Non‐invasive imaging of coronary arteries in living mice using high‐resolution echocardiography

Comparative assessment of coronary physiology using transthoracic pulsed-wave Doppler and myocardial contrast echocardiography in rats

BACKGROUND

Coronary physiology assessment in rodents by ultrasound is an excellent noninvasive and easy to perform technique, including pulsed-wave Doppler (PWD) and myocardial contrast echocardiography (MCE). Both techniques and the corresponding calculated parameters were investigated in this study at rest as well as their response to pharmacologically induced stress.

METHODS

Left ventricular myocardial function was assessed in eight anaesthetised rats using transthoracic echocardiography. Coronary physiology was assessed by both PWD of the left coronary artery and MCE using a bolus method. Measurements were performed at rest and under stimulation with adenosine and dobutamine. Effects of stimulation on the calculated parameters were evaluated and rated by effect size (η²).

RESULTS

Changes could be demonstrated by selected parameters of PWD and MCE. The clearest effect in PWD was found for diastolic peak velocity (η² = 0.58). It increased from 528 ± 110 mm/s (mean ± standard deviation) at rest to 839 ± 342 mm/s (p = 0.001) with adenosine and 1093 ± 302 mm/s with dobutamine (p = 0.001). The most distinct effect from MCE was found for the normalised wash-in rate (η² = 0.58). It increased from 1.95 ± 0.35% at rest to 3.87 ± 0.85% with adenosine (p = 0.001) and 3.72 ± 1.03% with dobutamine (p = 0.001).

CONCLUSION

Induced changes in coronary physiology by adenosine and dobutamine could successfully be monitored using MCE and PWD in anaesthetised rats. Due to the low invasiveness of the measurements, this protocol could be used for longitudinal animal studies.

Coronary micro-computed tomography angiography in mice

Coronary computed tomography angiography is an established technique in clinical practice and a valuable tool in the diagnosis of coronary artery disease in humans. Imaging of coronaries in preclinical research, i.e. in small animals, is very difficult due to the high demands on spatial and temporal resolution. Mice exhibit heart rates of up to 600 beats per minute motivating the need for highest detector framerates while the coronaries show diameters below 100 μm indicating the requirement for highest spatial resolution. We herein use a custom built micro-CT equipped with dedicated reconstruction algorithms to illustrate that coronary imaging in mice is possible. The scanner provides a spatial and temporal resolution sufficient for imaging of smallest, moving anatomical structures and the dedicated reconstruction algorithms reduced radiation dose to less than 1 Gy but do not yet allow for longitudinal studies. Imaging studies were performed in ten mice administered with a blood-pool contrast agent. Results show that the course of the left coronary artery can be visualized in all mice and all major branches can be identified for the first time using micro-CT. This reduces the gap in cardiac imaging between clinical practice and preclinical research.

Time Course of Isoflurane-Induced Vasodilation: A Doppler Ultrasound Study of the Left Coronary Artery in Mice

Isoflurane is widely used as vasodilator in studies of coronary flow reserve (CFR) in small animals, but the protocols have not been standardized. This study assessed the time course of the increase in isoflurane-induced flow in the mouse coronary artery by pulsed-wave Doppler measurements at 1% isoflurane concentration maintained for 6 min and then increased to 2.5% for 30 min. Velocity-time integral and velocity peak values were best fitted by the sigmoid model, which allowed derivation of the mean time (Tt90 = 14 min) of high-isoflurane needed to reach 90% of the hyperemic plateau value. In subsequent experiments, CFR was measured at 4 min (mean time of literature data) and 14 min of hyperemic response. The 4-min CFR was significantly lower than the 14 -min CFR, and the Bland-Altman plot revealed significant bias of the 4-min CFR against the 14-min CFR. This result suggests that measurements of flow velocity at times shorter than 14 min may be inappropriate for expressing the effective value of CFR.

[18F]fluorodeoxyglucose uptake in atherosclerotic plaques is associated with reduced coronary flow reserve in mice

OBJECTIVES

Coronary microvascular dysfunction, observed as impaired coronary vasodilator capacity, is an early manifestation of coronary artery disease. Inflammation plays an important role in different stages of atherogenesis. To study the role of vessel wall inflammation in the development of coronary dysfunction, we compared [(18)F]fluorodeoxyglucose (FDG) uptake in the aorta and coronary flow reserve (CFR) in atherosclerotic mice.

