Oxygen distribution in microcirculation after arginine vasopressin-induced arteriolar vasoconstriction

The microvascular distribution of oxygen was studied in the arterioles and venules of the awake hamster window chamber preparation to determine the contribution of vascular smooth muscle contraction to oxygen consumption of the microvascular wall during arginine vasopressin (AVP)-induced vasoconstriction. AVP was infused intravenously at the clinical dosage (0.0001 IU.kg(-1).min(-1)) and caused a significant arteriolar constriction, decreased microvascular flow and functional capillary density, and a substantial rise in arteriolar vessel wall transmural Po(2) difference. AVP caused tissue Po(2) to be significantly lowered from 25.4 +/- 7.4 to 7.2 +/- 5.8 mmHg; however, total oxygen extraction by the microcirculation increased by 25%. The increased extraction, lowered tissue Po(2), and increased wall oxygen concentration gradient are compatible with the hypothesis that vasoconstriction significantly increases vessel wall oxygen consumption, which in this model appears to constitute an important oxygen-consuming compartment. This conclusion was supported by the finding that the small percentage of the vessels that dilated in these experiments had a vessel wall oxygen gradient that was smaller than control and which was not determined by changes in tissue Po(2). These findings show that AVP administration, which reduces oxygen supply by vasoconstriction, may further impair tissue oxygenation by the additional oxygen consumption of the microcirculation.     

Microvascular oxygen distribution in awake hamster window chamber model during hyperoxia

The microvascular effects and hemodynamic events following exposure to normobaric hyperoxia (because of inspiration of 100% O2) were studied in the awake hamster window chamber model and compared with normoxia. Hyperoxia increased arterial blood Po2 to 477.9 +/- 19.9 from 60.0 +/- 1.2 mmHg (P < 0.05). Heart rate and blood pressure were unaltered, whereas cardiac index was reduced from 196 +/- 13 to 144 +/- 31 ml.min-1.kg-1 (P < 0.05) in hyperoxia. Direct measurements in the microcirculation showed there was arteriolar vasoconstriction, reduction of microvascular flow (83% of control, P < 0.05), and functional capillary density (FCD, 74 +/- 16% of control), the latter change being significant (P < 0.05). Calculations of oxygen delivery and oxygen consumption based on the measured changes in microvascular blood flow velocity and diameter and estimates of oxygen saturation corrected for the Bohr effect due to the lowered pH and increased Pco2 showed that oxygen transport in the microvascular network did not change between normal and hyperoxic condition. The congruence of systemic and microvascular hemodynamics events found with hyperoxia suggests that the microvascular findings are common to most tissues in the organism, and that hyperoxia, due to vasoconstriction and the decrease of FCD, causes a maldistribution of perfusion in the microcirculation.     

PO2 measurements in the microcirculation using phosphorescence quenching microscopy at high magnification

In phosphorescence quenching microscopy (PQM), the multiple excitation of a reference volume produces the integration of oxygen consumption artifacts caused by individual flashes. We analyzed the performance of two types of PQM instruments to explain reported data on Po2 in the microcirculation. The combination of a large excitation area (LEA) and high flash rate produces a large oxygen photoconsumption artifact manifested differently in stationary and flowing fluids. A LEA instrument strongly depresses Po2 in a motionless tissue, but less in flowing blood, creating an apparent transmural Po2 drop in arterioles. The proposed model explains the mechanisms responsible for producing apparent transmural and longitudinal Po2 gradients in arterioles, a Po2 rise in venules, a hypothetical high respiration rate in the arteriolar wall and mesenteric tissue, a low Po2 in lymphatic microvessels, and both low and uniform tissue Po2. This alternative explanation for reported paradoxical results of Po2 distribution in the microcirculation obviates the need to revise the dominant role of capillaries in oxygen transport to tissue. Finding a way to eliminate the photoconsumption artifact is crucial for accurate microscopic oxygen measurements in microvascular networks and tissue. The PQM technique that employs a small excitation area (SEA) together with a low flash rate was specially designed to avoid accumulated oxygen photoconsumption in flowing blood and lymph. The related scanning SEA instrument provides artifact-free Po2 measurements in stationary tissue and motionless fluids. Thus the SEA technique significantly improves the accuracy of microscopic Po2 measurements in the microcirculation using the PQM.     

