Fetal-maternal nitrite exchange in sheep: Experimental data, a computational model and an estimate of placental nitrite permeability

Abstract

Introduction: Nitrite conveys NO-bioactivity that may contribute to the high-flow, low-resistance character of the fetal circulation. Fetal blood nitrite concentrations depend partly on placental permeability which has not been determined experimentally. We aimed to extract the placental permeability-surface (PS) product for nitrite in sheep from a computational model.

Methods: An eight-compartment computational model of the fetal-maternal unit was constructed (Matlab(®) (R2013b (8.2.0.701), MathWorks Inc., Natick, MA). Taking into account fetal and maternal body weights, four variables (PS, the rate of nitrite metabolism within red cells, and two nitrite distribution volumes, one with and one without nitrite metabolism), were varied to obtain optimal fits to the experimental plasma nitrite profiles observed following the infusion of nitrite into either the fetus (n = 7) or the ewe (n = 8).

Results: The model was able to replicate the average and individual nitrite-time profiles (r(2) > 0.93) following both fetal and maternal nitrite infusions with reasonable variation of the four fitting parameters. Simulated transplacental nitrite fluxes were able to predict umbilical arterial-venous nitrite concentration differences that agreed with experimental values. The predicted PS values for a 3 kg sheep fetus were 0.024 ± 0.005 l∙min(-1) in the fetal-maternal direction and 0.025 ± 0.003 l∙min(-1) in the maternal-fetal direction (mean ± SEM). These values are many-fold higher than the reported PS product for chloride anions across the sheep placenta.

Conclusion: The result suggests a transfer of nitrite across the sheep placenta that is not exclusively by simple diffusion through water-filled channels.

Keywords: Nitrite; Placental transfer; Sheep; Simulation.

Fig. 1

Structure of the model. Shown here is the arrangement for fetal nitrite infusion because nitrite infusion to the maternal plasma (Kinfm) and “placental” transfer from maternal plasma to fetal plasma (Kplacm) are inactivated. Circles (ovals) indicate nitrite fluxes into the fetus (Kinf, μmol·min⁻¹) or mother (Kinfm, μmol·min⁻¹), out of the red cell volume (Kexf, Kexm) or distribution volumes B (KexDistf, KexDistm) and across the placenta (Kplacm, Kplacf). Squares indicate nitrite movements between compartments in the fetus or the ewe. FetPlasmaNitrite, FetRBCNitrite, MatPlasmaNitrite etc. refer to the amount of nitrite (μmol) within a compartment. The “passive” distribution volumes A (FetDistVolA, MatDistVolA) do not convert nitrite, in contrast to the “active” distribution volumes B (FetDistVolB, MatDistVolB) that do. Equal concentrations are reached inside the blood volumes within about 10 seconds. See text for more details.

Fig. 2

Average nitrite profiles following nitrite infusion to the fetus (A) or the ewe (B). Symbols (closed circles: fetus; open circles: ewe) indicate experimental profiles and lines (dashed lines: fetus; solid lines: ewe) simulated nitrite profiles. Experimental data points are the mean of 7 (A) and 8 (B) animals, respectively. Note that the right y-axis in Panel A gives the appropriate nitrite concentration values for maternal data. Spikes in simulated profiles reflect the bolus infusion of nitrite that have no experimental equivalent. Arrows indicate injection/infusion of nitrite. Bars on symbols are ± SEM.

Fig. 3

Experimental (A) and simulated (B) individual nitrite profiles following nitrite infusion to the fetus. Line patterns (B) and symbols (A) correspond to experiments in individual sheep. The corresponding four fitted model parameters are presented in Table 2. Maternal nitrite profiles are not shown but were taken into account in the fitting procedure.

Fig. 4

Experimental (A) and simulated (B) individual nitrite profiles following nitrite infusion to the ewe. Line patterns (B) and symbols (A) correspond to experiments in individual sheep. The corresponding four fitted model parameters are presented in Table 3. Fetal nitrite profiles are not shown but were taken into account in the fitting procedure.

Fig. 5

Simulated nitrite fluxes for fetal (A) and maternal (B) nitrite infusions. Dashed lines (--) represent fluxes from the red cell volume compartments into nitrite conversion, solid black lines are transplacental fluxes, and the interrupted lines (-) are fluxes from the secondary distribution volume into nitrite conversion. The sum of the three fluxes (-..) must equal the infusion rate (gray line) into the fetus or the ewe, respectively, at steady-state concentrations. Note that on the fetal side most of the nitrite is metabolized in the red cell volume, whereas on the maternal side nitrite is predicted to be removed mostly in the secondary distribution volume.

Fig. 6

Experimental (symbols, taken from Figure 1 in [28]) and simulated “chloride” time courses (continuous lines) after a single injection of chloride (continuous infusion inactivated in the model) into a 3 kg sheep fetus [28]. The amount of “chloride” injected is adjusted to result in an initial fetal chloride concentration close to that observed by Thornburg, et al [28]. The model uses the values of all parameters and compartments of the average of our fetal infusion experiments (cf. Fig. 2A) but “nitrite conversion” is inactivated completely. The “chloride” curves (dashed and solid lines) are calculated with a PS values of 0.0018 l min⁻¹ [28] whereas the “nitrite curves” (dotted lines) are based on a PS value of 0.024 l min⁻¹, the proposed nitrite permeability of the sheep placenta. Regarding “nitrite permeability”, equilibrium is reached after about 10 hours, whereas it takes about 6 days to obtain equilibrium when simulation is based on “chloride permeability”. It is noteworthy that it is necessary for a reasonable fit to assume extravascular compartments for chloride also as is proposed for nitrite. Considering the unknown compartment volumes (esp. maternal body weight) of the experiment [28], the fit between the experimental data [28] and the simulated curves appears to be reasonably good.