Influence of a prolactin- and ACTH-secreting tumour on oestrous cyclicity, the pro-oestrous surges of LH and prolactin and ovarian hypertrophy in the rat.

These studies were designed to investigate the effects of a prolactin- and ACTH-secreting tumour (7315a) on reproductive cyclicity, pro-oestrous surges of LH and prolactin and ovarian hypertrophy in the rat. Normal adult Buffalo rats, which are syngeneic to the 7315a tumour, were found to have a significant pro-oestrous prolactin surge and a relatively low-level preovulatory LH surge. Within 14 days of s.c. injection of dispersed tumour cells, a small tumour was detectable by palpation in one rat, but measurable tumours were not observed until day 18. The pro-oestrous surge of LH, but not of prolactin, was effectively suppressed by day 17. Cessation of reproductive cyclicity (anoestrus) was apparent within 19.4 +/- 1.1 days of injection of tumour cells. Removal of a single ovary showed that there was no change in ovarian weight before anoestrus in the tumour-bearing animals. Subsequent hypertrophy of the second ovary was augmented during the early stages of tumour development (days 8 and 13), and this corresponded to the unexpected finding that the gonadotrophin surges on days 8 and 13 were significantly increased when compared with controls. Ovarian hypertrophy was not significantly different from that in controls after suppression of the gonadotrophin surge (day 17). The results suggest that inhibition of the preovulatory LH surge might be a critical event in the tumour-induced cessation of reproductive cyclicity. The fact that pre-surge concentrations of prolactin did not increase substantially until the conspicuous onset of tumour prolactin secretion on day 21 indicated that the preovulatory LH surge might be inhibited by relatively low levels of tonically secreted prolactin.