Restoring Atrial T-Tubules Augments Systolic Ca Upon Recovery From Heart Failure

BACKGROUND: Transverse (t)-tubules drive the rapid and synchronous Ca 2+ rise in cardiac myocytes. The virtual complete atrial t-tubule loss in heart failure (HF) decreases Ca 2+ release. It is unknown if or how atrial t-tubules can be restored and how this affects systolic Ca 2+.

METHODS: HF was induced in sheep by rapid ventricular pacing and recovered following termination of rapid pacing. Serial block-face scanning electron microscopy and confocal imaging were used to study t-tubule ultrastructure. Function was assessed using patchclamp, Ca 2+, and confocal imaging. Candidate proteins involved in atrial t-tubule recovery were identified by western blot and expressed in rat neonatal ventricular myocytes to determine if they altered t-tubule structure.

RESULTS: Atrial t-tubules were lost in HF but reappeared following recovery from HF. Recovered t-tubules were disordered, adopting distinct morphologies with increased t-tubule length and branching. T-tubule disorder was associated with mitochondrial disorder. Recovered t-tubules were functional, triggering Ca 2+ release in the cell interior. Systolic Ca 2+, I Ca-L, sarcoplasmic reticulum Ca 2+ content, and SERCA function were restored following recovery from HF. Confocal microscopy showed fragmentation of ryanodine receptor staining and movement away from the z-line in HF, which was reversed following recovery from HF. Acute detubulation, to remove recovered t-tubules, confirmed their key role in restoration of the systolic Ca 2+ transient, the rate of Ca 2+ removal, and the peak L-type Ca 2+ current. The abundance of telethonin and myotubularin decreased during HF and increased during recovery. Transfection with these proteins altered the density and structure of tubules in neonatal myocytes. Myotubularin had a greater effect, increasing tubule length and branching, replicating that seen in the recovery atria.

CONCLUSIONS: We show that recovery from HF restores atrial t-tubules, and this promotes recovery of I Ca-L, sarcoplasmic reticulum Ca 2+ content, and systolic Ca 2+. We demonstrate an important role for myotubularin in t-tubule restoration. Our findings reveal a new and viable therapeutic strategy.