Relative role of T-tubules disruption and decreased SERCA2 on contractile dynamics of isolated rat ventricular myocytes

Abstract

Aims: Ventricular myocytes (VM) depolarization activates L-type Ca²⁺ channels (LCC) allowing Ca²⁺ influx (I_Ca) to synchronize sarcoplasmic reticulum (SR) Ca²⁺ release, via Ca²⁺-release channels (RyR2). The resulting whole-cell Ca²⁺ transient triggers contraction, while cytosolic Ca²⁺ removal by SR Ca²⁺ pump (SERCA2) and sarcolemmal Na⁺/Ca²⁺ exchanger (NCX) allows relaxation. In diseased hearts, extensive VM remodeling causes heterogeneous, blunted and slow Ca²⁺ transients. Among remodeling changes are: A) T-tubules disorganization. B) Diminished SERCA2 and low SR Ca²⁺. However, those often overlap, hindering their relative contribution to contractile dysfunction (CD). Furthermore, few studies have assessed their specific impact on the spatiotemporal Ca²⁺ transient properties and contractile dynamics simultaneously. Therefore, we sought to perform a quantitative comparison of how heterogeneous and slow Ca²⁺ transients, with different underlying determinants, affect contractile performance.

Methods: We used two experimental models: A) formamide-induced acute "detubulation", where VM retain functional RyR2 and SERCA2, but lack T-tubules-associated LCC and NCX. B) Intact VM from hypothyroid rats, presenting decreased SERCA2 and SR Ca²⁺, but maintained T-tubules. By confocal imaging of Fluo-4-loaded VM, under field-stimulation, simultaneously acquired Ca²⁺ transients and shortening, allowing direct correlations.

Key findings: We found near-linear correlations among key parameters of altered Ca²⁺ transients, caused independently by T-tubules disruption or decreased SR Ca²⁺, and shortening and relaxation, SIGNIFICANCE: Unrelated structural and molecular alterations converge in similarly abnormal Ca²⁺ transients and CD, highlighting the importance of independently reproduce disease-specific alterations, to quantitatively assess their impact on Ca²⁺ signaling and contractility, which would be valuable to determine potential disease-specific therapeutic targets.

Keywords: Calcium release desynchronization; Contractile dysfunction; RyR2 sensitivity; SERCA2; Transverse tubules remodeling.