States of pathological sodium retention and edema formation such as congestive heart failure, nephrotic syndrome, and cirrhosis of the liver are characterized by renal resistance to the natriuretic action of atrial natriuretic peptide (ANP). This abnormality has been suggested to be the mediator of the impaired sodium excretion leading to positive sodium balance and the development of edema. A number of mechanisms have been argued to contribute to the renal resistance to ANP in these conditions, including activation of antinatriuretic pathways such as the renin-angiotensin system and sympathetic nerve activity, reduced delivery of filtrate to ANP-responsive sites in the inner medullary collecting duct (IMCD), and impaired binding of ANP to its renal receptors. We have developed evidence that another mechanism contributes to renal ANP resistance in experimental nephrotic syndrome and liver cirrhosis. This mechanism involves a heightened activity of a specific phosphodiesterase (PDE) enzyme in renal target cells for ANP action such that ANFs intracellular second messenger cyclic guanosine-3',5'-monophosphate (cGMP), normally formed after ANP binds to its biologically active receptors, is rapidly catabolized before it can exert its full cellular actions. ANP responsiveness of renal cells in vitro, and of natriuresis in vivo, is restored by pharmacologic inhibitors selective for PDES. We shall determine the role of heightened PDE5 activity in the renal resistance to ANP observed in rats with experimental nephrosis or liver cirrhosis by (1) measuring the rate of cGMP hydrolysis in homogenates of glomeruli and IMCD cells isolated from these rats in the presence of selective PDE inhibitors and after inimunoprecipitation of PDE5; (2) quantitating the amount of PDES enzyme protein in glomeruli and IMCD cells from these animals by Western analysis, localizing its distribution along the nephron by immunohistochemistry, and determining if phosphorylation contributes to heightened PDE5 activity; (3) measuring PDE5 gene expression in glomeruli and IMCD cells from nephrotic and cirrhotic rats by quantitating mRNA abundance, and localizing the nephron sites expressing PDE5 mRNA by in situ hybridization; and (4) determining the contributions of the renal nerves and endothelin to increased PDE5 activity in renal targets for ANP action by measuring PDE5 activity and protein level in denervated kidneys from nephrotic and cirrhotic rats, studying the effect of endothelin receptor antagonism on ANP responsiveness in vivo and in vitroand on PDE5 activity, and quantitating PDE5 activity and protein level in IMCD cell cultures exposed to endothelin and aipha-adrenergic agonists. These experiments will describe fully the role of increased PDE5 activity in renal ANP resistance, and thereby shed light on the pathogenesis of edema formation. This in turn will suggest new options for treatment of this often vexing clinical condition.
