Effects of Cysteamine on Pituitary, MtTW15Tumor, and Serum Prolactin Levels Measured by Rat Lymphoma Cell Bioassay and Radioimmunoassay*

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Abstract
Cysteamine (CSH), a sulfhydryl compound reduces both serum and anterior pituitary (AP) prolactin [PRL] measured by RIA [radioimmunoassay]. The Nb2 lymphoma cell bioassay (BIO) was used for PRL to evaluate possible CSH-related changes in PRL levels in sera and tissues of male and MtTW15 mammosomatotropic tumor-bearing female rats. Experimental animals received a single s.c. injection of CSH (300 mg/kg), and samples were collected 0.5-24 h later. Since CSH and serum from CSH rats were toxic in BIO, samples were dialyzed before assay. All samples were evaluated for PRL and growth hormone [GH] by RIA as well. A significant decrease (P < 0.05) in BIO serum PRL was evident in male rats 0.5 h after CSH; levels remained low for 24 h. Serum PRL by RIA was significantly depressed at 4 h but not at 0.5 h or 24 h. PRL in AP extracts was decreased (60-90%) at all times by BIO and RIA. Significant decreases of BIO- and RIA-detectable PRL were recorded in serum and tissues (AP and tumors) at 4 h in tumor rats. Sequentially bled (0.5-4 h) CSH-treated tumor-bearing rats showed 50 and 80% reductions in serum PRL at 1 and 4 h by both BIO and RIA. CSH had no effect on GH levels in sera and tissues of any animal studied at any time interval. Earlier reports on CSH-induced decreases in RIA-detectable PRL were substantiated. Such changes cannot be attributed to assay effects alone, as significant decreases in circulating and stored PRL (both AP and tumor) were evident by BIO. Results with tissue extracts were the most dramatic. They suggest an action of CSH or a metabolic intermediate with stored PRL which reduces both extractable PRL and hormone release. Such an effect of CSH on PRL extraction has been suggested by others. Whatever the mechanism, it appears to be relatively specific, since GH cells were not affected.