Facilitation and Ca2+-dependent Inactivation Are Modified by Mutation of the Cav1.2 Channel IQ Motif

The heart muscle responds to physiological needs with a short-term modulation of cardiac contractility. This process is determined mainly by properties of the cardiac L-type Ca2+ channel (Ca(v)1.2), including facilitation and Ca2+-dependent inactivation (CDI). Both facilitation and CDI involve the interaction of calmodulin with the IQ motif of the Ca(v)1.2 channel, especially with Ile-1624. To verify this hypothesis, we created a mouse line in which Ile-1624 was mutated to Glu (Ca(v)1.2(I1624E) mice). Homozygous Ca(v)1.2(I1624E) mice were not viable. Therefore, we inactivated the floxed Ca(v)1.2 gene of heterozygous Ca(v)1.2(I1624E) mice by the alpha-myosin heavy chain-MerCreMer system. The resulting I/E mice were studied at day 10 after treatment with tamoxifen. Electrophysiological recordings in ventricular cardiomyocytes revealed a reduced Ca(v)1.2 current (I-Ca) density in I/E mice. Steady-state inactivation and recovery from inactivation were modified in I/E versus control mice. In addition, voltage-dependent facilitation was almost abolished in I/E mice. The time course of I-Ca inactivation in I/E mice was not influenced by the use of Ba2+ as a charge carrier. Using 1,2-bis(o-aminophenoxy)ethane-N,N,N ',N '-tetraacetic acid as a chelating agent for intracellular Ca2+, inactivation of ICa was slowed down in control but not I/E mice. The results show that the I/E mutation abolishes Ca2+/calmodulin-dependent regulation of Ca(v)1.2. The Ca(v)1.2(I1624E) mutation transforms the channel to a phenotype mimicking CDI.