Date

6-21-2006

UMMS Affiliation

Graduate School of Biomedical Sciences, MD/PhD Program

Document Type

Dissertation, Doctoral

Subjects

Myocardial Contraction; Receptor, Adenosine A1; Receptor, Adenosine A2A; Adenosine; Phosphoprotein Phosphatase; Heart; Mice; Academic Dissertations; Dissertations, UMMS

Disciplines

Life Sciences | Medicine and Health Sciences | Physiology

Abstract

Adenosine is a nucleoside that plays an important role in the regulation of contractility in the heart. Adenosine receptors are G-protein coupled and those implicated in regulation of contractility are presumed to act via modulating the activity of adenylyl cyclase and cAMP content of cardiomyocytes. Adenosine A1 receptors (A1R) reduce the contractile response of the myocardium to β-adrenergic stimulation. This is known as anti adrenergic action. The A2A adenosine receptor (A2AR) has the opposite effect of increasing contractile responsiveness of the myocardium. The A2AR also appears to attenuate the effects of A1R. The effects of these receptors have been primarily studied in the rat heart and with the utilization of cardiomyocyte preparations. With the increasing use of receptor knockout murine models and murine models of various pathological states, it is of importance to comprehensively study the effects of adenosine receptors on regulation of contractility in the murine heart. The following studies examine the adenosinergic regulation of myocardial contractility in isolated murine hearts. In addition, adenosinergic control of contractility is examined in hearts isolated from A2AR knockout animals. Responses to adenosinergic stimulation in murine isolated hearts are found to be comparable to those observed in the rat, with A1R exhibiting an anti adrenergic action and A2AR conversely enhancing contractility. A significant part of the A2AR effect was found to occur via inhibition of the A1R antiadrenergic action. A part of the anti adrenergic action of A1R has previously been shown to be the result of protein phosphatase 2A activation and localization to membranes. Additional experiments in the present study examine the effect of adenosinergic signaling on PP2A in myocardial extracts from wild type and A2AR knockout hearts. A2AR activation was found to decrease the activity of PP2A and enhance localization of the active enzyme to the cytosol; away from its presumed sites of action. In the A2AR knockout the response to A1R activation was enhanced compared with the wild type and basal PP2A activity was reduced. It is concluded that A2AR modulation of PP2A activity may account for the attenuation of the A1R effect by A2AR observed in the contractile studies.