Stunning and myocardial contractile autoregulation studied on the isolated isovolumic blood-perfused dog heart

AIM: To study, for the first time, the effects of stunning on homeometric and heterometric autoregulation. METHODS AND RESULTS: Ischaemia (15 min)/reperfusion (30 min) was induced in the isovolumic blood-perfused dog heart preparation. Heart rate elevations (n = 9) from 60 to 200 beats min-1, in steps of 20 beats min-1, promoted the same inotropic stimulation in control (C) and stunning (S), indicating that ischaemia/reperfusion does not affect the changes in calcium kinetics elicited by the Bowditch effect. Sudden ventricular dilation (VD) (n = 10) evoked an instantaneous increase in developed pressure (Delta1DP) followed by a continuous slow performance increase (Delta2DP) in C and S. Delta1DP (C: 35 +/- 2.2 mmHg; S: 27 +/- 2.1 mmHg; P = 0.002) and Delta2DP (C: 20 +/- 1.6 mmHg; S: 14 +/- 1.3 mmHg; P = 0.002) decreased proportionally, while Delta2/Delta1DP (C: 0.57 +/- 0.13; S: 0.58 +/- 0.14) and slow response time course (T/2) were unchanged (C: 55 +/- 6.6 s; S: 57 +/- 7.7 s) after ischaemia/reperfusion. The reduction of Delta1DP can be understood as a decline of the myofilaments calcium responsiveness, the main pathophysiological effect of stunning. The reason for the weakening of Delta2DP, due to intracellular calcium gain, was not determined but it was supposed that its complete manifestation could be restricted by cyclic adenosine monophosphate (cAMP) myocardial content reduction. As reported by others, Delta2DP depends on myocardial cAMP, and it has been shown that myocardial cAMP is decreased after ischaemia/reperfusion. CONCLUSIONS: Contractile depression due to stunning has no effect on the inotropic stimulation generated by the Bowditch phenomenon. Immediate and time-dependent enhancements of contraction evoked by sudden VD are proportionally reduced and the slow response time course is unaffected in the stunned myocardium.     

Regional myocardial heat-shock protein (HSP70) concentrations under different blood flow conditions

 The concentration of heat-shock proteins of 70 kD (HSP70) in heart tissue has been shown to increase during transient myocardial ischaemia and to persist during several hours of reperfusion. In this study the relationship between the local myocardial HSP70 concentration and blood flow was addressed for control physiological conditions and acute myocardial ischaemia. A specific aim of this study was to address the question of whether low flow areas under control physiological conditions have undergone a transient ischaemia during the preceding hours and thus may be in a state of hibernation or stunning. In 12 anaesthetized, open-chest beagle dogs (6 control and 6 with 60-min coronary artery stenosis) heart rate, mean aortic pressure, mean arterial partial pressure of O₂ and partial pressure of CO₂ averaged 85±16 beats/min, 94±14 mmHg, 102±17 mmHg and 39±6 mmHg, respectively. Regional HSP70 and myocardial blood flow (RMBF) were measured using an HSP70-enzyme-linked immunosorbent assay and the tracer microsphere technique, respectively, in samples of 250 mg wet mass. In the control group the mean RMBF was 1.06±0.59 ml·min^–1·g^–1 and the local HSP70 concentration was 7.08±1.03 μg/mg cytosolic protein. Myocardial HSP70 showed a blood flow-independent regional biological heterogeneity, equivalent to a coefficient of variation of 0.31. Local HSP70 concentrations did not differ (P>0.05) between control low and high flow samples, 6.16±1.0 vs 6.08±0.75 μg/mg cytosolic protein, respectively. However, after 60 min of coronary artery occlusion the local HSP70 concentration increased from 7.08 ±1.03 to 13.43±3.19 μg/mg cytosolic protein (P<0.001). There was a significant inverse relationship between the percent reduction of local blood flow and HSP70 (r=–0.56, P<0.001). From these results it is concluded that: (1) low flow samples under control physiological conditions are unlikely to be in a state of hibernation or stunning since their HSP70 concentration is normal and (2) the increase in the local HSP70 concentration during myocardial ischaemia reflects the degree of impairment of O₂ delivery. Received: 29 May 1998 / Received after revision: 14 August 1998 / Accepted: 25 August 1998     

High afterload during 10 min of regional ischaemia affects diastolic creep but not systolic function in reperfused (stunned) myocardium