METHODS

We studied healthy young C57BL/6 mice fed a normal diet (n = 7) as well as hypercholesterolemic low-density lipoprotein receptor-disrupted/apolipoprotein B100-expressing (LDLR(-/-)ApoB(100/100)) mice (n = 15) and hypercholesterolemic and diabetic LDLR(-/-)ApoB(100/100)insulinlike growth factor II-overexpressing mice (n = 14) fed a western-type diet, aged 4 to 6 months. Doppler sonography was used to measure CFR as the ratio of coronary flow velocity during isoflurane-induced hyperemia and at rest. Uptake of [(18)F]FDG into the aorta was measured by autoradiography of tissue sections.

RESULTS

Histologic sections showed extensive atherosclerosis in the aorta, but coronary arteries were not obstructed. Both hyperemic coronary flow velocity and CFR were reduced (P < .05) in hypercholesterolemic mice with and without diabetes in comparison to healthy young C57BL/6 controls. Among hypercholesterolemic mice, both hyperemic flow velocity and CFR inversely correlated with atherosclerotic plaque [(18)F]FDG uptake in the aorta (r = -0.73; P < .001; r = -0.63; P = .001, respectively). In a multivariate analysis, including animal weight, aortic plaque burden, plasma glucose, plasma cholesterol, and [(18)F]FDG uptake in atherosclerotic plaques, only [(18)F]FDG uptake remained an independent predictor of reduced CFR (β = 0.736; P = .001).

CONCLUSIONS

The inflammatory activity in atherosclerotic plaques of the aorta independently predicts reduced CFR in atherosclerotic mice without obstructive coronary artery disease. This finding suggests that atherosclerotic inflammation contributes to coronary dysfunction.

Evaluation of bi-ventricular coronary flow patterns using high-frequency ultrasound in mice with transverse aortic constriction

Using high-frequency color and pulsed Doppler ultrasound, we evaluated the flow patterns of the left (LCA), septal (SCA) and right (RCA) coronary arteries in mice with and without transverse aortic constriction (TAC). Fifty-two male C57BL/6J mice were subjected to TAC or a corresponding sham operation. At 2 and 8 wk post-surgery, Doppler flow spectra from the three coronary arteries, together with morphologic and functional parameters of the left and right ventricles, were measured. Histology was performed to evaluate myocyte size and neo-angiogenesis in both ventricles. In sham-operated mice, the LCA and SCA both exhibited low-flow waveforms during systole and dominantly higher-flow waveforms during diastole. The RCA exhibited generally lower flow velocity, with similar systolic and diastolic waveforms. TAC significantly increased the systolic flow velocities of all coronary arteries, but enhanced the flow mainly in the LCA and SCA. In the left ventricle, coronary flow reserve was partially preserved 2 wk post-TAC, but decreased at 8 wk, consistent with changes in neo-angiogenesis and systolic function. In contrast, no significant change was found in the coronary flow reserve, structure or function of the right ventricle. This study has established a protocol for evaluating the flow pattern in three principal coronary arteries in mice using Doppler ultrasound and illustrated the difference among three vessels at baseline. In mice with TAC, the difference in the associating pattern of coronary flow dynamics with the myocardial structure and function between the left and right ventricles provides further insights into ventricular remodeling under pressure overload.

Coronary flow reserve from mouse to man--from mechanistic understanding to future interventions

Myocardial ischemia is recognized as an important mechanism increasing the risk for cardiovascular events in both symptomatic and asymptomatic patients. In addition to obstructive coronary diseases, systemic inflammation, macro- and microvascular function are additional important mechanisms contributing to the ischemic myocardium. Accumulating evidence indicates that coronary flow reserve (CFR) is a quantitative measurement of ischemia including integrated information on structure and function of the coronary artery at all levels. Not surprisingly, CFR has been shown to confer strong prognostic value for hard cardiovascular (CV) events in a number of relevant patient cohorts. Using high-resolution imaging, it is now possible to study coronary arteries from mouse to man. Therefore, CFR may be an important translational tool to risk-stratify patients and to perform both preclinical and clinical proof-of-concept studies before investing in large-scale outcome trials, thus improving the translational value for novel CV targets.

