Characterisation of cardiovascular hemodynamics and tetrahydrobiopterin levels in murine septic shock.

Anna Starr, Manasi Nandi. King’s College London, London, UK.

Sepsis, a leading cause of death in intensive care patients worldwide, occurs when the body elicits an overwhelming inflammatory response to a microbial infection. Initial hypotension can escalate to circulatory collapse (septic shock) with ensuing multiple organ failure and death. Excessive nitric oxide (NO) production by inducible nitric oxide synthase (iNOS) contributes to the profound hypotension observed. Tetrahydrobiopterin (BH4) is an essential cofactor for NOS and is produced from GTP by GTP cyclohydrolase 1 (GCH1). Pharmacological regulation of vascular BH4 levels by targeting GCH1 may limit the pathophysiological NO production that occurs in septic shock patients. Thus, the aim of this study was to investigate the temporal cardiovascular changes and concurrent alterations in BH4 bioavailability using two in vivo models of septic shock.

C57BL6 mice (male; 12 wks) received either intravenous lipopolysaccharide (LPS; 12.5mg/kg; i.v) or surgical ligation and puncture of the caecum with a 19 gauge needle (CLP). All mice received 15µg/kg buprenorphine and 10 ml/kg saline fluid resuscitation. Blood pressure was recorded continuously in conscious mice by radiotelemetry. Temporal changes (0, 6 and 24hr post LPS or CLP) in cardiac function were measured by echocardiography, performed on anaesthetised (isofluorane, 1.5%) mice using an ultrasound system. Cardiac output (CO) was calculated from left ventricular internal dimensions from M-mode images. Furthermore, BH4 was measured in multiple tissue homogenates from control and septic animals using Coulochem III electro-chemical detection.

Both septic shock models led to cardiovascular dysfunction characterised by hypotension and a reduction in CO. Significant increases in BH4 levels were also detected in heart and aortic homogenates in both models, compared to control.

Table 1. Changes in CO, MAP and vascular tissue BH4 levels in control and septic mice. (mean ± s.e.m, * p<0.05 vs. baseline, 1-way ANOVA followed by Dunnett’s test).

Both preclinical models of septic shock resulted in cardiovascular dysfunction consistent with a profound inflammatory response and excessive NO production. The marked increases in cardiac and aortic BH4 concomitant with the impaired CO and reduced MAP that are observed following sepsis indicate that limitation of the NOS cofactor, BH4, may offer a potential therapeutic target for the treatment of circulatory collapse observed in septic shock patients.