Regulation of prolactin secretion by dopamine and thyrotropin-releasing hormone in lactating rat adenohypophyses: influence of intracellular age of the hormone.
We studied the effect of dopamine (DA) on the rate of processing of PRL after biosynthesis and, with TRH, on the secretion of PRL of different intracellular ages. In these studies anterior pituitary (AP) PRL of lactating rats was pulse labeled either in vivo with [3H]leucine (3 microCi/g BW, injected iv 0.2, 1, 4, 8, 16, or 24 h before removing the AP for incubation) or in vitro with [3H]- or [14C]leucine (5 microCi/ml Krebs-Ringer bicarbonate buffer for 5 min), followed by a chase period of 15-240 min of AP fragments in medium 199. Also, to determine if endogenous DA influenced PRL synthesis, the rate of [14C]-leucine incorporation into PRL was determined in AP fragments from alpha-methyl-p-tyrosine (alpha MpT)-pretreated (200 mg/kg BW) rats. Tissue and medium PRL levels were quantified by polyacrylamide gel electrophoresis densitometric and liquid scintillation techniques. In APs from alpha MpT-treated animals [14C]-leucine incorporation into PRL increased about 30% above control values after the 5-min pulse period, and the release of labeled PRL from alpha MpT-treated APs was about 80% higher than control values after 4 h of incubation. On the other hand, when DA (50 microM) was present in the incubation medium, AP concentrations and release of in vitro synthesized [3H]PRL were significantly decreased (45-55% compared to control values; P less than 0.001). The [3H]PRL concentration, but not that of total, i.e. unlabeled PRL, fell within the AP as the time from in vivo pulse labeling to removal of the pituitary gland increased from 8 to 24 h, thus suggesting that a loss of labeled hormone occurred as it aged within the gland. Also, it was found that biosynthesis and/or processing of PRL were markedly depressed in APs from rats whose pups were removed for 24 h. Under basal conditions, in vitro secretion of [3H]PRLs during the first 30-60 min of incubation consisted primarily of mature [3H]PRL, i.e. those labeled 4 and 8 h previously, whereas newly synthesized (labeled 0.2 and 1 h previously) and old [3H]PRL (labeled 16 and 24 h previously) were secreted at much lower rates. These data confirm previous in vivo and in vitro results on the sequential release of different age PRLs. DA (17 microM) had a significantly greater inhibitory effect on newly synthesized and older stored PRL than on PRL labeled 4-8 h previously. The converse was true with regard to the PRL stimulatory effects of TRH; it provoked greater stimulation of PRL labeled 4-8 h before incubation, thus suggesting an interdependence of the actions of DA or TRH with intracellular age of the hormone.
Mena
F
F
Departamento de Fisiología, Universidad Nacional Autónoma de México, Mexico City.
Clapp
C
C
Aguayo
D
D
Morales
M T
MT
Grosvenor
C E
CE
Martínez de la Escalera
G
G
D000818:Animals; D004298:Dopamine / Q000378:metabolism*; D005260:Female; D007425:Intracellular Membranes / Q000378:metabolism*; D007774:Lactation / Q000378:metabolism*; D009994:Osmolar Concentration; D010903:Pituitary Gland, Anterior / Q000378:metabolism*; D011247:Pregnancy; D011388:Prolactin / Q000096:biosynthesis / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013973:Thyrotropin-Releasing Hormone / Q000502:physiology*; D013997:Time Factors
D013973:Thyrotropin-Releasing Hormone; D011388:Prolactin; D004298:Dopamine