Abstract 503: Paracrine Factors from Cardiac Fibroblasts Slow Conduction Velocity and Prolong the Action Potential Duration of Cardiomyocytes
By acting as passive current sinks or conduction obstacles, cardiac fibroblasts directly slow conduction and increase electrical anisotropy in fibrotic cardiac tissue, leading to increased arrhythmogenicity. We studied if indirect paracrine action by cardiac fibroblasts can alter electrical properties and related gene expression in cardiomyocytes. Confluent cardiomyocyte cultures were incubated for 24 h in serum-free defined media conditioned 24 h by cardiac fibroblasts. Age-matched unconditioned cultures served as controls. The cell membrane potentials were optically mapped at 504 sites using voltage sensitive dye, Di-4 ANEPPS. Remarkably, a two-fold decrease in conduction velocity and three-fold increase in action potential duration were measured. Observed changes in conditioned cultures were not caused by increased cell apoptosis or fibroblast proliferation. Fresh medium with concentrated fibroblast paracrine factors induced the same effect. Consistent with electrophysiological changes, conditioned cultures had decreased mRNA expression for fast sodium (SCN5A, 3.8 fold), inward rectifier potassium (Kir2.1, 6.6 fold) and transient outward potassium (Kv4.3, below detection limit) channels, but unchanged expression of Connexin 43 and 45. Expressions of myosin heavy chain-beta and alpha were respectively increased 17 fold and decreased 2.2 fold. Importantly, media conditioned by the same number of fibroblasts in the presence of cardiomyocytes had no effect on cardiac electrical properties, suggesting that under normal conditions, cardiomyocytes may secrete protective factors that inhibit the negative paracrine action of cardiac fibroblasts.