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A cell suspension containing more than 90% lactotrophs can be prepared from enzymically dispersed adenohypophyses obtained from male rats pretreated with estradiol. The lactotrophs are separated from the mixed cell population by centrifugation on a discontinuous density gradient prepared from a commercial preparation of colloidal silica (Percoll, Pharmacia). The method allows isopycnic separation of these delicate cells under very mild conditions; normal ionic strength and normal pH were maintained throughout the gradient, centrifugal acceleration did not exceed 1600 X g, and all procedures were done at room temperature. Histological verification that at least 90% of the cells were lactotrophs was done using specific immunoperoxidase staining. The functional capability of the lactotrophs was established by measuring the dose--response to the dopamine agonist bromocriptine and to thyrotropin-releasing hormone (TRH). Bromocriptine decreased spontaneous release in a dose-related way over the concentration range of 10(-10) to 10(-8) M. TRH, which causes an in vivo release of prolactin (PRL) in estrogen-primed rats, produced a dose-related increase in the release of PRL over the concentration range of 3 X 10(-10) to 3 X 10(-8) M after the high spontaneous release had been previously reduced by bromocriptine (3 X 10(-8) M).

Dispersed rat lactotrophs were treated with bromocriptine (10(-10) M) for either 30 min or 3 h to investigate its effect on cell morphology using light and electron microscopy and ultrastructural morphometry. After 30 min, lactotrophs treated with bromocriptine exhibited an increase in storage granule size (p less than 0.05). Crinophagy was also evident and a significant increase in lysosome volume density was observed (p less than 0.001). However, no significant change in prolactin content in the media was detected until 3 h of incubation in bromocriptine (p less than 0.05). At this time a nonsignificant decrease in Golgi region volume density was also observed.