Heterogeneity in the bioactivity of LH secreted by peripubertal rats

Treatment of immature rats with 5 iu equine chorionic gonadotrophin (eCG) o n day 25 typically stimulates a preovulatory surge of LH on day 27 and ovul ation on day 28. In rats weighing > 60 g at the time of treatment, an LH su rge and ovulation occurred in 75% of the animals but, in rats weighing < 60 g, only 13% ovulated even though 69% showed an LH surge. Previous findings have shown that exogenous LH can stimulate ovulation in the rats < 60 g, i ndicating that the anovulation was not due to ovarian immaturity, but rathe r to an abnormal form of LH. Thus, it was important to determine whether th e bioactivity of LH released at the time of the surge differs in rats < 60 g compared with rats > 60 g. Experiments showed that LH from both groups of eCG-treated animals were equipotent in stimulating testosterone production from incubated Leydig cells and progesterone production from cultured gran ulosa cells. Similarly the surge of progesterone in vivo, which occurs co-i ncident with the LH surge, was of similar magnitude in both groups of anima ls. Since prostaglandin synthesis increases at the time of ovulation and is also stimulated by LH, it was investigated whether the activity of ovarian phospholipase A,, the rate limiting enzyme in prostaglandin synthesis, and ovarian prostaglandin E-2 concentrations differed in the animals > 60 g an d < 60 g. Phospholipase A, activities were similar in both groups of animal s at the time of the LH surge, as were the prostaglandin E-2 concentrations . However in all animals that ovulated (15/20 in rats > 60 g and 2/15 in ra ts < 60 g), there was a threefold increase in ovarian prostaglandin E-2 con centrations. The results show that, in underweight animals, the bioactivity of LH, in terms of its ability to stimulate steroidogenesis and phospholip ase A, activity, is similar to that released by animals > 60 g; however, th e LH produced by the underweight animals fails to induce ovulation by faili ng to increase, either directly or indirectly, prostaglandin E-2 production . Comparison of the profiles of plasma LH collected at the time of the LH s urge on an anionic ion exchange column indicates that the LH from rats < 60 g possesses significantly less of the neutral or basic glycoform of LH tha n that from rats > 60 g. This finding provides a further index that the bio potency of LH produced by underweight animals is different from that of rat s > 60 g.