Prolactin and testosterone are synergistic in stimulating growth of the rat prostate. The lateral lobe is more sensitive to this synergism than the ventral and dorsal lobes. To investigate whether prolactin acts directly in the rat prostate or indirectly through another systemic mediator, anterior pituitary grafts (1 mm3) were implanted in the lateral prostate of castrated Sprague‐Dawley rats in whom a 0.5 cm or 1.0 cm testosterone‐filled silastic tubing was implanted subcutaneously at the same time. Rats were randomly assigned to receive either the pituitary or a muscle chip of similar size grafted beneath the fascia lateral to the lateral prostate. Twenty‐one days later, serum prolactin levels were not elevated in pituitary‐grafted animals and were not significantly different from those in muscle‐grafted rats. The mean lateral prostate weight on the grafted side in pituitary‐implanted rats with 1.0 cm testosterone tubing was 43% heavier than either that of the contralateral side or the corresponding weights in muscle‐implanted rats. In pituitary‐implanted rats with 0.5 cm testosterone tubing, the mean lateral prostate weight on the grafted side was 60% heavier than either that of the contralateral side or that of the corresponding weights in muscle‐implanted rats. The weight of the ventral and dorsal lobes of the prostate was not significantly affected by the presence of pituitary grafts in one of the lateral lobes. The local effect of prolactin on the lateral prostate was further demonstrated by an overall decline in tissue concentrations of dihydrotestosterone in the grafted side. These results provided evidence to indicate that there was a direct effect of prolactin on growth of the lateral prostate in rats.
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