Enhanced vascular responses to vasopressin in a rat model of faecal peritonitis

Barrett LK1,2, Taylor V2, Singer M2, and Clapp LH1
1Department of Medicine and 2Wolfson Institute for Biomedical Research, University College London, London, UK

Introduction
In patients, prolonged septic shock is associated with a decreased pressor response to norepinephrine (NE), but an enhanced sensitivity to vasopressin (VP) (Landry et al., 2001). The mechanisms underlying these changes are unknown. We have established a rat model of faecal peritonitis that mimics this clinical scenario and have used wire myography to examine the ex-vivo contractile responses to NE and VP in resistance vessels taken from these animals.

Methods
Sepsis was induced in conscious male Wistar rats (300g) by intraperitoneal injection of faecal slurry. Paired sham controls received no injection, but were otherwise treated the same. Continuous fluid resuscitation was administered via a central venous line. After 24 hours of sepsis, animals were sacrificed, and mesenteric resistance arteries (200μ i.d.) dissected and mounted on a wire myograph for assessment of isometric contractions to NE (10-8-10-5M) and VP (10-11-3x10-8M). Tissues were maintained at 37oC in a physiological salt solution and bubbled with 5%CO2/95%O2. The effect of endothelial removal on the VP concentration-response curve was investigated. In addition, Ca2+ mobilisation pathways were examined by removing and returning extracellular Ca2+, and by using the voltage-gated calcium channel (VGCC) blocker, nifedipine. Data are expressed as mean±SEM and statistical significance assessed using 2-way ANOVA with repeated measures and Bonferroni post-hoc testing.

Results
In arteries from septic rats, NE was unable to produce sustained contractions, whereas VP was able to do so. Both agonists produced sustained contractions in vessels from sham controls. When peak tension responses were analysed, the efficacy of NE to constrict arteries from septic rats was significantly ( p<0.001) reduced (Emax 3.0±0.3, n=13 in septic, vs 4.7±0.2mN, n=9 in shams). In contrast, Emax for VP was maintained and the potency significantly ( p<0.001) increased (pD2 9.1±0.04 vs 8.7±0.05 in septic and sham respectively). Removal of the endothelium further increased VP potency in septic vessels (pD2 9.4±0.10 p=0.02), but had no effect on sham arteries (pD2 8.8±0.06). Contractions to NE and VP were highly dependent on extracellular Ca2+ in control tissues (75-80% reduction in Ca2+free conditions), and NE, but not VP, responses were marginally more sensitive to Ca2+ removal in septic tissues (90% reduction). Nifedipine reduced VP contractions by 75% in sham arteries but by only 45% in septic arteries, whereas NE contractions were reduced by 90% in both groups.

Conclusion
The enhanced potency of VP to constrict resistance arteries taken from septic rats may be explained by differential changes occurring to VP receptor populations on the vascular smooth muscle and endothelial cells and/or the ability of this agonist to mobilise calcium by pathways other than the VGCC.

Landry et al., (2001) N Engl. J Med. 345 : 588-95