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In late gestation, Ca2+ transport across the human placenta must increase in response to the demands of accelerating bone mineralization of the fetus. This is an ATP-dependent transport against a concentration gradient across the basal or the fetal-facing plasma membrane of the syncytiotrophoblast. The aims of the present study were to determine the relationship between ATP-dependent Ca2+ transport and gestational age in the third trimester and to identify the specific isoforms of plasma membrane Ca2+ ATPase (PMCA) present in human syncytiotrophoblast. Basal membrane vesicles were isolated from normal placentas and from placentas obtained from preterm deliveries with no other complications (32-37 wk of gestation). We studied the uptake of 45Ca2+ into basal membrane vesicles in the absence and presence of ATP by using rapid filtration techniques. Western blot was used to assess the protein expression of the PMCA isoforms 1-4. Isoforms 1 and 4 of PMCA were identified in basal membrane of human placenta. The ATP-dependent Ca2+ transport increased linearly during the third trimester (r = 0.571, p = 0.0015, n = 28). However, PMCA protein expression was unaltered during the same period of gestation. Our results show that PMCA in the fetal-facing plasma membrane of the human syncytiotrophoblast is markedly activated toward the end of pregnancy. We suggest that these changes are critical in supplying the rapidly growing fetus with sufficient Ca2+ for bone mineralization.

ATP-Dependent Ca2+ Transport Is Up-Regulated during Third Trimester in Human Syncytiotrophoblast Basal Membranes