[Specific binding of cortisol to plasma protein in the guinea-pig from birth to weaning]

We have estimated two parameters of guinea-pig adrenal function during the neonatal period: --the concentration of plasma cortisol by competitive binding to protein; --the association constant, Ka, and total capacity of CBG binding (NS) by the thin-layer gel filtration technique. The concentration of plasma cortisol, very high at birth, declined markedly between neonatal days 1 and 10 and then remained constant between days 10 and 20, the time of weaning. Ka was 0.25.10(7).M-1 in the newborn and the adult of both sexes. This value was lower than that of other species. NS, which was very high at birth (111 +/- 5 micrograms/100 ml in the male; 101 +/- 5 micrograms/100 ml in the female), decreased rapidly during the first two neonatal days. At that time, it reached adult levels in the female (45 +/- 5 micrograms/100 ml) and remained stable. Adult binding capacity (16 +/- 2 micrograms/100 ml) was reached in the male after neonatal day 20.