Endothelin-1 is not involved in hemoglobin associated vasoactivities

Evaluation of Angiotensin Converting Enzyme (ACE)-Like Activity of Acellular Hemoglobin

Despite the tremendous progress in research on hemoglobin (Hb) cellular and molecular responses, the current understanding of Hb's overall intrinsic toxicity is still limited. The complete mechanism of Hb-induced vasoconstriction has not yet been established, particularly the involvement of the renin-angiotensin system (RAS). Some studies emphasized that Hb may augment the vascular responsiveness to angiotensin (Ang)-II. It was also reported that Hb, as well as Ang-II, influences the synthesis of 8-iso prostaglandin F2 alpha, which has an impact on renal flow and possibly RAS. Hb in the presence of H(2)O(2) gains enzymatic activity. Thus, it is possible that Hb directly and/or indirectly can activate RAS. In this study, we monitored the effect of ferrous- and ferryl-Hb, and H(2)O(2) alone, on conversion of Ang-I to its active metabolites. The structural and immunological identity of the resulting products were evaluated by reversed phase C-18 HPLC and ELISA, respectively. Additionally, ACE-like activity of Hbs was measured spectrophotometrically by determining their ability to react with the ACE substrate, the synthetic tripeptide N-[3-(2-furyl)acryloyl]-L-phenylalanylglycylglycine. Results indicate that while ferrous-Hb can serve as a receptor for Ang-I, its ferryl form possesses ACE-like activity, being able to convert, within minutes, Ang-I to Ang-II, Ang-III, Ang-IV, Ang (1-7) and other unresolved fragments. H(2)O(2) itself had a very limited hydrolyzing effect on Ang-I. Based on this study, it can be concluded that ACE-like activity of Hb with rapid formation of active angiotensins may be a contributor to the still unexplained vasoconstrictive response observed immediately after Hb administration.

Modulation of the NO/cGMP pathway reduces the vasoconstriction induced by acellular and PEGylated haemoglobin

Activation of the NO/cGMP pathway modulates smooth muscle cells relaxation and hence vasoconstriction, a major hindrance for the use of cell-free haemoglobin (Hb) as blood substitute, despite conjugation with 5-kDa maleimide poly(ethylene)-glycol (PEG) reduces vasoconstriction in vivo. We aimed at assessing how a recently developed PEGylated-Hb (Deoxy-PEGHb) and manipulation of the NO/cGMP pathway enable modulation of vasoconstriction in isolated rat hearts. Hearts were Langendorff-perfused with oxygenated Krebs-Henseleit (15 ml/min) while monitoring the coronary pressure (CPP) after injection (1 min) of 50 nM norepinephrine followed by a 1 microM Hb or Deoxy-PEGHb bolus, without altering the flow. Deoxy-PEGHb induced less vasoconstriction than Hb. Although the presence of PEG could contribute to vasoconstriction, Deoxy-PEGHb did not contain appreciable amounts of free PEG. Whereas reducing endothelial NO release by 0.2 mM L-NAME increased vasoconstriction, abolishing NO scavenging by Hb using its cyanomet derivative almost completely blunted it. Furthermore, maintaining intracellular cyclic GMP by inhibiting phosphodiesterase-5 with 0.02 mM sildenafil enabled control of Hb-induced vasoconstriction. We conclude that, although PEG-Hb represents a possible approach to limit Hb-induced vasoconstriction, manipulating the NO/cGMP pathway may provide a powerful way to circumvent this problem.

Pharmacological and physicochemical factors in the pressor effects of conjugated haemoglobin-based oxygen carriers in vivo

BACKGROUND

The hypertension induced by haemoglobin-based oxygen carriers could be a result of different pharmacological and physicochemical factors.

OBJECTIVE

To investigate whether production of superoxide anion (O2*-) and release of endothelin could be the factors responsible.

METHODS

We studied the variation in mean arterial pressure (MAP) in guinea pigs by carrying out a 50% isovolaemic exchange transfusion with conjugated oxyhaemoglobin (non-oxidized form) or conjugated methaemoglobin (fully oxidized form) in the presence or absence of BQ-788 (5 nmol/l), an endothelin receptor type B (ETR-B) antagonist. At key timepoints of variation in MAP, the plasma concentrations of O2*- were measured. The presence of conjugated oxyhaemoglobin and increases in ETR-B concentrations inside the vascular wall were investigated in different vessels, using western blotting.

RESULTS

We found that the administration of conjugated oxyhaemoglobin induced a significant increase in MAP, whereas conjugated methaemoglobin had no significant haemodynamic effect. Pretreatment with BQ-788 attenuated the increase in MAP induced by conjugated oxyhaemoglobin. This haemoglobin induced the production of high concentrations of O2*- that declined towards control values after 120 min and decreased in the presence of BQ-788. Western blot analysis showed that the presence of conjugated oxyhaemoglobin inside the vascular wall was time-dependent and correlated with increased ETR-B.

CONCLUSION

These results show that the release of O2*- during auto-oxidation of conjugated oxyhaemoglobin is associated with the observed increase in MAP, which may be a result of the vasoconstriction caused by an increase in activation of ETR-B. This activation may be caused by the massive release of endothelin induced by the production of O2*-.