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The effect of polymerised bovine haemoglobin on the responses of rat aorta to phenylephrine and acetylcholine Haemoglobin-based oxygen-carriers (HBOC) are being developed as “blood substitutes”. Early cross-linked haemoglobin solutions caused marked vasoconstriction by scavenging nitric oxide (NO; Hart et al. 1997). Polymerised haemoglobin solutions, such as the bovine product Oxyglobin, are said to have lesser vascular effects (Day 2003). Oxyglobin is currently approved for use in dogs and in vivo studies have shown that it increases systemic vascular resistance (Driessen et al 2001). However the extent and mechanism of this vasoconstriction has not been characterised in vitro. The aim of this study was to quantify the vasoconstrictive effect of Oxyglobin on isolated rings of rat aorta and to determine if this effect is mediated via inactivation of NO. The actions of Oxyglobin (Oxy) were investigated on endothelium-intact rings of aorta from female Wistar rats (180g), killed by stunning and exsanguination. The rings were mounted under 1 g of tension, bathed in Krebs’ solution (95% O2 / 5% CO2, 37°C). After equilibration, cumulative concentration response curves to phenylephrine (PE; 1nM - 10µM) and acetylcholine (ACh; 1nM - 10µM) were obtained in control tissues and following incubation (20 minutes) with Oxy (1µM) and/or the NO synthase inhibitor L-NAME (100µM). In a separate series of experiments the acute effects of Oxy were investigated by addition of Oxy (1µM) and/or L-NAME 100µM) to either PE (1µM) constricted or ACh (1µM) relaxed rings. Responses to PE are expressed in grams tension. Responses to ACh are expressed as % change in tone. Data are mean ± s.e.mean, n ≥ 5. Data are analysed by ANOVA (Bonferroni post-test). In aortic rings incubation with Oxy significantly enhanced the contractile response to PE (response at 100nM: 0.93±0.15g vs. 0.51±0.13g control, p<0.001) but to a lesser extent than L-NAME alone or in combination with Oxy (1.29±0.13g, 1.26±0.05g respectively, p<0.001 compared to Oxy alone). Similarly the response to ACh was inhibited by Oxy alone (response at 100nM: -24±8% vs. -81±2% control, p<0.001) but to a lesser extent than L-NAME alone or in combination with Oxy (1±6%, 9±7% respectively, p<0.001 compared to Oxy alone). In contrast, acute addition of Oxy to PE-constricted rings caused a significant increase in tone (0.52±0.04g vs –0.09±0.04g control, p<0.001) this was not different from the rise induced by L-NAME alone or in combination with Oxy (0.39±0.10g, 0.49±0.16g respectively). Similarly acute addition of Oxy to ACh-relaxed rings completely reversed the relaxation and constricted the vessels beyond the original PE-induced level of tone (77±16% vs. -68±9% control, p<0.001), again this was not different from the reversal induced by L-NAME alone or in combination with Oxy (54±10%, 46±22% respectively). These data clearly demonstrate that the HBOC Oxyglobin has a significant vasoconstrictor effect and that scavenging of NO is the likely mechanism. This has implications for the clinical use of Oxyglobin, since NO scavenging may be detrimental in some patients but beneficial in others (e.g. in septic shock).
Day TK (2003). J Vet Emerg Crit Care 13: 77-92. |
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