Progesterone concentration in rabbit uterine flushings before implantation

Connell, 1973). The protein is known to bind steroids, particularly progesterone (Urzua et al., 1970; Arthur & Daniel, 1972; Beato & Baier, 1975), and its role in early rabbit embryonic develop-ment may be that of a carrier protein for progesterone (Arthur, Cowan & Daniel, 1972; El Banna & Daniel, 1972a, b; Beato & Baier, 1975). Progesterone can be detected in 5- and 6-day post coitum rabbit blastocysts, 7-, 8- and 9-day post coitum blastocoelic fluid and in uterine washings from 5-day pseudopregnant rabbits (Beier, 1968b; Seamark & Lutwak-Mann, 1972). We now report measure-ments of the concentration of progesterone in non-pregnant and pregnant rabbit uterine flushings and plasma. Methods Mature virgin rabbits of the New Zealand breed were killed by cervical dislocation at oestrus and 1-6 àayspost coitum. The uteri were flushed with 50 ml 0-9 % (w/v) sodium chloride solution through each horn from the oviducal end. The washings were centrifuged at 12,000 g · at 3°C for 10 min, and the supernatant fluid was frozen at —20°C until analysed. An average of 7 embryos/animal was recovered. Blood samples were obtained by cardiac puncture with heparin as anticoagulant. The blood was centrifuged at 6000 g for 10 min, and the plasma was frozen. There were two animals i