Compartmentalization of cardiac A2A-rezeptors

Adenosine has long been known to exert many actions on the heart: it is well understood, how myocardial A 1 - adenosine receptors elicit negative chronotropic and dromotropic effects (and stimulation of myocardial A 1 - receptors is exploited in clinical pharmacotherapy to abolish paroxysmal supraventricular tachycardia). Similarly, adenosine is known to regulate coronary blood flow via stimulation of A 2A-receptors. In contrast, it is not clear, why the myocardium expresses A2A-receptors. In fact, their functional relevance has been questioned. In the current application, I propose two models (which are not mutually exclusive) to address the discrepancies and divergent observations in the published literature. The first model assumes that myocardial A 2A-receptors predominantly reside within the cell and that these receptors can be mobilized and traffic to the cell surface due to the action of the deubiquitinating enzyme USP4. The working hypothesis underlying the second model postulates that the A2A-receptors are localized to specific subcompartments of the plasma membrane. The aim of the current proposal is to verify the predictions of these models and explore their physiological implications. Specifically, I propose to test, whether (i) USP4 enhances the cell surface expression of A2A-receptors in cardiac myocytes (ii) hypoxic stimuli increase the cell surface expression of A2A-receptors in cardiac myocytes (iii) there is a link between (i) and (ii), i.e. whether hypoxia increases the expression of USP4 (iv) this enhanced cell surface expression of A2A-receptors translates into increased cAMP accumulation and enhanced functional responses (i.e. on L-type Ca2+-channels and on the speed of cell shortening as an index for changes in contractility) (v) the mobility of the A2A-receptor is restricted when compared to that of the ß 1 -adrenergic receptor (vi) cAMP accumulation in response to A2A-receptor stimulation is locally confined. It is evident that the insights generated in the current grant application may have repercussions on our general understanding of how the activity of G protein-coupled receptors is regulated. In addition, the insights may be pertinent to understand how signals are compartmentalized in the heart and how hypoxia can give rise to cardiac arrhythmia. Similarly, adenosine is known to participate in myocardial preconditioning and myocardial A2A- receptors may be relevant under certain conditions. Thus, the work is eventually likely to have clinical implications.