Inositol trisphosphate mediates thyrotropin-releasing hormone mobilization of nonmitochondrial calcium in rat mammotropic pituitary cells

Prolactin in the brushtail possum (Trichosurus vulpecula): development of homologous radioimmunoassay using recombinant possum prolactin

We report the production of recombinant possum prolactin (posPrl), and its use in the development and validation of a highly specific homologous radioimmunoassay for the measurement of prolactin (Prl) in brushtail possums. This enabled the subsequent investigation of some basic mechanisms involved in the regulation of Prl secretion in this species. Recombinant posPrl spanning the entire coding region was expressed in Escherichia coli, resulting in a 199 amino acid protein with a molecular weight approximately 23 kDa. The potency of posPrl was 45.3 +/- 4.8% that of ovine Prl in a radioreceptor assay using possum mammary gland receptors and induced a 3.4 +/- 0.8-fold increase in progesterone secretion in primary possum granulosa cells. Antiserum (G27) was raised against recombinant posPrl and was highly specific for possum Prl (approximately 30% binding at 1:60,000 final dilution), and exhibited negligible cross-reactivity (<0.0001%) with possum growth hormone. Serial dilutions of pituitary gland extracts, and plasma samples from male and female possums gave parallel inhibition curves to recombinant posPrl standards in the assay. Biological validation of the RIA included treating possums with drugs known to alter Prl secretion in other mammals. In seasonally anoestrous female possums, administration of 20 microg thyrotropin-releasing hormone (TRH) resulted in a 15-fold increase (P < 0.01) in plasma Prl concentrations. In mid-late lactating female possums, a bolus of cabergoline (dopamine agonist; 75 microg) reduced (P < 0.05) plasma Prl levels to baseline for 24 h, while repeated administration (6 x 75 microg at 12 h intervals) suppressed (P < 0.01) plasma Prl concentrations until 24h after the last injection. Prolonged inhibition of Prl levels subsequently caused marked (P < 0.01) attenuation in rate of bodyweight increase of pouch young. The amplitude of the Prl surge in response to a bolus of TRH (15 microg) was 5-fold lower in cabergoline-treated, compared to control mid-late lactating possums. In conclusion, we report the development and validation of a robust and sensitive RIA for measuring Prl concentrations in the plasma of brushtail possums.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Characterization of DNA regions mediating the ability of Ca2+/calmodulin dependent protein kinase II to stimulate prolactin promoter activity

The ability of Ca2+/calmodulin-dependent protein kinases (CaMKs) to regulate transcription of the rat prolactin (PRL) gene has been examined. We found that KN-62, a potent inhibitor of CaM kinases, blunted the ability of TRH to activate the prolactin promoter. Transfection experiments using expression plasmids for constitutively active forms of CaMKI, CaMKII, or CaMKIV show that CaMKII is the most effective activator of prolactin promoter expression. Deletion studies demonstrated that the upstream boundary of sequences necessary to respond to CaMKII is located within the distal enhancer of the prolactin gene. Neither the distal enhancer alone nor the proximal region of the prolactin gene are sufficient to mediate a response to CaMKII. Mutational analysis suggests that several Pit-1 binding sites contribute to CaMKII responsiveness. These findings suggest that CaMKII responsiveness of the prolactin promoter requires multiple factor binding sites in both the distal and proximal regions of the gene.

Capacitative Ca2+ entry contributes to the Ca2+ influx induced by thyrotropin-releasing hormone (TRH) in GH3 pituitary cells

Treatment of GH3 cells with either hypothalamic peptide thyrotropin-releasing hormone (TRH), the endomembrane Ca2+-ATPase inhibitor thapsigargin or the Ca2+ ionophore ionomycin mobilized, with different kinetics, essentially all of the Ca2+ pool from the intracellular Ca2+ stores. Any of the above- described treatments induced a sustained increase in intracellular Ca2+ concentration ([Ca2+]i), which was dependent on extracellular Ca2+ and was prevented by Ni2+ but not by dihydropyridines (DHPs), suggesting that it was due to capacitative Ca2+ entry via activation of a plasma membrane pathway which opened upon the emptying of the intracellular Ca2+ stores. The increase of the plasma membrane permeability to Ca2+ correlated negatively with the filling degree of the intracellular Ca2+ stores and was reversed by refilling of the stores. The mechanism of capacitative Ca2+ entry into GH3 cells differed from similar mechanisms described in several types of blood cells in that the pathway was poorly permeable to Mn2+ and not sensitive to cytochrome P450 inhibitors. In GH3 cells, TRH induced a transient [Ca2+]i increase due to Ca2+ release from the stores (phase 1) followed by a sustained [Ca2+]i increase due to Ca2+ entry (phase 2). At the single-cell level, phase 2 was composed of a DHP-insensitive sustained [Ca2+]i increase, due to activation of capacitative Ca2+ entry, superimposed upon which DHP- sensitive [Ca2+]i oscillations took place. The two components of the TRH-induced Ca2+ entry differed also in that [Ca2+]i oscillations remained for several minutes after TRH removal, whereas the sustained [Ca2+]i increase dropped quickly to prestimulatory levels, following the same time course as the refilling of the stores. The drop was prevented when the refilling was inhibited by thapsigargin. It is concluded that, even though the mechanisms of capacitative Ca2+ entry may show differences from cell to cell, it is also present and may contribute to the regulation of physiological functions in excitable cells such as GH3. There, capacitative Ca2+ entry cooperates with voltage-gated Ca2+ channels to generate the [Ca2+]i increase seen during phase 2 of TRH action. This contribution of capacitative Ca2+ entry may be relevant to the enhancement of prolactin secretion induced by TRH.

