We have demonstrated that in the isolated perfused rat heart, EGF increases contractility and heart rate by elevating cAMP accumulation. Moreover, despite the presence of functional EGF receptors in the myocytes and non-myocytes derived from rat hearts, EGF increases cAMP accumulation only in myocytes. The increase in cAMP accumulation is the result of activation of cardiac adenylyl cyclase by a mechanism involving the participation of G protein which, employing a specific antibody, we have identified to be Gsalpha. More recently, we have demonstrated that the EGF receptor protein tyrosine kinase (EGFRK) activity is important for EGF-elicited stimulation of cardiac adenylyl cyclase. Since the EGFRK and Gsalpha are necessary for EGF to stimulate cardiac adenylyl cyclase, our studies will investigate mechanism(s) which allow interactions between the EGFRK and adenylyl cyclase signaling components. Moreover, since EGF increases cAMP accumulation and stimulation of adenylyl cyclase in some cells and tissues only, we will investigate whether expression of specific isoforms of the signaling elements (Gsalpha and/or adenylyl cyclase) determine the responsiveness of adenylyl cyclase to EGF. The first hypothesis to be investigated under aim l is that EGFRK phosphorylates Gsalpha. Phosphorylation of Gsalpha may be manifested as increased Gsalpha activity of increased coupling between Gsalpha and adenylyl cyclase. The second hypothesis to be tested (aim 2) is based on the premise that in addition to phosphorylation of adenylyl cyclase signaling elements, upon autophosphorylation of the EGF receptor, and alteration in its conformation, region(s) within the EGF receptor are exposed which can directly interact with - Gsalpha. Finally, studies in aim 3 are proposed to address the hypothesis that expression of specific adenylyl cyclase isozymes and/or Gsalpha isoforms confer specificity to the ability of EGF to stimulate adenylyl cyclase.
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