182 PLACENTAL IMPERMEABILITY TO PARATHORMONE (PTH) 1–125 IN THE EWE

Abstract

Studies altering plasma calcium levels have demonstrated that the maternal (M) and fetal (F) parathyroid glands appear to function automonously and suggest a lack of placental permeability to PTH. To determine whether the ovine placenta is permeable to PTH I-125, catheters were placed into the F abdominal aorta and pedal vein and M abdominal aorta and jugular vein in 5 date bred ewes at 121 to 135 days gestation. The studies were conducted 24 to 48 hours after surgery when the M and F arterial blood gases and pH returned to normal. Purified intact bovine PTH (1-84) was iodinated by the chloramine-T method. The placenta transfer of tracer quantities of PTH I-125 was studied in 5 animals in the M→F and 4 in the F→M direction. Simultaneous serial F and M plasma samples were obtained during the 60 minute study period and precipitated with 15% trichloracetic acid and the results expressed as percent dose/liter plasma. During the initial 20 minutes of the study, the t 1/2 was 21 minutes (r= -0.99) in the F and 21 minutes in the M (r=-0.99). In the M→F studies, no more than 0.3% of the injection dose/liter plasma was detected in the F and in the F→M studies, no more than 0.5% of the injected dose/liter plasma was detected in the M at 15 to 30 minutes post injection. Thus, during the 3rd trimester, the ovine placenta is impermeable to PTH I-125. The t 1/2 of PTH I-125 is similar in both the ovine F and M.