Role of nitrite in regulation of fetal cephalic circulation in sheep

Abstract

Key pointsr Recent evidence in adult humans demonstrates that nitrite, at physiological concentrations, can be converted into vasodilating amounts of NO, thus constituting an alternative to NO production by NO synthases.r Nitrite reacts with deoxyhaemoglobin to produce NO, a reaction proposed to mediate the vasodilating effects of nitrite. We have demonstrated previously that the rate of this reaction is ß2-fold faster with fetal haemoglobin than adult haemoglobin. Thus, we hypothesized that nitrite would be a potent vasodilator in the cephalic vasculature served by the carotid artery in the fetal sheep.r In conflict with human adult forearm studies, we find that nitrite is not a vasodilator in the fetal sheep cephalic vasculature, despite the fact that nitrite is converted to NO more efficiently by fetal haemoglobin.r The results suggest that the vasodilatory effects of nitrite are age-and species-specific, and that the reaction of nitrite with deoxyhaemoglobin is not rate limiting with respect to its ability to decrease vascular tone.Abstract Nitrite has been postulated to provide a reservoir for conversion to nitric oxide (NO), especially in tissues with reduced oxygen levels as in the fetus. Nitrite would thus provide local vasodilatation and restore a balance between oxygen supply and need, a putative mechanism of importance especially in the brain. The current experiments test the hypothesis that exogenous nitrite acts as a vasodilator in the cephalic vasculature of the intact, near term fetal sheep. Fetuses were first instrumented to measure arterial blood pressure and carotid artery blood flow and then studied 4-5 days later while in utero without anaesthesia. Initially L-nitro-arginine (LNNA) was given to block endogenous NO production. Carotid resistance to flow increased 2-fold from 0.54 ± 0.01 (SEM) to 1.20 ± 0.08 mmHg min ml −1 (in 13 fetuses, P < 0.001), indicating NO tonically reduces cerebral vascular tone. Sodium nitrite (or saline as control) was then infused in increasing step-doses from 0.01 to 33 μM in half-log increments over a period of 2 h. Carotid artery pressure, blood flow and vascular resistance did not change compared to fetuses receiving saline, even at plasma nitrite concentrations two orders of magnitude above the physiological range. The results indicate that while cephalic vascular tone is controlled by endogenous nitric oxide synthase activity, exogenously administered nitrite is not a vasodilator at physiological concentrations in the vasculature served by the carotid artery of fetal sheep.

Local and systemic vasodilatory effects of low molecular weight S-nitrosothiols

“…Indeed, in this study, approximately half of the L-cysNO and GSNO disappearance was accounted for by increases in plasma nitrite to concentrations of ~0.4 μmol/L. We have recently reported, however, that in sheep nitrite is a poor vasodilator even at concentrations orders of magnitude greater than those observed in the present study[22, 56]. Thus it seems more likely that the vasodilation observed in this study was not due to increases in circulating nitrite.…”

Role of blood and vascular smooth muscle in the vasoactivity of nitrite

“…A major difference between previous human studies and the present study is the use of L-NAME and hexamethonium and the anesthesia required for the sheep. We have previously observed a lack of vasodilatory response to nitrite in chronically instrumented sheep fetuses that were not anesthetized (35). Although this indicates the lack of response to nitrite in the present study may be specific to sheep than due to anesthesia, studies in unanesthetized adult sheep have not been conducted to rule out this possibility.…”

“…The results of this study show that, in contrast with previous studies in the human forearm but in agreement with our study on cephalic blood flow in fetal sheep (35), blood vessels in the hind limb of adult sheep do not vasodilate in response to increases in arterial nitrite concentration in the physiologic range. This observation holds true even when blood oxyhemoglobin saturations are within the optimal range for the reduction of nitrite to NO by deoxyhemoglobin.…”

“…Also, hexamethonium did not dilate the vasculature to a maximum, as evidenced by the strong vasodilatory response to intra-arterial infusion of NO, and thus vessels retained a potential for further relaxation during nitrite infusions. Finally, hexamethonium was not used in the previous fetal sheep studies where a lack of effect of nitrite was also observed (35).…”

“…We recently observed that nitrite infusion into the carotid artery of near-term fetal sheep does not increase the conductance of the cephalic vascular bed (35), a result that was unexpected particularly given that fetal deoxyhemoglobin reduces nitrite to NO twice as fast as adult deoxyhemoglobin (6). We therefore decided to measure the effect of nitrite on resistance to flow in the femoral artery of adult sheep.…”