Microlymphatic and tissue oxygen tension in the rat mesentery

Oxygen phosphorescence quenching was used to measure tissue Po(2) of lymphatic vessels of 43.6 +/- 23.1 microm (mean +/- SD) diameter in tissue locations of the rat mesentery classified according to anatomic location. Lymph and adipose tissue Po(2) were 20.6 +/- 9.1 and 34.1 +/- 7.8 mmHg, respectively, with the difference being statistically significant. Rare microlymphatic vessels in connective tissue not surrounded by microvessels had a Po(2) of 0.8 +/- 0.2 mmHg, whereas the surrounding tissue Po(2) was 3.0 +/- 3.2 mmHg, with both values being significantly lower than those of adipose tissue. Lower of lymph fluid Po(2) relative to the surrounding tissue was also evident in paired measurements of Po(2) in the lymphatic vessels and perilymphatic adipose tissue, which was significantly lower than the Po(2) in paired adipose tissue. The Po(2) of the lymphatic fluid of the mesenteric microlymphatics is consistently lower than that of the surrounding adipose tissue by approximately 11 mmHg; therefore, lymph fluid has the lowest Po(2) of this tissue. The disparity between lymph and tissue Po(2) is attributed to the microlymphatic vessel wall and lymphocyte oxygen consumption.     

Role of endothelial nitric oxide in microvascular oxygen delivery and consumption

Nitric oxide (NO) is an important signaling molecule modulating diverse processes such as vasodilation, neurotransmission, long-term potentiation, and immune responses. The endothelium contributes a significant fraction of NO from endothelial NO synthase (eNOS). The objective of this work was to analyze the role of eNOS in the modulation of oxygen supply to the tissues and in adaptation to maintain oxygenation uncompromised. Oxygen delivery and consumption were measured in the microcirculation of homozygous mutant endothelial nitric oxide synthase-deficient (eNOS(-/-)) and wild-type mice. Animals were implanted with a dorsal window chamber, allowing us to assess the intact microvascular system. Hemodynamics and oxygen tension were assessed in the microcirculation of conscious animals. The eNOS(-/-) mice had significantly higher blood pressure and lower heart rate (146 +/- 8 mm Hg, 401 +/- 17 bpm) than wild type (127 +/- 6 mm Hg, 428 +/- 20 bpm). Microvascular hemodynamic parameters were not significantly different between groups. The eNOS(-/-) animals delivered less oxygen to the microcirculation and released more oxygen to the tissue; both differences were statistically significant compared to wild type. The arteriolar vessel wall oxygen gradient, a measure of vascular smooth muscle cells and endothelial cell wall oxygen consumption, was significantly lower for eNOS(-/-) than for wild type, suggesting that the inhibition of eNOS is an antianoxia (oxygen sparing) mechanism. Finally, the findings of the study support the argument that NO availability limits oxygen consumption by the tissue.     

Oxygen release from arterioles with normal flow and no-flow conditions.

The rate of oxygen release from arterioles ( approximately 55 microm diameter) was measured in the hamster window chamber model during flow and no-flow conditions. Flow was stopped by microvascular transcutaneous occlusion using a glass pipette held by a manipulator. The reduction of the intra-arteriolar oxygen tension (Po2) was measured by the phosphorescence quenching of preinfused Pd-porphyrin, 100 microm downstream from the occlusion. Oxygen release from arterioles was found to be 53% greater during flow than no-flow conditions (2.6 vs. 1.7 x 10(-5) ml O2.cm(-2).s(-1), P < 0.05). Acute hemodilution with dextran 70 was used to reduce vessel oxygen content, significantly increase wall shear stress (14%, P < 0.05), reduce Hct to 28.4% (SD 1.0) [vs. 48.8% (SD 1.8) at baseline], lower oxygen supply by the arterioles (10%, P < 0.05), and increase oxygen release from the arterioles (39%, P < 0.05). Hemodilution also increased microcirculation oxygen extraction (33% greater than nonhemodilution, P < 0.05) and oxygen consumption by the vessel wall, as shown by an increase in vessel wall oxygen gradient [difference in Po2 between the blood and the tissue side of the arteriolar wall, nonhemodiluted 16.2 Torr (SD 1.0) vs. hemodiluted 18.3 Torr (SD 1.4), P < 0.05]. Oxygen released by the arterioles during flow vs. nonflow was increased significantly after hemodilution (3.6 vs. 1.8 x 10(-5) ml O2.cm(-2).s(-1), P < 0.05). The oxygen cost induced by wall shear stress, suggested by our findings, may be >15% of the total oxygen delivery to tissues by arterioles during flow in this preparation.     

