Objectives: Women have longer rate-corrected QT intervals (QTc) and are at higher risk for developing life-threatening torsades de pointes ventricular arrhythmias than men, especially after taking medications that block cardiac human ether-a-go-go-related gene (HERG)-encoded K(+) channels. The purpose of the present study was to determine if the male sex steroid hormone, dihydrotestosterone (DHT), influences QT intervals in orchiectomized (Orch) male rabbits.
Methods: ECG and whole-cell patch-clamp analyses were employed to evaluate cardiac repolarization and K(+) currents in hearts isolated from orchiectomized (Orch) male New Zealand White rabbits receiving subcutaneous sustained release pellets for either dihydrotestosterone (DHT) or placebo. The efficacy of the treatment paradigm was monitored by measuring plasma DHT concentrations before and after the treatment period (10-14 days).
Results: The results show that rate- and drug-induced QT-lengthening is attenuated in hearts from DHT-treated rabbits relative to placebo-treated controls. No significant changes in QRS were observed in response to DHT, thereby indicating that DHT influences QT primarily through an effect on ventricular repolarization. In addition, hearts from DHT-treated rabbits displayed significantly less QT lengthening in response to quinidine challenge compared to placebo controls. Current densities for two important cardiac repolarizing K(+) currents, I(K1) and I(Kr), were found to be significantly increased in ventricular myocardium of DHT-treated rabbits. Further, the half-maximal voltage of activation (V(1/2)) for I(Kr) was significantly shifted to more negative potentials in myocytes from DHT vs. placebo hearts (21.2+/-1.2 vs. 30.2+/-1.4 mV, respectively, n=12, P<0.001). Corresponding changes in rabbit ether-a-go-go-related gene (RERG) mRNA were not found when examined by Northern blot hybridization.
Conclusions: These results suggest that the presence of male sex steroid hormones in male rabbits helps to suppress rate- and drug-induced delays in cardiac repolarization. DHT action produces increased current densities for I(K1) and I(Kr) and a left-shift in the V(1/2) for I(Kr) that could account, at least in part, for the observed QTc differences between males and females. Since little change was seen in ventricular RERG gene expression, DHT action in the heart may influence I(Kr) via post-transcriptional and/or post-translational mechanisms.