Effects of thyrotropin-releasing hormone and phorbol ester on dopamine release from dispersed rat tuberoinfundibular dopaminergic neurons

We have investigated the intracellular mechanisms underlying thyrotropin-releasing hormone (TRH)-mediated [3H]dopamine ([3H]DA) release from dispersed rat tuberoinfundibular dopaminergic (TIDA) neurons. The specific binding of [3H]Me-TRH to these cells is characterized by a single, high-affinity binding site (Kd = 1.2 nM) with a Bmax value of 178 fmol/mg protein. Thyrotropin-releasing hormone markedly increased [3H]DA release and intracellular free calcium concentration ([Ca2+]i) in TIDA neurons, and its effect was abolished by treatment with EGTA (5 mM) or chlordiazepoxide, a specific TRH receptor antagonist (10 microM). Furthermore, to examine the involvement of protein kinase C on [3H]DA release, we investigated the effect of phorbol myristate acetate (PMA), which is known to activate protein kinase C directly. Phorbol myristate acetate induced a significant increase in [3H]DA release in a concentration-dependent manner. Treatment with TRH (1 microM) plus PMA (100 nM) resulted in an additive increase in [3H]DA release. Thyrotropin-releasing hormone (1 microM) still increased [3H]DA release even after preincubation with PMA (500 nM) for 24 h, but PMA (100 nM) did not under the same conditions. These results suggest that TRH may induce DA release in dispersed rat TIDA cells by increasing calcium influx and activating the protein kinase C system.

Effects of high extracellular calcium concentrations on phosphoinositide turnover and inositol phosphate metabolism in dispersed bovine parathyroid cells

We previously showed that high extracellular calcium (Ca2+) concentrations raise the levels of inositol phosphates in bovine parathyroid cells, presumably via the G protein-coupled, "receptor-like" mechanism through which Ca2+ is thought to regulate these cells. To date, however, there are limited data showing Ca(2+)-evoked hydrolysis of phosphoinositides with attendant increases in the levels of the biologically active 1,4,5 isomer of inositol trisphosphate (IP3) that would be predicted to arise from such a receptor-mediated process. In the present studies we used HPLC and TLC, respectively, to quantify the high Ca(2+)-induced changes in various inositol phosphates, including the isomers of IP3, and phosphoinositides in bovine parathyroid cells prelabeled with [3H]inositol. In the absence of lithium, high Ca2+ dose dependently elevated the levels of inositol-1,4,5-trisphosphate [I(1,4,5)P3], with a maximal, 4- to 5-fold increase within 5 s; the levels of inositol 1,3,4-trisphosphate [I(1,3,4)P3] first rose significantly at 5-10 s and remained 5- to 10-fold elevated for at least 30 minutes. These changes were accompanied by reciprocal 29-36% decreases in PIP2 (within 5-10 s, the earliest time points examined), PIP (within 60 s), and PI (within 60 s). These results document that, as in other cells responding to more classic "Ca(2+)-mobilizing" hormones, the high Ca(2+)-evoked increases in inositol phosphates in bovine parathyroid cells arise from the hydrolysis of phosphoinositides, leading to the rapid accumulation of the active isomer of IP3. The latter presumably underlies the concomitant spike in the cytosolic calcium concentration (Ca(i)) in parathyroid cells.

Muscarinic receptors, phosphoinositide metabolism and intracellular calcium in neuronal cells

1. We have utilised SH-SY5Y human neuroblastoma cells and primary cultures of rat neonatal cerebellar granule cells, both expressing M3 muscarinic receptors, to examine agonist driven polyphosphoinositide hydrolysis and alterations in intracellular calcium. 2. Stimulation of SH-SY5Y cells leads to a biphasic increase in intracellular calcium, the initial peak being due to the release of calcium from an intracellular store and the second maintained phase being due to calcium entry across the plasma membrane. The channel involved does not appear to be voltage sensitive, to involve a pertussis toxin sensitive G protein, or be opened by inositol polyphosphates. 3. Muscarinic receptor stimulation also leads to increased inositol polyphosphate formation in SH-SY5Y cells. Ins(1,4,5)P3 mass formation was biphasic in profile whereas Ins(1,3,4,5)P4 mass formation was slower and monophasic in profile. These data are consistent with substantial activity of 5-phosphatase (dephosphorylating Ins(1,4,5)P3 to Ins(1,4)P2) and 3-kinase (phosphorylating Ins(1,4,5)P3 to Ins(1,3,4,5)P4) in SH-SY5Y cells. 4. In order to better understand the role of Ins(1,4,5)P3 and its metabolites in calcium homeostasis we have examined the ability of a variety of natural and synthetic analogues to release intracellular sequestered calcium. The Ins(1,4,5)P3 calcium mobilizing receptor displays a remarkable degree of stereo- and positional selectivity with the most potent agonist to date being Ins(1,4,5)P3 (EC50 = 0.09 microM). 5. As an alternative to the continuous SH-SY5Y neuroblastoma (tumour derived) cell line we have used the primary cultured cerebellar granule cell. These cells also display a biphasic increase in Ins(1,4,5)P3 mass and a subsequent release of intracellular stored calcium. In our hands carbachol appears to increase calcium influx, a response which is only visible in the absence of magnesium.

