GTPCH is not a rate-limiting factor for hemorrhagic shock-induced hepatic nitric oxide biosynthesis

Background: Hemorrhagic shock upregulates inducible nitric oxide (NO) synthase (iNOS) expression and the resultant NO overproduction. Liver is one of the major organs that is responsible for increased NO production after trauma-hemorrhage and resuscitation. Guanosine triphosphate cyclohydrolase I (GTPCH) is the rate-limiting enzyme for the synthesis of tetrahydrobiopterin (BH₄), a necessary co-factor for iNOS activity. Very little is known about the effects of hemorrhagic shock on hepatic GTPCH expression. Methods: Fifteen male Sprague-Dawley rats were randomly assigned to one of three groups, i.e. a sham instrumented (Sham) group, a sustained hemorrhagic shock (HS) group, and a hemorrhagic shock with resuscitation (HS/RES) group (n = 5 in each group). Controlled hemorrhagic shock was induced and the mean arterial pressure (MAP) was kept between 40-45 mmHg for sixty minutes in both HS and HS/RES groups. Then resuscitation with infusion of shed autologous blood and normal saline was performed in HS/RES group. Microdialysis probes were put in the liver and the right atrium for collection of serial samples. NO concentrations in dialysate samples were measured using chemiluminescence. Hepatic iNOS and GTPCH mRNA concentrations were analyzed using semi-quantitative reverse transcription and polymerase chain reaction (RT-PCR). Results: Hemorrhagic shock induced both the hepatic and circulating NO biosynthesis as well as hepatic iNOS mRNA expression. Resuscitation with shed blood/normal saline normalized this upregulation. However, no difference was found in mean hepatic GTPCH mRNA concentrations between groups in this experiment. Conclusions: We provide the evidence that hemorrhagic shock-induced NO biosynthesis involves upregulation of iNOS transcription in liver tissue and GTPCH transcription is unaffected by either hemorrhagic shock or resuscitation. Furthermore, microdialysis is an ideal technique for serial sampling and that events can be followed.