Dissociation of local nitric oxide concentration and vasoconstriction in the presence of cell-free hemoglobin oxygen carriers

Abstract

Cell-free hemoglobin's (CFH) high affinity for nitric oxide (NO) could limit CFH's use as an oxygen-carrying blood replacement fluid because it scavenges NO, causing vasoconstriction and hypertension. However, the extent to which perivascular NO levels change following intravascular administration of hemoglobin (Hb) with different molecular dimensions correlates with vasoconstrictive responses in the microcirculation is unknown. The study objective was to determine vasoconstrictive effects following bolus infusions of (1) alphaalpha cross-linked Hb; (2) polymerized bovine Hb; or (3) polyethylene glycol-decorated Hb (PEG-Hb), by measurements of in vivo microvessel diameter, blood flow, perivascular NO concentration, and systemic hemodynamic parameters. All CFHs caused reductions in perivascular NO levels, not correlated to microvascular responses. PEG-Hb (largest molecular volume) maintained blood flow, while the others caused vasoconstriction and reduced perfusion. All solutions increased mean arterial pressure due to vasoconstriction and blood volume expansion, except for PEG-Hb, which increased blood pressure due to blood volume expansion and maintenance of cardiac output. In conclusion, perivascular NO reduction is similar for all Hb solutions because NO binding affinities are similar; however, effects on vascular resistance are related to the type of molecular modification, molecular volume, and oxygen affinity.

Figure 1.

Changes in MAP and HR for each study group. (A) NOS inhibition. Both the increase in MAP and the decrease in HR were statistically significant from baseline at all time points. (B) CFH concentration. MAP was statistically increased by PBH and PBH4 compared with saline at all time points, but increases were not statistically different from each other. (C) CFH molecular configuration. MAP was statistically increased by all CFHs compared with saline (P < .05), but no statistical differences were obtained between the groups at each time point. All data are shown as mean ± SEM. Parameters are presented as change relative to baseline, thus no change from baseline would be denoted as 0, while 0.1 would mean a 10% increase from baseline. MAP changes within each experimental group were all statistically significantly different from baseline except for saline (P < .05; this significance is not denoted in the graphs). The asterisk is used to denote the statistical significance at each time point between the experimental groups (P < .05).

Figure 2.

Response at the microvascular level: vessel diameter, blood flow, and perivascular NO levels. (A) NOS inhibition. Significant vasoconstriction, loss of perfusion, and reduced perivascular NO levels were obtained (P < .05). (B) CFH concentration. The microvascular response was not a function of the CFH concentration. Significant decrease in arteriolar diameter, microvascular blood flow, and perivascular NO were obtained with both PBH and PBH4 (P < .05). (C) CFH molecular configuration. The smaller CFH molecules (ααHb and PBH4) caused arteriolar constriction leading to reduced microvascular perfusion compared with baseline and MP4. MP4 was not vasoconstrictive and had increased arteriolar blood flow. All CFHs reduced perivascular NO to similar levels relative to saline (P < .05). and ▪ denote the arterioles and venules, respectively. Blood flow data are shown as mean ± SEM. Parameters are present as relative to baseline, thus no change from baseline would be denoted as 1, while 1.1 would mean a 10% increase from baseline. ^* and ^** denote a statistical significant difference relative to saline and MP4, respectively.

Figure 3.

Changes to MAP and arteriolar diameter as a function of perivascular NO levels. Reduction of arteriolar perivascular NO by the introduction of CFH caused a concomitant pressure response, while the change in vascular resistance at the microvascular level represented by arteriolar vasoconstriction was not present with the MP4 experimental group (A). MP4 reduced perivascular NO and increased MAP similarly in magnitude to the other CFHs, however the slight arteriolar dilation caused a statistically significant increase in arteriolar blood flow (B). Data are presented as mean ± SEM. Mean arteriolar pressure is 30 minutes after the introduction of CFH. The normal levels for each parameter are marked with dotted lines; the intersection, by a star.