The effects of Ca2+ and calmodulin on adenylyl cyclase activity in plasma membranes derived from neural and non-neural cells

The regulation of adenylyl cyclase activity by varying concentrations of Ca2+ was examined in plasma membrane preparations derived from a number of neural and non-neural cells. Enzyme activity in neural tissue (i.e. cerebellum) neural-derived pheochromocytoma PC12 cells and certain endocrine cells (i.e. pancreatic RINm5f and parathyroid cells) was stimulated by physiologic concentrations of Ca2+ by a calmodulin (CaM)-dependent mechanism. In contrast, adenylyl cyclase activity in non-neural cells (e.g. platelets and GH3 cells) was not stimulated by Ca2+. In these latter sources, enzyme activity was inhibited by increasing concentrations of Ca2+, independent of CaM. In liver membranes, Ca2+ and/or CaM did not alter adenylyl cyclase activity. These results demonstrate that the effects exerted by physiologic concentrations of Ca2+ on adenylyl cyclase activity range from CaM-dependent stimulation of activity to no effect, to CaM-independent inhibition of activity. The actions of Ca2+ on adenylyl cyclase may be major contributors to the various synergistic or antagonistic interactions that are seen between cAMP-generating and Ca(2+)-mobilizing systems.

PMA-sensitive protein kinase C is not necessary in TRH-stimulated prolactin release from female rat primary pituitary cells

In GH3 cells and other clonal rat pituitary tumor cells, TRH has been shown to mediate its effects on prolactin release via a rise of cytosolic Ca2+ and activation of protein kinase C. In this study, we examined the role of protein kinase C in TRH-stimulated prolactin release from female rat primary pituitary cell culture. Both TRH and PMA stimulated prolactin release in a dose-dependent manner. When present together at maximal concentrations, TRH and PMA produced an effect which was slightly less than additive. Pretreatment of rat pituitary cells with 10(-6) M PMA for 24 hrs completely down-regulated protein kinase C, since such PMA-pretreated cells did not release prolactin in response to a second dose of PMA. Interestingly, protein kinase C down-regulation had no effect on TRH-induced prolactin release from rat pituitary cells. In contrast, PMA-pretreated GH3 cells did not respond to a subsequent stimulation by either PMA or TRH. Pretreatment of rat pituitary cells with TRH (10(-7) M, 24 hrs) inhibited the subsequent response to TRH, but not PMA. Forskolin, an adenylate cyclase activator, stimulated prolactin release by itself and in a synergistic manner when incubated together with TRH or PMA. The synergistic effects of forskolin on prolactin release was greater in the presence of PMA than TRH. Down-regulation of protein kinase C by PMA pretreatment abolished the synergistic effect produced by PMA and forskolin but had no effect on those generated by TRH and forskolin. sn-1,2-Dioctanylglycerol (DOG) pretreatment attenuated the subsequent response to DOG and PMA but not TRH. The effect of TRH, but not PMA, on prolactin release required the presence of extracellular Ca2+. In conclusion, the mechanism by which TRH causes prolactin release from rat primary pituitary cells is different from that of GH3 cells; the former is a protein kinase C-independent process whereas the latter is at least partially dependent upon the activation of protein kinase C.

Agonist/inositol trisphosphate-induced release of calcium from murine keratinocytes: a possible link with keratinocyte differentiation

Extracellular calcium concentrations markedly affect the pattern of proliferation and differentiation in cultured keratinocytes. When medium contains 0.1 mM calcium or above, the cells lose their proliferative ability, rapidly stratify, and terminally differentiate. Because 1,25(OH)2D3 (a modulator of Ca++ homeostasis) enhances the differentiation of keratinocytes, we investigated whether a link exists between 1,25(OH)2D3-induced release of inositol-1,4,5-trisphosphate (Ins(1,4,5)P3) from PtdIns 4,5-P2 and intracellular calcium [Ca++]i release from keratinocytes. Specifically, primary culture of keratinocytes were loaded with fluorescence dye Fura-2AM (10 microM) and changes in fluorescence intensity were monitored at the excitation wavelengths of 340 and 380 nm and emission wavelength of 505 nm. Additions of two agonists, 1,25(OH)2D3 (1.2 x 10(-9) M) and 13-Cis retinoic acid (0.2 x 10(-9) M), to dye-loaded keratinocytes induced rapid release of [Ca++]i, respectively, followed by gradual return to the prestimulated state. Addition of Ins(1,4,5)P3 (10 microM) to saponin-treated (leaky) keratinocytes also resulted in a rapid release of [Ca++]i. In contrast, the addition of inositol-1,3,4,5-tetrakisphosphate Ins(1,3,4,5)P4 at similar concentrations exerted negligible effect. Taken together, these results support the view that 1,25(OH)2D3-induced [Ca++]i release in keratinocytes may be via the Ins(1,4,5)P3-induced early release of intracellular [Ca++]i. This may explain, at least in part, 1,25(OH)2D3-enhanced keratinocyte differentiation.

