Inhibition of mitochondrial Na ⁺-dependent Ca ²⁺ efflux by 17β-estradiol in the rat hippocampus

Our results, as well as those of others, have indicated that 17β-estradiol (E2) exerts its nongenomic effects in neuronal cells by affecting plasma membrane Ca ²⁺ flux. In neuronal cells mitochondria possess Ca ²⁺ buffering properties as they both sequester and release Ca ²⁺. The goal of this study was to examine the rapid non-genomic effect of E2 on mitochondrial Ca ²⁺ transport in hippocampal synaptosomes from ovariectomised rats. In addition, we aimed to determine if, and to what extent, E2 receptors participated in mitochondrial Ca ²⁺ transport modulation by E2 in vitro. E2-specific binding and Ca ²⁺ transport was monitored. At physiological E2 concentrations (0.1-1.5 nmol/L), specific E2 binding to mitochondria isolated from hippocampal synaptosomes was detected with a B _max. and K _m of 37.6±2.6 fmol/mg protein and 0.69±0.14 nmol/L of free E2, respectively. The main mitochondrial Ca ²⁺ influx mechanism is the Ruthenium Red-sensitive uniporter driven by mitochondrial membrane potential. Despite no effect of E2 on Ca ²⁺ influx, a physiological E2 concentration (0.5 nmol/L) protected mitochondrial membrane potential and consequently Ca ²⁺ influx from the uncoupling agent carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (1 μmol/L). In neuronal cells the predominant mitochondrial Ca ²⁺ efflux mechanism is the Na ⁺/Ca ²⁺ exchanger. E2 caused Ca ²⁺ efflux inhibition (by 46%) coupled with increased affinity of the Na ⁺/Ca ²⁺ exchanger for Na ⁺. Using E2 receptor (ERα and ERβ) antagonists and agonists, we confirmed ERβ's involvement in E2-induced mitochondrial membrane potential protection as well as Ca ²⁺ efflux inhibition. In summary, our results indicate that the non-genomic neuromodulatory role of E2 in rat hippocampus is achieved by affecting mitochondrial Ca ²⁺ transport via, in part, mitochondrial ERβ.