Protein binding of plasma cortisol in the foetal lamb near term

Binding of cortisol to plasma proteins was studied in the foetal lamb by equilibrium dialysis at 37 \s=deg\C.At 122 days of pregnancy the mean level of transcortin expressed as cortisol-binding capacity was 28 \m=+-\6 (s.d.) ng cortisol/ml plasma. During the last 14 days of preg-nancy there was a progressive increase in transcortin-binding capacity to 85 \m=+-\14 ng cortisol/ ml plasma. A sharp increase in the concentration of both protein-bound and unbound cortisol was observed over the same period. A rise in the concentration of total cortisol from around 3 to 42 ng/ml was associated with an increase in unbound cortisol from 0\m=.\2to a maximum of 2\m=.\1ng/ml. The concentration of albumin-bound cortisol was approximately equal to that of unbound cortisol. The mean value for the transcortin\p=n-\cortisolaffinity constant was 1\m=.\15\m=x\108 l/mol. It is concluded that an increase in transcortin-binding capacity is partly responsible for the prepartum increase of corticosteroid levels observed in normal foetal lambs