Biphasic response of the rat lateral prostate to increasing levels of serum prolactin

Our previous work has shown that an increase in serum prolactin (Prl) levels produced by two pituitary grafts specifically enhances testosterone-stimulated growth of the rat lateral prostate. Since Prl has been shown in several mammalian systems to have biphasic effects, i.e., stimulatory at low doses and inhibitory or without effect at high levels, the present study was undertaken to determine whether the prostate gland would respond differentially to increasing Prl levels. Adult male rats were castrated, given s.c. testosterone implants and grafted with 0, 1, 2, 4 or 6 pituitaries under the renal capsule. Three weeks later all animals were sacrificed. The three prostate lobes were removed for analysis and blood was collected for Prl and testosterone radioimmunoassay. There were no significant differences in ventral and dorsal lobe parameters among the 5 groups whereas the lateral lobe showed a marked tropic response. As serum Prl levels increased, the weight of the lateral prostate first increased and then diminished. Mean weights (mg ± SEM) were: 40 ± 3, 65 ± 3, 76 ± 6, 70 ± 4 and 59 ± 3 for animals with 0, 1, 2, 4 and 6 grafts, respectively. Protein and DNA levels showed the same response pattern. To ensure that this response was a result of elevated Prl, the study was repeated with daily injections of bromoergocriptine (CB-154). The mean weights after 3 weeks in the control and CB-154-treated animals, respectively were: 0 graft, 44 ± 3 and 43 ± 2; 2 grafts, 81 ± 4 and 41 ± 3; 6 grafts, 56 ± 3 and 44 ± 3. Again, a biphasic response occurred in control animals. In animals treated with CB-154, serum Prl levels were undetectable and no increase in lateral lobe weight, protein or DNA content was observed. The present results demonstrate that the rat prostatic response to Prl is biphasic in that the stimulatory effect seen at low to moderately elevated Prl levels diminishes with higher levels of the hormone.