Regulation of Renal Cortical Adenosine Levels

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DESCRIPTION (provided by applicant): Because the renal cortex expresses high affinity type 1 adenosine receptors that modulate preglomerular microvascular tone, renin release and sodium reabsorption, it is important to understand how renal cortical interstitial levels of adenosine are regulated. Because our comprehension of the regulation of renal cortical interstitial adenosine is rudimentary, the overall purpose of this proposal is to further our knowledge in this regard. The venous drainage of the pancreas empties directly into the portal circulation, an anatomical arrangement that maximizes concentrations, and therefore effects, of pancreatic hormones on hepatocytes. Glucagon is a pancreatic hormone secreted into the portal circulation. Importantly, glucagon is a powerful stimulant of hepatic adenylyl cyclase, and activation of hepatic adenylyl cyclase causes release of large quantities of cyclic AMP into the systemic circulation. We hypothesize that systemic cyclic AMP (secreted from the liver in response to glucagon) is delivered to the renal cortex via the dense peritubular capillary network and is metabolized in the renal cortex to adenosine via the sequential actions of ectophosphodiesterase (converts cyclic AMP to AMP) and ecto-5'-nucleotidase (converts AMP to adenosine). In this view, the liver secretes an endocrine pro-hormone (cyclic AMP) that is metabolized locally in the target tissue (renal cortical interstitial space) to a biologically active hormone (adenosine) via a specific set of enzymes (ecto-phosphodiesterase and ecto-5'-nucleotidase). The specific goal of this proposal is to test this innovative hypothesis using our newly developed and unique LC/MS ion trapping assay for purines. The proposed mechanism will be addressed both in vitro and in vivo. In vitro we will determine whether cyclic AMP added to the basolateral aspect of proximal convoluted tubules is rapidly metabolized to adenosine by the proposed enzymes. In vivo we will determine whether the appropriate maneuvers appropriately influence the levels of cyclic AMP and adenosine in the renal cortical interstitial compartment by a mechanism involving the proposed enzymes. This work may identify a novel pathway by which the pancreas and liver regulate renal cortical interstitial levels of adenosine and may reveal important mechanistic insights into diseases such as the hepatorenal syndrome and the metabolic syndrome X.