The relation between the age of the corpus luteum (CL) and the luteolytic effect of an LH-antiserum (LH-AS): comparison of hysterectomized pseudopregnant rats with intact pregnant rats for their response to LH-AS treatment at four stages of CL activity.

At one of four stages during the period of corpus luteum (CL) activity (day 9, day 12, day 15, or day 18) groups of hysterectomized pseudopregnant (PSP) rats and of intact pregnant (PRG) rats were compared for the effect of a single sc injection of an antiserum to LH (LH-AS) on progesterone secretion. Groups of control rats were injected sc with normal horse serum (NHS) at these same stages, and in all rats, the progesterone level in jugular blood serum was measured by RIA on the day of treatment and 24 and 72 h after treatment. Among the PRG rats, the controls' progesterone levels rose to a peak on day 15, and then slowly declined. LH-AS on day 9 induced abortion and a rapid, drastic and permanent fall in the progesterone level in all rats. On day 12, it induced a similar fall in progesterone, and abortion, in 4 of 10 rats; in the 6 which remained pregnant, a much less severe fall occurred 24 h after treatment, and by 72 h the level had returned to close to the initial one. On days 15 or 18, LH-AS induced neither abortion nor a significant change in the progesterone level from that seen in the controls. Among the PSP rats, the controls' progesterone levels tended to fall progressively after day 9; the average length of diestrus was about 21 days. At each of the four stages the LH-AS induced a rapid, drastic and permanent fall in the progesterone level. Early termination of the diestrus was easily discernible in the groups injected on days 9 or 12, but was obscured in the other groups because of the similarity in length of the expected remaining diestrus and the duration of the neutralizing effect of the LH-AS on LH in the circulation. The PSP rats' CL, thus, once they become dependent on LH (about day 9), remain so to the end of PSP. The PRG rats' CL seem to lose this dependency after day 12, but the possibility could not be eliminated that the dependency may shift from LH to a placental LH-like hormone.