The effect of afterload during regional ischaemia on myocardial stunning was studied in 15 pentobarbital anaesthetized cats. 10 min occlusion of the left anterior descending artery (LAD) was followed by 60 min of reperfusion. Afterload was decreased by intravenous infusion of nitroglycerine 3–8 μg kg^-1 min^-1 in group I (n=8); left ventricular peak systolic pressure (LVSP) 84±4 mmHg (mean±SEM) during coronary artery occlusion. In group II (n=7) LVSP was increased to 188±10 mmHg by inflating an intraaortic balloon during coronary artery occlusion. Regional function in the LAD perfused region was evaluated by cross-oriented sonomicrometry. Myocardial tissue blood flow was evaluated by radio-labelled microspheres. Afterload alterations did not affect regional systolic shortening (10.8±2.0% vs. 11.0±1.5% in group I and II, respectively, after 60 min of reperfusion). However, increased end-diastolic dimensions (diastolic creep) in both the circumferential and longitudinal segments were markedly more pronounced in the high afterload group (group II). Also important, the markedly increased myocardial tissue blood flow during reperfusion in group II as compared with group I (2.30±0.18 vs.  1.34±0.08 mL min^-1 g^-1 and 2.58±0.23 vs. 1.49±0.07 mL min^-1 g^-1 in subepicardial and subendocardial layers in the LAD perfused region) suggests that increased diastolic creep increased metabolic demands. This study indicates that passive stretching of the ischaemic area during coronary artery occlusion is an important mechanism behind diastolic creep.     

Histamine release and its effects in ischaemia-reperfusion injury of the isolated rat heart

Histamine is released from the heart during ischaemia-reperfusion injury. As histamine has cardiac effects, we investigated the role of histamine in ischaemia-reperfusion injury of isolated rat hearts. A Langendorff-model with 30 min global (37 ^oC) ischaemia followed by 60 min reperfusion was employed. The effects of ischaemia alone (n= 10, group 1.1+n = 10, group 2.1, 2 different series), and ischaemia with H₁- and H₂-receptor blockade with cimetidine (10 μM, n= 10), chlorpheniramine (10 μm, n= 8), terfenadine (10 μM, n= 8), and promethazin (10 μM, n= 9), or both cimetidine and chlorpheniramine (n = 8), were studied. Histamine was measured in the coronary effluent and cardiac tissue of group 1.1. Release of histamine increased from 6.5 ± 1 pmol min^-1 before ischaemia to 19 ± 3 pmol min^-1 at the start of reperfusion. Ischaemia decreased left ventricular developed pressure to 18 ± 11 % (1.1) and 50 ± 11 % (2.1) of initial value (mean ± SEM) at the start of reperfusion. Left ventricular end-diastolic pressure increased from 0 to 79 ± 8 mmHg (1.1) and 39 ± 9 (2.1) mmHg, while left ventricular systolic pressure was unchanged (101 ± 12% in 1.1 and 101 ± 10% in 2.1). Severe arrhythmias were induced in 90 (1.1) and 30 (2.1)% of the hearts, while coronary flow decreased during reperfusion. H₂-blockade did not modify the changes in left ventricular pressures, coronary flow, or heart rate induced by ischaemia. Three different Hj-blockers increased left ventricular systolic pressure, inhibited the decrease of developed pressure, attenuated the increase of end-diastolic pressure, and totally inhibited reperfusion arrhythmias. The effect of both blockers together was similar to that of H₁-blockers alone. Coronary flow was increased during reperfusion in two of the groups with Hj-blocker compared with ischaemic controls. Increased release of histamine from ischaemic-reperfused rat hearts concurred with depression of left ventricular function and arrhythmias during early reperfusion. Cardiac dysfunction during reperfusion was attenuated by three different Hj-receptor blockers.     

The novel non-peptide selective endothelin A receptor antagonist LU 135 252 protects against myocardial ischaemic and reperfusion injury in the pig

The aim of the study was to investigate the efficacy of the novel non-peptide selective endothelin A (ET_A) receptor antagonist LU 135 252 to limit the extent of myocardial ischaemic and reperfusion injury. Administration of LU 135 252 (1 and 5 mg kg^–1 i.v.) to anaesthetised pigs reduced mean arterial pressure (MAP) from 91 ± 4 to 79 ± 3 mmHg (P < 0.05) and 96 ± 3–82 ± 3 mmHg (P < 0.01), respectively. Heart rate, coronary blood flow and coronary vascular resistance were not affected by LU 135 252. The infarct size induced by 45-min ligation of the left anterior descending coronary artery (LAD) followed by 4-h reperfusion in pigs was 81 ± 5% of the area at risk in control animals given vehicle (n = 8). In pigs receiving 1 mg kg^–1 (n = 6) or 5 mg kg^–1 (n = 8) of LU 135 252 i.v. 20 min before ischaemia the infarct size was reduced to 64 ± 3% (P < 0.05) and 35 ± 4% (P < 0.001), respectively, of the area at risk. During the reperfusion period there was a non-significant trend towards a higher coronary blood flow and a lower coronary vascular resistance in the groups given LU 135 252 compared to controls. Myocardial overflow of ET-like immunoreactivity was increased during the reperfusion period but it was not affected by administration of LU 135 252. It is concluded that administration of the selective ET_A receptor antagonist LU 135 252 effectively protects the myocardium from ischaemia/reperfusion injury, indicating that the ET_A receptor subtype is involved in the development of ischaemia/reperfusion injury.     

