3-isobutylmethylxanthine inhibits hepatic urea synthesis: protection by agmatine.
We previously showed that agmatine stimulated hepatic ureagenesis. In this study, we sought to determine whether the action of agmatine is mediated via cAMP signaling. A pilot experiment demonstrated that the phosphodiesterase inhibitor, 3-isobutylmethylxanthine (IBMX), inhibited urea synthesis albeit increased [cAMP]. Thus, we hypothesized that IBMX inhibits hepatic urea synthesis independent of [cAMP]. We further theorized that agmatine would negate the IBMX action and improve ureagenesis. Experiments were carried out with isolated mitochondria and (15)NH(4)Cl to trace [(15)N]citrulline production or [5-(15)N]glutamine and a rat liver perfusion system to trace ureagenesis. The results demonstrate that IBMX induced the following: (i) inhibition of the mitochondrial respiratory chain and diminished O(2) consumption during liver perfusion; (ii) depletion of the phosphorylation potential and overall hepatic energetic capacity; (iii) inhibition of [(15)N]citrulline synthesis; and (iv) inhibition of urea output in liver perfusion with little effect on [N-acetylglutamate]. The results indicate that IBMX directly and specifically inhibited complex I of the respiratory chain and carbamoyl-phosphate synthase-I (CPS-I), with an EC(50) about 0.6 mm despite a significant elevation of hepatic [cAMP]. Perfusion of agmatine with IBMX stimulated O(2) consumption, restored hepatic phosphorylation potential, and significantly stimulated ureagenesis. The action of agmatine may signify a cascade effect initiated by increased oxidative phosphorylation and greater ATP synthesis. In addition, agmatine may prevent IBMX from binding to one or more active site(s) of CPS-I and thus protect against inhibition of CPS-I. Together, the data may suggest a new experimental application of IBMX in studies of CPS-I malfunction and the use of agmatine as intervention therapy.
Nissim
Itzhak
I
Children's Hospital of Philadelphia, Division of Child Development, Department of Pediatrics, University of Pennsylvania School of Medicine, 34th Street and Civic Center Boulevard, Philadelphia, PA 19104, USA.
Horyn
Oksana
O
Nissim
Ilana
I
Daikhin
Yevgeny
Y
Wehrli
Suzanne L
SL
Yudkoff
Marc
M
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology*; D000376:Agmatine / Q000494:pharmacology*; D000818:Animals; D002222:Carbamoyl-Phosphate Synthase (Ammonia) / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism; D004579:Electron Transport / Q000187:drug effects; D008099:Liver / Q000378:metabolism*; D008297:Male; D008930:Mitochondria, Liver / Q000378:metabolism*; D010101:Oxygen Consumption / Q000187:drug effects; D010477:Perfusion; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology*; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D014508:Urea / Q000378:metabolism*
D010726:Phosphodiesterase Inhibitors; D000376:Agmatine; D014508:Urea; D000242:Cyclic AMP; D010727:Phosphoric Diester Hydrolases; D002222:Carbamoyl-Phosphate Synthase (Ammonia); D015056:1-Methyl-3-isobutylxanthine