Doppler velocity measurements from large and small arteries of mice

With the growth of genetic engineering, mice have become increasingly common as models of human diseases, and this has stimulated the development of techniques to assess the murine cardiovascular system. Our group has developed nonimaging and dedicated Doppler techniques for measuring blood velocity in the large and small peripheral arteries of anesthetized mice. We translated technology originally designed for human vessels for use in smaller mouse vessels at higher heart rates by using higher ultrasonic frequencies, smaller transducers, and higher-speed signal processing. With these methods one can measure cardiac filling and ejection velocities, velocity pulse arrival times for determining pulse wave velocity, peripheral blood velocity and vessel wall motion waveforms, jet velocities for the calculation of the pressure drop across stenoses, and left main coronary velocity for the estimation of coronary flow reserve. These noninvasive methods are convenient and easy to apply, but care must be taken in interpreting measurements due to Doppler sample volume size and angle of incidence. Doppler methods have been used to characterize and evaluate numerous cardiovascular phenotypes in mice and have been particularly useful in evaluating the cardiac and vascular remodeling that occur following transverse aortic constriction. Although duplex ultrasonic echo-Doppler instruments are being applied to mice, dedicated Doppler systems are more suitable for some applications. The magnitudes and waveforms of blood velocities from both cardiac and peripheral sites are similar in mice and humans, such that much of what is learned using Doppler technology in mice may be translated back to humans.

In vivo MR angiography and velocity measurement in mice coronary arteries at 9.4 T: assessment of coronary flow velocity reserve

PURPOSE

To demonstrate the feasibility of coronary magnetic resonance (MR) angiography in living mice and to evaluate a dynamic MR angiographic method for coronary flow measurement at 9.4-T field strength.

MATERIALS AND METHODS

This study was conducted according to European law and was in full compliance with National Institutes of Health recommendations for animal care and a local institutional animal care committee. Mice were anesthetized by using isoflurane. First, time-of-flight MR angiography was performed in 10 mice to measure coronary diameters at 80-mum isotropic resolution. Second, left coronary artery (LCA) velocity measurements were performed at seven cardiac phases in nine other mice to assess the velocity curve profile. Third, coronary velocities were measured at the middiastolic phase in 13 mice at rest and during adenosine-induced hyperemia to calculate coronary flow velocity reserve (CFVR). The Pearson coefficient compared the correlation between isoflurane dose and CFVR. Paired t tests compared R-R intervals and respiratory rates between rest and hyperemia.

RESULTS

Proximal diameters were, respectively, 404 mum +/- 34 [standard deviation] and 259 mum +/- 22 for the LCAs and the right coronary arteries, which were in accordance with reported values. The velocity curve profile throughout the cardiac cycle was similar to values from the literature. Baseline and hyperemic velocities were, respectively, 19.0 cm/sec +/- 4.4 and 33.7 cm/sec +/- 4.7 (P<.001), resulting in a CFVR of 1.77 +/- 0.19. CFVR did not correlate with isoflurane dose (r = 0.05, P = .88). R-R intervals shortened by 2.5% during hyperemia (P = .04). Respiratory rates showed no difference between rest and hyperemia (P = .39).

CONCLUSION

High-spatial-resolution three-dimensional coronary MR angiography is feasible in living mice. Dynamic MR angiography depicts coronary velocity changes throughout the cardiac cycle and between rest and maximum hyperemia, providing a tool for CFVR assessment.