A comparison of pulse oximetry and cerebral oxygenation in children with severe sleep apnea–hypopnea syndrome: a pilot study

Near infrared spectroscopy (NIRS) has been used to assess the impact of obstructive sleep apnea–hypopnea syndrome (OSAHS) on cerebral oxygenation. However, the relationship between the variations in the cerebral tissue oxygen saturation (ΔTOI) and pulse oximetry (ΔSpO₂) has not been assessed in children with OSAHS. Consecutive clinically stable children with severe OSAHS [apnea–hypopnea index (AHI) >15 events h⁻¹] diagnosed during a night‐time polygraphy with simultaneous recording of cerebral oxygenation with NIRS (NIRO‐200NX, Hamamatsu Photonics KK) were included between September 2015 and June 2016. Maximal ΔSpO₂ (SpO₂ drop from the value preceding desaturation to nadir) and concomitant variations in transcutaneous carbon dioxide (ΔPtcCO₂), maximal ΔTOI and maximal variations in cerebral oxygenated (O₂Hb) and deoxygenated (HHb) haemoglobin were reported. The relationships between ΔSpO₂, ΔPtcCO₂ and ΔTOI, ΔO₂Hb and ΔHHb were investigated. The data from five children (three boys, aged 9.6 ± 6.7 years, AHI 16–91 events h⁻¹) were analysed. Strong correlations were found between ΔSpO₂ and ΔTOI (r = 0.887, P < 0.001), but also with ΔO₂Hb and ΔHHb with a particular pattern in the youngest child with a dark skin pigmentation. Mean ΔSpO₂ was 20 ± 17% and mean ΔTOI was 8 ± 7%. Maximal ΔSpO₂ of approximately 70% were coupled with ΔTOI of no more than 35%. ΔPtcCO₂ correlated only weakly with the cerebral oxygenation indexes. This pilot study shows a strong relationship between pulse oximetry and cerebral oxygenation in children with OSAHS, with lower changes in TOI compared to SpO₂. Future studies should address the clinical impact of respiratory events on cerebral oxygenation and its consequences.     

Inhibition of elastase improves myocardial function after repetitive ischaemia and myocardial infarction in the rat heart

 We investigated the potential of inhibition of elastase, a granulocyte-derived proteolytic enzyme, in ameliorating the effects of myocardial stunning caused by repetitive ischaemia (RI) and myocardial infarction (MI) for the first time in an in situ, perfused, rat heart model. The effects of the elastase-inhibitors Elafin (EL, 10 mg/kg/h) and ICI 200,880 (ICI,5 mg/kg/h) on myocardial blood flow (MBF, H₂ clearance), regional myocardial function (FT, pulsed doppler) and neutrophil extravasation (myocardial myeloperoxidase activity, MPO) were investigated in RI (5×10 min ligature of the anterior descending ramus (LAD), 5×20 min reperfusion) and MI (50 min LAD ligature, 60 min reperfusion). Under control conditions, MBF and FT were significantly reduced and MPO was significantly increased after RI (n=8) and MI (n=8) in the ischaemic area compared with baseline. Pretreatment with EL (n=7) or ICI (n=7) did not improve MBF significantly and did not influence the successive attenuation of peak values of reactive hyperaemia. However, both EL and ICI significantly improved FT and significantly reduced MPO after RI and MI compared with control conditions. Additionally, both the area at risk and MI size were reduced significantly by both inhibitors. These results demonstrate that elastase inhibitors significantly improve the reduction of FT both in myocardial stunning and in myocardial infarction in the rat without significant improvement of MBF. It is concluded that elastase inhibitors exert a cardioprotective effect against reperfusion injury, probably by inhibition of leukocyte extravasation as indicated by the decrease in MPO activity. Received: 3 June 1996 / Received after revision: 29 October 1996 / Accepted: 1 November 1996     

Pulmonary ischemia/reperfusion injury: A quantitative study of structure and function in isolated heart‐lungs of the rat

