Fetal Control of Calcium Transport across the Rat Placenta

Control of maternofetal calcium transfer across the in situ perfused rat placenta at day 21 of gestation (term 23 d) was investigated in both intact fetuses and those parathyroidectomized by decapitation on day 19. Decapitation resulted in significant fetal hypocalcemia. Injection of fetuses subcutaneously through the uterine wall with 0.43 µg bovine (b) PTH(l-84), 20 ng 1,25(OH)2D3or 10 µL of the appropriate diluent resulted 2 h later in a raised fetal blood ionized Ca concentration only with bPTH(l-84) in both normal and decapitated fetuses. Fetal decapitation caused a significant (p<0.001) fall in the clearance of 45Ca across the placenta (Kmf45Ca), which was significantly (p<0.05) reversed after fetal bPTH(l-84) and 1,25 dihydroxy vitamin D3(1,25(OH)2D3) injection, but not back to normal levels. There was no effect of either hormone on Kmf45Ca in placentas from intact fetuses, or on Kmf51Cr-EDTA (used as an extracellular marker) in either group. When 4 ng/mL rNIe8,21, Tyr34 PTH(l-34), 50 pg/mL 1,25(OH)2D3or the appropriate diluent was perfused through placentas the only response observed was a significant (p<0.05) increase in Kmf45Ca with 1,25(OH)2D3perfusion in placentas from decapitated fetuses, Kmf51Cr-EDTA being unchanged. Finally, perfusion with 10-5M forskolin (an activator of adenylate cyclase) stimulated Kmf45Ca in placentas from both normal and decapitated fetuses. Although there was also some effect on Kmf51Cr-EDTA in the latter, there was none in the placentas from normal fetuses, and here the effect on Kmf45Ca was dose dependent with an initial response at 10-6M. It is therefore suggested that fetal PTH and 1,25(OH)2D3may play a permissive role in the control of maternofetal calcium transfer and that other hormones which act via cAMP may be involved in the acute regulation of calcium transfer under normal conditions