The rate of O₂ loss from mesenteric arterioles is not unusually high

The O(2) disappearance curve (ODC) recorded in an arteriole after the rapid arrest of blood flow reflects the complex interaction among the dissociation of O(2) from hemoglobin, O(2) diffusivity, and rate of respiration in the vascular wall and surrounding tissue. In this study, the analysis of experimental ODCs allowed the estimation of parameters of O(2) transport and O(2) consumption in the microcirculation of the mesentery. We collected ODCs from rapidly arrested blood inside rat mesenteric arterioles using scanning phosphorescence quenching microscopy (PQM). The technique was used to prevent the artifact of accumulated O(2) photoconsumption in stationary media. The observed ODC signatures were close to linear, in contrast to the reported exponential decline of intra-arteriolar Po(2). The rate of Po(2) decrease was 0.43 mmHg/s in 20-μm-diameter arterioles. The duration of the ODC was 290 s, much longer than the 12.8 s reported by other investigators. The arterioles associated with lymphatic microvessels had a higher O(2) disappearance rate of 0.73 mmHg/s. The O(2) flux from arterioles, calculated from the average O(2) disappearance rate, was 0.21 nl O(2)·cm(-2)·s(-1), two orders of magnitude lower than reported in the literature. The physical upper limit of the O(2) consumption rate by the arteriolar wall, calculated from the condition that all O(2) is consumed by the wall, was 452 nl O(2)·cm(-3)·s(-1). From consideration of the microvascular tissue volume fraction in the rat mesentery of 6%, the estimated respiration rate of the vessel wall was ～30 nl O(2)·cm(-3)·s(-1). This result was three orders of magnitude lower than the respiration rate in rat mesenteric arterioles reported by other investigators. Our results demonstrate that O(2) loss from mesenteric arterioles is small and that the O(2) consumption by the arteriolar wall is not unusually large.     

Nitric oxide regulation of microvascular oxygen exchange during hypoxia and hyperoxia.

The objective of this work was to test the hypothesis that the limitation of nitric oxide (NO) availability accentuates microvascular reactivity to oxygen. The awake hamster chamber window model was rendered hypoxic and hyperoxic by ventilation with 10 and 100% oxygen. Systemic and microvascular parameters were determined in the two conditions and compared with normoxia in a group receiving the NO scavenger nitronyl nitroxide and a control group receiving only the vehicle (saline). Mean arterial blood pressure did not change with different gas mixtures during infusion of the vehicle, but it increased significantly in the NO-depleted group. NO scavenging increased the reactivity of microvessels to the changed oxygen supply, causing the arteriolar wall to significantly increase oxygen consumption. Tissue Po2 was correspondingly significantly reduced during NO scavenger infusion. The present findings support the hypothesis that microvascular oxygen consumption is proportional to oxygen-induced vasoconstriction. The effect of oxygen on vascular tone is modulated by NO. As a consequence, NO acts as a regulator of the vessel wall oxygen consumption. The vessel wall consumes oxygen in proportion to the local Po2, and an impairment of NO availability renders the circulation more sensitive to changes in the oxygen supply.     

Terminal lymphatics: the potential "lethal corner" in the distribution of tissue pO2