Early graft dysfunction after lung transplantation is a significant and unpredictable problem. Our study aimed at a detailed investigation of structure‐function correlations in a rat isolated heart‐lung model of ischemia/reperfusion injury. Variable degrees of injury were induced by preservation with potassium‐modified Euro‐Collins solutions, 2 hr of cold ischemia, and 40 min of reperfusion. Pulmonary artery pressure (P_pa), pulmonary vascular resistance (PVR), peak inspiratory pressure (PIP), and perfusate gases (ΔP_O2, ΔP_CO2) were recorded during reperfusion. Right lungs were used to calculate W/D‐weight ratios. Nineteen experimental and six control left lungs were fixed for light and electron microscopy by vascular perfusion. Systematic random samples were analyzed by stereology to determine absolute and relative volumes of lung structures, the amount of interstitial and intraalveolar edema, and the extent of epithelial injury. Lectin‐ and immunohistochemistry using established epithelial cell markers were performed in three animals per group to reveal sites of severe focal damage. Experimental lungs showed a wide range in severity of ischemia/reperfusion injury. Intraalveolar edema fluid amounted to 77–909 mm³ with a mean of 448±250 mm³ as compared with 22±22 mm³ in control lungs (P<0.001). Perfusate oxygenation (ΔP_O2) decreased from 30.5±15.2 to 21.7±15.2 mm Hg (P=0.05) recorded after 5 and 40 minutes of reperfusion. In experimental lungs, a surface fraction of 1% to 58% of total type I pneumocyte surface was damaged. Intraalveolar edema per gas exchange region (Vv ape,P) and ΔP_O2 were related according to ΔP_O2 = 96 − 60 × log₁₀(Vv ape,P) [mm Hg]. The extent of epithelial injury did not correlate with ΔP_O2 nor with intraalveolar edema, but increased significantly with PVR. Lectin‐ and immunohistochemistry revealed focal severe damage to the alveolar epithelium at the border of perivascular cuffs. We conclude that ischemia/reperfusion‐associated respiratory compromise is a direct function of the amount of intraalveolar edema, however, it is not determined by the actual extent of diffuse alveolar epithelial damage at the air‐blood‐barrier but by the presence of focal severe epithelial damage at the perivascular/alveolar interface. Anat Rec 255:84–99, 1999. © 1999 Wiley‐Liss, Inc.     

Muscle microvascular oxygenation in chronic heart failure: role of nitric oxide availability

Aim: To test the hypothesis that diminished vascular nitric oxide availability might explain the inability of individuals with chronic heart failure (CHF) to maintain the microvascular PO₂’s (PO_2mv ∝ O₂ delivery‐to‐uptake ratio) seen in healthy animals. Methods: We superfused sodium nitroprusside (SNP; 300 μm ), Krebs–Henseleit (control, CON) and l ‐nitro arginine methyl ester (l ‐NAME; 1.5 mm ) onto the spinotrapezius muscle and measured PO_2mv by phosphorescence quenching in female Sprague–Dawley rats (n = 26) at rest and during twitch contractions (1 Hz). Seven rats served as controls (Sham) while CHF was induced by myocardial infarction. CHF rats were grouped as moderate (MOD; n = 15) and severe CHF (SEV; n = 4) according to morphological data and baseline PO_2mv. Results: In contrast to Sham and MOD,l ‐NAME did not affect the PO_2mv response (dynamics and steady‐state) of SEV when compared with CON. SNP restored the PO_2mv profile of SEV to that seen in Sham animals during CON. Specifically, the effect of l ‐NAME expressed as Δ(l ‐NAME – CON) were: Baseline PO_2mv [in mmHg, ΔSham = ?7.0 ± 1.6 (P < 0.05); ΔSEV =?1.2 ± 2.1], end‐contractions PO_2mv [in mmHg, ΔSham = ?5.0 ± 1.0 (P < 0.05); ΔSEV = ?2.5 ± 0.5] and time constant of PO_2mv decrease [in s, ΔSham = ?6.5 ± 3.0 (P < 0.05); ΔSEV = ?3.2 ± 1.8]. Conclusion: These data provide the first direct evidence that the pathological profiles of PO_2mv associated with severe CHF can be explained, in part, by a diminished vascular NO availability.     

Expression and activity of the glutamate transporter EAAT2 in cardiac hypertrophy: implications for ischaemia reperfusion injury

The expression and activity of the glutamate transporter, excitatory amino acid transporter 2 (EAAT2), in cardiac hypertrophy were investigated with respect to glutamate’s potential as a cardioprotective agent. Sarcolemmal vesicles (SV) isolated from hypertrophic hearts of male spontaneously hypertensive rats (SHR) or normotrophic hearts from age-matched male Wistar Kyoto rats (WKY) were used to measure the relative level of EAAT2 expression by Western blotting and the initial rate of 0–0.3 mM l-[¹⁴C]glutamate uptake. The effects of 20-min global normothermic ischaemia ±0.5 mM glutamate on cardiac function were measured in isolated working SHR/WKY hearts. In a separate series of hearts, glutamate, lactate and ATP levels were measured. Both the level of EAAT2 expression and the V _max for sodium-dependent l-[¹⁴C]glutamate uptake were significantly greater in SHR SV compared to WKY SV. The reperfusion cardiac output (CO) of SHR hearts was significantly worse than that of the WKY hearts (24.3±2.2 ml/min vs 39.8±3.3 ml/min, n=7/9±SE, p<0.01). The addition of 0.5 mM l-glutamate improved the SHR reperfusion CO to 45.2±5 ml/min, (n=6±SE, p<0.01) but had no effect on WKYs (46.2±3.8 ml/min, n=6±SE). SHR with 0.5 mM l-glutamate had higher glutamate levels at the start of ischaemia, plus higher glutamate and ATP levels at the end of ischaemia compared to any other group. These results suggest that increased glutamate transporter expression and activity in the SHR hearts helped facilitate glutamate entry into the SHR cardiomyocytes leading to improved myocardial metabolism during ischaemia and better functional recovery on reperfusion.     

