A comparative study of altered hemodynamics in iliac vein compression syndrome.

Introduction: Iliac vein compression syndrome (IVCS) is present in over 20% of the population and is associated with left leg pain, swelling, and thrombosis. IVCS symptoms are thought to be induced by altered pelvic hemodynamics, however, there currently exists a knowledge gap on the hemodynamic differences between IVCS and healthy patients. To elucidate those differences, we carried out a patient-specific, computational modeling comparative study. Methods: Computed tomography and ultrasound velocity and area data were used to build and validate computational models for a cohort of IVCS (N = 4, Subject group) and control (N = 4, Control group) patients. Flow, cross-sectional area, and shear rate were compared between the right common iliac vein (RCIV) and left common iliac vein (LCIV) for each group and between the Subject and Control groups for the same vessel. Results: For the IVCS patients, LCIV mean shear rate was higher than RCIV mean shear rate (550 ± 103 s-1 vs. 113 ± 48 s-1, p = 0.0009). Furthermore, LCIV mean shear rate was higher in the Subject group than in the Control group (550 ± 103 s-1 vs. 75 ± 37 s-1, p = 0.0001). Lastly, the LCIV/RCIV shear rate ratio was 4.6 times greater in the Subject group than in the Control group (6.56 ± 0.9 vs. 1.43 ± 0.6, p = 0.00008). Discussion: Our analyses revealed that IVCS patients have elevated shear rates which may explain a higher thrombosis risk and suggest that their thrombus initiation process may share aspects of arterial thrombosis. We have identified hemodynamic metrics that revealed profound differences between IVCS patients and Controls, and between RCIV and LCIV in the IVCS patients. Based on these metrics, we propose that non-invasive measurement of shear rate may aid with stratification of patients with moderate compression in which treatment is highly variable. More investigation is needed to assess the prognostic value of shear rate and shear rate ratio as clinical metrics and to understand the mechanisms of thrombus formation in IVCS patients.

An ultrasound imaging and computational fluid dynamics protocol to assess hemodynamics in iliac vein compression syndrome.

OBJECTIVE: Elevated shear rates are known to play a role in arterial thrombosis; however, shear rates have not been thoroughly investigated in patients with iliac vein compression syndrome (IVCS) owing to imaging limitations and assumptions on the low shear nature of venous flows. This study was undertaken to develop a standardized protocol that quantifies IVCS shear rates and can aid in the diagnosis and treatment of patients with moderate yet symptomatic compression. METHODS: Study patients with and without IVCS had their iliac vein hemodynamics measured via duplex ultrasound (US) at two of the following three vessel locations: infrarenal inferior vena cava (IVC), right common iliac vein, and left common iliac vein, in addition to acquiring data at the right and left external iliac veins. US velocity spectra were multiplied by a weighted cross-sectional area calculated from US and computed tomography (CT) data to create flow waveforms. Flow waveforms were then scaled to enforce conservation of flow across the IVC and common iliac veins. A three-dimensional (3D), patient-specific model of the iliac vein anatomy was constructed from CT and US examination. Flow waveforms and the 3D model were used as a basis to run a computational fluid dynamics (CFD) simulation. Owing to collateral vessel flow and discrepancies between CT and US area measurements, flows in internal iliac veins and cross-sectional areas of the common iliac veins were calibrated iteratively against target common iliac flow. Simulation results on mean velocity were validated against US data at measurement locations. Simulation results were postprocessed to derive spatial and temporal values of quantities such as velocity and shear rate. RESULTS: Using our modeling protocol, we were able to build CFD models of the iliac veins that matched common iliac flow splits within 2% and measured US velocities within 10%. Proof-of-concept analyses (1 subject, 1 control) have revealed that patients with IVCS may experience elevated shear rates in the compressed left common iliac vein, more typical of the arterial rather than the venous circulation. These results encourage us to extend this protocol to a larger group of patients with IVCS and controls. CONCLUSIONS: We developed a protocol that obtains hemodynamic measurements of the IVC and iliac veins from US, creates patient-specific 3D reconstructions of the venous anatomy using CT and US examinations, and computes shear rates using calibrated CFD methods. Proof-of-concept results have indicated that patients with IVCS may experience elevated shear rates in the compressed left common iliac vein. Larger cohorts are needed to assess the relationship between venous compression and shear rates in patients with IVCS as compared with controls with noncompressed iliac veins. Further studies using this protocol may also give promising insights into whether or not to treat patients with moderate, yet symptomatic compression.