Protein disulfide isomerase increases in myocardial endothelial cells in mice exposed to chronic hypoxia: a stimulatory role in angiogenesis

Previous studies have shown that exposure to chronic hypoxia protects against myocardial infarction, but little is known about the cellular and molecular mechanisms involved. Here we observed that chronic hypoxia for 3 wk resulted in improved survival of mice (from 64% to 83%), reduced infarction size (from 45 +/- 4% to 32 +/- 4%, P < 0.05), increased cardiac ejection fraction (from 19 +/- 4% to 35 +/- 5%, P < 0.05), coronary flow velocity under adenosine-induced hyperemia (from 58 +/- 2 to 75 +/- 5 cm/s, P < 0.05), myocardial capillary density (from 3,772 +/- 162 to 4,760 +/- 197 capillaries/mm(2), P < 0.01), and arteriolar density (from 8.04 +/- 0.76 to 10.34 +/- 0.69 arterioles/mm(2), P < 0.05) 3 wk after myocardial infarction. With two-dimensional gel electrophoresis, we identified that protein disulfide isomerase (PDI) was highly upregulated in hypoxic myocardial capillary endothelial cells. The loss of PDI function in endothelial cells by small interfering RNA significantly increased the number of apoptotic cells (by 3.4-fold at hypoxia, P < 0.01) and reduced migration (by 52% at hypoxia, P < 0.001) and adhesion to collagen I (by 42% at hypoxia, P < 0.01). In addition, the specific inhibition of PDI by PDI small interfering RNA (by 46%, P < 0.01) and bacitracin (by 72%, P < 0.001) reduced the formation of tubular structures by endothelial cells. Our data indicate that chronic hypoxic exposure improves coronary blood flow and protects the myocardium against infarction. These beneficial effects may be partly explained by the increased endothelial expression of PDI, which protects cells against apoptosis and increases cellular migration, adhesion, and tubular formation. The increased PDI expression in endothelial cells may be a novel mechanism to protect the myocardium against myocardial ischemic diseases.

In vivo quantification of blood velocity in mouse carotid and pulmonary arteries by ECG-triggered 3D time-resolved magnetic resonance angiography

Blood flow velocity is a functional parameter of fundamental importance in diagnosis and follow-up of various vascular diseases. Vascular pathologies can be efficiently studied in animal models, especially in small rodents. ECG-gated magnetic resonance imaging (MRI) assessment of blood velocity in small animals is a challenge because of limited spatial resolution and high-frequency physiological parameters. Here it is shown that a bright-blood cine-3D-MRI method can be used to measure blood velocity at specific times of the cardiac cycle in mouse pulmonary and carotid arteries. The method used a series of time-of-flight (TOF) acquisitions in a volume of interest at different times after signal cancellation in the same volume. This scheme was repeated at different periods of the cardiac cycle by varying the delay between the ECG R-wave peak and signal cancellation. Velocity values in mouse pulmonary artery varied from 35 cm/s in systole to 0-10 cm/s in diastole. A similar pattern was displayed in carotid arteries (18 and 2.5 cm/s, in systole and diastole, respectively). Results are discussed in terms of efficiency, limitation, and comparison with other methods.

Adenosine induces dilation of epicardial coronary arteries in mice: relationship between coronary flow velocity reserve and coronary flow reserve in vivo using transthoracic echocardiography

For an accurate estimate of volumetric coronary flow reserve (CFR) using Doppler-assessed flow velocity measurement, it is important to take into consideration potential diameter change during coronary hyperemia. Using ultrasound techniques, left coronary artery (LCA) flow velocity and LCA lumen diameter (LCA(D)) were measured simultaneously for the first time to measure coronary flow during baseline and adenosine-induced hyperemic condition in isoflurane-anesthetized C57BL/6 (n = 38) and in old apolipoprotein E-gene deficient (ApoE(-/-)) mice (n = 44) mice. LCA(D) increased significantly and to a similar extent during adenosine infusion in both groups (3.7 +/- 1.1 %, p < 0.003 for C57BL/6; 4.2 +/- 0.9 %, p < 0.00003 for ApoE(-/-)). Yet, a positive correlation was still found between velocity-based coronary flow velocity reserve (CFVR) and volumetric CFR in both strains (R(2) = 0.77, p < 0.001 for C57BL/6; R(2) = 0.80, p < 0.001 for ApoE(-/-)). Coronary reserve was higher in C57BL/6 mice than in ApoE(-/-) mice (CFR 1.93 +/- 0.17 vs. 1.47 +/- 0.07, p < 0.05; CFVR 1.73 +/- 0.13 vs. 1.28 +/- 0.07, p < 0.01). Thus, ultrasound techniques can be used to measure volumetric flow in the LCA and flow-based CFR measurements of intact, living mice. The positive correlation between CFR and CFVR, together with the lower method variability of the latter, makes CFVR a more robust protocol for assessing mouse in-vivo coronary artery function. Therefore, the CFVR protocol will probably work well in most settings.