Influence of coronary venous retroinfusion and vasodilatation on regional myocardial blood flow measurement with microspheres. An analysis of ‘microsphere loss’ from ischaemic and reperfused porcine hearts

The influence of coronary venous retroinfusion and a vasoselective calcium antagonist felodipine on the microsphere loss in a porcine model of myocardial ischaemia and reperfusion was studied. Sixteen open-chest pigs underwent 45 min of myocardial ischaemia induced by occlusion of the left anterior descending coronary artery followed by 4 h of reperfusion. Either felodipine (felo-retro group, 7 nmol kg^-1, n= 6) or the corresponding amount of vehicle (vehicle-retro group, n= 5) was infused retrogradely into the coronary veins over 30 min, starting 5 min before reperfusion. In a third group, the same amount of felodipine was administered intravenously (felo-iv group, n= 5). Myocardial regional blood flow was measured with radiolabelled microspheres (ø= 15 μm) injected before ischaemia to investigate a possible loss during ischaemia. In the felo-retro group, the apparent blood flow in the ischaemic areas, expressed as a percentage of the corresponding values in the non-ischaemic areas (%-flow), were 73±15, 73±11 and 75±19 in the subendocardial, midmyocardial and subepicardial layers, respectively. The corresponding percentage flows were 64±11, 70±11 and 62±9 in the vehicle-retro group and 75±18, 77±15 and 76±11 in the felo-iv group. The differences between the groups were not satistically significant. It is concluded that in this open-chest preparation microsphere loss observed in the ischaemic and reperfused myocardium is not increased by coronary venous retroinfusion or by a concomitantly administered vasodilative agent like felodipine.     

Renal nerve stimulation restores tubuloglomerular feedback after acute renal denervation

Renal nerves play an important role in the setting of the sensitivity of the tubuloglomerular feedback (TGF) mechanism. We recently reported a time-dependent resetting of TGF to a lower sensitivity 3–4 h after acute unilateral renal denervation (aDNX). This effect persisted after 1 week, but was then less pronounced. To determine whether normal TGF sensitivity could be restored in aDNX kidneys by low-frequency renal nerve stimulation (RNS), the following experiments were performed. Rats with aDNX were prepared for micropuncture. In one experimental group proximal tubular free flow (P_t) and stop flow pressures (P_sf) were measured during RNS at frequencies of 2, 4 and 6 Hz. In another series of experiments the TGF sensitivity was evaluated from the P_sf responses at different loop perfusion rates after 20 min of RNS at a frequency of 2 Hz. The maximal drop in P_sf (ΔP_sf) and the tubular flow rate at which half the maximal response in ΔP_sf was observed (turning point, TP), were recorded. At RNS frequencies of 2, 4 and 6 Hz, P_t decreased from the control level of 14.1 ± 0.8–13.1 ± 1.0, 12.4 ± 1.1 and 11.2 ± 0.8 mmHg (decrease 21%, P < 0.05), respectively, while at zero perfusion and during RNS at 2 and 4 Hz P_sf decreased from 42.5 ± 1.6 to 38.2 ± 1.4 and 32.8 ± 4.3 mmHg (decrease 23%, P < 0.05), respectively. The TGF characteristics were found to be reset from the normal sensitivity with TP of 19.0 ± 1.1 nL min^–1 and ΔP_sf of 8.7 ± 0.9 mmHg to TP of 28.3 ± 2.4 nL min^–1 (increase 49%, P < 0.05) and ΔP_sf of 5.8 ± 1.2 mmHg (decrease 33%) after aDNX. After 20 min of RNS at 2 Hz TP was normalized and ΔP_sf was 33% higher. Thus the present findings indicate that the resetting of the TGF sensitivity that occurred 2–3 h after aDNX could be partially restored by 20 min of RNS at a frequency of 2 Hz. These results imply that renal nerves have an important impact on the setting of the sensitivity of the TGF mechanism.     

