Sex differences in the cardiac beta-adrenergic receptor contractile response

Premenopausal females are at a reduced risk for developing cardiovascular disease as compared to males; this sex difference is not present following menopause. beta-adrenergic receptors are pharmaceutical targets in the treatment of cardiovascular disease, and sex differences in beta-adrenergic responsiveness have been demonstrated. However, limited studies have addressed the mechanism(s) underlying these differences. To investigate these sex differences, studies were performed using isolated perfused hearts from male, intact female and ovariectomized female mice, as well as male and female ventricular myocytes. Female hearts exhibited blunted contractile responses to the beta-adrenergic receptor agonist isoproterenol (ISO) compared to males but not ovariectomized females. The role of the A1 adenosine receptor (A1AR) in antagonizing the beta-adrenergic contractile response was investigated using the A 1AR agonist CCPA and A1AR knockout (KO) mice. Intact females showed an enhanced A1AR anti-adrenergic effect compared to males and ovariectomized females. The beta-adrenergic contractile response was potentiated in male and female A1ARKO hearts, with sex differences no longer present. Forskolin (FSK), IBMX and CPT-cAMP dose response studies were performed in the isolated heart to investigate the role of adenylyl cyclase, phosphodiesterase and the cAMP signaling cascade, respectively, in generating sex differences in the beta-adrenergic contractile response. There were essentially no sex differences in the contractile responses to FSK, IBMX or CPT-cAMP. There were no sex differences in the expression of the beta 1R, beta2R, or A1AR gene in ventricular myocytes, nor were there differences in the expression of Gsalpha protein, Gi-2alpha protein, PDE4D, PP1 or PP2A as determined with western blotting. Adenylyl cyclase V/VI was expressed in significantly higher levels in female myocytes. Females showed enhanced cAMP accumulation upon ISO treatment with and without PDE4 inhibition. No sex differences were present in troponin I phosphorylation at 10 or 100 nM ISO, while females displayed enhanced phospholamban phosphorylation only at 100 nM ISO. Females appear to show compensatory increases in adenylyl cyclase expression, cAMP accumulation and phospholamban phosphorylation. The mechanism(s) for the reduced beta-adrenergic contractile response in female hearts remains unresolved.