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

AIMS

Ventricular myocytes (VM) depolarization activates L-type Ca channels (LCC) allowing Ca influx (I) to synchronize sarcoplasmic reticulum (SR) Ca release, via Ca-release channels (RyR2). The resulting whole-cell Ca transient triggers contraction, while cytosolic Ca removal by SR Ca pump (SERCA2) and sarcolemmal Na/Ca exchanger (NCX) allows relaxation. In diseased hearts, extensive VM remodeling causes heterogeneous, blunted and slow Ca transients. Among remodeling changes are: A) T-tubules disorganization. B) Diminished SERCA2 and low SR Ca. However, those often overlap, hindering their relative contribution to contractile dysfunction (CD). Furthermore, few studies have assessed their specific impact on the spatiotemporal Ca transient properties and contractile dynamics simultaneously. Therefore, we sought to perform a quantitative comparison of how heterogeneous and slow Ca 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 Ca, but maintained T-tubules. By confocal imaging of Fluo-4-loaded VM, under field-stimulation, simultaneously acquired Ca transients and shortening, allowing direct correlations.

KEY FINDINGS

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