Peroxisomal fatty acid oxidation capacity is more resistant to ischaemic and reperfusion injury than mitochondrial fatty acid oxidation capacity in feline hearts

We investigated ischaemic and postischaemic mitochondrial and peroxisomal fatty acid oxidation capacity, ATP levels and regional function in 40 anaesthetized open chest cats subjected to 10 or 40 min of regional myocardial ischaemia with or without 3 h of reperfusion (n=10 in each situation). Following 10 min of ischaemia, the mitochondrial fatty acid oxidation capacity measured in tissue extracts from ischaemic tissue (nmol min^-1 mg protein^-1) was reduced in both subepi- and subendocardium, but was normalized in reperfused tissue extracts from both wall layers (0.29±0.03 and 0.30±0.04 vs. 0.57±0.05 and 0.59±0.05, P<0.05). Peroxisomal fatty acid oxidation capacity in tissue extracts was unaffected by ischaemia and reperfusion. ATP levels and regional function measured in the LAD region was partly restored transmurally. After 40 min of LAD occlusion, mitochondrial fatty acid oxidation capacity was reduced, with higher activity in subepi- than in subendocardium (0.27±0.05 vs. 0.19±0.04, P<0.05). Reperfusion did not restore mitochondrial fatty acid oxidation capacity. Peroxisomal fatty acid oxidation capacity was increased in the ischaemic subendocardium compared with levels in non-ischaemic subendocardium (0.53±0.02 vs. 0.45±0.03, P<0.05), with normalization at the end of reperfusion. ATP levels were non-uniformly reduced during ischaemia and not repleted during reperfusion. Regional function recovered in circumferential segments but not in longitudinal segments following 40 min of ischaemia. In conclusion fatty acid oxidation enzymes seem to be more resistant to ischaemia in peroxisomes than in mitochondria. Mitochondrial fatty acid oxidation is fully reversible following shortlasting ischaemia, but remains depressed following prolonged ischaemia and reperfusion.     

Blood oxygenation level-dependent (BOLD) total and extravascular signal changes and ΔR2* in human visual cortex at 1.5, 3.0 and 7.0 T

The characterisation of the extravascular (EV) contribution to the blood oxygenation level‐dependent (BOLD) effect is important for understanding the spatial specificity of BOLD contrast and for modelling approaches that aim to extract quantitative metabolic parameters from the BOLD signal. Using bipolar crusher gradients, total (b = 0 s/mm²) and predominantly EV (b = 100 s/mm²) gradient echo BOLD ΔR₂* and signal changes (ΔS/S) in response to visual stimulation (flashing checkerboard; f = 8 Hz) were investigated sequentially (within < 3 h) at 1.5, 3.0 and 7.0 T in the same subgroup of healthy volunteers (n = 7) and at identical spatial resolutions (3.5 × 3.5 × 3.5 mm³). Total ΔR₂* (z‐score analysis) values were ?0.61 ± 0.10 s^?1 (1.5 T), ?0.74 ± 0.05 s^?1 (3.0 T) and ?1.37 ± 0.12 s^?1 (7.0 T), whereas EV ΔR₂* values were ?0.28 ± 0.07 s^?1 (1.5 T), ?0.52 ± 0.07 s^?1 (3.0 T) and ?1.25 ± 0.11 s^?1 (7.0 T). Although EV ΔR₂* increased linearly with field, as expected, it was found that EV ΔS/S increased less than linearly with field in a manner that varied with TE choice. Furthermore, unlike ΔR₂*, total and EV ΔS/S did not converge at 7.0 T. These trends were similar whether a z‐score analysis or occipital lobe‐based region‐of‐interest approach was used for voxel selection. These findings suggest that calibrated BOLD approaches may benefit from an EV ΔR₂* measurement as opposed to a ΔS/S measurement at a single TE. Copyright © 2010 John Wiley & Sons, Ltd.     

