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

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 alpha(1), 1.2/L775P/T1066Y mutant introduced through lentiviral infection into neonate cardiomyocytes triggers Ca2+ transients in a manner independent of Ca2+ influx. In contrast, the alpha(1)1.2/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.