Heterogeneous Turnover of Adenohypophysial Prolactin1

Swearingen, Karen C.

doi:10.1210/endo-89-6-1380

Cited by 82 publications

(28 citation statements)

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“…3) demonstrated that provision in this scheme for reutilization of proinsulin or degradation of insulin was unnecessary. Preferential secretion is not unique to insulin, and also has been observed for placental lactogen (30), prolactin (31), parathyroid hormone (32), pancreatic amylase (33), gonadotropin (34), vasopressin (35), thyroglobulin (36), and acetyl choline (37). For most of these secretory products, preferential secretion can be estimated to occur from a labile compartment not too dissimilar in size from the approximately 33% of the total storage compartment reported here.…”

Section: Discussionsupporting

confidence: 66%

Heterogeneity and compartmental properties of insulin storage and secretion in rat islets.

Gold¹,

Landahl²,

Gishizky³

et al. 1982

J. Clin. Invest.

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A B S T R A C T To investigate compartmental properties of insulin storage and secretion, isolated rat islets were used for pulse-labeling experiments, after which proinsulin and insulin were purified rigorously. Processing of proinsulin to insulin neared completion by 3 h without additional loss of either radioactive peptide by cellular or extracellular proteolysis. The amount of labeled hormone rapidly diminished in islets; it was secreted at a higher fractional rate than immunoreactive insulin, resulting in secreted insulin's having a higher specific activity than the average cellular insulin. Newly synthesized insulin, therefore, was secreted preferentially. Changes in the specific activity of secreted and cellular insulin with time were consistent with changes predicted for islets containing 33% of their total insulin in a glucose-labile compartment. Predictions were based on steady-state analysis of a simple storage-limited representation of B cell function. Islets from either the dorsal or ventral part of the pancreas also contained 33% of their total insulin in a glucose-labile compartment. The same compartment was mobilized by 20 mM glucose, 50 mM potassium + 2 mM glucose, or 20 mM glucose + 1 mM 3-isobutylmethylxanthine as indicated by the specific activity ratio of secreted vs. cellular insulin, even though average secretion rates with these stimuli differed by more than threefold. In the absence of calcium, the effectiveness of 20 mM glucose as a secretagogue declined markedly, and the older stored insulin was preferentially mobilized because secreted insulin had a lower rather than a higher specific activity than cellular insulin. Results provide insight into the mechanisms of nonrandom mobilization and secretion of insulin from the B cell.

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Section: Discussionsupporting

confidence: 66%

Heterogeneity and compartmental properties of insulin storage and secretion in rat islets.

Gold¹,

Landahl²,

Gishizky³

et al. 1982

J. Clin. Invest.

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“…In agreement with data from others [38,41], the earliest time that newly synthesized PRLs could be detected in the medium in our study was at 30 min of incubation. It is pos sible that this early release and that occurring during the first 2 h of incubation may be due to the early and rapid release of newly synthesized PRL granules [41].…”

Section: Discussionsupporting

confidence: 94%

“…Previous results obtained with nonlactating rat AP tis sues or cells have indicated that newly synthesized PRL is preferentially released over older hormone in vitro [21, 38,41] and that such preferential release may result from func tional heterogeneity among lactotrophs [41]. However, we consider that this apparent conflict may be explained by the different methods employed to define in vivo and in vitro newly synthesized and mature hormone and by taking into account that releasability and secretory patterns of PRLs of different age change as a function of time during incubation.…”

Section: Discussionmentioning

confidence: 99%

Differential Effects of Thyrotropin-Releasing Hormone on in vitro Release of in vivo or in vitro Newly Synthesized and Mature Prolactin by Lactating Rat Adenohypophyses

