The impact of estrogen and the cAMP/PKA pathway on male-female differences in SR calcium release in murine cardiomyocytes
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This study investigated the cellular mechanisms involved in the initiation and regulation of Ca2+ release and contraction in ventricular myocytes from male and female mice. Results showed that Ca2+ transients were smaller in myocytes from females in comparison to males, although Ca2+ currents were similar. This was accompanied by a reduction in the gain of sarcoplasmic reticulum (SR) Ca2+ release due to smaller subcellular SR Ca2+ release events (Ca2+ sparks). As cardiac contraction is regulated by the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) pathway, its contributions were compared between the sexes. Results showed that basal cAMP levels were lower in females and that PKA inhibition with H-89 abolished basal sex differences in Ca2+ release. Interestingly, the expression of phosphodiesterase type 4B (PDE4B) in female ventricles was increased in comparison to males, which could explain lower cAMP levels in females. Inhibition of PDE4 with rolipram also removed male-female differences in SR Ca2+ release. These results suggest that estrogen may suppress SR Ca2+ release through cAMP/PKA-dependent mechanisms. This was further investigated by comparing SR Ca2+ release mechanisms in ovariectomized (OVX) female mice with sham-operated controls. Ca2+ transients, contractions and SR Ca2+ stores were increased by OVX. These differences were abolished by PKA inhibition or dialysis of sham and OVX myocytes with the same cAMP concentration. Although basal cAMP levels were similar in sham and OVX cells, treatment with a non-selective PDE inhibitor caused a larger increase in OVX than sham, which suggests that cAMP production was increased. Further, PDE4A expression was increased by OVX, suggesting that the breakdown of cAMP was higher. These findings suggest that estrogen may suppress SR Ca2+ release by PKA-dependent mechanisms and that estrogen affects these mechanisms at least in part by regulating cAMP production and breakdown in cardiomyocytes. Lower SR Ca2+ release in pre-menopausal females may limit the ability of female hearts to augment contractile function in response to exercise, although this may also protect against cardiovascular disease by reducing Ca2+ overload. Loss of these protective effects of estrogen may help explain the increased incidence of cardiovascular disease in post- menopausal women.