BACKGROUND: Terminal lymphatic fluid is the compartment furthest removed from the oxygen supply, and therefore should present the lowest pO(2) in the tissue due to oxygen consumption by the tissue and the lymphatic vessel wall. METHODS AND RESULTS: The distribution of pO(2) was determined in the tissue, the lymphatic microvessels, and arterioles and venules of the hamster chamber window model, which is studied without anesthesia with the tissue isolated from the environment. Lymphatic fluid pO(2) was measured with the phosphorescence oxygen quenching method. Small terminal lymphatic fluid pO(2) was 18.4 +/- 2.6 mmHg, and 18.0 +/- 2.4 mmHg in collecting lymphatics. Tissue pO(2) averaged 24.6 +/- 2.7 mmHg. The significant difference between tissue and intralymphatic pO(2) was due in part to the presence of an oxygen gradient across the lymphatic wall, which ranged from 3.7 +/- 1.3 mmHg for terminal lymphatics, to 6.0 +/- 1.2 mmHg for collecting lymphatics. This gradient is assumed to be due to the oxygen consumption by the cellular component of the lymphatic wall. CONCLUSION: The increased vessels wall gradient found in collecting lymphatics was reconciled by the findings that these microlymphatic vessels tend to be contiguous to the arterioles, whereas the terminal lymphatics are dispersed in the tissue. These findings indicate that terminal lymphatic present the lowest oxygen tension in the tissue, and therefore are the locations at risk to develop anoxia when the microvascular oxygen supply becomes limited.     

Erythrocyte-associated transients in capillary PO2: an isovolemic hemodilution study in the rat spinotrapezius muscle

Mathematical simulations of oxygen delivery to tissue from capillaries that take into account the particulate nature of blood flow predict the existence of oxygen tension (Po(2)) gradients between erythrocytes (RBCs). As RBCs and plasma alternately pass an observation point, these gradients are manifested as rapid fluctuations in Po(2), also known as erythrocyte-associated transients (EATs). The impact of hemodilution on EATs and oxygen delivery at the capillary level of the microcirculation has yet to be elucidated. Therefore, in the present study, phosphorescence quenching microscopy was used to measure EATs and Po(2) in capillaries of the rat spinotrapezius muscle at the following systemic hematocrits (Hct(sys)): normal (39%) and after moderate (HES1; 27%) or severe (HES2; 15%) isovolemic hemodilution using a 6% hetastarch solution. A 532-nm laser, generating 10-micros pulses concentrated onto a 0.9-microm spot, was used to obtain plasma Po(2) values 100 times/s at points along surface capillaries of the muscle. Mean capillary Po(2) (Pc(O(2)); means +/- SE) significantly decreased between conditions (normal: 56 +/- 2 mmHg, n = 45; HES1: 47 +/- 2 mmHg, n = 62; HES2: 27 +/- 2 mmHg, n = 52, where n = capillary number). In addition, the magnitude of Po(2) transients (DeltaPo(2)) significantly decreased with hemodilution (normal: 19 +/- 1 mmHg, n = 45; HES1: 11 +/- 1 mmHg, n = 62; HES2: 6 +/- 1 mmHg, n = 52). Results suggest that the decrease in Pc(O(2)) and DeltaPo(2) with hemodilution is primarily dependent on Hct(sys) and subsequent microvascular compensations.     

Prolonged tissue PO2 reduction after contraction in spinotrapezius muscle of spontaneously hypertensive rats

We tested the hypothesis that a deficit in oxygen extraction or an increase in oxygen demand after skeletal muscle contraction leads to delayed recovery of tissue oxygen tension (Po(2)) in the skeletal muscle of hypertensive rats compared with normotensive rats. Blood flow and Po(2) recovery at various sites in the spinotrapezius muscle of spontaneously hypertensive rats (SHRs) were evaluated after a 3-min period of muscle contraction and were compared with corresponding values in Wistar-Kyoto rats (WKYs). The recovery of tissue Po(2) [75 +/- 7 (SHRs) vs. 99 +/- 12% (WKYs) of resting values] and venular Po(2) [72 +/- 13 (SHRs) vs. 104 +/- 10% (WKYs) of resting values] were significantly depressed in the SHRs 30 s postcontraction. The delayed recovery persisted for 120 s postcontraction for both tissue [86 +/- 11 (SHRs) vs. 119 +/- 13% (WKYs) of resting values] and venular [74 +/- 2 (SHRs) vs. 100 +/- 9% (WKYs) of resting values] Po(2) levels. There was no significant difference in the recovery of arteriolar Po(2) between the two groups 30 s postcontraction [95 +/- 7 (SHRs) vs. 84 +/- 8% (WKYs) of resting values]. Values for resting diameter of arcade arterioles in the two groups were not different [52 +/- 3 (SHRs) vs. 51 +/- 3 microm (WKYs)], but the arteriolar diameter after the 3-min contraction period was greater in the SHRs (71 +/- 4 microm) than the WKYs (66 +/- 4). Likewise, red blood cell (RBC) velocity [5.8 +/- 0.3 (SHRs) vs. 4.7 +/- 0.2 mm/s (WKYs)] and blood flow [23.0 +/- 0.8 (SHRs) vs. 16.0 +/- 1.0 nl/s (WKYs)] measurements were significantly greater in the SHRs at 30 s postcontraction. The delayed recovery of tissue Po(2) in the SHRs compared with the WKYs can be explained by a decrease in oxygen diffusion from the rarefied microvascular network due to the increased RBC velocity and the shorter residence time in the microcirculation and the consequent disequilibrium for oxygen between plasma and RBCs. The delayed recovery of venular Po(2) in the SHRs is consistent with this explanation, as venular Po(2) is slowly restored to baseline by release of oxygen from the RBCs. This leaves the arterioles in the primary role as oxygen suppliers to restore Po(2) in the tissue after muscle contraction.     