Free-hand ultrasound scanning approaches for volume quantification of the mouse heart Left ventricle

Two approaches for free-hand motion tracking that enable volumetric quantification of the murine heart were investigated. One approach used an instrumented, multijointed articulated arm attached to a 14 MHz ultrasound transducer array. A second approach used an E-beam transducer--a modified linear transducer array containing a main imaging array adjacent to three perpendicular tracking arrays. Motion between successive B-mode image frames was computed by tracking image speckle in each tracking array. Both tracking systems produced accurate results in a phantom validation study (4.50% error and 3.75% error for estimates derived using the articulated arm and E-beam tracking techniques, respectively). The tracking approaches also were tested in vivo on three mice. Results were compared to values obtained by mounting each mouse on a micromanipulator, adjusting its position by 0.5-mm increments, and acquiring B-mode images using a high-resolution ultrasound scanner. Left ventricular end diastolic volume (LVEDV) estimates differed from values obtained using the high-resolution scanner by a mean error of 18.2% and 2.60% for eight scans conducted on each of two mice using the articulated arm, and a mean error of 13.6%, 6.53%, and 12.58% for eight scans conducted on each of three mice using the E-beam.

Effects of isoflurane on coronary blood flow velocity in young, old and ApoE(-/-) mice measured by Doppler ultrasound

The commonly used anesthetic agent isoflurane (ISO) is a potent coronary vasodilator that could potentially be used in the assessment of coronary reserve, but its effects on coronary blood flow in mice are unknown. Coronary reserve is reduced by age, coronary artery disease and other cardiac pathologies in man, and some of these conditions can now be modeled in mice. Accordingly, we used Doppler ultrasound to measure coronary flow velocity in mice anesthetized with low (1%) and high (2.5%) levels of ISO to generate baseline (B) and elevated hyperemic (H) coronary flows, respectively. A 20-MHz Doppler probe was mounted in a micromanipulator and pointed trans-thoracically toward the origin of the left main coronary arteries of 10 6-wk (Young [Y]), 10 2-y (Old [O]) and 20 2-y apolipoprotein-E null (ApoE(-/-)) atherosclerotic (A) mice. In each mouse, we measured (B) and (H) peak diastolic velocities. B was 35.4 +/- 1.4 cm/s (Y), 24.8 +/- 1.6 (O) and 51.7 +/- 6.4 (A); H was 83.5 +/- 1.3 (Y), 86.5 +/- 1.9 (O) and 120 +/- 16.9 (A) and H/B was 2.4 +/- 0.1 (Y), 3.6 +/- 0.2 (O) and 2.5 +/- 0.2 (A). The differences in baseline velocities and H/B between O and Y and between A and O were significant (p < 0.01), whereas the differences in hyperemic velocities were not (p > 0.05). H/B was higher in old mice as a result of decreased baseline flow rather than increased hyperemic flow velocity. In contrast, ApoE(-/-) mice have increased baseline and hyperemic velocities, perhaps because of coronary lesions. The differences in baseline velocities between young and old mice could be the result of age-related changes in basal metabolism or to differential sensitivity to isoflurane. We conclude that Doppler ultrasound combined with coronary vasodilation via isoflurane could provide a convenient and noninvasive method to estimate coronary reserve in mice, but also that care must be taken when assessing coronary flow in mice under isoflurane anesthesia because of its potent coronary vasodilator properties.

Proximal to Middle Left Coronary Artery Flow Velocity Ratio, As Assessed Using Color Doppler Echocardiography, Predicts Coronary Artery Atherosclerosis in Mice

Background— We aimed to establish a completely noninvasive technique to assess coronary artery atherosclerosis in living mice using proximal to middle left coronary artery (LCA) velocity ratio as assessed with color Doppler echocardiography (CDE).