Phorbol esters modulate the high Ca2(+)-stimulated accumulation of inositol phosphates in bovine parathyroid cells

We examined the effects of TPA on the high Ca2(+)-stimulated accumulation of inositol phosphates in bovine parathyroid cells to determine whether protein kinase C modulates phosphoinositide turnover in a fashion similar to that observed in other cell types stimulated by more classic Ca2+ mobilizing hormones. Following exposure of parathyroid cells to TPA (10(-6) M) for 10 or 30 minutes, there was a time- and dose-dependent inhibition of the accumulation of inositol monophosphate (IP), inositol bisphosphate (IP2) and inositol trisphosphate (IP3) stimulated by 3 mM Ca2+. Half the maximal observed inhibition took place at 1-10 nM TPA, with 50-60% inhibition of high Ca2(+)-stimulated accumulation of inositol phosphates at 10(-6) M TPA. The active phorbol ester, 4 beta-phorbol didecanoate, produced similar effects; the inactive derivative, 4 alpha-phorbol didecanoate, was without effect. When parathyroid cells were exposed to TPA (10(-6) M) for varying times and were then incubated with high (3 mM) Ca2+, inhibition of inositol phosphate accumulation was observed with 10 or 30 minutes preincubation. In contrast, preincubation of cells with TPA for 3 or 18 h markedly enhanced the high (3 mM) Ca2(+)-induced increase in inositol phosphates. In cells preincubated with TPA for 18 h, binding sites for [3H]phorbol dibutyrate and total protein kinase C (PKC) activity were reduced by greater than 95% and by 71%, respectively, consistent with downregulation of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of inositol 1,4,5-trisphosphate on calcium release from the endoplasmic reticulum and Golgi apparatus in mouse mammary epithelial cells: a comparison during pregnancy and lactation

It has been established that inositol 1,4,5-trisphosphate(IP3) is responsible for the mobilization of calcium(Ca2+) from intracellular locations in a wide variety of tissues, and that this response triggers the stimulation of several hormones and neurotransmitters. However, these phenomena have yet to be examined in the mammary epithelium. Ca2+ uptake from the medium into the endoplasmic reticulum(ER) and Golgi apparatus in vitro in both pregnant and lactating mouse mammary epithelial cells was studied and a strong Ca2+ release from these organelles into the medium with the use of IP3 was shown. The Ca2+ uptake and its release due to IP3 was also usually greater during pregnancy than lactation.

Arachidonic acid and its metabolites increase cytosolic free calcium in bovine luteal cells

We studied the effects of arachidonic acid and its metabolites on intracellular free calcium concentrations ([Ca2+]i) in highly purified bovine luteal cell preparations. Corpora lutea were collected from Holstein heifers between days 10 and 12 of the estrous cycle. The cells were dispersed and small and large cells were separated by unit gravity sedimentation and flow cytometry. The [Ca2+]i was determined by spectrofluorometry in luteal cells loaded with the fluorescent Ca2+ probe, Fura-2. Arachidonic acid elicited a dose-dependent increase in [Ca2+]i in both small and large luteal cells, having an effect at concentrations as low as 5 microM; and was maximally effective at 50 microM. Several other fatty acids failed to exert a similar response. Addition of nordihydroguaiaretic acid (NDGA) or indomethacin failed to suppress the effects of arachidonic acid. In fact, the presence of both inhibitors resulted in increases of [Ca2+]i, with NDGA exerting a greater stimulation of [Ca2+]i than indomethacin. Prostaglandin F2 alpha (PGF2 alpha) as well as prostaglandin E2 (PGE2) increased [Ca2+]i in the small luteal cells. These results support the idea that arachidonic acid exerts a direct action in mobilizing [Ca2+]i, in the luteal cells. Furthermore, they demonstrate that the cyclooxygenase (PGF2 alpha and PGE2) and lipoxygenase products of arachidonic acid metabolism also play a role in increasing [Ca2+]i in bovine luteal cells. Since the bovine corpus luteum contains large quantities of arachidonic acid, these findings suggest that this compound may regulate calcium-dependent functions of the corpus luteum, including steroid and peptide hormone production and secretion.

The mechanisms by which vasoactive intestinal peptide (VIP) and thyrotropin releasing hormone (TRH) stimulate prolactin release from pituitary cells

The effect of vasoactive intestinal peptide (VIP) on prolactin (PRL) secretion from pituitary cells is reviewed and compared to the effect of thyrotropin releasing hormone (TRH). These two peptides induced different secretion profiles from parafused lactotrophs in culture. TRH was found to increase PRL secretion within 4 s and induced a biphasic secretion pattern, while VIP induced a monophasic secretion pattern after a lag time of 45-60 s. The secretion profiles are compared to changes in adenylate cyclase activity, production of inositol polyphosphates, changes in intracellular calcium concentrations and changes in electrophysiological properties of the cell membrane.

