B. S. Ward

Placental-specific Igf2 knockout mice exhibit hypocalcemia and adaptive changes in placental calcium transport

Evidence is emerging that the ability of the placenta to supply nutrients to the developing fetus adapts according to fetal demand. To examine this adaptation further, we tested the hypothesis that placental maternofetal transport of calcium adapts according to fetal calcium requirements. We used a mouse model of fetal growth restriction, the placental-specific Igf2 knockout (P0) mouse, shown previously to transiently adapt placental System-A amino acid transporter activity relative to fetal growth. Fetal and placental weights in P0 mice were reduced when compared with WT at both embryonic day 17 (E17) and E19. Ionized calcium concentration [Ca2+] was significantly lower in P0 fetal blood compared with both WT and maternal blood at E17 and E19, reflecting a reversal of the fetomaternal [Ca2+] gradient. Fetal calcium content was reduced in P0 mice at E17 but not at E19. Unidirectional maternofetal calcium clearance (Ca K mf) was not different between WT and P0 at E17 but increased in P0 at E19. Expression of the intracellular calcium-binding protein calbindin-D9K, previously shown to be rate-limiting for calcium transport, was increased in P0 relative to WT placentas between E17 and E19. These data show an increased placental transport of calcium from E17 to E19 in P0 compared to WT. We suggest that this is an adaptation in response to the reduced fetal calcium accumulation earlier in gestation and speculate that the ability of the placenta to adapt its supply capacity according to fetal demand may stretch across other essential nutrients.

Increased maternofetal calcium flux in parathyroid hormone‐related protein‐null mice

The role of parathyroid hormone‐related protein (PTHrP) in fetal calcium homeostasis and placental calcium transport was examined in mice homozygous for the deletion of the PTHrP gene (PTHrP−/− null; NL) compared to PTHrP+/+ (wild‐type; WT) and PTHrP+/− (heterozygous; HZ) littermates. Fetal blood ionized calcium was significantly reduced in NL fetuses compared to WT and HZ groups at 18 days of pregnancy (dp) with abolition of the fetomaternal calcium gradient. In situ placental perfusion of the umbilical circulation at 18 dp was used to measure unidirectional clearance of 45Ca across the placenta in maternofetal (CaKmf) and fetoplacental (CaKfp) directions; CaKfp was < 5% of CaKmf for all genotypes. At 18 dp, CaKmf across perfused placenta and intact placenta (CaKmf(intact)) were similar and concordant with net calcium accretion rates in vivo. CaKmf was significantly raised in NL fetuses compared to WT and HZ littermates. Calcium accretion was significantly elevated in NL fetuses by 19 dp. Placental calbindin‐D9K expression in NL fetuses was marginally enhanced (P < 0.07) but expression of TRPV6/ECaC2 and plasma membrane Ca2+‐ATPase (PMCA) isoforms 1 and 4 were unaltered. We conclude that PTHrP is an important regulator of fetal calcium homeostasis with its predominant effect being on unidirectional maternofetal transfer, probably mediated by modifying placental calbindin‐D9K expression. In situ perfusion of mouse placenta is a robust methodology for allowing detailed dissection of placental transfer mechanisms in genetically modified mice.

Histochemical localization of phosphatases in the pig placenta: II. Potassium-dependent and potassium-independent p-nitrophenyl phosphatases at high pH; relation to sodium-potassium-dependent adenosine triphosphatase

Histochemical localization by Mg2+ capture methods of K+-dependent, ouabain-sensitive phosphatase activity in the pig placenta shows that strong Na+,K+-dependent adenosine triphosphatase (Na+,K+-ATPase) activity is restricted to the basal zone of the columnar epithelium covering the areolar chorionic villi. It is proposed that active Na+ absorption at this epithelium may be the source of the ouabain-sensitive, fetal-side-positive potential difference which can be measured across the placental membrane in vitro. The one-step procedure for Na+,K+-ATPase localization is unsatisfactory in this organ as any specific ATPase reaction is swamped by activity probably attributable to uteroferrin and other non-specific phosphatases.

Site of catecholamine modulation of feto-maternal electric potential difference in the pig

Feto‐maternal vascular (PD(F‐M)) and amniotic maternal (PD(A‐M)) potential differences were measured simultaneously on seven occasions in six conscious pigs of 100–106 days gestation. Resting values of PD(F‐M) and PD(A‐M) were not significantly different although the range was wide. Fetal intravascular injection of 20 microg adrenaline, but not of saline, was associated with a prompt reversible change, of equal magnitude, in both PD(F‐M) and PD(A‐M). In some experiments polarity was reversed. Feto‐amniotic potential difference did not change. There was no change in fetal plasma K+ and Na+ concentrations. Because of the simultaneous and equal alterations in PD(F‐M) and PD(A‐M) following adrenaline and the anatomical configuration of the pig conceptus, we conclude that the catecholamine modifiable component of PD(F‐M) is generated by electrogenesis in the pig placenta, probably by its chorionic (trophoblastic) cell layer.

Histochemical localization of phosphotases in the pig placenta: I. Non-specific phosphatases and their relation to uteroferrin

The uterine glands, glandular secretions and areolar chorionic epithelium of the pig placenta in late gestation show high levels of histochemical phosphatase activity at acid pH, particularly towards aromatic monophosphates. The properties of this phosphatase allow its identification with the placental iron carrier protein, uteroferrin, rather than with lysosomal acid phosphatase. Alkaline phosphatase activity is restricted to the microvasculature on the maternal side of the placenta and is entirely distinct from uteroferrin phosphatase activity.