Preliminary characterization of a placental factor inhibiting breathing in fetal sheep

Abstract

Previous studies have revealed a placental extract that inhibits breathing in fetal sheep. In the present study of 29 chronically instrumented sheep at 132±1 days of gestation, infusion of the 1-10 kDa extract inhibited breathing in 76% of the experiments whereas Krebs’ solution inhibited it in 24%. It retained this activity after 6 months of freezing, after lyophilization, and upon lowering the pH during purication from 8·0 to 4·0, but it inhibited breathing in only 35% when the pH was lowered to 2·0. A signicant dose-dependent effect was observed from a 16-fold dilution to a 4-fold concentration. Treatment of the extract with proteinase K or boiling reduced the activity to 30% or 26% inhibition, respectively. The activity was not adsorbed to an ion-exchange column at pH 7·0 or 8· 0, but it was at pH 9· 0 and it eluted with increasing NaCl concentrations. On a polyacrylamide gel the activity was eluted at a K_av of 0· 66 (82% inhibition), corresponding to between 2·5 and 4·5 kDa. These ndings suggest that a peptide produced by the placenta, with a molecular mass between 2· 5 and 4·5 kDa, inhibits fetal breathing.