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We investigated the subcellular localization of PRL and GH in the pituitary gland of the female Japanese house bat by the double immunolabeling procedure using the protein A-gold method combined with electron microscopy and demonstrated a seasonal alteration in the distribution of PRL within the cells. The seasonal changes were related to the different phases of the bats' reproductive cycles. Mammosomatotrophs (MS cells) containing both PRL and GH were constantly present throughout the reproductive cycles of the female bats and they were remarkably hypertrophied during pregnancy. The distribution pattern of PRL and GH within the MS cells was extremely variable owing to the different phases of reproductive cycles of the bats. We divided MS cells into the following four types: the cell containing 1) only mixed granules containing both PRL and GH, 2) both mixed and PRL granules, 3) mixed, PRL and GH granules, 4) both mixed and GH granules. While pure PRL-containing cells were observed in pregnant and lactating bats, they were not observed in bats obtained during prehibernation, midhibernation, and arousal periods. These results suggest that MS cells in the female bats hypertrophy during pregnancy and that some of them may be transformed into PRL-producing cells.

By a double immunocytochemical labeling procedure, using the protein A-gold method combined with electron microscopy, the co-localization pattern of growth hormone (GH) and prolactin (PRL) was detected in the anterior pituitary cells of female rats and female musk shrews. Two types of co-localization of GH and PRL were demonstrated. First, GH- and PRL-containing secretory granules were intermixed within closely aggregated and interdigitated cell clusters that were composed of GH and PRL cells. This phenomenon was characteristically seen in pregnant rats and pregnant musk shrews. Therefore, the occurrence of an intermixture of GH and PRL granules might be related to an enhanced cellular function for PRL synthesis. In another pattern of co-localization of GH and PRL, both hormones were co-packaged in the same secretory granules within a single cell. Such cells were scarce, small, irregularly shaped, and observed only in pregnant rats. These mixed GH-PRL cells contained not only mixed GH-PRL granules but also granules containing only GH or PRL. This suggests that these bihormonal cells are able to synthesize, synchronously or asynchronously, GH and PRL. Furthermore, granule extrusion from the mixed cells was clearly shown in this study. It seems likely that the mixed GH-PRL cells reveal active cellular function in the pituitary gland of the pregnant rat.

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