Male and female individuals display differences in the cardiovascular phenotype, e.g. the QT interval in the electrocardiogram (ECG) is shorter in men than in women. Sex hormones like androgens and oestrogens may be responsible for these gender specific cardiovascular characteristics. A cardioprotective action of oestrogens has been reported in various studies, especially since premenopausal women suffer less often from heart diseases than men (Review: Babiker et al., 2002). Oestrogens operate via oestrogen receptor alpha (ERα) and beta (ERβ) which both act as transcription factors. ERα and ERβ deficient mice were investigated here to characterize the influence of ER dependent signalling on the cardiovascular system. In ERα deficient male and female mice and their homozygous (+/+) littermates (WT) heart weight (HW), peripheral and left ventricular blood pressure and ECGs were measured. ECGs and blood pressure were recorded under anaesthesia as approved by the local committee for animal research (3% isoflurane for handling, 1% for recording). After blood pressure recording hearts were excised from anaesthetised ERα and ERβ deficient mice and their WT littermates. Single cardiac myocytes were isolated from hearts retrogradely perfused by trypsin and collagenase. In externally stimulated cells sarcomere shortening was monitored optically at frequencies between 0.5 and 10 Hz. Action potentials (APs) as well as L-type calcium current (I_Ca,L) were recorded by patch clamp electrodes in whole cell mode. All single cell experiments were performed at 36°C. APs were elicited at the same stimulation frequencies as shortenings. Western blot analyses were used to examine ERα dependent expression of sarcoplasmic reticulum Ca-ATPase 2a (SERCA) and phospholamban (PL). Body weight (BW) and HW were significantly smaller in female WT compared to male WT. ERα deficiency increased BW and HW in females but decreased HW in males resulting in equilibration of HW and HW/tibia length differences. In the ECG the QTc interval was shorter in male than in female WT hearts. ERα deficiency did not affect QTc in males, but in ERα deficient females the QTc interval was shortened to the level of males. Mean arterial blood pressure (MABP) as well as left ventricular systolic pressure (LVSP) were higher in male than in female WT. ERα deficiency lowered MABP and LVSP in both genders and pressures of ERα deficient males approached those of female WT. In WT isolated cells sarcomere shortening exhibited a biphasic shortening frequency relation with minimum around 2 Hz. Gender differences in shortening were not detected among WT cells. ERα deficiency reduced sarcomere shortening in both genders, but more pronounced in females. ERβ deficiency, however, did not affect shortening frequency relation neither in male nor in female cells, therefore ERβ deficient hearts were not analysed further. In murine cardiac cells increasing stimulation frequency is accompanied by a prolongation of AP duration (APD), e.g. APD₉₀ in female WT was 34 ± 10 ms (± SD; n=32) at 0.5 Hz and 49 ± 15 ms (± SD; n=18) at 6 Hz. AP duration in WT cardiac myocytes was not different among genders. In contrast to QT interval APD was not affected by ERα deficiency. In WT cells of both genders I_Ca,L exhibited the typical bell shaped voltage dependency with a maximum of 10 ± 3 pA/pF (± SD; n=12) at 0 mV clamp potential. In male cardiomyocytes ERα deficiency lead to an increase in I_Ca,L current amplitude, whereas I_Ca,L in female cells remained unchanged. Expression of SERCA and PL did not differ between female WT and ERα deficient hearts, but both proteins were reduced in male ERα deficient hearts compared to male WT hearts. The increase of I_Ca,L in male ERα deficient cells seems to be counterbalanced by a down regulation of SERCA and PL. Gender differences in heart weight, peripheral and LV blood pressure as well as in QT interval were abolished by ERα deficiency. Decreased sarcomere shortening of isolated ERα deficient cardiomyocytes seems to be in agreement with reduced LV pressure. However, neither APD nor changes I_Ca,L are able to explain the in vivo recorded differences nor the reduced sarcomere shortening. Interestingly ERα deficiency causes significant changes in cardiac phenotype of both female and male mice. Taken together, ERα dependent signalling plays an important role in the development of gender based differences in cardiac phenotype.