Methods and Results— Three groups of apolipoprotein E/low-density lipoprotein receptor double-knockout (apoE/LDLr dko) mice 10, 40, and 80 weeks of age and 3 additional age-matched groups of C57BL/6 mice were examined under anesthesia. Coronary flow velocity in proximal (V prox ) and middle part (V mid ) of LCA was measured using CDE. A 40-MHz ultrasound biomicroscope (UBM) was used to visualize lumen and outer vessel diameter in the proximal LCA. Flow velocity in the proximal LCA increased significantly with age and remained constant in the middle part in the apoE/LDLr dko mice, whereas velocities at both the sites remained unchanged in C57 mice. CDE-assessed flow velocity ratio (V prox /V mid ) increased significantly with age in apoE/LDLr dko mice ( P =0.0055) and correlated significantly to percentage wall thickness, as assessed by UBM ( P =0.0044; r =0.65) and histology ( P =0.0002; r =0.78). Wall thickness increased with age in the apoE/LDLr dko mice as measured with UBM ( P =0.0093; r =0.49), which was also confirmed with histology ( P <0.0001; r =0.73).

Conclusions— CDE and UBM are useful noninvasive tools to quantify mouse coronary artery atherosclerosis in vivo.

Coronary flow reserve and heart failure in experimental coxsackievirus myocarditis. A transthoracic Doppler echocardiography study

The objective of this study was to apply transthoracic Doppler echocardiography (TTDE) in mice to study coronary flow reserve (CFR), an index of coronary microvascular function, in mild and severe forms of experimental viral myocarditis. Regarding methodology, BALB/c mice were infected with cardiotropic coxsackieviruses causing either a mild (Nancy strain) or a severe (Woodruff strain) myocarditis. Left ventricular dimensions, fractional shortening, and CFR (ratio of left coronary artery flow velocity during maximal adenosine-induced vasodilatation to rest) were measured by TTDE before infection and again 1 or 2 wk after infection. As a result, the resting flow velocity did not change after infection. In contrast, CFR reduced significantly 1 wk after infection with either virus variant [from 2.5 (SD 0.3) to 1.4 (SD 0.1) in severe and from 2.4 (SD 0.4) to 2.1 (SD 0.3) in mild myocarditis], being significantly lower in the severe than mild myocarditis. CFR remained low in severe myocarditis 2 wk after infection. Fractional shortening decreased to the same levels 1 wk after infection with either virus variant [from 0.54 (SD 0.02) to 0.43 (SD 0.03) in severe and from 0.51 (SD 0.03) to 0.44 (SD 0.02) in mild myocarditis, P < 0.05]. However, 2 wk after infection, mice with severe myocarditis had enlarged left ventricles and lower fractional shortening [0.31 (SD 0.03)] than mice with mild myocarditis [0.47 (SD 0.02), P < 0.01]. In conclusion, CFR measured with TTDE is reduced in coxsackievirus myocarditis in mice. Low CFR is associated with progressive heart failure, indicating that dysfunction of coronary microcirculation is a determinant of poor outcome in viral myocarditis.

Voluntary physical exercise and coronary flow velocity reserve: a transthoracic colour Doppler echocardiography study in spontaneously hypertensive rats

In the present study, we have developed and demonstrated a coronary artery imaging protocol in rats using transthoracic high-frequency CDE (colour Doppler echocardiography) to investigate the potential direct effects of exercise on CFVR (coronary flow velocity reserve). SHR (spontaneously hypertensive rats) performed voluntary exercise for 6 weeks. Rats were then submitted to ultrasonographic examination and CFVR measurements. The LAD (left anterior descending coronary artery) was visualized using transthoracic CDE in a modified parasternal long-axis view. Doppler measurement was made in mid-LAD during baseline and adenosine-induced hyperaemic condition. Gene and protein expression in cardiac tissue were studied using real-time PCR and immunohistochemistry. Adenosine infusion significantly (P<0.001, as determined by ANOVA) decreased HR, without affecting blood pressure in anaesthetized SHR. A significantly greater adenosine dose-dependent response was seen in exercised rats compared with controls (P=0.02, as determined by ANOVA). The baseline flow velocity in mid-LAD was 0.33±0.06 and 0.41±0.14 m/s in the exercised and control animals respectively (P value was not significant). The maximum adenosine-induced response was reached at a dose of 140 μg·kg−1 of body weight·min−1, and CFVR averaged at 2.6±0.53 and 1.5±0.24 in exercised and control animals respectively (P<0.01). Gene expression of CuZnSOD was up-regulated by 21% in exercised animals compared with controls (1.1±0.16 compared with 0.89±0.09; P<0.01), whereas eNOS expression was unchanged. In conclusion, CFVR in rats can be non-invasively assessed using CDE with high feasibility. Physical exercise is associated with improved CFVR and antioxidative capacity in SHR.

