Voltage-Driven Ca²⁺ Binding at the L‑Type Ca²⁺ Channel Triggers Cardiac Excitation–Contraction Coupling Prior to Ca²⁺ Influx

Abstract

The activation of the ryanodine Ca²⁺ release channels (RyR2) by the entry of Ca²⁺ through the L-type Ca²⁺ channels (Cav1.2) is believed to be the primary mechanism of excitation–contraction (EC) coupling in cardiac cells. This proposed mechanism of Ca²⁺-induced Ca²⁺ release (CICR) cannot fully account for the lack of a termination signal for this positive feedback process. Using Cav1.2 channel mutants, we demonstrate that the Ca²⁺-impermeable α₁1.2/L775P/T1066Y mutant introduced through lentiviral infection into neonate cardiomyocytes triggers Ca²⁺ transients in a manner independent of Ca²⁺ influx. In contrast, the α₁1.2/L775P/T1066Y/4A mutant, in which the Ca²⁺-binding site of the channel was destroyed, supports neither the spontaneous nor the electrically evoked contractions. Ca²⁺ bound at the channel selectivity filter appears to initiate a signal that is conveyed directly from the channel pore to RyR2, triggering contraction of cardiomyocytes prior to Ca²⁺ influx. Thus, RyR2 is activated in response to a conformational change in the L-type channel during membrane depolarization and not through interaction with Ca²⁺ ions diffusing in the junctional gap space. Accordingly, termination of the RyR2 activity is achieved when the signal stops upon the return of the L-channel to the resting state. We propose a new model in which the physical link between Cav1.2 and RyR2 allows propagation of a conformational change induced at the open pore of the channel to directly activate RyR2. These results highlight Cav1.2 as a signaling protein and provide a mechanism for terminating the release of Ca²⁺ from RyR2 through protein–protein interactions. In this model, the L-type channel is a master regulator of both initiation and termination of EC coupling in neonate cardiomyocytes