Catecholamines were quantified in hypophysial portal plasma obtained from cycling diestrous rats and from noncycling diestrous rats with hyperprolactinemia due to PRLsecreting pituitary explants or a PRL-secreting pituitary tumor. In rats bearing PRL-secreting tumors, the plasma concentration of PRL was 4450 ± 938 ng/ml (mean ± SE), whereas in cycling diestrous animals the plasma level of PRL was 16 ± 2.2 ng/ml. The concentration of dopamine in hypophysial portal plasma of tumor-bearing rats (2.0 ± 0.3 ng/ml) was significantly greater (P < 0.05) than that in diestrous rats (1.1 ± 0.2 ng/ml). The anterior pituitary content of PRL was 18 ± 1.5 μg in cycling diestrous animals and 9 ± 0.8 μg in tumor-bearing rats. The adrenals from tumor-bearing animals were much heavier (155 ± 14 mg) than those from cycling diestrous rats (58 ± 1.2 mg). The concentration of dopamine in portal plasma from tumor-bearing adrenalectomized rats was 2.0 ± 0.5 ng/ml, similar to that in portal plasma from tumor-bearing intact animals. The arterial plasma concentration of PRL in rats bearing four anterior pituitary explants maintained under the renal capsule was 178 ± 20 ng/ ml. In these same animals, the pituitary content of PRL was 11 ± 1.6 μg, which was significantly less (P μ 0.01) than that in cycling diestrous animals (19 ± 1.9 fig). The dopamine concentration in portal plasma from cycling diestrous rats was 1.3 ± 0.23 ng/ml and was 1.3 ± 0.36 ng/ml in the rats bearing ectopic pituitary glands. It is concluded that agents secreted by the tumor, possibly PRL and/or an ACTH-like substance, can lead to an increased secretion of dopamine by the hypothalamus.
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