Peripherally Restricted Dual Cannabinoid Receptor Agonist Modifies Mitochondrial Signaling and Inhibits Atrial Electrical Remodeling in a Tachypaced Ex Vivo Rat Model

Abstract

Atrial fibrillation (AF) leads to rate‐dependent changes, which progressively increase the risk for recurrence and persistence of the arrhythmia (collectively defined as atrial remodelling). Atrial effective refractory period (AERP) shortening and conduction velocity prolongation are hallmarks of atrial remodelling markedly promoting AF substrate. The molecular mechanisms underlying atrial remodelling are not fully elucidated and therapeutic strategies to reduce AF burden by preventing atrial remodelling are not yet available. Cannabinoid (CB) receptor ligands exert cardioprotective effects. We hypothesized that CB13, a peripherally restricted dual CB receptor agonist with limited brain penetration, would reduce atrial remodelling.Sprague Dawley (SD) rat hearts were excised and hung on a Langendorff set‐up, and supplemented with CB13 (1μM) or without (control) (paced and non‐paced). Coronary pressure of 60–80 mmHg was maintained. Electrophysiological (EP) parameters were measured by placing a mini‐hook platinum‐iridium quadrupole electrode on the right atrium (RA) for pacing and atrial recording (CB13 n=8; control n=8). RA were tachypaced for 90 min at double diastolic threshold and a cycle length of 5 ms higher than failed 1:1 atrial capture. S1S2 stimulation protocol was used to measure AERP, defined as the longest interval with failed 1:1 capture. The effects of CB13 on atrial refractoriness and AF substrate were assessed by analysis of high resolution electrogram recordings and protein expression via western blot.Post‐tachypacing EP recordings showed reduced AERP in control compared to pre‐tachypacing (54.38±3.05 vs. 44.75±3.67 ms; p<0.01). AERP post‐tachypacing in CB13‐treated hearts remained unchanged compared to pre‐tachypacing (55.25±3.46 vs. 55.50±2.67 ms; ns). Thus, CB13‐treatment resulted in preservation of AERP compared to control (103.6±9.2 vs. 82.3±4.4 % of baseline; p<0.05). Protein expression of PGC1α was significantly increased by CB13 compared to controls (paced and non‐paced). Upregulation of mitochondrial biogenesis may be related to increased phosphorylation of AMPKα at residue threonine 172 (compared to total AMPKα, p=0.12, CB13 vs. control paced). Furthermore, gap junction connexin 43 was significantly downregulated in paced controls vs. non‐paced controls, while CB13 remained upregulated (no significance vs. controls).CB13 exposure during tachypacing prevents AERP reduction and gap junction downregulation probably through mechanisms involving mitochondrial biogenesis. Thus, by inhibiting atrial remodeling CB13 and the subsequent activation of CB receptors may be a viable treatment strategy for AF.Support or Funding InformationThis work was supported by studentships from the Canadian Institutes of Health Research and Mitacs (D.L.)