Rat serum inhibits prolactin release from rat pituitary mammotrophs in culture

Rat pituitary cells maintained in culture for 3 days in alpha-MEM supplemented with 17% horse serum produce and release 4-8X the quantity of prolactin (PRL) originally contained in the cells at the time of plating. Addition of small (200 microliter) quantities of rat serum repeatedly suppressed PRL release by approximately 50%. Inhibition was related to dose of rat serum. Other studies showed (a) that the cells could recover from serum-induced inhibition, (b) that the effect was independent of the age of the pituitary and/or serum donor, (c) that GH release was unaffected by rat serum addition, (d) that the effect could not be attributed to proteases in the serum preparations and (e) that addition of T3 to the culture medium prevented the inhibitory response. The inhibitory material(s) in rat serum is stable on freezing; is non-dialyzable; is heat-labile; is stable over 3 days incubation at 37 degrees C; and is associated with molecules between 85 000 and 146 000 dalton. Inhibitory activity was generated upon incubating whole clotted blood at 37 degrees C for 23 h. It is suggested that the inhibitory material might (a) be released from platelets during clotting or (b) be generated from an inactive precursor molecule cleaved by proteases released during the clotting process.