Relationships between cardiac hyperactivity, oxygen tension, and release of vasodilator material in coronary autoregulation of guinea-pig hearts

Increases in cardiac activity induce autoregulatory coronary vasodilation. The intermediate steps which trigger this process are thought to be myocardial hypoxia which induces the release of vasodilator mediator(s). The present study examines the relationships between mechanical activity, oxygen tension, and release of vasodilator material in isolated perfused hearts. Guinea-pig isolated hearts were perfused in series, the effluent from donor hearts being regassed prior to entry to recipient hearts. Histamine (1 microgram) and isoproterenol (10 ng) increased the rate and tension of donor hearts and produced predominant coronary vasodilator responses which were followed by the appearance of vasodilator material in the recipient (falls in perfusion pressure, 9.8 +/- 1.1 and 9.1 +/- 2.5 mmHg) (1 mmHg = 133.322 Pa). Exposure of donor hearts to hypoxia also caused vasodilatation and release of vasodilator material (fall in pressure, 11.4 +/- 1.6 mmHg). Pacing-induced tachycardia (6 Hz) of donor hearts promoted the release of vasodilator material, the fall in recipient heart pressure being 11.5 +/- 1.8 mmHg. This was abolished by beta-adrenoceptor blockade and when donor hearts were from reserpine-pretreated guinea pigs. In was concluded that pacing released endogenous catecholamines which in turn released the vasodilator material. Pacing per se did not cause vasodilatation or release of the vasodilator. The Po2 of perfusates from donor hearts was reduced by pacing at 5 Hz (25.7 +/- 5.2 mmHg) and by isoproterenol (10 ng, 32.0 +/- 3.7 mmHg), indicative of an elevated oxygen extraction. The isoproterenol-induced falls in Po2 were abolished by beta-adrenoceptor blockade. However, the pacing-induced falls in Po2 persisted, the values occurring before (25.7 +/- 5.2 mmHg) and after propranolol (45.7 +/- 4.5 mmHg) and before (32.1 +/- 1.1 mmHg) and after practolol (27.3 +/- 4.1 mmHg) not differing significantly (p greater than 0.05). These falls in perfusate Po2 were not accompanied by coronary vasodilatation or release of vasoactive material. Perfusate Po2 changes could therefore be dissociated from the coronary vasodilatation and vasoactive material release, suggesting that hypoxia may not be a prerequisite for the metabolic autoregulatory vasodilatation in response to myocardial hyperactivity induced by cardiac stimulants.     

Vascular wall energetics in arterioles during nitric oxide-dependent and -independent vasodilation.