Inositol 1,4,5-trisphosphate- and arachidonic acid-induced calcium mobilization in T and B lymphocytes

Inositol triphosphate (IP3) formation and increase in intracytoplasmic calcium are mediators of signal transduction in lymphocytes. It has been proposed that IP3 induces Ca2+ release from intracellular stores. It is in order to study the relationship between these two events that we have analyzed the effect of IP3 addition on Ca2+ mobilization in permeabilized resting T and B lymphocytes, EBV-B lymphocytes, and HTLV1-T lymphocytes. IP3 induces a rapid and significant release of Ca2+ from the endoplasmic reticulum in a dose-dependent manner. Ca2+ release is more sensitive to IP3 addition in cycling cells (EBV-B lymphocytes and HTLV1-T lymphocytes) than in resting T and B lymphocytes. Arachidonic acid (AA) induces Ca2+ release from the endoplasmic reticulum (ER) in a manner similar to that of IP3. Neither component has an effect on Ca2+ accumulated in mitochondria, and they have no additive effects suggesting that they act on a similar Ca2+ pool. These results directly demonstrate that in T and B human lymphocytes IP3 mobilizes Ca2+ from ER as in other cellular systems and that other potential second messengers, namely AA, could play a significant role in the internal mobilization of calcium during T and B lymphocyte activation.

Calcium release induced by inositol 1,4,5-trisphosphate in thymocyte microsomes. Inhibition by barium and strontium

The properties of calcium transport in microsomes and the effect of inositol 1,4,5-trisphosphate (IP3) on accumulated calcium were studied in rat thymocytes. Active calcium transport shows an apparent affinity constant for calcium of 0.2 +/- 0.01 microM and a maximal velocity of 2.3 +/- 0.6 nmol/mg/30 min (mean +/- SD). IP3 was able to induce release of calcium only in the absence of oxalate. At 6 microM ambient free calcium, half-maximal effect of IP3 was attained at 2 microM and maximal calcium release was produced by IP3 concentrations over 5 microM. Barium and strontium did not modify calcium uptake by microsomes but markedly inhibited the action of IP3.

Adenosine A1 receptors inhibit both adenylate cyclase activity and TRH-activated Ca2+ channels by a pertussis toxin-sensitive mechanism in GH3 cells

The present study has examined the effects of adenosine A1 receptors on second messenger processes in GH3 cells. A1 receptors are present which are shown to inhibit adenylate cyclase in a GTP-requiring manner. Hormone (VIP) stimulation is also absolutely required for the observation of inhibition. Adenosine A1 receptor analogues also inhibit TRH-stimulated [Ca2+]i-mobilization in GH3 cells. Both effects of the adenosine receptor agonists are apparently mediated by pertussis toxin substrates, of which there are two--41,000 and 40,000 daltons respectively--in these cells. Somatostatin exerts analogous effects to the adenosine agonists in GH3 cells. Thus it may turn out that a general property of 'cyclase inhibitory receptors' is also to inhibit [Ca2+]i-mobilization in the same cells, when such mechanisms are present.

Distribution of thyrotropin-releasing hormone (TRH) in the hippocampal formation as determined by radioimmunoassay

The distribution of thyrotropin-releasing hormone (TRH) in the hippocampal formation was determined using a radioimmunoassay (RIA) specific for TRH. RIA of hippocampal subregions revealed that the CA3 region of the hippocampal formation contained the highest amount of TRH, followed by intermediate levels in region CA1 and the dentate gyrus. The hilus and subiculum contained the lowest levels. The issue of whether hippocampal TRH is derived from extrinsic and/or intrinsic sources was evaluated by making lesions of the major subcortical afferent to the hippocampus, the fornix pathway. Analysis of the hippocampal formation by RIA revealed that the ventral hippocampus contains higher levels of TRH than the dorsal hippocampus (6.01 +/- 0.62 pg/mg tissue weight vs 1.11 +/- 0.19 pg/mg tissue weight). Lesions of the fornix produced significant decreases in ventral TRH to 52.9% of its control level and in dorsal TRH to 28.8% of its control level. The results from these studies suggest that (1) there is a differential distribution of TRH in the hippocampal formation, (2) the hippocampal formation might be composed of extrinsic and intrinsic sources of TRH, and (3) extrinsic sources of TRH might enter the hippocampus via the fornix pathway. In addition (4) the greater post-lesion decrement in ventral vs dorsal hippocampal TRH suggests that TRH fibers traversing the fornix innervate the ventral hippocampal formation in preference to its dorsal counterpart.

