Maternal and neonatal biometrics
There was no significant difference observed in maternal age, weight, parity, smoking and alcohol or substance use between groups (Table 1). Gestational age was significantly different between term and preterm births (P < 0.001). Mean gestational age in preterm males was not significantly different from preterm females (male, 336 ± 13 weeks vs. female, 335 ± 11 weeks), and no differences were observed between male and female term gestational age (male, 392 ± 02 weeks vs. female, 396 ± 02 weeks).
As expected, term male infants had a greater birth weight than female infants. Term infants had a greater birth weight than same-sex preterm infants (term male, 3579 ± 84 g vs. preterm male, 2299 ± 191 g; term female, 3434 ± 79 g vs. preterm female, 1969 ± 283 g, P < 0.01 for effect of sex and term vs. preterm). Term placentas were heavier than same-sex preterm placentas (term male, 620 ± 82 g vs. preterm male, 540 ± 72 g, P < 0.05) and term female, 580 ± 79 g vs. preterm female, 517 ± 91 g, P < 0.05). There was no effect of sex on placental weight within either gestational age group (Table 1). All women enrolled in the study delivered live-born singleton infants.
Placental chorionic plate vessel responsiveness to Bradykinin
Bradykinin (10 ρmol/l–1 μmol/l) produced a concentration-dependent vasodilation in all vessels which was not different between groups or sex within group. U46619 (0.1–100 μmol/l) produced a concentration-dependent vasoconstriction in all vessels which was not different between groups (Fig. 1).
MgSO4 dose-response in placental chorionic plate vessels
Following pre-constriction with U46619 (100 μmol/l), the vessels were treated with incremental concentrations of MgSO4 (0.05–0.25 mmol/l). There was no overall effect of treatment, term or sex. However, a significant interaction effect (interaction effect = treatment × sex, P = 0.03, F = 11.61, df = 2) was observed. This is likely due to preterm male vessels having significantly less percentage of relaxation than all other groups following MgSO4 administration. Following post hoc analysis, the placental chorionic plate vessels from preterm male placentae had a reduced vasodilatory response (P < 0.01) compared to all other groups following MgSO4 treatment (0.05, 0.125, 0.15 and 0.20 mmol/l). No further differences were observed at MgSO4 concentrations of 0.25 mmol/l as all vessels across all groups appeared to reach the point of saturation with further doses having no effect on percentage of relaxation (Fig. 2).
Bradykinin-induced relaxation in placental chorionic plate vessels incubated with MgSO4
Following pre-constriction with U46619 (100 μmol/l) and MgSO4 treatment (0.2 mmol/l), bradykinin-induced vasodilation was observed in a concentration-dependent manner in all vessels (Fig. 3). There was no overall effect of treatment, term or sex. However, a significant interaction effect (interaction effect = treatment × sex, P = 0.02, F = 9.376, df = 2) was observed. Following post hoc analysis, significantly less (P < 0.05) percentage of relaxation was observed in chorionic plate vessels from preterm male placental vessels when compared to all other groups.
Bradykinin-induced relaxation in placental chorionic plate vessels incubated with INDO, L-NAME and MgSO4
Following pre-constriction with U46619 (100 μmol/l) and administration of MgSO4 (0.2 mmol/l), INDO (10 μmol/l) and L-NAME (0.1 mmol/l), there was no overall effect of treatment, term or sex. However, a significant interaction effect (interaction effect = treatment × sex, P < 0.01, F = 8.213, df = 2) was observed. Following post hoc analysis, significantly less (P < 0.01) percentage of relaxation was significantly attenuated in male preterm placental vessels when compared to female preterm vessels and both male and female term placental vessels at all −log concentrations of bradykinin (P < 0.01) (Fig. 4).
Bradykinin-induced relaxation in placental chorionic plate vessels incubated with TRAM-34, apamin, INDO and MgSO4
In the presence of MgSO4 (0.4 mmol/l), TRAM-34 (1 μM), apamin (3 μM) and INDO (10 μmol/l) Bradykinin-induced vasodilatation did not differ between groups or sex within group (Fig. 5a).
Bradykinin-induced relaxation in placental chorionic plate vessels incubated with INDO and MgSO4
In the presence of INDO (10 μmol/l) and MgSO4 (0.2 mmol/l), bradykinin-induced vasodilatation was reduced in all groups, but did not differ between groups or sex within group (Fig. 5b).
Bradykinin-induced relaxation in placental chorionic plate vessels incubated with TRAM-34 and apamin and MgSO4
In the presence of MgSO4 (0.2 mmol/l), TRAM-34 (1 μM) and apamin (3 μM) bradykinin-induced vasodilatation did not differ between group/sex within group (Fig. 5c).
Vascular calcium concentration
An overall effect of MgSO4 treatment was observed (main effect of treatment, P < 0.001, F = 9.87, df = 1) on vascular calcium concentration. MgSO4 significantly reduced vascular calcium concentration in male and female vessels of both groups (MgSO4 treated vs. MgSO4 untreated; main effect of term, P < 0.001, F = 5.35, df = 1). This equated to an average reduction in vascular calcium concentration of ~40 % (male preterm, 26.6 ± 2.4 vs. female preterm, 23.7 ± 0.9 μg/mg, male term, 22.5 ± 2.1 μg/mg and female term, 23 ± 2.2 μg/mg) following MgSO4 treatment. An overall effect of sex could also be observed (main effect of sex, P = 0.05, F = 18.15, df = 2) in vascular calcium concentrations. In male preterm placental vascular tissue, calcium concentrations were significantly higher when compared to female preterm vascular tissue following MgSO4 (male, 9.2 ± 1.5 μg/mg vs. female, 5.2 ± 0.7 μg/mg, male term, 6.1 ± 2.0 μg/mg, female term, 5.1 ± 1.4 μg/mg). No further effects of term or interactions were observed. Following post hoc analysis, intravascular calcium prior to MgSO4 treatment was significantly (P < 0.05) increased in male preterm vessels when compared to all other groups. Following MgSO4 treatment, male preterm vascular calcium concentrations were further significantly increased (P < 0.01) when compared to all other groups (Fig. 6).