The objective of this study was to evaluate whether the nitric oxide (NO) released from vascular endothelial cells would decrease vessel wall oxygen consumption by decreasing the energy expenditure of mechanical work by vascular smooth muscle. The oxygen consumption rate of arteriolar walls in rat cremaster muscle was determined in vivo during NO-dependent and -independent vasodilation on the basis of the intra- and perivascular oxygen tension (Po2) measured by phosphorescence quenching laser microscopy. NO-dependent vasodilation was induced by increased NO production due to increased blood flow, whereas NO-independent vasodilation was induced by topical administration of papaverine. The energy efficiency of vessel walls was evaluated by the variable ratio of circumferential wall stress (amount of mechanical work) to vessel wall oxygen consumption rate (energy cost) in the arteriole between normal and vasodilated conditions. NO-dependent and -independent dilation increased arteriolar diameters by 13 and 17%, respectively, relative to the values under normal condition. Vessel wall oxygen consumption decreased significantly during both NO-dependent and -independent vasodilation compared with that under normal condition. However, vessel wall oxygen consumption during NO-independent vasodilation was significantly lower than that during NO-dependent vasodilation. On the other hand, there was no significant difference between the energy efficiency of vessel walls during NO-dependent and -independent vasodilation, suggesting the decrease in vessel wall oxygen consumption produced by NO to be related to reduced mechanical work of vascular smooth muscle.     

Resveratrol reduction of infarct size in Long-Evans rats subjected to focal cerebral ischemia

Although vasomotion has been considered a feature of the microvascular bed under physiological conditions, it has also been observed following hypotension in several tissues. In this work, 158 mesenteric microvessels of 36 rats were investigated quantitatively in normovolemic and hemorrhaged animals, focussing on diameter changes, particularly vasomotion incidence and characteristics. The femoral arteries of Wistar rats (body weight BW = 188 +/- 23 g, mean +/- SD) anesthetized with pentobarbital were cannulated for arterial pressure (AP) monitoring and blood withdrawal. The protocol consisted of 15 min control and 30 min of hemorrhagic hypotension (AP = 52 +/- 5 mmHg, hemorrhaged vol. = 17 +/- 4 ml/kg BW). During control normovolemic conditions, analysis of mesenteric microcirculation using intravital videomicroscopy revealed neither arteriolar nor venular vasomotion. During hemorrhagic hypotension (HH) microvascular blood flow reduced to 25% of control. While venules did not show diameter changes during HH, arterioles contracted to 85 +/- 20% of control and arteriolar vasomotion appeared in 42% of the animals and 27% of the arterioles. The amplitude of arteriolar diameter change during HH relative to mean diameter and to control diameter averaged 65 +/- 24% (range: 32-129%) and 41 +/- 10% (range: 25-62%), respectively. Vasomotion analysis showed two major frequency components: 1.7 +/- 0.8 and 7.0 +/- 5.2 cycles/min. Arterioles showing vasomotion had a mean control diameter larger than the remaining arterioles and showed the largest constriction during HH. We conclude that hemorrhagic hypotension does not change venular diameter but induces arteriolar constriction and vasomotion in rat mesentery. This activity is expressed as slow waves with high amplitude and fast waves with low amplitude, and is dependent on vessel size.     

Jejunal tissue oxygenation and microvascular flow motion during hemorrhage and resuscitation

The relationship between flow motion and tissue oxygenation was investigated during hemorrhage/retransfusion with and without dopamine in 14 pigs. During 45% bleed, jejunal microvascular hemoglobin O(2) saturation (HBjO(2)) and mucosal tissue Po(2) (Po(2)muc) were recorded in seven control and seven dopamine-treated animals. Mean arterial pressure and systemic O(2) delivery decreased during hemorrhage and returned to baseline after retransfusion. Hemorrhage decreased Po(2)muc from 33 +/- 2.8 to 13 +/- 1.6 mmHg and HBjO(2) from 53 +/- 4.9% to 32 +/- 3.9%, respectively, in control animals. During reperfusion, Po(2)muc and HBjO(2) remained low. Dopamine increased Po(2)muc from 28 +/- 4.3 to 45 +/- 4.6 mmHg and HBjO(2) from 54 +/- 5.7% to 69 +/- 1.5% and attenuated the decrease in Po(2)muc and HBjO(2) during hemorrhage. After retransfusion, dopamine restored Po(2)muc and HBjO(2) to baseline. Control animals developed rhythmic HBjO(2) oscillations with increasing amplitude (frequency, 4.5 to 7.6 cycles/min) and showed an inverse relationship between Po(2)muc and HBjO(2) oscillation amplitude. Dopamine prevented regular flow motion. The association between decreased Po(2)muc and increased oscillations in HBjO(2) after normalization of systemic hemodynamics and O(2) transport in control animals suggests a cause-and-effect relationship between low tissue Po(2) and flow motion activity within the jejunal microcirculation.     

