smooth muscle, which had been contracted with prostaglandin F 2α_(2 × 10 −6 M) and no change in tension was exhibit. However, addition of calcium ionophore A23187 (10 −6 M) to the cardiac chambers’ perfusion line induced vasodilation of the bioassay coronary ring, 61.4 ± 7.4% versus 70.49 ± 6.1% of initial prostaglandin F 2α contraction for the left and right cardiac chambers perfusate, respectively (mean ± SEM, n = 10, p > 0.05). Production of vasodilator was blocked totally in the left heart but, only partially blocked in the right heart by adding indomethacin (10 −5 M) to the perfusate, respectively, 95.2 ± 2.2% versus 41.5 ± 4.8% (mean ± SEM, n = 10, p < 0.05). 6-Keto prostaglandin F 1α, measured in the endocardial superfusion effluent was also higher for the left cardiac chambers than for the right at the time of stimulation with the A23187, respectively, 25385.88 ± 5495 pg/ml ( n = 8) versus 13,132.45 ± 1839.82 pg/ml ( n = 8), ( p < 0.05). These results showed that cyclooxygenase pathway plays major role in generating vasoactive substances for the left cardiac chamber endocardium; while it is not the main pathway for the right ventricular endocardium at which EDNO and PgI 2 could act together and potentiate their antithrombogenic activities in isolated perfused rabbit heart. This may be an explanation for the intraventricular thrombus mostly seen in left ventricle rather than in right ventricle as a complication of myocardial infarction.

The venous valve agger and plasma noradrenaline-mediated venodilator feedback

To investigate the structure and function of the venous valve agger, a fibroelastic structure located at the base of every valve in veins that are responsive to noradrenaline (NA). Constant flow perfusion studies of the responses of in situ and in vitro canine lateral saphenous vein segments to NA and electrical stimulation, supplemented by histological studies of the segments. University Departments of Physiology and Histology. Segments of canine lateral saphenous vein. In vitro=84, in situ=60 plus. The agger is a crescentic fibroelastic sleeve, spanning the vein wall very obliquely through which the local vasa venarum network drains. It has a dedicated musculature which, when the tone rises, contracts and stretches the fibroelastic of the agger and opens the drainage channels to reflux. Each agger has four muscles, two each of which insert on the concave and convex margins of the agger. They pull in opposite directions when they contract. The agger forms part of a complex that, in conjunction with its dedicated musculature, a reversible transmural pressure gradient and physiological turbulence in the valve sinuses, positively facilitates drainage from the local segment of the vasa venarum network when venous tone is normal; and when venous tone is elevated it pumps and sucks blood from the lumen of the vein to perfuse the vasa venarum network. When the plasma NA in the perfusate diffuses from the network, it causes a localized venodilator feedback effect that restores the elevated tone of the vein to normal. The feedback effect is potent, being estimated to reduce the venonstrictor effect of neuronal NA by about 50% when flow is laminar and considerably more when flow is turbulent. There is evidence that plasma NA may constitute the lateral inhibitory component of the NA chemostimulus of the smooth muscle cell, neuronal NA constituting the excitor component. A chronic breakdown in agger function is believed to be the cause of varicose veins.

Contractile effects of adenosine, coronary flow and perfusion pressure in murine myocardium

