Electrical pacing counteracts intrinsic shortening of action potential duration of neonatal rat ventricular cells in culture

Previous studies have demonstrated the relationship between the functional electrophysiological properties of cultured neonatal rat ventricular myocytes (NRVMs) and the ability of the substrate to induce and sustain arrhythmia. The goal of this study was to examine the effects of chronic pacing at a constant rate akin to that in vivo, on the functional electrophysiological properties of NRVM monolayers. Confluent NRVM monolayers grown on 20 mm diameter cover slips were left either unpaced or were stimulated at 3 Hz for the duration of the culture, and were optically mapped on days 4, 6, or 8. Action potential duration at 80% repolarization (APD₈₀), conduction velocity (CV), and Kv4.3 (I_to) and NCX protein expression were measured. The effects of the excitation-contraction uncoupler 2,3-butadione monoxime (BDM) were also investigated. The 2 Hz APD₈₀ of non-paced monolayers decreased significantly on days 6 (137.1 ± 13.9 ms) and 8 (109.8 ± 9.0 ms) compared with day 4 (197.0 ± 11.8 ms), while that of paced monolayers did not (206.8 ± 9.7, 209.1 ± 9.2, and 210.6 ± 9.9 ms, respectively). The 2 Hz CV of non-paced monolayers increased significantly on days 6 (26.0 ± 1.6 cm/s) and 8 (26.5 ± 1.0 cm/s) compared with day 4 (20.0 ± 1.0 cm/s), while that of paced monolayers did not change significantly (26.0 ± 2.0, 26.0 ± 1.0, and 23.8 ± 1.2 cm/s, respectively). The restitution curves of APD₈₀ and CV of paced monolayers were also unchanging from days 4 through 8. Despite the unchanging APD₈₀ and CV, a decrease in Kv4.3 expression and an increase in NCX expression were observed in paced compared with non-paced monolayers. Cessation of pacing or administration of BDM caused a reversal of phenotype back to that of non-paced monolayers. In summary, chronic electrical stimulation of confluent NRVM monolayers results in stabilization of APD₈₀ and an advancement of the developmental rise of CV that is mediated by electromechanical coupling. These effects produce a steadier functional phenotype that may be beneficial for electrophysiological studies.