Alginate plasma expander maintains perfusion and plasma viscosity during extreme hemodilution

Extreme hemodilution was performed in the hamster chamber window model using 6% Dextran 70, lowering systemic hematocrit by 60%. Animals were subsequently divided into three groups and hemodiluted to a hematocrit of 11% using 6% Dextran 70, 6% Dextran 500, and a 4% Dextran 70 + 0.7% alginate solution (n = 6 each group). Final plasma viscosities were 1.4 +/- 0.2, 2.2 +/- 0.1, and 2.7 +/- 0.2 cp, respectively, (P < 0.05, high viscosity vs. low viscosity). Blood viscosities were 2.1 +/- 0.2, 2.9 +/- 0.4, and 3.9 +/- 0.3 cp, respectively. The lowest blood and plasma viscosity group had a significantly lower functional capillary density, 37 +/- 16%, whereas the two high-viscosity solutions were 71 +/- 15% and 76 +/- 12% (P < 0.05, high viscosity vs. low viscosity), respectively. Arteriolar and venular flow in the Dextran 500 and alginate groups was higher than baseline (i.e., normal nontreated animals), whereas the low-viscosity group showed a reduction in flow. These microvascular changes were paralleled by changes in base excess, which was negative for the Dextran 70 group and positive for the other groups. However, tissue Po(2) was uniformly low for all groups (average of 1.4 mmHg). Calculation of tissue oxygen consumption in the window chamber based on the microvascular data, flow, and intravascular Po(2) showed that only the alginate + Dextran 70 solution-exchanged animals returned to baseline oxygen consumption, whereas the other groups were lower than baseline (P < 0.05). These results show that hemodilution performed with high-viscosity plasma expanders yields systemic arterial pressures and functional capillary densities that are significantly higher (P < 0.05) than those obtained with 6% Dextran 70, a fluid whose viscosity is similar to that of plasma. A condition for obtaining these results is that the oncotic pressure of the plasma expander be titrated to near normal, so that autotransfusion of fluid from the tissue into the vascular compartment does not reduce the effects of increasing plasma viscosity and increased shear stress on the microvascular wall.     

Cardioprotection by HO-4038, a novel verapamil derivative, targeted against ischemia and reperfusion-mediated acute myocardial infarction

Many cardiac interventional procedures, such as coronary angioplasty, stenting, and thrombolysis, attempt to reintroduce blood flow (reperfusion) to an ischemic region of myocardium. However, the reperfusion is accompanied by a complex cascade of cellular and molecular events resulting in oxidative damage, termed myocardial ischemia-reperfusion (I/R) injury. In this study, we evaluated the ability of HO-4038, an N-hydroxypiperidine derivative of verapamil, on the modulation of myocardial tissue oxygenation (Po(2)), I/R injury, and key signaling molecules involved in cardioprotection in an in vivo rat model of acute myocardial infarction (MI). MI was created in rats by ligating the left anterior descending coronary artery (LAD) for 30 min followed by 24 h of reperfusion. Verapamil or HO-4038 was infused through the jugular vein 10 min before the induction of ischemia. Myocardial Po(2) and the free-radical scavenging ability of HO-4038 were measured using electron paramagnetic resonance spectroscopy. HO-4038 showed a significantly better scavenging ability of reactive oxygen radicals compared with verapamil. The cardiac contractile functions in the I/R hearts were significantly higher recovery in HO-4038 compared with the verapamil group. A significant decrease in the plasma levels of creatine kinase and lactate dehydrogenase was observed in the HO-4038 group compared with the verapamil or untreated I/R groups. The left ventricular infarct size was significantly less in the HO-4038 (23 +/- 2%) compared with the untreated I/R (36 +/- 4%) group. HO-4038 significantly attenuated the hyperoxygenation (36 +/- 1 mmHg) during reperfusion compared with the untreated I/R group (44 +/- 2 mmHg). The HO-4038-treated group also markedly attenuated superoxide production, increased nitric oxide generation, and enhanced Akt and Bcl-2 levels in the reperfused myocardium. Overall, the results demonstrated that HO-4038 significantly protected hearts against I/R-induced cardiac dysfunction and damage through the combined beneficial actions of calcium-channel blocking, antioxidant, and prosurvival signaling activities.     

