Cardiac remodelling in AF is associated with functional heterogeneity within the atria, giving rise to ectopic and re-entrant activity. Significant atrial remodelling is seen following exposure to chronic hypoxia (CH). This is likely attributable, in part, to dysregulated cardiac autonomic control, of which the noradrenaline transporter (NAT) is a key regulator. Therefore, we investigated regional differences in left atrial NAT function in normoxic (N) and CH-exposed rats (FiO₂=0.12, 9-10 days). Regional innervation density was also compared in N and CH within the left atrium. Since CH is associated with adaptive changes in metabolic function, we also investigated the effect of mitochondrial inhibition on NAT dynamics.

To investigate single-terminal NAT dynamics, we employed a recently developed confocal fluorescence technique using the NAT-specific neurotransmitter uptake assay (NTUA; Molecular Devices, USA) (Cao et al. 2020). Experiments and procedures were performed in accordance with the UK Animals (Scientific Procedures) Act 1986. Hearts were excised from male Wistar rats (200-300g) under non-recovery terminal inhalation isoflurane (3-5% in O₂, flow rate 1.5L min^-1) with death confirmed by cervical dislocation. Ventricles were dissected free and atria were transferred to a superfusion chamber. Tissues were dissected into left atrial appendage (LAA), left atrial posterior wall (LAPW) and pulmonary veins (PV). Sections were pinned flat and transferred to a confocal microscope. Immediately after NTUA loading, image stacks were captured over 15 minutes to allow calculation of single-terminal NTUA uptake (n=6 terminals per region). Imaging was performed for N tissues (n=5 rats), CH tissues (n=3-5 rats), and in an additional subset of normoxic rats where sodium azide (10µM), a complex IV inhibitor, was added to the NTUA solution (n=4-5 rats). Additional images were acquired at low magnification to allow calculation of innervation density. Two-way ANOVA was performed for statistical analysis with Bonferroni testing for multiple comparisons. Values are expressed as mean ±SEM.

CH rats demonstrated a relative right ventricular hypertrophy compared to N rats (p<0.0001). Analysis of uptake traces in all tissues revealed a two-phase association characterized by an initial 6-minute linear uptake followed by a plateau phase. Maximum fluorescence (F_Max) and linear uptake gradients (F_L) were not significantly different between atrial regions in N tissues (p>0.05). In CH animals, LAA terminals showed increased F_Max (p=0.004) and F_L (p=0.021) compared to N. NAT dynamics the LAPW and PV were unchanged in CH (p>0.05). CH was also associated with a generalised increase in atrial innervation density, independent of region (p=0.034). Sodium azide exposure was also associated with an increase in F_L in the LAA (p=0.030), as well as an increase in F_Max in the LAPW (p<0.001).

Our data demonstrate functional variability in the left atrial response to CH and mitochondrial inhibition. Further investigation is required to determine the role of hypoxic dysregulation of the NAT in arrhythmogenesis.