Optimal Cutting Temperature Medium‐Embedding Is a Valid Method for Storing and Preparing Myocardial Biopsies Preceding Myofilament Function‐Assessment

Background and objective Optimal Cutting Temperature (OCT) media is commonly used for histological characterisation. Myocardial tissues from clinical biopsies are precious and thus utilising them for both histological and functional assessment is an attractive prospect. This study was designed to assess whether OCT media stored samples can be used for preparing myocardial tissue for myofilament function‐assessment. Methods Liquid nitrogen (LN) – and OCT‐stored myocardium from mouse was compared using a permeabilised fibre set‐up for functional characterisation of thin and thick filament‐interaction including Ca 2+ sensitivity (pCa 50 ), cooperativity along the thin filament (nH), cross‐bridge cycling rate (k tr ), Ca 2+ activated maximum tension (F max )/cross sectional area, cross‐bridge recruitment upon Ca 2+ activation, as well as passive strain (k strain ) and the relative titin contribution to passive stiffness. LN samples were prepared in a conventional fashion and OCT‐stored tissue was first sectioned in parallel with papillary muscle longitudinal axis before chemical demembranisation and further trimming under the microscope. Results Ca 2+ sensitivity (pCa 50 ): LN (n=8): 5.75 ±0.019, OCT (n=8): 5.69 ± 0.022 SEM (p>0.05). Cooperativity along the thin filament (nH): LN (n=8): 2.63 ± 0.24, OCT (n=8): 2.53 ± 0.25 SEM (p>0.05). F max : LN (n=8): 19.6 ± 1.9, OCT (n=8): 11.6 ± 1.7 mN/mm 2 SEM (p<0.05). K tr : LN (n=8): 37.5±1.6, OCT (n=8): 37.9 ± 1.6 s −1 SEM (p<0.05). Relative cross‐bridge recruitment upon full Ca 2+ activation: LN (n=8): 0.32 ± 0.015, OCT (n=7): 0.31 ± 0.014 SEM (p<0.05). k strain : LN (n=7): 13.9 ± 0.67, OCT (n=7): 11.4 ± 0.49 L −1 , p<0.01). Relative titin contribution to stiffness at 20 % stretch from slack: LN (n=6): 0.52 ± 0.07, OCT (n=7): 0.80 ± 0.03 SEM, (p<0.01). Conclusions OCT‐storage and cryostat sectioning before mounting fibre bundles on a force‐measuring rig can be used for investigating relative measures of function depending on active force development, ie pCa‐tension relationship, cooperativity along the thin filament and cross‐bridge cycling rate. There appears to be no negative effect on the amount of cross‐bridge recruitment upon Ca 2+ activation compared to a conventionally LN‐stored and prepared sample. Absolute values such as F max , as well as passive strain, need to be interpreted with great caution as they clearly differ between mode of preparation and thus these parameters are not suited for investigation. We speculate this may be due to how cutting of fibre bundles using a cryostat, to an unknown degree, disrespects the natural continuity of the bundles and thus hampers force relay. Support or Funding Information This work was supported by: Swedish Stately Grant for Medical Research (ALF‐means) & Lars Hierta Memorial Fund