Acetylcholinesterase exhibits trypsin-like and metalloexopeptidase-like activity in cleaving a model peptide.

Acetylcholinesterase (EC 3.1.1.7) has been shown to possess an intrinsic peptidase activity. [Chubb et al. (1983), Neuroscience 10, 1369-1383]. To examine this activity further, the breakdown of a model hexapeptide (leu-trp-met-arg-phe-ala) LWMRFA was studied. Affinity-purified eel acetylcholinesterase rapidly cleaved the hexapeptide in a trypsin-like manner to produce two peptides (LWMR and FA). Acetylcholinesterase more slowly cleaved the C-terminal alanine residue from the peptide to yield LWMRF. Although the enzyme showed preference for cleaving the hexapeptide at its C-terminal, it was also able to cleave the N-terminal leucine residue form the tryptic product LWMR. Hydrolysis of the peptide at the tryptic site (arg4-phe5) was strongly inhibited by the trypsin inhibitor diisopropylfluorophosphate. Cleavage of the C-terminal alanine was only poorly inhibited by diisopropylfluorophosphate, but more strongly inhibited by metal-ion chelating agents, and it was increased in the presence of Zn2+ and Co2+. The pH optimum for cleavage at the tryptic site was 6, while that for the carboxypeptidase site was 8-9. These results show that acetylcholinesterase can hydrolyse peptides like a trypsin-like endopeptidase and a Zn2+- or Co2+-dependent exopeptidase, and they suggest that these two peptidase activities are associated with two separate active sites on the acetylcholinesterase molecule. As both peptidase activities eluted with acetylcholinesterase from a TSK 4000SW column when it was chromatographed by high-performance liquid chromatography, it is unlikely that the presence of either peptidase activity could be attributable to a contaminant in the acetylcholinesterase preparation. We suggest that acetylcholinesterase may be involved in the breakdown of bioactive peptides or their precursors in neuroendocrine cells.

Acetylcholinesterase hydrolyses chromogranin A to yield low molecular weight peptides.

The major soluble protein of bovine chromaffin granules chromogranin A was purified by reverse-phase high performance liquid chromatography. Brief incubations with either acetylcholinesterase or trypsin cleaved chromogranin A to yield two chromogranin-immunoreactive polypeptides which were similar in molecular weight to two of the major endogenous chromogranin polypeptides. A number of peptidase inhibitors which strongly inhibited tryptic digestion of chromogranin A also inhibited the acetylcholinesterase digestion, although they were less potent. More prolonged digestion of chromogranin A with acetylcholinesterase produced a large number of peptides which were similar to some of the endogenous chromogranin peptides in their elution profile by high performance liquid chromatography. In contrast, complete tryptic digestion of chromogranin A yielded peptides with a totally different elution profile. The experiments indicate that acetylcholinesterase possesses a peptidase activity which is similar, but not identical to trypsin, and suggest that a second non-tryptic activity is also present. They also suggest that acetylcholinesterase, an enzyme found in chromaffin cells, may process chromogranin A to yield lower molecular weight chromogranins in bovine chromaffin cells.

Acetylcholinesterase generates enkephalin-like immunoreactivity when it degrades the soluble proteins (chromogranins) from adrenal chromaffin granules.

Acetylcholinesterase was purified by passage through 3 affinity columns. The enzyme so purified was found to be homogeneous by electrophoresis and the peptidase and AChE activities co-eluted from a high pressure liquid chromatography column. The purified AChE degraded the chromogranins, the soluble proteins from the adrenal chromaffin granules, at a rate of nearly 8 micrograms/microgram AChE/h. The rate was fastest with the largest chromogranins, but proteins across the whole molecular weight spectrum were hydrolyzed. Immunoassay of extracts after incubation with AChE showed that enkephalin-like material had been produced. Incubations were also done with chromogranins that had been fractionated by size exclusion chromatography. The AChE degraded protein in all fractions and generated enkephalin-like immunoreactive material in fractions where it was produced by sequential treatment with trypsin and carboxypeptidase B. It seems likely, therefore, that AChE can hydrolyze some of the enkephalin precursors that are sensitive to trypsin and carboxypeptidase B, but the one-step nature of its action suggests a mode of action with fewer restrictions. It is concluded that AChE can hydrolyze proteins of widely differing sizes and the data add to the evidence that AChE is able to hydrolyze enkephalin precursors resulting in the generation of immunoreactive peptide.