Regulation of the Cardiac L-Type Calcium Channel by the Cyclic Nucleotide Cross-Talk Signaling Network

Abstract

Regulation of L-type Calcium (Ca2+) channel (LCC) gating is critical to shaping the cardiac action potential (AP) and excitation-contraction coupling (ECC), the process by which electrical excitation leads to mechanical contraction of cardiac myocytes. The cyclic nucleotide (cN) cross-talk signaling network, which encompasses the β-adrenergic and the Nitric Oxide (NO)/cGMP/Protein Kinase G (PKG) pathways and their interaction through distinctively-regulated phosphodiesterase isoenzymes (PDEs), regulates LCC current via Protein Kinase A (PKA)- and PKG-mediated phosphorylation. Due to tightly-coupled biochemical reactions, it remains to be clarified how LCC gating is ultimately regulated by this signaling network. In addition, the large number of potential ECC-related phosphorylation targets of PKA and PKG makes it difficult to quantify and isolate the effect of LCC regulation. We have developed a multi-scale, biophysically-detailed computational model of LCC regulation by the cN signaling network based on a wide range of experimental data. Changes in LCC current properties arising from interaction with PKA and PKG are modeled by redistributing the number of channels among four gating modes, corresponding to the four channel phosphorylation states: non-, PKA-, PKG-, and PKA-and-PKG- phosphorylated. The model exhibits experimentally observed single LCC gating characteristics, as well as whole-cell LCC current. Simulations predict how LCC gating modes are redistributed in response to a variety of simulation protocols. Results distinguish LCC regulation exerted between two mechanisms which are difficult to resolve by experimentation alone: 1) LCC interaction with activated PKA and PKG and 2) regulation of PKA and PKG activation as a result of cN cross-talk. Consequently, the model reveals underlying mechanisms that explain LCC current observations under various stimulation scenarios of the signaling network. These results provide insights into how cN signals are integrated via LCC regulation.