Renal response to volume depletion and expansion in Milan hypertensive rats

In previous studies on Milan hypertensive (MHS) rats, we found an impaired tubuloglomerular feedback (TGF) response before, during and after development of hypertension. In the present study MHS rats and rats of the Milan normotensive strain (MNS) were investigated after 24 hours of volume depletion (VD) and subsequently after 5% isotonic volume expansion (VE) with respect to whole kidney function, interstitial hydrostatic (P_int) and oncotic (II_int) pressures, stop-flow pressure characteristics of TGF and changes in early proximal flow rate in response to increased loop of Henle flow. MHS rats had higher mean arterial blood pressure (P_a) than MNS rats (129 vs. 101 mmHg) both after VD and after subsequent VE. No difference in glomerular filtration rate (GFR) was found. Both strains had a low urine flow rate (1.5 μl min^-1) during VD, which increased fourfold after VE. The interstitium was significantly more dehydrated in MHS, as indicated by a more negative net interstitial pressure (P_int–±_int t than in MNS (-1.3 ± 0.3 vs. ± 0.0 ± 0.5 mmHg) after VE. The TGF mechanism was more activated in MHS during volume depletion, as indicated by a larger drop in stop-flow pressure (P_sf) in response to loop of Henle perfusion (7.1 ± 0.7 vs. 4.7 ± 0.2 mmHg, P < 0.05). However, during VD the loop of Henle flow that elicited half maximal response in P_sf, the turning point (TP), was equally low in MHS and MNS (13.5 ± 0.6 and 14.3 ± 0.4, respectively). After VE, however, TP increased significantly more in MNS to (32.6 ± 2.1 nl min^-1) then in MHS (to 21.8 ± 0.9 nl min^-1, P < 0.05). It is concluded that the blunting of the TGF resetting in response to VE in MHS rats may well be of importance in the development of hypertension in the MHS strain.     

Pretreatment with insulin before ischaemia reduces infarct size in Langendorff‐perfused rat hearts

Aim: To compare the possible role of Akt and mammalian target of rapamycin (mTOR) in mediating cardioprotection against ischaemia under three different conditions: (1) During ischaemic preconditioning (IPC), (2) when insulin was given as a pretreatment agent (InsPC) and (3) when insulin was given as a reperfusion cell survival agent (Ins_R). Methods: Isolated perfused rat hearts were subjected to IPC (3 × 5 min) or InsPC (50 mU mL^?1; 3 × 5 min), before 30 min of regional ischaemia followed by 120 min of reperfusion ± 1L‐6‐hydroxymethyl‐chiro‐inositol‐2‐[(R)‐2‐O‐methyl‐3‐O‐octadecylcarbonate] (HIMO) (20 μm ; Akt inhibitor) or rapamycin (1 nm ; mTOR inhibitor). In addition, insulin (3 mU mL^?1) was given at the onset of reperfusion, ±HIMO or rapamycin. Risk zone (R) and infarct size (I) were determined with Evans blue and tetrazolium staining respectively. Western blot analysis was performed on tissue from Langendorff‐perfused rat hearts and cell lysates from cultured HL1 cells. Results: IPC, InsPC and Ins_R treatment resulted in a significant reduction in infarct size compared to controls (all P < 0.05). This protective effect of IPC and insulin was abolished by the inhibitors. However, the putative Akt inhibitor, although capable of abolishing cardioprotection induced by insulin, was not able to inhibit insulin‐induced phosphorylation of Akt in Langendorff‐perfused rat hearts and cultured HL1 cells. The target for this compound therefore remains to be determined. Conclusion: IPC and insulin (either as InsPC or Ins_R) appear to activate mTOR, and this kinase seems to play an essential role in cardioprotection against ischaemia and reperfusion injury as rapamycin blocked the protection.     

Effect of fetal anaemia on myocardial ischaemia-reperfusion injury and coronary vasoreactivity in adult sheep

Aims: We investigated whether chronic fetal anaemia affects myocardial infarct in adulthood and elicits functional modifications in adult coronary vasoreactivity. Methods: Seven‐month‐old sheep that were made anaemic in utero and transfused to normal haematocrit before birth were studied. Infarct size was determined by tetrazolium after 1‐h ischaemia (occlusion of the mid of left anterior descending artery) and 2‐h reperfusion. The dose–response to vasoconstrictors and vasodilators was assessed in small resistance coronary arteries. Results: There were no significant differences between the animals previously subjected to in utero anaemia and the control animals regarding the percentage infarct size and the area‐at‐risk to the left ventricle. The ventricular function (dP/dt) was preserved. The percentage infarct size of the area‐at‐risk (70.7 ± 3.5%) was larger than that in the controls (49.8 ± 4.5%) (P = 0.006). The vascular responses were not altered. Endothelium‐dependent relaxation to bradykinin (96.0 ± 2.6% vs. 98.8 ± 1.0%) was not affected by PGI₂ inhibitor (94.6 ± 2.6% vs. 98.5 ± 1.0%) but significantly reduced by the inhibition of nitric oxide (NO) in both anaemic (P < 0.05) and control (P < 0.001) groups with a significant right shift of EC₅₀ (P < 0.01). The non‐NO–non‐PGI₂‐mediated relaxation was slightly potentiated in anaemic animals. Conclusions: Exposing fetal sheep to in utero anaemia in late gestation for 3 weeks may increase the susceptibility of adult hearts to ischaemia–reperfusion injury without major alterations in coronary vasomotor responsiveness. The impact of in utero anaemia at earlier period of pregnancy and on the earlier or later life of the adult is yet to be further investigated.     

