Regional genomic regulation of cardiac sodium-calcium exchanger by oestrogen

Non-technical summary Women of child bearing age are known to have longer QT intervals on their electrocardiogram recordings due to sex-differences in the heart that are mediated by the sex hormone oestrogen. This results in a higher risk of lethal arrhythmias in women when their QT interval is prolonged further through drugs or inherited congenital diseases (i.e. long QT syndrome). Using rabbit hearts, which exhibit the same sex-differences as in humans, it is here shown for the first time that oestrogen up-regulates the sodium-calcium exchanger (NCX), a critical protein that regulates calcium ions in the heart and thereby alters the force of contraction, but only at the base of the ventricles (where the heart has its widest cross section). This effect accounts for the greater risk of long QT-related arrhythmias in women.Female rabbit hearts are more susceptible to torsade de pointes (TdP) in acquired long QT type 2 than males, in-part due to higher L-type Ca2+ current (I-Ca,I-L) at the base of the heart. In principle, higher Ca2+ influx via I-Ca,I-L should be balanced by higher efflux, perhaps mediated by parallel sex differences of sodium-calcium exchange (NCX) current (I-NCX). We now show that NCX1, like Cav1.2 alpha, is greater at the base of female than male left ventricular epicardium and greater at the base than at the apex in both sexes. In voltage-clamp studies, inward (0, +20 mV, P < 0.04) and outward (-80, -60, -40, -20 mV, P < 0.01) I-NCX densities were significantly higher (1.5-2 fold) in female base compared to apex and male (base and apex) myocytes. Myocytes were incubated +/- 17 beta-oestradiol (E2 = 1 nm) and I-NCX was measured on days 0, 1, 2 and 3. Inward and outward I-NCX decreased over 2 days in female base myocytes becoming similar to I-NCX at the apex. E2 incubation (24 h) increased NCX1 (50%) and I-NCX (similar to 3-fold at 60 mV) in female base but not endocardium, apex or in male base myocytes. I-NCX upregulation by E2 was blunted by an oestrogen receptor (ER) antagonist (fulvestrant, 1 mu m), and inhibition of transcription (actinomycin D, 5 mu g ml-1) or translation (cycloheximide, 20 mu g ml-1). Dofetilide (an I-Kr blocker) induced early afterdepolarizations (EADs) in female base myocytes cultured for 1 day if incubated with E2, but not without E2 or with E2+KB-R4973 (an I-NCX inhibitor), E2+fulvestrant or E2 with apex myocytes. Thus, E2 upregulates NCX1 by a genomic mechanism mediated by ERs, and de novo mRNA and protein biosynthesis, in a sex- and region-dependent manner which contributes to the enhanced propensity to EADs and TdP in female hearts.