Prolactin Released in vitro from the Pituitary of Lactating, Pregnant, and Steroid-Treated Female or Male Rats Stimulates Prolactin Secretion from Pituitary Lactotropes of Male Rats

Abstract

We have previously shown that soluble factor(s) in conditioned media (CM) from the central and peripheral regions of the anterior pituitary (AP) gland of lactating rats promoted the in vitro dose-related release of prolactin (PRL) from pituitary glands of male rats. In the present experiments we sought to determine whether CM from rats in different physiological states provoked similar effects (like those of lactating rats), and the nature of the factors, whether 23K PRL or other variants of the hormone, were responsible for these effects. Stimulatory effects were induced by CM from pregnant females and steroid-treated castrated males or females, but not from untreated castrated rats, intact males, or by a PRL standard. More potent effects occurred with CM from APs of early- than from mid- or late-lactating rats, and from rats unsuckled for 8 or 16 h than from those unsuckled for 32 h. With respect to the nature of factor(s) responsible for these effects, immunoprecipitation of PRL from the CM of lactating females and of steroid-treated, castrated males eliminated, whereas dephosphorylation or deglycosylation of CM of lactating rats greatly increased its effects upon PRL release. Also, electrophoretic analysis and Western blotting of the CM proteins under native and denaturing conditions revealed a variety of PRL variants, ranging from 14 to <90 kDa, in CM from lactating rats, and the main effects on PRL release were provoked by the 23- to 46-kDa PRL variants. These results indicate that specific effects upon male rat lactotropes may be exerted by PRL variants released from APs of lactating and non-lactating rats.

Autocrine and Paracrine Regulation of Prolactin Secretion by Prolactin Variants and by Hypothalamic Hormones

“…In previous reports (Huerta-Ocampo et al, 2007;Mena et al, 2010) we showed that conditioned media (CM) and PRL variants i.e., from 7-14 and 70-97 kDa, from lactating rat APs, characterized by Western blotting and eluted from SDS-PAGE, promoted the in vitro vesicular release of the hormone from preformed, mature PRL granules of male rat APs, and that such release was independent of PRL synthesis (Huerta-Ocampo et al, 2007). Autocrine and paracrine types of actions have also been shown to occur within the AP (Denef, 2008;Freeman et al, 2000;Lorenson and Walker, 2001;Schwartz & Cherny, 1992;Schwartz, 2000;Welsch et al, 1968;Diaz, et al, 2002;Huerta-Ocampo et al, 2007;Mena et al, 2010), and were demonstrated when the central and peripheral AP regions of lactating rats were incubated in vitro with CM from pituitaries of lactating, pregnant and steroid-treated castrated males or females, but not from untreated castrated rats, intact male rats or by a PRL Standard (Huerta-Ocampo et al, 2007;Mena et al, 2010).…”

Prolactin and the physiological regulation of its secretion. A review

“…Functionally, lactotrophs are also heterogeneous with respect to basal hormone release, electrical activity and responsiveness to releasing inhibitory factors, including regional differences in exposure to hypothalamic or posterior pituitary factors (BenJonathan et al, 1996;Mena et al, 2010).…”

“…Prolactin variants, ranging from 14 to <90 kDa, released in vitro from the pituitary of lactating, pregnant and steroid-treated female or male rats stimulates PRL secretion from pituitary lactotrophs of male rats (Mena et al, 2010). The main effects on PRL release are provoked by the 23 to 46 kDa PRL variants (Mena et al, 2010).…”

“…The main effects on PRL release are provoked by the 23 to 46 kDa PRL variants (Mena et al, 2010). Glycosylated PRL is detected in pituitary and plasma, composing 1%, 15% and 50% of total PRL in bovine, human and porcine pituitaries, respectively (Sinha, 1995).…”

“…It varies among species and their heterogeneity accounts for differences in bioactivity, immunoreactivity, receptor bindings and often decreases PRL bioactivity (Ben-Jonathan et al, 1996;Mena et al, 2010).…”