Heterogeneous Turnover of Adenohypophysial Prolactin1

Abstract

Adenohypophyses (AP) of female rats were exposed to 3H-leucine either in vivo or in vitro, then incubated in vitro. The prolactin (PI) in the tissues and incubation medium was separated and quantified by disc electrophoresis (DE) and densitometry. The PI bands were cut from the gel columns and counted by liquid scintillation procedures. In all experiments the specific activity of prolactin (cpm/μg) in the medium became significantly greater than that in the tissue, and was as high as 2.7 times that of the tissue in some cases. This indicated that prolactin was secreted in vitro from more than one pool, with the newly synthesized (labeled) hormone being rapidly released. One rapid turnover pool was indicated by an increase of as much as 100% in medium PI specific activity between 40 and 70 minutes of incubation of AP labeled in vivo one hour before incubation. This early increase did not occur with AP labeled in vivo 4 hours. Another less rapid turnover pool was suggested by a greater discrepancy between medium and tissue PI specific activity in incubations of AP labeled in vivo 4 hours prior to incubation (medium specific activity 2.6 times that of tissue after 4 hours incubation) than in those of AP labeled in vivo 1 hour prior to incubation (medium specific activity 1.5 times that of tissue after 4 hrs). The total gain in labeled prolactin (cpm/mg AP) in the medium of AP incubated with cycloheximide for 4 hours was as much as 2.6 times greater than the total decrease in the tissues. In addition, the tissue decrease and medium increase of labeled PI for the individual incubation periods was not equal and concomitant. These findings suggest the existence in the AP of a form of PI which is not detectable by the DE assay method, but which became detectable in the medium. The DE nondetectable prolactin may be a presecretory form of the hormone. These results are discussed in relation to other recent reports on heterogeneity of hormone turnover and form, and a model for PI synthesis and secretion is proposed. (Endocrinology89: 1380, 1971)