Transmural gradients of oxygen tension on cerebral microvessels in rats

Using modified oxygen needle microelectrodes, vital microscopy with video-recording facilities, measurements of tissue oxygen tension (PO2) profiles near the cortical arterioles and transmural PO2 gradients on pial arterioles of the rat were performed. At control transmural PO2 gradient averaged 1.17 +/- 0.06 mm Hg/microm (mean +/- SEM, n = 40). Local dilatation of the arteriolar wall (microapplication of sodium nitroprusside approximately 2 x 10(-7) M) resulted in marked drop of the transmural PO2 gradient to 0.68 +/- 0.04 mm Hg/microm (p < 0.001, n = 38). The important finding of the study is the dependence of the transmural PO2 gradient on the vascular tone of pial arterioles. The data presented allow to conclude that O2 consumption of the arteriolar wall lies within the range for surrounding tissue and O2 consumption of the endothelial layer and, apparently, has no substantial impact on transmural PO2 gradient.     

Elevated plasma viscosity in extreme hemodilution increases perivascular nitric oxide concentration and microvascular perfusion

We tested the hypothesis that high-viscosity (HV) plasma in extreme hemodilution causes wall shear stress to be greater than low-viscosity (LV) plasma, leading to enhanced production of nitric oxide (NO). The perivascular concentration of NO was measured in arterioles and venules and the tissue of the hamster chamber window model, subjected to acute extreme hemodilution, with a hematocrit (Hct) of 11% using Dextran 500 (n = 6) or Dextran 70 (n = 5) with final plasma viscosities of 1.99 +/- 0.11 and 1.33 +/- 0.04 cp, respectively. HV plasma significantly increased the periarteriolar, perivenular, and tissue NO concentration by 2.0, 1.9, and 1.4 times the control (n = 7). The NO concentration with LV plasma was not statistically different from control. Arteriolar shear stress was significantly increased in HV plasma relative to LV plasma in arterioles but not in venules. Aortic endothelial NO synthase (eNOS) protein expression was increased with HV plasma but not with LV plasma. There was a weak correlation between perivascular NO concentration and the locally calculated shear stress induced by the procedures, when blood viscosity was corrected according to Hct values previously determined in studies of microvascular Hct distribution. The finding that the periarteriolar and venular NO concentration in HV plasma was the same although arteriolar shear stress was significantly greater than venular shear stress maybe be due to differences in vessel wall metabolism between arterioles and venules and the presence of NO transport through the blood stream in the microcirculation. Results support the concept that in extreme hemodilution HV plasma maintains functional capillary density through a NO-mediated vasodilatation.     

Oxygen regulation and limitation to cellular respiration in mouse skeletal muscle in vivo

In skeletal muscle, intracellular Po2 can fall to as low as 2-3 mmHg. This study tested whether oxygen regulates cellular respiration in this range of oxygen tensions through direct coupling between phosphorylation potential and intracellular Po2. Oxygen may also behave as a simple substrate in cellular respiration that is near saturating levels over most of the physiological range. A novel optical spectroscopic method was used to measure tissue oxygen consumption (Mo2) and intracellular Po2 using the decline in hemoglobin and myoglobin saturation in the ischemic hindlimb muscle of Swiss-Webster mice. 31P magnetic resonance spectroscopic determinations yielded phosphocreatine concentration ([PCr]) and pH in the same muscle volume. Intracellular Po2 fell to <2 mmHg during the ischemic period without a change in the muscle [PCr] or pH. The constant phosphorylation state despite the decline in intracellular Po2 rejects the hypothesis that direct coupling between these two variables results in a regulatory role for oxygen in cellular respiration. A second set of experiments tested the relationship between intracellular Po2 and Mo2. In vivo Mo2 in mouse skeletal muscle was increased by systemic treatment with 2 and 4 mg/kg body wt 2,4-dinitrophenol to partially uncouple mitochondria. Mo2 was not dependent on intracellular Po2 above 3 mmHg in the three groups despite a threefold increase in Mo2. These results indicate that Mo2 and the phosphorylation state of the cell are independent of intracellular Po2 throughout the physiological range of oxygen tensions. Therefore, we reject a regulatory role for oxygen in cellular respiration and conclude that oxygen acts as a simple substrate for respiration under physiological conditions.