Fluorofenidone enhances cardiac contractility by stimulating CICR and Ca_V1.2

Fluorofenidone (AKF-PD) is a novel pyridone derivative that inhibits fibrosis and inflammation in many tissues. Accordingly, it has been effective in disease models, such as liver failure, nephropathy, and pulmonary fibrosis. However, its potential role in cardiac physiology and pathology has yet to be elucidated. Thus, this paper investigated a possible functional impact of AKF-PD on adult rat cardiac myocytes. Cells were kept in culture for 1-2 days under either control conditions or the presence of AKF-PD (500 μM). They were next examined concerning cell contractility, intracellular Ca²⁺ homeostasis, and activity of voltage-gated Ca²⁺ channels. Remarkably, AKF-PD enhanced the percentage of cell shortening and rates of both contraction and relaxation by nearly 100%. A stimulus in Ca²⁺-induced Ca²⁺ release (CICR) most likely accounts for these effects because AKF-PD also increased the magnitude of electrically evoked Ca²⁺ transients. Of note, the compound did not alter the peak value of caffeine-elicited Ca²⁺ transients, indicating stimulation of CICR at constant sarcoplasmic reticulum Ca²⁺ load. Since CICR is triggered by the entry of Ca²⁺ through Ca_V1.2 (I_Ca), a possible effect on these Ca²⁺ channels was also investigated. AKF-PD increased the magnitude of both I_Ca and maximal macroscopic Ca²⁺ conductance (G_max) by about 50%. However, no differences were found in either voltage dependence of inactivation or the amount of maximal immobilization-resistant charge movement (Q_max). Thus, the effect on I_Ca could be explained by a higher channel's open probability (P_o) rather than a greater abundance of channel proteins. Additional data indicate that AKF-PD reduces the rate of Ca²⁺ extrusion in the presence of caffeine, suggesting inhibition of the Na/Ca exchanger. Overall, these results indicate that AKF-PD upregulates the P_o of Ca_V1.2 and then sequentially enhances I_Ca, CICR, and contractility. Therefore, the novel compound is also a candidate to be tested in cardiac disease models.