The effects of thyrotropin-releasing hormone and potassium depolarization on phosphoinositide metabolism and cytoplasmic calcium in bovine pituitary cells

Addition of thyrotropin-releasing hormone (TRH) (10 nM to 10 microM) to bovine anterior pituitary cells labelled with [3H]inositol decreased the radioactivity in inositol-containing lipids and increased it in inositol phosphates. TRH also increased the cytoplasmic calcium concentration biphasically. At TRH concentrations below 10 nM, the increase was sustained and sensitive to inhibitors of calcium influx through voltage-gated channels, whereas concentrations over 10 nM elicited in addition a rapid transient increase in calcium, which was relatively insensitive to such inhibition. Incubation of the cells in medium containing 25 mM KCl increased the cytoplasmic calcium concentration by stimulating influx through voltage-gated channels, and markedly enhanced the initial transient increase of calcium seen at TRH concentrations above 10 nM. It did not affect the generation of InsP3 and it also enhanced the calcium response to ionomycin. It is suggested that stimulation of calcium entry through voltage-gated channels can increase the amount of calcium available for mobilisation by TRH.

Diacylglycerol, but not inositol 1,4,5-trisphosphate, accounts for platelet-derived growth factor-stimulated proliferation of BALB 3T3 cells

Recently we found that an intracellular event related to phosphatidylinositol 4,5-bisphosphate (PIP2) is crucial for platelet-derived growth factor (PDGF)-induced mitogenesis in fibroblastic cells (Matuoka, K., et al.: Science 239:640-643, 1988). In the present study we examined the mitogenic effects of PIP2 and its hydrolysis products introduced into the cytoplasm of BALB 3T3 cells by micro-injection to confirm the role of PIP2 hydrolysis in PDGF stimulation of cell proliferation. Injection of 1,2-dioleylglycerol (diolein) into serum-deprived quiescent cells induced DNA synthesis with the same time course as that induced by exposure of the cells to PDGF and, in the presence of PDGF, caused no additional increase in the cell population entering S phase. The injection of PIP2, inositol 1,4,5-trisphosphate, or 1,2-dioleylphosphatidic acid into the cells did not induce mitogenesis. Consistent results were obtained in experiments in which the cells were exposed to 1-oleyl-2-acetylglycerol (OAG) and ionomycin; namely, OAG stimulated proliferation of BALB 3T3 cells, but ionomycin did not induce any mitogenesis. Desensitization of the protein kinase C pathway by prolonged exposure of the cells to phorbol ester abolished the induction of cell proliferation by subsequent injection of diolein or exposure to phorbol ester or OAG as well as by PDGF challenge. These findings strongly suggest that activation of the protein kinase C system following formation of diacylglycerol by PIP2 hydrolysis is mainly responsible for the mitogenic action of PDGF on BALB 3T3 cells.

Estradiol attenuates prolactin secretion and phosphoinositide hydrolysis in MMQ cells

We previously isolated a clonal cell line, designated MMQ, which only secretes prolactin (PRL) and whose secretory process is nonresponsive to thyrotropin releasing hormone (TRH) and angiotensin II (AII). In the present study, we injected MMQ cells into rats to determine whether the tumor cells would become responsive to secretagogues when subsequently propagated in vitro. We also investigated what effects in vivo administration of 17 beta-estradiol would have on secretagogue-induced PRL release and on intracellular biochemical mechanisms in these cells. MMQ cells were implanted subcutaneously in the backs of female rats. One group was injected with 100 micrograms polyestradiol phosphate (PEP) every 5 days, a second with saline. The inoculants grew into solid tumors within 3 weeks. The day after the tumors were removed and enzymatically dispersed, the cells, now designated MMQt cells, were perifused in vitro. Basal PRL released by MMQt cells was approximately 1 ng/min/10(7) cells and perifusions with 100 nM TRH or AII for 5 min significantly increased PRL release above baseline (integrated areas: 1.8 +/- 0.4 and 5.2 +/- 1.3 ng/10(7) cell, respectively; P less than 0.01). Two ng/ml maitotoxin (MTX), a calcium channel activator, increased PRL release (38.2 +/- 6.7 ng/10(7) cells; P less than 0.01). In PEP-treated perifused MMQt cells, basal in vitro PRL release was not different from that observed in the control group, but the responses to TRH, AII and MTX were greatly attenuated (TRH: 0.6 +/- 0.1, AII: 1.3 +/- 0.2 and MTX: 9.2 +/- 2.5 ng/10(7) cells).(ABSTRACT TRUNCATED AT 250 WORDS)

Bradykinin and inositol 1,4,5-trisphosphate-stimulated calcium release from intracellular stores in cultured bovine endothelial cells

The relative importance of intracellular and extracellular Ca2+ in the release of endothelium-derived relaxing factor (EDRF) and the mechanisms involved in the release of intracellular Ca2+ were investigated in cultured bovine endothelial cells. The release of EDRF by bradykinin, determined by bioassay, was dose-dependent showing an EC50 of 4 x 10(-10) M. The bradykinin-induced EDRF release from endothelial cells was maintained in the presence of extracellular Ca2+. However, in the absence of external Ca2+, bradykinin-induced EDRF release was both attenuated and transient. In cells loaded to isotopic equilibrium with 45Ca, bradykinin increased the 45Ca efflux into both calcium-containing and calcium-free solutions, with an EC50 for the increase in 45Ca efflux induced by bradykinin of 1.3 x 10(-9) M. The involvement of an intracellular Ca2+ store and the participation of a second messenger in its release were investigated in saponin-permeabilized endothelial cells. In saponin-permeabilized cells, ATP-sensitive calcium uptake was Ca2+,Mg2+ -ATPase-dependent. The ATP-sensitive uptake of calcium at different free Ca2+ concentrations showed at least two compartments involved in the uptake of Ca2+. The 45Ca uptake into the compartment with the lowest affinity and highest capacity could be inhibited by sodium azide, suggesting that this uptake was into mitochondria. The majority of the 45Ca uptake into the azide-insensitive store could be released by inositol-1,4,5-trisphosphate (IP3). The IP3-induced release was not affected by apyrase or exogenous GTP. The EC50 for the release of Ca2+ by IP3 was 1.0 microM and was unaffected by an inhibitor of IP3 breakdown (2,3-diphosphoglyceric acid).(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of sarcoplasmic reticulum in skinned muscle cultures

