Rationale

Flow cytometry (FCM) of ventricular myocytes (VMs) is an emerging technology in adult cardiac research that is challenged by the wide variety of VM shapes and sizes. Cellular variability and cytometer flow cell size can affect cytometer performance. These two factors of variance limit assay validity and reproducibility across laboratories. Washing and filtering of ventricular cells in suspension are routinely done to prevent cell clumping and minimize data variability without the appropriate standardization. We hypothesize that washing and filtering arbitrarily biases towards sampling smaller VMs than what actually exist in the adult heart.

Objective

To determine the impact of washing and filtering on adult ventricular cells for cell sizing and FCM.

Methods and results

Left ventricular cardiac cells in single-cell suspension were harvested from New Zealand White rabbits and fixed prior to analysis. Each ventricular sample was aliquoted before washing or filtering through a 40, 70, 100 or 200μm mesh. The outcomes of the study are VM volume by Coulter Multisizer and light-scatter signatures by FCM. Data are presented as mean±SD. Myocyte volumes without washing or filtering (NF) served as the "gold standard" within the sample and ranged from 11,017 to 46,926μm3. Filtering each animal sample through a 200μm mesh caused no variation in the post-filtration volume (1.01+0.01 fold vs. NF, n = 4 rabbits, p = 0.999) with an intra-assay coefficient of variation (%CV) of <5% for all 4 samples. Filtering each sample through a 40, 70 or 100μm mesh invariably reduced the post-filtration volume by 41±10%, 9.0±0.8% and 8.8±0.8% respectively (n = 4 rabbits, p<0.0001), and increased the %CV (18% to 1.3%). The high light-scatter signature by FCM, a simple parameter for the identification of ventricular myocytes, was measured after washing and filtering. Washing discarded VMs and filtering cells through a 40 or 100μm mesh reduced larger VM by 46% or 11% respectively (n = 6 from 2 rabbits, p<0.001).

Conclusion

Washing and filtering VM suspensions through meshes 100μm or less biases myocyte volumes to smaller sizes, excludes larger cells, and increases VM variability. These findings indicate that validity and reproducibility across laboratories can be compromised unless cell preparation is standardized. We propose no wash prior to fixation and a 200μm mesh for filtrations to provide a reproducible standard for VM studies using FCM.