THE INFLUENCE OF OESTROGEN ADMINISTRATION IN VIVO ON IN VITRO PROLACTIN RELEASE

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Abstract
The influence of estrogen administered to the ovariectomized rat on the interaction between dopamine (DA) and thyrotropin releasing hormone (TRH) on the release of radioimmunoassayable (RIA) and [3H]Leu incorporated into prolactin ([3H]PRL) was examined in vitro. Dopamine had a more marked suppressing effect on newly synthetized PRL (80%), as determined [3H]PRL, than on total PRL (50%), as determined by RIA-PRL. The administration of 5 .mu.g of estradiol benzoate (EB) for 7 days resulted in blocking the suppressing effect of DA when RIA-PRL was measured but not when [3H]PRL was measured. The administration of 5 .mu.g of EB enabled TRH to partially override the suppressing effect of DA and the degree of response was more marked when RIA-PRL was measured than when [3H]PRL was measured. The administration of 50 .mu.g of EB for 3 days enabled TRH to override the DA blockade of PRL release to levels comparable to that of the control when RIA-PRL was measured but had little to no effect on [3H]PRL. The results are discussed in relation to the 2 storage pools of PRL in the pituitary. DA apparently acts predominantly to suppress the newly synthetized, rapidly releasable pool. Estrogen acts to block DA action on the older more stable PRL pool. The ability of TRH to override the DA blockade of PRL release depends upon the presence of estrogen; here TRH acts predominantly on the older more stable pool of PRL. Estrogen''s action on disrupting the DA suppression of PRL release appears to be related to the time of day the hormone is administered subsequent to when the pituitary is exposed to DA in vitro.