Radioimmunologic assessment of the level of circulating LH antibodies after passive immunization of the rat: relation to the level of LH secretion.

A double antibody radioimmunologic technique was used to estimate circulating levels of "free" LH antibodies (Ab) (i.e., Ab available for binding to radioiodinated rat LH in vitro) after a single injection of an equine antiserum to bovine LH (LH-AS) into cyclic (diestrus-1), early pregnant (d 6 or d 8), short-term (1 d) or long-term (12 d) ovariectomized, or 8-d hypophysectomized female rats. In both cyclic and pregnant rats given 0.5 ml LH-AS sc the blood Ab levles peaked 1 to 2 days after injection and then decreased exponentially (half-time: 14.6-17.0 h) and equally: vaginal estrus also appeared in the cyclic and the aborting pregnant rats on the 6th day, and ovulation on the 7th day after LH-AS treatment, when the blood levels had fallen to very low values. The rate of exponential decrease in blood Ab levels was not affected by other routes of administration (iv or ip) of LH-AS, or by sc doses of 1.0 or 0.25 ml, although the peak levels and their duration were related to dose and route of administration. Ovariectomy significantly increased the rate of exponential Ab decrease (half-time: 9.8 h in rats tested 12 days after operation) and hypophysectomy markedly diminished it (half-time: 63.0 h in rats tested 8 days after operation). Daily treatment of hypophysectomized rats with 20 mu/day of ovine LH from the time of LH-AS injection increased the rate of exponential Ab decrease to one approaching that of the intact rats (half-time: 20.5 h). Free LH antibodies thus seem to disappear from the circulation at a rate proportional to the amount of LH in the circulation.