CONSTITUCION DE LA SOCIEDAD, NOMBRAMIENTO DE GERENTE GENERAL

After having demonstrated the mechanisms underlying the transfer of amino acids from the syncytiotrophoblast to the fetal compartment, the next step was to explore how syncytiotrophoblast concentrations are established as a result of uptake of amino acids from the maternal circulation. As shown in Figure 5.1, the transport of amino acids across the MVM is mediated by the accumulative transporter and the exchanger. While the accumulative transporters actively pump the amino acids into the syncytiotrophoblast, the exchangers are responsible for equalising the composition of the

Table 5.5: Physiological amino acid concentrations for each group at the BM: Fetal values represent both the initial and inlet concentrations, while the syncytiotrophoblast concentrations were kept constant throughout the simulation. From Table 5.2, amino acid group BMEx comprises of AcEx and Ex, while BMExF comprises of Ex and AcEx.

Amino acid Syncytiotrophoblast Fetal

group compartment compartment BMEx 5199 µmol l−1 1627 µmol l−1 (ratio of total) (0.62) (0.69)

BMExF 3132 µmol l−1 721 µmol l−1 (ratio of total) (0.38) (0.31)

Figure 5.5: Physiological model simulations of transport interactions across the BM for equal exchanger and facilitative transporter activity (solid), and results for 10x increased facilitative transport activity (dashed line) using compartmental model with inputs from Table 5.5.

amino acids. Amino acid substrates were categorised for the model into two groups as: 1) accumulative and exchange transporter substrate, MVMAcEx, and 2) exchanger-only substrate, MVMEx. The compartmental model was then implemented to explore the uptake interactions across the MVM, assuming a constant inlet concentration of amino acids into the maternal compartment with values shown in Table 5.6. Furthermore, transport across the BM was disabled to clearly demonstrate the potential for uptake across the MVM. As shown in Figure 5.6, the model simulated uptake in to the initially empty syncytiotrophoblast compartment (i.e. zero-trans), which showed the high accumulation of both amino acids in the syncytiotrophoblast compartment over time. The accumulative transporters and exchangers configuration allowed uptake of both amino acid MVMEx and MVMAcEx, while the exchanger only substrate would not have been transferred if only the accumulative transporters were considered. Note that the initial dip in concentrations on the maternal side was the result of placental uptake,

which goes to zero as steady state is approached. The uptake profile of MVMEx slightly lagged behind, which was due to initial absence of exchangeable MVMEx in the placental compartment. Once MVMAcEx was pumped in to the placenta, then the exchangers start to operate in conjunction with the accumulative transporters. Steady states were reached when the difference in accumulative transporter substrate and sodium electro- chemical potential were equilibrated and the fraction of exchanger substrates equalised in both compartments.

Table 5.6: Amino acid concentration inputs for each group at the MVM: The maternal values represent inlet and initial concentrations, while in the syncytiotrophoblast compartment is zero.

Amino acid Maternal Syncytiotrophoblast group compartment compartment MVMAcEx 100 µmol l−1 0 µmol l−1 (ratio of total) (0.5)

MVMEx 100 µmol l−1 0 µmol l−1 (ratio of total) (0.5)

Figure 5.6: Syncytiotrophoblast amino acid concentrations for the simplified model simulations of transport across the MVM with conditions from Table 5.6. The concentrations of amino acid group MVMAcEx and MVMEx in the maternal vein and placental syncytiotrophoblast layer were simulated using the compartmental model in Figure 5.1.

The model simulated the effects of varying the transporter activity, which is shown in Figure 5.7. The results suggested that changes in the activities did not affect the steady levels, but only the uptake rate and thus the speed at which the equilibrium was

reached. In similar manner as in the previous section, high relative exchanger activity resulted in bringing amino acids profiles together (shown in Figure 5.7a). However, a small lag in the transfer of MVMEx was still present. For low relative exchange activity, the transfer of MVMEx into syncytiotrophoblast compartment occurred at a slower rate. Also, it reduced the amount of MVMAcEx exchanged back to the maternal side, which resulted in the faster accumulation of MVMAcEx in the syncytiotrophoblast compartment. Moreover, the model showed that with increased relative accumulative activity (Figure 5.7b), the uptake rate of MVMEx also increased along with MVMAcEx. Note that, the effects of MVMEx uptake rate lagging behind MVMAcEx was also amplified when relative accumulative activity increased.

