Abstract-In erythrocytes, S-nitrosohemoglobin (SNO-Hb) arises from S-nitrosylation of oxygenated hemoglobin (Hb). Ithas been shown that SNO-Hb behaves as a nitric oxide (NO) donor at low oxygen tensions. This property, in combination with oxygen transport capacity, suggests that SNO-Hb may have unique potential to reoxygenate hypoxic tissues. The present study was designed to test the idea that the allosteric properties of SNO-Hb could be manipulated to enhance oxygen delivery in a hypoxic tumor. Using Laser Doppler flowmetry, we showed that SNO-Hb infusion to animals breathing 21% O 2 reduced tumor perfusion without affecting blood pressure and heart rate. Raising the pO 2 (100% O 2 ) slowed the release of NO bioactivity from SNO-Hb (ie, prolonged the plasma half-life of the SNO in Hb), preserved tumor perfusion, and raised the blood pressure. In contrast, native Hb reduced both tumor perfusion and heart rate independently of the oxygen concentration of the inhaled gas, and did not elicit hypertensive effects. Window chamber (to image tumor arteriolar reactivity in vivo) and hemodynamic measurements indicated that the preservation of tissue perfusion by micromolar concentrations of SNO-Hb is a composite effect created by reduced peripheral vascular resistance and direct inhibition of the baroreceptor reflex, leading to increased blood pressure. Overall, these results indicate that the properties of SNO-Hb are attributable to allosteric control of NO release by oxygen in central as well as peripheral issues. Key Words: nitric oxide Ⅲ hemoglobin Ⅲ oxygen Ⅲ hemodynamics Ⅲ blood flow H emoglobin (Hb) of red blood cells (RBC) is a tetrameric protein composed of 2 ␣ and 2 ␤ chains, each containing a heme prosthetic group. One ␣ and 1 ␤ chain is combined in stable ␣␤ dimers, and 2 dimers are more loosely associated to form tetramers. The physiological role of Hb depends on its ability to reversibly bind O 2 at its heme iron centers. This transport capacity is governed by a cycle of allosteric transitions in which Hb assumes the R (relaxed, high O 2 affinity) conformation to bind O 2 in the lungs and, on partial deoxygenation, the T (tense, low O 2 affinity) conformation to efficiently deliver O 2 to peripheral tissues. This transition also controls the reactivity of 2 conserved cysteines on the ␤ chains (Cys␤93). Thiols of the cysteines can react with the potent vasodilator nitric oxide (NO) to form S-nitrosohemoglobin (SNO-Hb) in the R conformation, and preferentially unload SNO in the T conformation. [1][2][3] It has therefore been proposed that Hb would carry NO equivalents from the lungs to the periphery, thereby bringing tissue blood flow in line with oxygen demand. Although mechanisms are debated, 4 -7 some consensus has been reached, namely that Hb can act as an oxygen sensor and oxygen-dependent transducer of NO bioactivity, 7-9 and the direct coupling between SNO and O 2 content of Hb has been recently affirmed in intact human erythrocytes (Doctor et al, unpublished observations).Outside the red blood cell (R...

Summary Carbogen (95% O 2 and 5% CO 2 ) has been used in preference to 100% oxygen (O 2 ) as a radiosensitizer, because it is believed that CO 2 blocks O 2 -induced vasoconstriction. However, recent work suggests that both normal and tumour arterioles of dorsal flap window chambers exhibit the opposite: no vasoconstriction vs constriction for O 2 vs carbogen breathing respectively. We hypothesized that CO 2 content might cause vasoconstriction and investigated the effects of three O 2 -CO 2 breathing mixtures on tumour arteriolar diameter (TAD) and blood flow (TBF). Fischer 344 rats with R3230Ac tumours transplanted into window chambers breathed either 1%, 5%, or 10% CO 2 + O 2 . Intravital microscopy and laser Doppler flowmetry were used to measure TAD and TBF respectively. Animals breathing 1% CO 2 had increased mean arterial pressure (MAP), no change in heart rate (HR), transient reduction in TAD and no change in TBF. Rats breathing 5% CO 2 (carbogen) had transiently increased MAP, decreased HR, reduced TAD and a sustained 25% TBF decrease. Animals exposed to 10% CO 2 experienced a transient decrease in MAP, no HR change, reduced TAD and a 30-40% transient TBF decrease. The effects on MAP, HR, TAD and TBF were not CO 2 dose-dependent, suggesting that complex physiologic mechanisms are involved. Nevertheless, when ≥ 5% CO 2 was breathed, there was clear vasoconstriction and TBF reduction in this model. This suggests that the effects of hypercarbic gases on TBF are site-dependent and that use of carbogen as a radiosensitizer may be counterproductive in certain situations.

The effect of sodium nitroprusside-induced hypotension on the perfusion of the R3230 adenocarcinoma during local 42 degrees C hyperthermia was studied using a combination of intravital microscopy and laser Doppler flowmetry. Fischer 344 rats were implanted with dorsal skin flap window chambers containing the R3230Ac tumor and allocated to three treatment groups (34 degrees C with nitroprusside, 42 degrees C with nitroprusside, and 42 degrees C with 0.9% saline). After baseline observation at 34 degrees C, tumors were locally heated to 42 degrees C using a water bath and either 0.9% saline or nitroprusside sufficient to reduce blood pressure 20% below pretreatment baseline was infused. Nitroprusside at 34 degrees C decreased tumor vascular conductance 40% with no effect on the diameter of arterioles entering the tumor. The diameter of arterioles entering 42 degrees C heated tumors increased 35% independent of blood pressure change. Saline at 42 degrees C had no effect on tumor vascular conductance; however, nitroprusside at 42 degrees C increased tumor vascular conductance 55%. Local 42 degrees C tumor heating, combined with a moderate reduction in blood pressure with nitroprusside, overrides the vascular steal effect associated with reduced perfusion pressure alone and results in improved tumor perfusion. Observations of the effect of vasodilator substances on normothermic tumor perfusion cannot be extrapolated to situations where moderate hyperthermia is used.