The cardiac L-type calcium channel (CaV1.2) plays a major role in excitation-contraction coupling. Dysfunctions, such as based on genetic mutations, may lead to intracellular calcium overload and arrhythmias. In this study, we are investigating the impact of two different mutations affecting mostly CaV1.2 inactivation (G406R and L566P) on action potential (AP) and calcium content. Timothy syndrome (G406R) is characterized by a near-complete loss of voltage-dependent inactivation. As this mutation is found in exon 8A, only 11.5% of cardiac CaV1.2 channels express this mutation in heterozygous patients. The novel L566P mutation shows a much less pronounced loss of inactivation, but the mutation is not located in an alternatively spliced exon and thus in patients 50% of the channels are affected by the mutation. We integrated the patch clamp data of both mutations into the human ventricular myocyte model of ten Tusscher and Panfilov by adjusting the steady-state voltage-dependent inactivation and the respective inactivation time constants. 11.5% (G406R) or 50% (L566P) mutant current were added to respective fractions of wild-type (WT) current to represent heterozygous state. Although the recorded steady-state inactivation of both mutants appears quite different, the simulated heterozygous behavior is very similar up to a voltage of −10mV. AP prolongation is significantly increased in G406R (77ms) compared to L566P (20ms), mainly due to a higher resting calcium content in the sarcoplasmic reticulum (SR; WT: 3.1 μM; L566P: 3.2 μM; L566P: 4.4 μM). As a large fraction of the AP is above −10 mV, the increased loss of inactivation of G406R leads to a stronger influx of calcium via CaV1.2 compared to WT or L566P. This may explain both the longer QT interval and the higher arrhythmogenic potential of G406R observed in patients, as a higher SR calcium content increases the risk for spontaneous calcium releases and thus afterdepolarizations.