There is mixed evidence adenosine receptors (ARs) may enhance myocardial contractility, although this remains contentious. We assessed inotropic actions of adenosine (50 muM) and selective AR activation with 100 nM N (6)-cyclohexyladenosine (CHA; A(1)AR agonist), 25 nM 2-[p-(2-carboxyethyl) phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS-21680; A(2A)AR agonist) and 100 nM 2-chloro-N (6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA; A(3)AR agonist) in mouse hearts perfused at constant pressure, constant flow, or conditions of stable flow and pressure (following maximal nitroprusside-mediated dilatation at constant flow). Adenosine and CGS-21680 significantly (although modestly) increased force in constant-pressure perfused hearts (</=10 mmHg elevations in systolic pressure), effects paralleled by coronary vasodilatation (</=10 ml min(-1) g(-1) elevations in flow). Neither CHA nor Cl-IB-MECA altered force or flow. With constant-flow perfusion, adenosine and CGS-21680 reduced systolic pressure in parallel with perfusion pressure. When changes in coronary flow and pressure were prevented, CGS-21680 failed to alter contractility. However, adenosine still enhanced systolic pressure up to 10 mmHg. Relations between flow, perfusion pressure and ventricular force evidence substantial Gregg effects in murine myocardium: systolic force increases transiently by approximately 1 mmHg ml(-1) min(-1) g(-1) rise in flow during the first minutes of hyperaemia and in a sustained manner (by approximately 1 mmHg mmHg(-1)) during altered perfusion pressure. These effects contribute to inotropism with AR agonism when flow/pressure is uncontrolled. In summary, we find no evidence of direct A(1) or A(3)AR-mediated inotropic responses in intact myocardium. Inotropic actions of A(2A)AR agonism appear entirely Gregg-related. Nonetheless, the endogenous agonist adenosine exerts a modest inotropic action independently of flow and perfusion pressure. The basis of this response remains to be identified.

Factors affecting outflow facility calculations

Outflow facility (C), a ratio of outflow rate (F) to relevant pressure, is an important indication of outflow resistance in the ocular drainage pathway. The higher the C, the lower the outflow resistance. According to the classic Goldmann equation, C=F/(IOP-Pe)(IOP, intraocular pressure; Pe, episcleral venous pressure). Since anterior chamber (AC) perfusions directly measure outflow and are less affected by scleral creep and rigidity or changes in uveal blood content than repeated tonography in living eyes, they are widely used to determine C in living animals or cultured anterior segments from enucleated eyes. Barany (1964) initially developed the 2-level constant pressure perfusion technique for living monkey eyes to avoid possible influences of changes in eye parameters (e.g., Pe, spontaneous IOP [P0] or aqueous humor formation [AHF]) on C measurements. However, since these eye parameters do not exist in cultured anterior segments, 1-level constant pressure perfusion (Erickson-Lamy et al., 1991) and constant flow rate perfusion (Johnson and Tschumper, 1987) are typically used to determine C in organ culture. Based on different perfusion techniques, two basic formulas derived from the Goldmann equation are used to calculate C: (A) C=(F2-F1)/(P2-P1), which is usually used in 2-level constant pressure perfusion either in living animals or in cultured anterior segments (F1 or F2 represents F at P1 [lower perfusion pressure] or P2 [higher perfusion pressure]); (B) C=F/P, which is usually used in 1-level constant pressure perfusion or constant flow rate perfusion in cultured anterior segments (P, perfusion pressure; Pe is ignored in this formula because it is assumed to be zero in the cultured anterior segments). A comparison of C values calculated using Formula B and the data obtained during 2-level constant pressure perfusion in vivo or in vitro requires the following modifications: C=F1/(P1-P0) or C=F2/(P2-P0) where F1 or F2 is due to the change in pressure from P0 to P1 or P2 (P0 is assumed to be stable during short-term perfusions in vivo and is zero in vitro). C values from the same eye and the same perfusion are often quite different when calculated by the two formulas, since Formula A calculates C for the change in pressure from P1 to P2, whereas Formula B calculates C either for the change in pressure from P0 to P1 or P2 in living eyes or for a given pressure (P1 or P2) in cultured segments. However, the difference has not been clearly stated in previous studies, so that confusion might occur when comparing C values obtained by the two formulas. Additionally, C at different pressures may also be affected by factors physiologically unrelated to the formulas. To determine factors that may affect C values and to evaluate the significance of these factors in aqueous humor dynamics research, we compared C values calculated by Formulas A and B based on some previous perfusion experiments.