Figure 5.7: Model simulation of the effects of variation in transporter activity at the MVM. The model shows each amino acid concentrations in the maternal and syncytiotrophoblast compartment for the interactions across the MVM with input conditions given in Table 5.6 over a range of relative (a) exchange or (b) accumulative transport activities.

In addition, different maternal amino acid input levels (as shown in Table 5.7) in the maternal compartment were modelled, which is shown in Figure 5.8. By keeping the MVMAcEx level constant, the model suggested that an increase in MVMEx results in an increase in its concentration in the syncytiotrophoblast compartment and the uptake

of MVMAcEx reaches steady state slower. In particular when the maternal level of exchange-only MVMEx was higher (than MVMAcEx), it was shown that the uptake in the syncytiotrophoblast compartment of MVMEx could surpass that of the accumulative substrate, MVMAcEx. This was due to the continuation of exchange over time until syncytiotrophoblast concentration ratios matched those of the maternal inlet. Note that in all cases, the uptake levels in the syncytiotrophoblast compartment of both species were higher than the maternal levels.

Table 5.7: Amino acid concentrations for each group at the MVM to investigate variations in the composition: The maternal values represent inlet and initial concentrations, while the initial concentration in the syncytiotrophoblast compartment is zero.

Amino acid Maternal Syncytiotrophoblast group compartment compartment MVMAcEx 100 µmol l−1 0 µmol l−1 (ratio of total) (0.91,0.5,0.33)

MVMEx [10,100,200] µmol l−1 0 µmol l−1 (ratio of total) (0.09,0.5,0.67)

Figure 5.8: Model simulations showing the effects of various amino acid concentration levels on transport interactions across the MVM: The compartmental model simulated syncytiotrophoblast concentrations with various input conditions as given in Table 5.7.

For the physiological levels of the amino acid groups as prescribed in Table 5.1, the amino acids concentrations can be further grouped according to their transporter specificity at the MVM. MVMAcEx was then considered to consist of AcEx and AcExF, while MVMEx included Ex and ExF. With these values as inputs (summarised in Table 5.8), the model simulated the physiological uptake for these amino acid groups in to the syncytiotrophoblast, which suggested high uptake levels for both MVMAcEx

and MVMEx (shown in Figure 5.9). The uptake profile in the syncytiotrophoblast compartment of MVMEx and MVMAcEx showed monotonic increase. Although it was difficult to see, a slow increase of MVMAcEx initially was observed, which can be explained by the exchanger working against the accumulative transporter since the initial concentration ratio of MVMAcEX was higher in the syncytiotrophoblast. The model showed that the syncytiotrophoblast concentrations of both species continued to rise well above physiological values, however it is important to note that the model here only considered the MVM and did not include the efflux transport across the BM, which would lead to a lower equilibrium.

Table 5.8: Physiological amino acid concentrations for each group at the MVM: The maternal values represent inlet and initial concentrations, while the syncytiotrophoblast values represent the initial concentration. From Table 5.2, amino acid group MVMAcEx comprises of AcEx and AcExF, while MVMEx comprises of Ex and ExF.

Amino acid Maternal Syncytiotrophoblast group compartment compartment MVMAcEx 1041 µmol l−1 6400 µmol l−1 (ratio of total) (0.67) (0.77)

MVMEx 508 µmol l−1 1931 µmol l−1 (ratio of total) (0.33) (0.23)

Figure 5.9: Physiological model simulations of transport interactions across the MVM: The amino acid concentrations in the maternal vein and placental syncytiotrophoblast layer were simulated using the compartmental model with input conditions in Table 5.8.