Thyrotropin-releasing hormone causes loss of cellular calcium without calcium uptake by rat pituitary cells in culture. Studies using arsenazo III for direct measurement of calcium

Pituitary cell lines and their endocrine applications

The pituitary gland is an important component of the endocrine system, and together with the hypothalamus, exerts considerable influence over the functions of other endocrine glands. The hypothalamus either positively or negatively regulates hormonal productions in the pituitary through its release of various trophic hormones which act on specific cell types in the pituitary to secrete a variety of pituitary hormones that are important for growth and development, metabolism, reproductive and nervous system functions. The pituitary is divided into three sections-the anterior lobe which constitute the majority of the pituitary mass and is composed primarily of five hormone-producing cell types (thyrotropes, lactotropes, corticotropes, somatotropes and gonadotropes) each secreting thyrotropin, prolactin, ACTH, growth hormone and gonadotropins (FSH and LH) respectively. There is also a sixth cell type in the anterior lobe-the non-endocrine, agranular, folliculostellate cells. The intermediate lobe produces melanocyte-stimulating hormone and endorphins, whereas the posterior lobe secretes anti-diuretic hormone (vasopressin) and oxytocin. Representative cell lines of all the six cell types of the anterior pituitary have been established and have provided valuable information on genealogy of the various cell lineages, endocrine feedback control of hormone synthesis and secretions, intrapituitary interactions between the various cell types, as well as the role of specific transcription factors that determine each differentiated cell phenotype. In this review, we will discuss the morphology and function of the cell types that make up the anterior pituitary, and the characteristics of the various functional anterior pituitary cell systems that have been established to be representative of each anterior pituitary cell lineage.

The loss of 45Ca2+ associated with prolactin release from the tilapia (Oreochromis mossambicus) rostral pars distalis

The relationship between tritium 3H-labeled prolactin (PRL) release and the loss of tissue-associated 45Ca2+ was examined in the tilapia rostral pars distalis (RPD) using perifusion incubation under conditions which inhibit or stimulate PRL release. Depolarizing [K+] (56 mM) and hyposmotic medium (280 mOsmolal) increased both the release of [3H]PRL and the loss of 45Ca2+. The responses to high [K+] were faster and shorter in duration than those produced by reduced osmotic pressure. The depletion of Ca2+ from the incubation medium with 2 mM EGTA suppressed the [3H]PRL response evoked by high [K+] or reduced osmotic pressure. Exposing the tissues to Ca(2+)-depleted medium in the absence of high [K+] or reduced osmotic pressure produced a sharp, but brief, increase in 45Ca2+ loss. Cobalt (10(-3) M), a competitive inhibitor of calcium-mediated processes, inhibited the [3H]PRL response to hyposmotic medium and to high [K+]. Cobalt also diminished the increased loss of 45Ca2+ evoked by exposure to reduced osmotic pressure, but was ineffective in altering responses to high [K+]. Methoxyverapamil (D600; 10(-5) M), a blocker of certain voltage-sensitive Ca2+ channels, did not alter either the [3H]PRL or the 45Ca2+ responses to high [K+] and reduced osmotic pressure. Taken together with our earlier studies, the present findings suggest that exposure to high [K+] or hyposmotic medium produces rapid changes in the Ca2+ metabolism of the tilapia RPD that are linked to the stimulation of PRL secretion. Nevertheless, the increased 45Ca2+ loss, but not [3H]PRL release, upon exposure to Ca(2+)-depleted media suggests that Ca2+ loss may not always reflect intracellular events that lead to PRL release.

Ca2+ and hormones interact synergistically to stimulate rapidly both prolactin production and overall protein synthesis in pituitary tumor cells

