Abstract 440: The Cardiomyocyte Diastolic Stiffness and Contractility are Inversely Related to the Size of Titin: A Cellular Work Loop Study of Single Intact Cardiomyocytes

Abstract

Studying cardiac titin under physiological conditions can be at the level of the left ventricular (LV) chamber and the multicellular muscle level. However, those preparations are influenced by neurohormonal reflex, preload-afterload status and extracellular matrix. In this study, cellular work loop force measurement of the single intact cardiomyocyte was conducted under physiological conditions. Intact cell experiments were performed at 37°C under field-stimulation at 2 Hz frequency. The cell was attached to the force transducer and the piezo length controller which modulates the cell length by the feedback control system based on developed force. The force length relation (cellular work loop) of the single cell was generated. The end diastolic force length relation (EDFLR), the end systolic force length relation (ESFLR) and the preload recruitable stroke work (PRSW) were obtained. The Ttn ΔIAjxn -/- ( IA -/- ) mice, in which the I-band/A-band region of titin’s spring has been deleted were studied in comparison to α-MHC-Cre ; cRbm20 ΔRRM +/- ( cRbm20 +/- ) mice which are deficient in titin splicing and express large titin isoforms in the heart. The cellular diastolic stiffness (slope of EDFLR) was increased in IA -/- and was reduced in cRbm20 +/- (151.3, 104.0 and 91.5 μN/mm 2 in IA -/- , WT and cRbm20 +/- respectively). The contractility (slopes of ESFLR and PRSW) were 233.5 and 1215.6 uN/mm 2 in WT, and were increased in IA -/- (371.2 and 2076 μN/mm 2 ) but attenuated in cRbm20 +/- (202.9 and 907.7 μN/mm 2 ). Cellular work loop studies were also conducted in intact cells isolated from the mice that underwent transverse aortic constriction (TAC) surgery with deoxycorticosterone acetate (DOCA) implantation. TAC DOCA mice developed diastolic dysfunction at the LV chamber level. The result showed that the cells from TAC DOCA hearts had increased slopes of EDFLR, ESFLR and PRSW ( 157.9, 378.9 and 1699.0 μN/mm 2 ) compared to the cells from sham mice (98.6, 230.3, and 1203.9 μN/mm 2 ). Conclusions: Cellular work loops recapitulated the cardiac physiology at the LV chamber level. The result showed that the cellular diastolic stiffness and contractility were inversely related to the size of titin and that titin stiffness plays an important roles in both diastolic and systolic function.