The plasma membranes of chick or rat skeletal muscles, grown in cell culture, were made permeable with saponin in a solution lacking calcium. The cells were then supplied with a medium resembling the cytosol and the ATP-dependent Ca2+ sequestration was performed. Based on the low concentration of free Ca2+ in the medium (below 5 microM), the presence of mitochondrial inhibitors and the effect of drugs that interfere with sarcoplasmic reticulum (SR) function, we assume that the measured Ca2+ accumulation expresses SR function on the saponin-treated myotubes. The development of the SR in muscle cultures is augmented as myogenesis proceeds and depends on its occurrence. Whereas creatine kinase activity is elevated immediately following cell fusion, there is a delay of at least 1 day between myoblast fusion and the increase in Ca2+ accumulation in the SR. Thyroxine or triiodothyronine caused an inhibition of Ca2+ accumulation in rat or chick muscle cultures. This inhibition could explain some of the muscle abnormalities caused by excess of thyroid hormones. A comparison was made between a white-type (fast) and heterogeneous muscle, differentiated in cell culture. There was no significant difference in SR function, indicating the important role of innervation in specifying the properties of muscle fiber types.

Inhibitors of 1,2-diacylglycerol kinase potentiate the TRH-induced stimulation of Ca2+-activated K+ current

Transient activation of the outward K+ current caused by a rise in the cytosolic free Ca2+ concentration, [Ca2+]i was the predominant change in plasma membrane ion flux during the first phase of thyrotropin-releasing hormone (TRH) action on pituitary cells. Following the intracellular application of inhibitors of 1,2-diacylglycerol (DG) kinase, R59022 and 1-oleyl-2-acetyl glycerol (OAG) the outward K+ current response to TRH in cells of the pituitary line GH3B6 was potentiated. This potentiation was analyzed further with the combination of microfluorimetric and electrophysiological recording techniques. Receptor-induced changes in [Ca2+]i and ion channel activation were monitored simultaneously in the same cell. It was found that R59022 and OAG altered in parallel the TRH-induced transient rise in [Ca2+]i and outward K+ current. This resulted in a significant correlation between the kinetic parameters (speed of onset, duration) of the [Ca2+]i and the K+ current responses to TRH. Intracellular application of vanadate abolished the rapid start of the TRH response presumably by its block of Ca2+ uptake into the endoplasmic reticulum, leading to depletion of a Ca2+ pool mobilizable by inositol 1,4,5-trisphosphate (Ins(1,4,5)P3). The use of vanadate unmasked a slowly developing response to TRH, which was still potentiated by OAG and R59022. Together, these observations suggest that Ca2+ mobilization during the first phase of TRH action is mediated by two distinct processes, one of which is linked to receptor stimulation of DG production.

Distinct binding sites for Ins(1,4,5)P3 and Ins(1,3,4,5)P4 in bovine parathyroid glands

We utilized high specific activity, [32P]-labelled ligands to measure the binding of Ins(1,3,4,5)P4 and Ins(1,4,5)P3 to membranes prepared from bovine parathyroid glands. [32P]Ins(1,3,4,5)P4 bound rapidly and reversibly to parathyroid membranes, and the binding data could be fitted by the interaction of the ligand with two sites, one with Kd = 6.8 x 10(-9) M and Bmax = 26 fmol/mg protein and a second, lower affinity site, with Kd = 4.1 x 10(-7) M and Bmax = 400 fmol/mg protein. InsP5 was 10-20 fold less potent than InsP4, and Ins(1,3,4)P3 and Ins(1,4,5)P3 were nearly 1000-fold less potent in displacing [32P]Ins(1,3,4,5)P4. [32P]Ins(1,4,5)P3, on the other hand, bound to a single class of sites with Kd = 7.6 x 10(-9) M and Bmax = 34 fmol/mg. While the binding of [32P]Ins(1,4,5)P3 increased markedly on raising pH from 5 to 8, the binding of [32P]Ins(1,3,4,5)P4 decreased by 75% over this range of pH. Thus, [32P]-labelled Ins(1,3,4,5)P4 and Ins(1,4,5)P3 may be used to identify distinct binding sites which may represent physiologically relevant intracellular receptors for InsP3 and InsP4 in parathyroid cells.