Effects of Ca2+ and hormones on short-term protein synthesis were examined utilizing intact Ca2+-depleted and Ca2+-restored GH3 pituitary tumor cells as a model system. Amino acid incorporation by cells in complete growth medium during short incubations was markedly reduced by EGTA concentrations in excess of Ca2+. Thyrotropin-releasing hormone (TRH) rapidly enhanced amino acid incorporation and prolactin production, with both effects being reserved by EGTA in excess of extracellular Ca2+ or prevented by cellular Ca2+ depletion. Epidermal growth factor and phorbol myristate acetate (PMA) also stimulated amino acid incorporation and prolactin production; absolute increases in protein synthesis provided by these agents were significantly greater in Ca2+-restored than in Ca2+-depleted preparations. TRH and PMA concentrations which raised prolactin production were identical to those increasing the rate of amino acid incorporation into overall protein. The extracellular Ca2+ concentration dependencies of amino acid incorporation and prolactin production were similar and were unchanged by hormone. PMA, the most efficacious of the agents tested, and Ca2+ promoted incorporation of amino acid into the same spectrum of proteins. Stimulation of protein synthesis by hormones was not attributable to alterations in amino acid uptake, attachment to substrata, hormone binding, protein catabolism or transcription. Trifluoperazine selectively prevented the stimulation by Ca2+ of amino acid incorporation and prolactin production. Unlike total prolactin, the total protein content of GH3 cells during these short incubations was not altered by Ca2+, hormones or trifluoperazine. It is proposed that hormones and Ca2+, which have been demonstrated to regulate prolactin secretion and prolactin mRNA transcription in GH3 cells, also exert translational controls which serve to facilitate the overall expression of the prolactin gene.

Arachidonic acid mobilizes calcium and stimulates prolactin secretion from GH3 cells

Because arachidonic acid and/or its metabolites may be intracellular effectors of calcium-mediated secretion, we studied whether arachidonic acid added exogenously mobilizes calcium and stimulates prolactin secretion from GH3 cells, cloned rat pituitary cells. Arachidonic acid caused efflux of 45Ca from preloaded cells and stimulated prolactin secretion. The concentration dependencies of these effects were similar; stimulation was attained with 3 microM arachidonic acid. To determine indirectly whether these effects may be caused by arachidonic acid itself, not via conversion to metabolites, two experimental approaches were used. First, inhibitors of arachidonic acid metabolism, eicosatetraynoic acid and indomethacin, did not inhibit arachidonic acid-induced prolactin secretion. And second, alpha-linolenic acid, which cannot be converted to arachidonic acid, and linoleic acid, but not saturated fatty acids of equal chain length, stimulated 45Ca efflux and prolactin secretion. These data demonstrate that arachidonic acid added exogenously causes Ca2+ mobilization and prolactin secretion from GH3 cells and suggest that arachidonic acid itself, not via metabolism, may be a cellular regulator of prolactin secretion.

Different sodium requirements for 86Rb efflux and for growth hormone and prolactin secretion from bovine anterior pituitary cells

The sodium dependence of growth hormone and prolactin secretion and of 86Rb efflux from bovine anterior pituitary cells in response to acetylcholine and TRH was examined. Decreasing the external sodium concentration prevented the increases in the rates of 86Rb efflux and of growth hormone secretion caused by acetylcholine, or by TRH in the presence of IBMX. The growth hormone secretory response was less sensitive to sodium removal than was 86Rb efflux. However, even complete removal of extracellular sodium did not affect TRH-induced prolactin secretion. Ouabain and low extracellular potassium, which inhibit the sodium pump and increase intracellular sodium, prolonged the secretion of growth hormone in response to acetylcholine, but TRH-induced prolactin secretion was not affected. Inhibition of the sodium pump speeded the decline in the 86Rb efflux rate following stimulation by both acetylcholine and TRH. The results suggest that a sodium-dependent step is necessary for the efflux of 86Rb and for growth hormone secretion but not for prolactin secretion. The possible relationship between 86Rb efflux and hormone secretion from lactotrophs and somatotrophs is discussed.

Calcium influx is not required for thyrotropin-releasing hormone stimulation of prolactin release from GH3 cells

TRH stimulation of prolactin release from GH3 cells is dependent on Ca2+; however, whether TRH-induced influx of extracellular Ca2+ is required for stimulated secretion remains controversial. We studied prolactin release from cells incubated in medium containing 110 mM K+ and 2 mM EGTA which abolished the electrical and Ca2+ concentration gradients that usually promote Ca2+ influx. TRH caused prolactin release and 45Ca2+ efflux from cells incubated under these conditions. In static incubations, TRH stimulated prolactin secretion from 11.4 +/- 1.2 to 19 +/- 1.8 ng/ml in control incubations and from 3.2 +/- 0.6 to 6.2 +/- 0.8 ng/ml from cells incubated in medium with 120 mM K+ and 2 mM EGTA. We conclude that Ca2+ influx is not required for TRH stimulation of prolactin release from GH3 cells.

