In vivo androgen treatment shortens the QT interval and increases the densities of inward and delayed rectifier potassium currents in orchiectomized male rabbits.

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.

Differential rate responses to nicotine in rat heart: evidence for two classes of nicotinic receptors.

Nicotinic acetylcholine receptors are pentameric, typically being composed of two or more different subunits. To investigate which receptor subtypes are active in the heart, we initiated a series of experiments using an isolated perfused rat heart (Langendorff) preparation. Nicotine administration (100 microM) caused a brief decrease (-7 +/- 2%) followed by a much larger increase (17 +/- 5%) in heart rate that slowly returned to baseline within 10 to 15 min. The nicotine-induced decrease in heart rate could be abolished by an alpha7-specific antagonist, alpha-bungarotoxin (100 nM). In contrast, the nicotine-induced increase in heart rate persisted in the presence of alpha-bungarotoxin. These results suggest that the nicotinic acetylcholine receptors (nAChRs) that mediate the initial decrease in heart rate probably contain alpha7 subunits, whereas those that mediate the increase in heart rate probably do not contain alpha7 subunits. To investigate which subunits may contribute to the nicotine-induced increase in heart rate, we repeated our experiments with cytisine, an agonist at nAChRs that contain beta4 subunits. The cytisine results were similar to those obtained with nicotine, thereby suggesting that the nAChRs on sympathetic nerve terminals in the heart probably contain beta4 subunits. Thus, the results of this study show that pharmacologically distinct nAChRs are responsible for the differential effects of nicotine on heart rate. More specifically, our results suggest that alpha7 subunits participate in the initial nicotine-induced heart rate decrease, whereas beta4 subunits help to mediate the subsequent nicotine-induced rise in heart rate.