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Abstract
The biological activity and immunoreactivity of serum prolactin (PRL) has been shown
to fluctuate throughout the estrous cycle of the rat. Since the 24-kDa nonphosphorylated
and phosphorylated isoforms from several species have also been shown to differ in
their biological and immunoreactivities, we have investigated the possibility that
the 24-kDa monomer isoform profile varied throughout the estrous cycle of the rat.
The PRL isoform profile was assessed in homogenates of pituitaries and in short-term
incubation media. Comparisons between homogenates, which always contained isoforms
1, 2, 3, and 3’ (numbered according to increasing acidity), and media showed nonproportional
release of the isoforms at all stages. Of great interest were the release of isoform
1 (a nonphosphorylated form) only at estrus and the lack of release of isoform 3’
(a phosphorylated form) only in the afternoon of proestrus. This lack of release of
3’ was accompanied by a marked increase in the release of isoform 2 (the unmodified
polypeptide). These results suggest a unique function for isoform 1 during estrus
and a role for increased isoform 2 and absent isoform 3’ during the proestrus surge
of PRL. Moreover, they suggest that fluctuations in the biological activity and immunoreactivity
of serum PRL during the estrous cycle could be due, at least in part, to fluctuations
in the isoform profile.