A solubility shift occurs during depletion-transformation of prolactin within the lactating rat pituitary.

Abstract

We investigated the mechanism by which the depletion-transformation of PRL occurs within the anterior pituitary (AP) of the lacatating rat after suckling. The concentration of PRL in hemi-AP, extracted at pH 7.2 and assayed by polyacrylamide gel electrophoresis, fell quickly by 10 min of suckling. By 60 min, PRL had increased toward presuckling levels. PRL in the other halves of the same pituitary glands extracted at pH 10 did not decrease significantly by 10 min but then declined gradually over the next 50 min of suckling. The rate of loss in pH 10 extractable PRL from 0–60 min of suckling was quantitatively similar to the rate of release of PRL into the circulation during the same time. The PRL in the AP of other rats was labeled with [3H]leucine, and 8 h later the AP was removed and incubated in medium 199 buffered with 21.6 mM bicarbonate. The concentration of both total and [3H]leucinelabeled PRL extracted at pH 7.2 was reduced by about 40% during the first 30 min of incubation. This was followed by a repletion and then a second fall in concentration of both over the subsequent 60 min. Similar oscillations, but out of phase by 30 min, occurred in PRL from the AP of rats suckled for 15 min before removal of the AP for incubation. Specific activity of [3H] PRL in the AP from suckled and nonsuckled rats was low during depletion and high during repletion. The concentration of labeled and total PRL, and the specific activity of the labeled PRL, did not oscillate but gradually fell in parallel to the amount of the hormone which appeared in the medium when the PRL in the other halves of the same glands was extracted at pH 10. These results suggest that PRL is contained in the AP of the rat in a detectable (by gel electrophoresis) form, but upon either suckling or incubation of the AP of the nonsuckled rat, some of the PRL is “depleted” (i.e. becomes nondetectable). Alkaline extraction permits recovery of the nondetectable PRL, which suggests that during the initial depletion phase, PRL is transformed into a more insoluble form (releasable PRL), nonextractable at neutral pH. A reversal of this phenomenon apparently occurs during the repletion of PRL in vitro.