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Abstract-In congenital and acquired long QT type 2, women are more vulnerable than men to Torsade de Pointes. In prepubertal rabbits (and children), the arrhythmia phenotype is reversed; however, females still have longer action potential durations than males. Thus, sex differences in K ϩ channels and action potential durations alone cannot account for sex-dependent arrhythmia phenotypes. The L-type calcium current (I Ca,L ) is another determinant of action potential duration, Ca 2ϩ overload, early afterdepolarizations (EADs), and Torsade de Pointes. Therefore, sex, age, and regional differences in I Ca,L density and in EAD susceptibility were analyzed in epicardial left ventricular myocytes isolated from the apex and base of prepubertal and adult rabbit hearts. In prepubertal rabbits, peak I Ca,L at the base was 22% higher in males than females (6.4Ϯ0.5 versus 5.0Ϯ0.2 pA/pF; PϽ0.03) and higher than at the apex (6.4Ϯ0.5 versus 5.0Ϯ0.3 pA/pF; PϽ0.02). Sex differences were reversed in adults: I Ca,L at the base was 32% higher in females than males (9.5Ϯ0.7 versus 6.4Ϯ0.6 pA/pF; PϽ0.002) and 28% higher than the apex (9.5Ϯ0.7 versus 6.9Ϯ0.5 pA/pF; PϽ0.01). Apex-base differences in I Ca,L were not significant in adult male and prepubertal female hearts. Western blot analysis showed that Ca v 1.2␣ levels varied with sex, maturity, and apex-base, with differences similar to variations in I Ca,L ; optical mapping revealed that the earliest EADs fired at the base. Single myocyte experiments and Luo-Rudy simulations concur that I Ca,L elevation promotes EADs and is an important determinant of long QT type 2 arrhythmia phenotype, most likely by reducing repolarization reserve and by enhancing Ca 2ϩ overload and the propensity for I Ca,L reactivation. (Circ Res. 2008;102:e86 -e100.)

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