Role of l-arginine in preventing myocardial and endothelial injury following ischaemia/reperfusion in the rat isolated heart

The protective effect of l -arginine on ischaemia/reperfusion-induced myocardial injury was investigated in the rat isolated Langendorff perfused heart. Six groups of hearts subjected to 30 min global ischaemia and 30 min reperfusion received either vehicle, d -arginine, l -arginine, the nitric oxide (NO)-donor S-Nitroso-N-Acetyl-d, l -Penicillamine (SNAP), the inhibitor of NO formation N^G-nitro-l -arginine (l -NNA), or l -arginine plus l -NNA. The recoveries of left ventricular double product and coronary flow at the end of reperfusion were significantly higher in the l -arginine group (85±5 and 75±6%, respectively) than in the vehicle group (37±6 and 34±5%, respectively, P<0.05). During both the ischaemic and reperfusion periods, left ventricular end diastolic pressure was lower in the l -arginine group than in the vehicle group. Creatine kinase outflow and the area of no-reflow were smaller in the l -arginine treated hearts (P<0.01). There were no differences between vehicle and d -arginine treated groups. l -NNA did not affect recovery per se but abolished the protective actions of l -arginine. SNAP produced the same protective effects as l -arginine. Acetylcholine-induced endothelium-dependent vasodilation was reduced after ischaemia and reperfusion in the vehicle group but not in the l -arginine group. It is concluded that l -arginine reduces ischaemia/reperfusion-induced myocardial and endothelial injury. The results suggest that the beneficial effects of l -arginine are related to preserved synthesis and release of NO.     

Static autoregulation in humans: a review and reanalysis

IntroductionCerebral autoregulation (CA) is a theoretical construct characterized by the relationship between mean arterial pressure (MAP) and cerebral blood flow (CBF). We performed a comprehensive literature search to provide an up-to-date review on the static relationship between MAP and CBF.MethodsThe results are based on 40 studies (49 individual experimental protocols) in healthy subjects between 18 and 65 years. Exclusion criteria were: a ΔMAP <5%, hypoxia/hyperoxia or hypo/hypercapnia, and unstable levels (<2 min stages). The partial pressure of arterial CO₂ (PaCO₂) was measured in a subset of the included studies (n = 28); therefore, CBF was also adjusted to account for small changes in PaCO₂.ResultsThe linear regression coefficient between MAP and CBF (or velocity) of 0.82 ± 0.77%ΔCBF/%ΔMAP during decreases in MAP (n = 23 experiments) was significantly different than the relationship of 0.21 ± 0.47%ΔCBF/%ΔMAP during increases (n = 26 experiments; p < 0.001). After correction for increases/decreases in PaCO₂, the slopes were not significantly different: 0.64 ± 1.16%ΔCBF/%ΔMAP (n = 16) and 0.39 ± 0.30%ΔCBF/%ΔMAP (n = 12) for increased vs. decreased MAP changes, respectively (p = 0.60).ConclusionThe autoregulatory ability of the cerebral circulation appears to be more active in buffering increases in MAP as compared to reductions in MAP. However, the statistical finding of hysteresis is lost following an attempt to correct for PaCO₂.     

Cerebrovascular responses to haemorrhagic hypotension in anaesthetized cats. Effects of α-adrenoceptor antagonists

Haemorrhagic hypotension induces the phenomenon of cerebrovascular autoregulation and, concomitantly, involves an activation of the sympathetic nervous system. As brain vessels in cats have an atypical adrenoceptor distribution we studied the effects of an a-adrenoceptor antagonist on the autoregulatory response to haemorrhage. Cortical blood flow was studied by the H₂ technique in chloralose-anaesthetized cats subjected to a period of graded haemorrhage over 3 h. Three groups of cats were studied: control, i.e. those receiving saline (n= 10); yohimbine-treated (200/μg^-kg^-1 h^-1, n= 7); and prazosin-treated (50μg-kg^-1 h^-1, n= 6). In the control group, cortical blood flow remained relatively constant when mean arterial pressure was decreased from 102±1 mmHg (mean ± SE) to approximately 50.1 mmHg; thereafter, blood flow decreased with decreasing perfusion pressure. In the arterial pressure range 64-55 mmHg, cortical blood flow was significantly higher in the yohimbine group (109±12 ml-100 g^-1 min^-1) compared to the control group (69 ±6 ml-100 g^-1 min^-1) and remained higher in the yohimbine-treated cats at more extreme levels of hypotension. Blood flow did not fall significantly in the yohimbine-treated cats until mean arterial pressures of 31 ± 1 mmHg were attained. In the prazosin-treated cats, flow began to decrease at arterial pressures even greater than those observed in the control group. Thus, there is a sympathetic vasoconstriction of brain arteries that is primarily mediated by α₂-adrenoceptors in the feline cerebrovascular bed.