Abstract
The changes in hormonal responsiveness of corpora lutea produced by a single injection of hCG or ovine LH were studied in PMSG-treated immature rats with well-characterized corpora lutea of different ages. The hormonal response was measured as LH-stimulated accumulation of cyclic AMP in isolated, whole corpora lutea and, in some experiments, as LH stimulated luteal adenylate cyclase activity. Injection of hCG produced a complete desensitization (refractoriness) of very long duration (4–5 days) as has earlier been found in rats with heavily luteinized ovaries, while the injection of LH in equivalent doses, produced only a partial desensitization of much shorter duration (12–48 h). Morphological observations, after injection of trypan blue to a small number of the rats, showed that follicular growth, ovulation and formation of new corpora lutea could occur after both hCG and LH. The possibility that the corpora lutea which started to respond to LH stimulation 5–7 days after the hCG injection were not the ones originally desensitized by hCG but new corpora lutea, is discussed. The very rapid recovery of the LH sensitivity after desensitization with LH, can, however, not be explained in the same way. It represents a true reversal to the 'normal' hormonal sensitivity of the cells originally desensitized. The marked difference between hCG and LH in these desensitizing experiments is discussed in relation to earlier known differences between these two gonadotrophins, e.g. in binding properties to ovarian receptors and in rate of metabolism in the body.