Functional and Morphologic Imaging of Coronary Atherosclerosis in Living Mice Using High-Resolution Color Doppler Echocardiography and Ultrasound Biomicroscopy

OBJECTIVES

This study aimed to establish non-invasive methods of assessing coronary artery morphology in normal and atherosclerotic mice in vivo.

BACKGROUND

Coronary flow velocity reserve (CFVR) has been shown to correlate with coronary minimal lumen diameter (MLD) in patients with coronary artery stenosis. In mice, there are no existing non-invasive imaging techniques allowing quantitative measurement of the coronary artery morphology and function.

METHODS

Systemic hemodynamic effects of adenosine were studied in seven C57BL/6 mice. In 17 C57BL/6 mice, CFVR was measured in the mid left coronary artery (LCA) using either hypoxia- or adenosine-induced coronary hyperemia. Further, in another 10 atherosclerotic low-density lipoprotein receptor (LDLR)-/- mice, the hypoxia-induced CFVR was performed and proximal LCA MLD was measured using ultrasound biomicroscopy (UBM). Histologic sections of the LCA were collected.

RESULTS

The adenosine dose of 160 microg/kg/min induced maximal coronary hyperemia without any systemic hemodynamic effects. Adenosine and hypoxia-induced CFVR values averaged at 2.0 +/- 0.1 and 1.9 +/- 0.3, respectively, in C57BL/6 mice (p = NS). In LDLR-/- mice, CFVR and MLD ranged between 1.4 to 2.9 microm and 190 to 370 microm, respectively. Histology revealed proximal lumen-narrowing plaques in the LCA. Significant correlation was found between hypoxia-induced CFVR and the MLD (p < 0.005, R2 = 0.8707).

CONCLUSIONS

The CDE and UBM technique can be used to measure atherosclerosis-related lumen narrowing of the LCA in living mice. These non-invasive techniques may provide us with novel tools for following up disease status in mouse coronary arteries in a quantitative manner.

Voluntary physical exercise-induced vascular effects in spontaneously hypertensive rats

Forced training has been shown to have beneficial vascular effects in various animal exercise models. In the present study, we explored possible physiological and molecular effects of voluntary physical exercise on various vascular beds. SHR (spontaneously hypertensive rats) performed voluntary exercise for 5 weeks in a computerized wheel cage facility. Ex vivo myograph studies revealed an increased sensitivity of the ACh (acetylcholine)-mediated vasodilation in resistance arteries of the exercised animals (ED50=15.0+/-3.5 nmol/l) compared with the controls (ED50=37.0+/-8.8 nmol/l; P=0.05). The exercise/control difference was abolished after scavenging reactive oxygen radicals. In conduit arteries, ACh induced a similar vasodilatory response in both groups. The in vivo aortic wall stiffness, assessed by means of Doppler tissue echography, was significantly lower in the exercising animals than in controls. This was demonstrated by significantly increased peak systolic aortic wall velocity (P=0.03) and the velocity time integral (P=0.01) in exercising animals compared with controls. The relative gene expression of eNOS (endothelial nitric oxide synthase) was similar in both groups of animals, whereas Cu/ZnSOD (copper/zinc superoxide dismutase) gene expression was significantly increased (+111%; P=0.0007) in the exercising animal compared with controls. In conclusion, voluntary physical exercise differentially improves vascular function in various vascular beds. Increased vascular compliance and antioxidative capacity may contribute to the atheroprotective effects associated with physical exercise in conduit vessels.