Access Type

Open Access Dissertation

Date of Award

January 2012

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Physiology

First Advisor

Robert Lasley

Abstract

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 A1AR 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 &beta1R, &beta2R, or A1AR gene in ventricular myocytes, nor were there differences in the expression of Gsá protein, Gi-2á 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.

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