THE ROLE OF MICROTUBULES IN ATRIAL MECHANO-ARRHYTHMOGENESIS

Abstract

In the heart, electrical excitation causes mechanical contraction, while feedback of the heart’s mechanical state alters its electrical activity. With mechanical overload, this feedback can result in mechano-arrhythmogenesis (MA). In the ventricles, MA is driven by microtubule (MT) density/detyrosination in a transient receptor potential ankyrin 1 channel (TRPA1) dependent manner. Clinical evidence in the atria indicates arrhythmias are associated with increased TRP channels and pressure overload with MT densification. My thesis aimed to determine if atrial MA is increased by MT density/detyrosination in a TRPA1-dependent manner. This goal was unsuccessfully investigated using a chunk digested rabbit isolated atrial myocyte model. Modification of mechanical disruption, digestion time, enzyme concentration, calcium chelator exposure and calcium reintroduction were all shown to play a role in myocyte isolation. Unfortunately, extensive optimization did not reliably yield contractile myocytes. This goal was successfully investigated using a Langendorff-digested rabbit isolated atrial myocyte model that underwent transient stretch with carbon fibres (CF). MA incidence was assessed after pharmacologic MT stabilization, which promoted MT density and detyrosination with paclitaxel, detyrosination inhibition with parthenolide or selective antagonism of TRPA1 with HC-030031. Acute stretch characteristics were determined during pharmacologic manipulations. Contractile properties (rate and amount of contraction) were evaluated with sarcomere tracking. MT density and/or detyrosination were measured in right atrial samples from cardiac surgery patients with normal or elevated right ventricle systolic pressures by western blot. Here we found that increasing MT density and/or detyrosination with paclitaxel increases MA-incidence above a threshold level of stretch (p=0.0183). Co-incubation of paclitaxel with parthenolide or HC-030031 mitigates the increase in MA (p>0.999 and p=0.5640, respectively). Increased CF applied stretch increases the mechanical load per cross-sectional area, or cell stress. Myocyte contractile properties were improved by parthenolide. Patient pathologies may increase MT density and/or detyrosination. Our results demonstrate that MT density and/or detyrosination may play a critical role in atrial MA through activation of mechano-sensitive TRPA1. Parthenolide inhibiting detyrosination may decrease cytoskeletal linkages in atrial myocytes, thereby improving contraction. Future work will determine if pressure overload in patients is associated with MT density and/or detyrosination and leads to an increase in atrial MA.