Trifluoperazine blocks calcium-dependent action potentials and inhibits hormone release from rat pituitary tumour cells

The effects of trifluoperazine (TFP) on basal and stimulated release of prolactin (PRL) and growth hormone (GH) and on the electrical properties of the membrane were studied in clonal rat pituitary tumour cells in culture (GH3 cells). The basal GH release was inhibited maximally 50% by TFP (13-30 microM) and the K+- and thyroliberin (TRH)-induced stimulation of both PRL and GH release was blocked significantly. The sustained depolarization caused by elevated extracellular K+ concentration and the biphasic membrane potential response to TRH (normally leading to spontaneous action potentials) were not affected by TFP. However, TFP inhibited the Ca2+-dependent action potentials, probably by blocking the voltage sensitive Ca2+ channels in the membrane. We therefore suggest that TFP inhibits hormone release by blocking the uptake of extracellular Ca2+. This action of TFP is probably due to direct membrane effects which are independent of calmodulin.

Involvement of calcium ions in dopamine inhibition of prolactin secretion from sheep pituitary cells

The involvement of calcium in the regulation of prolactin secretion and a possible inhibitory mechanism of action for dopamine have been investigated. Basal prolactin secretion from cultured ovine pituitary cells was dependent on the concentration of calcium ions (Ca2+) in the medium and was inhibited by the presence of verapamil (10 microM). The divalent cation ionophore A23187 (1 microM) caused a rapid stimulation of prolactin release from the cells. The effect was essentially complete within 10 min and subsequently secretion of prolactin occurred at close to the basal rate. A23187 had no effect on cell cyclic AMP levels. Dopamine (0.1 microM) but not verapamil (10 microM) inhibited the A23187 (10 microM) induced release of prolactin. Inhibition of basal and A23187 (1 microM) stimulated prolactin secretion occurred over a similar range of dopamine concentrations. The dopamine receptor antagonist haloperidol (1 microM) reversed the inhibitory effect of dopamine (0.1 microM) on A23187-stimulated prolactin release. These results provide evidence to support the concept that control of Ca2+ handling by lactotrophs may be of fundamental importance in the regulation of prolactin secretion.

Regulation of mast cell histamine release by neurotensin

Neurotensin (NT), a neuropeptide found both centrally and peripherally, stimulated release of histamine from rat peritoneal mast cells in a dose-dependent manner. Release was evident by 10 nM and reached a plateau of 15-20% total cellular histamine by 10(-7)-10(-6) M NT. Optimal conditions for stimulation occurred at pH 6.5-7.5, 37 degrees C and at calcium concentrations of less than 1 mM. Release was complete within 2 minutes of peptide addition. Studies of histamine release by NT analogues indicted that the C-terminus is the biologically active portion of the molecule in this system, as is true of all other systems responsive to NT (1). D-Trp11-NT, which acts as a NT antagonist in several peripheral NT-sensitive tissues (2,3), also inhibited NT action on mast cells. Manipulations involving Ca2+ availability suggest that the mechanism of NT stimulation may involve use of intracellular Ca2+ to a greater extent than extracellular Ca2+. Lowering the extracellular Ca2+ concentration or blocking influx of extracellular Ca2+ with lanthanum (La3+), had little effect on NT-induced release, whereas Ca2+ depletion by treatment with ethylenediaminetetracetic acid (EDTA) or blockade of intracellular Ca2+ mobilization by N,N-(diethylamino)octyl 3,4,5-trimethoxybenzoate (TMB-8), inhibited the response to NT. Increasing cellular levels of adenosine 3',5'-cyclic monophosphate (cAMP), by treatment with 8-bromo-cAMP or stimulation with prostaglandin E2 (PGE2) in the presence of isobutylmethylxanthine (IBMX), served to reduce histamine release by NT, indicating that cAMP may play a role in NT stimulation.