COUNTERPOINT: ACTIVE VENOCONSTRICTION IS NOT IMPORTANT IN MAINTAINING OR RAISING END-DIASTOLIC VOLUME AND STROKE VOLUME DURING EXERCISE AND ORTHOSTASIS

Veins were considered merely as passive conduits of blood until Mellander ([13][1]) reported that stimulation of sympathetic nerves supplying the cat's hindquarters resulted in a decrease in regional blood volume. This and other work led Folkow and Mellander ([4][2]) to assert that veins were “at

Contractile effects of adenosine, coronary flow and perfusion pressure in murine myocardium

There is mixed evidence adenosine receptors (ARs) may enhance myocardial contractility, although this remains contentious. We assessed inotropic actions of adenosine (50 μM) and selective AR activation with 100 nM N 6-cyclohexyladenosine (CHA; A1AR agonist), 25 nM 2-[p-(2-carboxyethyl) phenethylamino]-5′-N-ethylcarboxamidoadenosine (CGS-21680; A2AAR agonist) and 100 nM 2-chloro-N 6-(3-iodobenzyl)-adenosine-5′-N-methyluronamide (Cl-IB-MECA; A3AR agonist) in mouse hearts perfused at constant pressure, constant flow, or conditions of stable flow and pressure (following maximal nitroprusside-mediated dilatation at constant flow). Adenosine and CGS-21680 significantly (although modestly) increased force in constant-pressure perfused hearts (≤10 mmHg elevations in systolic pressure), effects paralleled by coronary vasodilatation (≤10 ml min−1 g−1 elevations in flow). Neither CHA nor Cl-IB-MECA altered force or flow. With constant-flow perfusion, adenosine and CGS-21680 reduced systolic pressure in parallel with perfusion pressure. When changes in coronary flow and pressure were prevented, CGS-21680 failed to alter contractility. However, adenosine still enhanced systolic pressure up to 10 mmHg. Relations between flow, perfusion pressure and ventricular force evidence substantial Gregg effects in murine myocardium: systolic force increases transiently by approximately 1 mmHg ml−1 min−1 g−1 rise in flow during the first minutes of hyperaemia and in a sustained manner (by ∼1 mmHg mmHg−1) during altered perfusion pressure. These effects contribute to inotropism with AR agonism when flow/pressure is uncontrolled. In summary, we find no evidence of direct A1 or A3AR-mediated inotropic responses in intact myocardium. Inotropic actions of A2AAR agonism appear entirely Gregg-related. Nonetheless, the endogenous agonist adenosine exerts a modest inotropic action independently of flow and perfusion pressure. The basis of this response remains to be identified.

EFFECTS OF ISCHEMIC PRECONDITIONING ON RAT LUNG: ROLE OF NITRIC OXIDE

Recent studies suggest that ischemic preconditioning (IP) of the lung may have a protective effect in ischemia-reperfusion (I/R) injury. The purpose of the present study was to investigate the preconditioning hypothesis in rat pulmonary vascular bed and to examine the role of nitric oxide (NO) in IP. Isolated rat lung was perfused with Krebs-Henseleit solution containing indomethacin at a constant flow rate and perfusion pressure changes was recorded by a pressure transducer. In rat pulmonary vascular bed, 2 hours of hypothermic ischemia significantly attenuated histamine-induced vasodilator responses without affecting sodium nitroprusside (SNP) vasodilation when compared to sham values. However, 2 cycles of 5 minutes of ischemia and reperfusion that were applied prior to 2 hours of ischemia (IP protocol) prevented the attenuation of histamine-induced vasodilation. On the other hand, IP failed to prevent pulmonary edema after ischemia. Histopathological examination of rat lungs demonstrated that IP was able to protect endothelial cells and type II pneumocytes in lung. Moreover, in IP group, malondialdehyde (MDA) contents of the lung tissue were significantly lower and tissue glutathione (GSH) contents were significantly higher than those in I/R group. Administration of NO synthase inhibitor, NG-nitro-L-arginine-methyl ester (L-NAME) prior to the IP protocol abolished the protective effects of IP, but not affected the tissue malondialdehyde and glutathione levels. These results suggest that I/R impaired endothelium-dependent vasodilatory response, whereas endothelium-independent SNP-induced responses were preserved in rat pulmonary vascular bed. IP prevented the impairment of pulmonary vascular endothelium-dependent responses, and these effects may be partially mediated by NO.