et al. 1989

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We studied the basal and thyrotropin-releasing hormone (TRH)-stimulated in vitro release of newly synthesized and mature prolactin (PRL) by lactating rat pituitary fragments. Newly synthesized PRL was defined as that resulting from either in vivo pulse labelling with [³H]-leucine (injected i.v. 10 min before removing the pituitary glands for in vitro incubation); or in vitro labelled by a 5-min preincubation period of adenohypophyseal fragments. Mature PRL was defined as that labelled in vivo 8 h before incubation. Medium PRL was quantified by polyacrylamide-gel-electrophoresis densitometric and liquid scintillation techniques. Under basal conditions, both total (unlabelled) and 8-hour labelled (mature) PRLs showed an in vitro release pattern characterized by an initial period of faster and higher secretion that lasted for 30–60 min followed by a lower secretion rate that extended from 60 to 240 min of incubation. Secretion of newly synthesized PRLs was detected since the first 30 min, but did not reach maximum levels until 90–120 min of incubation. Accordingly, PRL release during the first 30–60 min of incubation consisted primarily of mature PRL, whereas secretion during the rest of incubation included the release of newly synthesized hormone. These results confirm previous data on the sequential, rather than preferential, release of different age PRLs. TRH (0.35 µM) stimulated (about 30% above control) the release of both total and mature PRLs. No effect could be detected upon the release of in vivo labelled new PRL, while the same dose of TRH strongly stimulated (about 100% above control) the release of in vitro labelled new PRL. These data show that TRH exerted different effects upon PRL secretion according to the age of the hormone since biosynthesis and whether new PRL was synthesized in vivo or in vitro. These findings suggest that qualitative differences between in vivo and in vitro synthesized PRL may exist, which may affect hormonal releasability to hypothalamic regulation.

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“…The use of a pulse label allowed us to examine the influence of DA and TRH on the release of newly synthesized PRL while RIA-PRL measured both the newly synthesized and the older storage pool of PRL. The suggestion that two storage pools exist for PRL, a newly synthesized, more readily releasable pool and an older, more stable pool has previously oo- been made by other investigators (Swearingen 1971;Morbi et al 1975). The per cent changes in response to the various experimental manipulation are summarized in Fig.…”

Section: Discussionmentioning

confidence: 95%

The Influence of Oestrogen Administration in Vivo on in Vitro Prolactin Release

Jaques¹,

Gala²

1979

Acta Endocrinologica

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The influence of oestrogen administered to the ovariectomized rat on the interaction between dopamine (DA) and thyrotrophin releasing hormone (TRH) on the release of radioimmunoassayable (RIA) and [3H] leucine incorporated into prolactin ([3H]PRL) was examined in vitro. Dopamine had a more marked suppressing effect on newly synthetized PRL (80 %), as determined [3H]PRL, than on total PRL (50 %), as determined by RIA\x=req-\ PRL. The administration of 5 \g=m\g of oestradiolbenzoate (OeB) for 7 days resulted in blocking the suppressing effect of DA when RIA-PRL was measured but not when [3H]PRL was measured. The administration of 5 \g=m\g of OeB enabled TRH to partially override the suppressing effect of DA and the degree of response was more marked when RIA-PRL was measured than when [3H]PRL was measured. The administration of 50 \g=m\g of OeB for 3 days enabled TRH to override the DA blockade of prolactin release to levels comparable to that of the control when RIA-PRL was measured but had little to no effect on [3H]PRL. The results are discussed in relation to the two storage pools of PRL in the pituitary and the data suggest that DA acts predominantly to suppress the newly synthetized, rapidly releasable pool. Oestrogen acts to block DA action on the older more stable PRL pool. The ability of TRH to override the DA blockade of PRL release depends upon the presence of oestrogen; here TRH acts predominantly on the older more stable pool of PRL. Oestrogen's action on disrupting the DA suppression of PRL release appears to be related to the time of day the hormone is administered subsequent to when the pituitary is exposed to DA in vitro.

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Heterogeneous Turnover of Adenohypophysial Prolactin1

Cited by 82 publications

References 1 publication

Heterogeneity and compartmental properties of insulin storage and secretion in rat islets.

Heterogeneity and compartmental properties of insulin storage and secretion in rat islets.

Differential Effects of Thyrotropin-Releasing Hormone on in vitro Release of in vivo or in vitro Newly Synthesized and Mature Prolactin by Lactating Rat Adenohypophyses

The Influence of Oestrogen Administration in Vivo on in Vitro Prolactin Release

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