A summary histogram of ER and ER protein density confirmed the uniform distribution of ERs. oestrogen acts by interacting with oestrogen receptors (ER) but not ER which may explain why hormone replacement therapy increases the risk of arrhythmia and offers a possible protective solution of using an oestrogen mimetic that selectively binds to Pirazolac ER. == Abstract == In type-2 long QT (LQT2), adult women and adolescent boys have a higher risk of lethal arrhythmias, called Torsades de pointes (TdP), compared to the opposite sex. In rabbit hearts, similar sex- and age-dependent TdP risks were attributed to higher expression levels of Pirazolac L-type Ca2+channels and Na+Ca2+exchanger, at the base of the female epicardium. Here, the effects of oestrogen and progesterone are investigated to elucidate the mechanisms wherebyICa,Ldensity is upregulated in adult female rabbit hearts.ICa,Ldensity was measured by the whole-cell patch-clamp technique on days 03 in cardiomyocytes isolated from the base and apex of adult female epicardium. PeakICa,Lwas 28% higher at the base than apex (P< 0.01) and decreased gradually (days 03), becoming similar to apex myocytes, which had stable currents for 3 days. Incubation with oestrogen (E2, 0.11.0 nm) increasedICa,L(2-fold) in female base but not endo-, apex or male myocytes. Progesterone (0.110 m) had no effect at base myocytes. An agonist of the - (PPT, 5 nm) but not the - (DPN, 5 nm) subtype oestrogen receptor (ER/ER) upregulatedICa,Llike E2. Western blots detected similar levels of ER and ER in male and female hearts at the base and apex. E2 increased Cav1.2 (immunocytochemistry) and mRNA (RT-PCR) levels but did not changeICa,Lkinetics.ICa,Lupregulation by E2 was suppressed by the ER antagonist ICI 182,780 (10 m) or by inhibition of transcription (actinomycin D, 4 m) or protein biosynthesis Rabbit polyclonal to CD80 (cycloheximide, 70 m). Therefore, E2 upregulatesICa,Lby a regional genomic mechanism involving ER which is a Pirazolac known determinant of sex differences in TdP risk in LQT2. == Introduction == The congenital form of long QT type 2 (LQT2) is caused by mutations of the K+channel protein HERG that result in a loss of function of the rapid component of delayed rectifying K+current,IKr, a prolongation of the action potential duration (APD) and QT interval (Moritaet al.2008)). Although the incidence of all forms of congenital LQT is rare (<1/5000), drug-induced LQT2 remains a serious public health problem because a wide range of cardiac and non-cardiac drugs suppressIKr, prolong APDs and promote early afterdepolarizations (EADs) that lead to Torsade de pointes (TdP) (Splawskiet al.2000;Vincent, 2000;Drici & Clement, 2001;Levineet al.2008;Moritaet al.2008)). Women are known to be at higher risk to congenital and acquired forms of TdP (Makkaret al.1993;Coker, 2008)) but in adolescents (<14 years old) before the surge of sex steroids, the risk of TdP is reversed, with boys being more susceptible to TdP (Goldenberget al.2008)). Rabbits exhibit the same sex differences in arrhythmia risk with adult females (>8 weeks) being more prone to TdP and the arrhythmia phenotype being reversed in young rabbits (Liuet al.2005)) (<42 days), before the surge of steroids (de Turckheimet al.1983)). In females, ovariectomy (OVX) reduced dofetilide-induced APD prolongation and EADs, whereas 17-oestradiol (E2) replacement promoted EADs (Driciet al.1996;Haraet al.1998;Phamet al.2001)). These studies suggest that E2 promotes TdP in female hearts. There is general agreement that TdP is initiated by EADs that are caused by the re-activation of L-type Ca2+channels. However, controversies persist regarding the mechanisms that re-activate the L-type Ca2+current (ICa,L) during long APs and whether or not an elevation of intracellular Ca2+() precedes and initiates EADs. Some studies found that the re-activation ofICa,Loccurred spontaneously, independent of Ca2+release from the sarcoplasmic reticulum (SR) because in ferret hearts, ryanodine and chelation of intracellular Ca2+interrupted delayed afterdepolarizations (DADs) but did not alter EADs (Marbanet al.1986)). Alternatively, long APDs can cause an imbalance between Ca2+influx and efflux, resulting in SR Ca2+overload which promotes spontaneous SR Ca2+release then activation of a forward-mode Na+Ca2+exchanger (NCX) current,INCX, which can depolarize the plateau potential to re-activateICa,L(Volderset al.2000)). Dual optical mapping of APs andtransients in the Langendorff rabbit model of drug-induced LQT2 revealed that adult females were more prone to EADs and TdP and that the arrhythmia phenotype was reversed in pre-pubertal hearts (Liuet al.2005)).elevation preceded EAD upstrokes at the origins of EADs and when paced at 1.2 s cycle length, markedoscillations preceded the occurrence of EADs (Choiet al.2002;Nemecet al.2010)). In pre-pubertal male and adult female hearts with LQT2, EADs originated at the base and not the apex of the epicardium (Simset al.2008)). In freshly isolated ventricular myocytes,.