MR angiography of collateral arteries in a hind limb ischemia model: comparison between blood pool agent Gadomer and small contrast agent Gd-DTPA.

The objective of this study was to compare the blood pool agent Gadomer with a small contrast agent for the visualization of ultra-small, collateral arteries (diameter<1 mm) with high resolution steady-state MR angiography (SS-MRA) in a rabbit hind limb ischemia model. Ten rabbits underwent unilateral femoral artery ligation. On days 14 and 21, high resolution SS-MRA (voxel size 0.49×0.49×0.50 mm(3)) was performed on a 3 Tesla clinical system after administration of either Gadomer (dose: 0.10 mmol/kg) or a small contrast agent (gadopentetate dimeglumine (Gd-DTPA), dose: 0.20 mmol/kg). All animals received both contrast agents on separate days. Selective intra-arterial x-ray angiograms (XRAs) were obtained in the ligated limb as a reference. The number of collaterals was counted by two independent observers. Image quality was evaluated with the contrast-to-noise ratio (CNR) in the femoral artery and collateral arteries. CNR for Gadomer was higher in both the femoral artery (Gadomer: 73±5 (mean ± SE); Gd-DTPA: 40±3; p<0.01) and collateral arteries (Gadomer: 18±4; Gd-DTPA: 9±1; p = 0.04). Neither day of acquisition nor contrast agent used influenced the number of identified collateral arteries (p = 0.30 and p = 0.14, respectively). An average of 4.5±1.0 (day 14, mean ± SD) and 5.3±1.2 (day 21) collaterals was found, which was comparable to XRA (5.6±1.7, averaged over days 14 and 21; p>0.10). Inter-observer variation was 24% and 18% for Gadomer and Gd-DTPA, respectively. In conclusion, blood pool agent Gadomer improved vessel conspicuity compared to Gd-DTPA. Steady-state MRA can be considered as an excellent non-invasive alternative to intra-arterial XRA for the visualization of ultra-small collateral arteries.

Optimized pharmacokinetic modeling for the detection of perfusion differences in skeletal muscle with DCE-MRI: effect of contrast agent size.

PURPOSE: The goal of this study was to optimize dynamic contrast-enhanced (DCE)-MRI analysis for differently sized contrast agents and to evaluate the sensitivity for microvascular differences in skeletal muscle. METHODS: In rabbits, pathophysiological perfusion differences between hind limbs were induced by unilateral femoral artery ligation. On days 14 and 21, DCE-MRI was performed using a medium-sized contrast agent (MCA) (Gadomer) or a small contrast agent (SCA) (Gd-DTPA). Acquisition protocols were adapted to the pharmacokinetic properties of the contrast agent. Model-based data analysis was optimized by selecting the optimal model, considering fit error, estimation uncertainty, and parameter interdependency from three two-compartment pharmacokinetic models (normal and extended generalized kinetic models and Patlak model). Model-based parameters were compared to the model-free parameter area-under-curve (AUC). Finally, the sensitivity of transfer constant Krans and AUC for physiological and pathophysiological microvascular differences was evaluated. RESULTS: For the MCA, the optimal model included Ktrans and plasma fraction nu(p). For the SCA, Ktrans and interstitial fraction nu(e) should be incorporated. For the MCA, Ktrans were (4.8 +/- 0.2) x 10(-3) min(-1) (mean standard error) and (3.6 +/- 0.1) x 10(-3) min(-1) for the red soleus and white tibialis muscle, respectively, p < 0.01. With the SCA, Ktrans were (81 +/- 5) x 10(-3) min(-1) (soleus) and (66 +/- 5) x 10(-3) min(-1) (tibialis) p < 0.01. In the ischemic limb, Ktrans was significantly decreased relative to the control limb (soleus: 15%-20%; tibialis: 5%-10%). Similar differences in AUC were found for both contrast agents. CONCLUSIONS: For optimal estimation of microvascular parameters, both model-based and model-free analysis should be adapted to the pharmacokinetic properties of the contrast agent. The detection of microvascular differences based on both Ktrans and AUC was most sensitive when the analysis strategy was tailored to the contrast agent used. The MCA was equally sensitive for microvascular differences as the SCA, with the advantage of improved spatial resolution.

Structural remodelling of cardiomyocytes in the border zone of infarcted rabbit heart.

Cardiomyocyte dedifferentiation, as detected in hibernating myocardium of chronic ischemic patients, is one of the characteristics seen at the border of myocardial infarcts in small and large animals. Our objectives were to study in detail the morphological changes occurring at the border zone of a rabbit myocardial infarction and its use as model for hibernating myocardium. Ligation of the left coronary artery (LAD) was performed on rabbit hearts and animals were sacrificed at 2, 4, 8 and 12 weeks post-infarction. These hearts together with a non-infarcted control heart were perfusion-fixed and tissue samples were embedded in epoxy resin. Hibernating cardiomyocytes were mainly distributed in the non-infarcted region adjacent to the border zone of infarcted myocardium but only in a limited number. In the border zone itself vacuolated cardiomyocytes surrounded by fibrotic tissue were frequently observed. Ultrastructural analysis of these vacuolated cells revealed the presence of a basal lamina inside the vacuoles adjacent to the surrounding membrane, the presence of pinocytotic vesicles and an association with cisternae of the sarcoplasmatic reticulum. Myocyte quantitative analyses revealed a gradual increase in vacuolar area/total cell area ratio and in collagen fibril deposition inside the vacuoles from 2 to 12 weeks post-infarction. Related to the remote zone, the increase in cell width of myocytes located in and adjacent to the border zone demonstrated cellular hypertrophy. These results indicate the occurrence of cardiomyocyte remodelling mechanisms in the border zone and adjacent regions of infarcted myocardium. It is suggested that the vacuoles represent plasma membrane invaginations and/or dilatations of T-tubular structures.

Partial cell fusion: a newly recognized type of communication between dedifferentiating cardiomyocytes and fibroblasts.

OBJECTIVE: Fibroblasts have been shown to couple to neonatal cardiomyocytes in heterocellular cultures through functional gap junctions. Our objective was to provide evidence for an additional type of heterocellular communication between fibroblasts and adult cardiomyocytes in vitro and in vivo. METHODS: The contact areas in heterocellular co-cultures were evaluated by specific labeling and the intercellular communication was studied using preloading of fibroblasts with tracer molecules. Heterocellular fibroblast-cardiomyocyte contacts present in the in vitro setting and in the border zone of a rabbit myocardial infarction in vivo were further examined by electron microscopy. RESULTS: Addition of fibroblasts preloaded with the fluorescent low molecular weight tracer calcein-AM to cultured myocytes indicated early dye transfer via connexin 43 functional gap junctions. At a later time-period after co-culturing, dye transfer of fibroblasts preloaded with the high molecular weight tracer dextran 10,000 suggested partial cell fusion. The membrane continuity giving rise to this partial cell fusion was confirmed by electron microscopy, clearly showing areas of intercytoplasmic contacts between fibroblasts and phenotypically adapted (dedifferentiated) cardiomyocytes. Fluorescein-labeled annexin V affinity studies revealed transient exposure of phosphatidylserine at the contact sites, suggesting that phosphatidylserine mediates the fusion process. Close contacts between cardiac fibroblasts and dedifferentiated cardiomyocytes accompanied by disruption of the basal lamina were observed in the border zone of a rabbit myocardial infarction in vivo. CONCLUSION: Our results suggest that the partial cell fusion-type of heterocellular communication in our co-culture model and the contacts observed in vivo may lead to new insights in cardiovascular disease.