Diacylglycerol modulates action potential frequency in GH3 pituitary cells: correlative biochemical and electrophysiological studies

We have investigated the involvement of enhanced phosphoinositide metabolism in mediating TRH-induced alteration of electrophysiological events related to prolactin secretion by GH3 cells (a line of pituitary origin). Patch-clamp recording (in the current clamp, whole-cell configuration) showed that a few seconds after TRH application there was a brief period (about 30 s) of membrane hyperpolarization followed by several minutes of increased calcium-dependent action potential frequency. In parallel experiments cells were labeled for 24 h with either [3H]myo-inositol or [3H]arachidonate. Application of TRH resulted in rapid increases in levels of inositol phosphates and diacylglycerol. The time course of elevation of inositol 1,4,5-triphosphate (maximal by 5 s) is compatible with an initial burst of intracellular calcium mobilization associated with a transient phase of TRH-induced prolactin release. Application of TRH was also followed by a rapid but more sustained (several minutes) period of elevated diglyceride accumulation; a time course corresponding to a prolonged period of prolactin release which is dependent on the influx of external calcium. A causal relationship between diglyceride release and increased action potential frequency was demonstrated since local application (via a U-tube apparatus) of either 2 microM phorbol ester (phorbol 12,13-dibutyrate or phorbol 12-myristate 13-acetate) or 60 microM 1-oleoyl-2-acetyl-glycerol to patch-clamped cells could mimic this aspect of the TRH effect. In contrast, the inactive phorbol ester, 4 alpha-phorbol, was unable to elicit this response.(ABSTRACT TRUNCATED AT 250 WORDS)

Phorbol esters and thyroliberin have distinct actions regarding stimulation of prolactin secretion and activation of adenylate cyclase in rat pituitary tumour cells (GH4C1 cells)

The phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA) enhances the effects of TRH on phase II of prolactin secretion as well as on hormone synthesis at both low and high TPA receptor occupancy. Furthermore TPA, but not the biologically inactive substance 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD), stimulates the particulate bound adenylate cyclase with a time course paralleling that of TRH activation. However, the combined additions of TRH and TPA activate this cyclase in an additive manner while the Gpp(NH)p- and the forskolin-sensitive enzyme are unaffected by TPA addition. Polymyxin B, which inhibits protein kinase C, abolishes activation of adenylate cyclase by TPA without interfering with the stimulatory action of TRH. Also, when phosphatase activity is preferentially inhibited by pretreatment of the cells with sodium vanadate, the TRH-sensitive cyclase is unaltered, while TPA activation is obliterated. Maximal stimulation of adenylate cyclase by cholera toxin pretreatment, obliterated the actions of TRH and TPA. Cells pretreated with pertussis toxin retained their TRH-sensitive cyclase, however, TPA-responsiveness was lost. We therefore suggest that the action of TPA as it relates to activation of adenylate cyclase, is probably mediated via the Gi component of the adenylate cyclase complex, while TRH stimulates the enzyme via the classical pathway involving the stimulatory GTP binding protein (Gs).

Regulation of growth hormone secretion and cyclic AMP metabolism in ovine pituitary cells: interactions involved in activation induced by growth hormone-releasing hormone and phorbol esters

Growth hormone-releasing hormone (GHRH) and the phorbol ester tetradecanoylphorbol acetate (TPA) each stimulated a rapid and extensive (up to 15-fold) increase in the secretion of growth hormone from cultured ovine anterior pituitary cells. Effects of the releasing hormone on growth hormone secretion were associated with a concurrent, large increase in cellular cyclic AMP accumulation. TPA induced a much smaller (26-78%), though still significant, increase in cellular cyclic AMP levels. Forskolin and isobutylmethylxanthine (IBMX) also stimulated growth hormone secretion and cyclic AMP accumulation. When combined with a maximally effective concentration of GHRH these compounds did not further elevate growth hormone secretion even though they induced further increases in cyclic AMP concentration; this is consistent with activation occurring via a common cyclic AMP-dependent pathway. In contrast TPA when combined with maximally effective concentrations of either GHRH, forskolin or IBMX caused additional release of growth hormone, suggesting that the TPA-induced secretion involved a cyclic AMP-independent process. However, TPA also markedly potentiated the cellular cyclic AMP accumulation due to each of these agents. That TPA induced stimulation of basal and GHRH-stimulated cyclic AMP levels measured in the presence of IBMX suggests an action affecting cyclic AMP synthesis. Carbachol had no effect on basal or GHRH-stimulated growth hormone secretion or cyclic AMP levels. The two actions of TPA, one on secretion and one on cyclic AMP metabolism, may result from activation of some common event possibly involving protein kinase C. Our results suggest that GHRH and TPA activate independent pathways regulating growth hormone secretion.

Bombesin stimulates inositol polyphosphate production in GH4C1 pituitary tumor cells: comparison with TRH

The hormones bombesin and thyrotropin-releasing hormone (TRH) stimulated formation of inositol- monophosphate, bisphosphate, trisphosphate and tetrakisphosphate with parallel time courses in GH4C1 cells, while a more polar inositol polyphosphate peak, consisting of inositol-pentakisphosphate and perhaps also inositol-hexakisphosphate, was unaffected by either hormone. Although bombesin and TRH had similar potencies in stimulating inositol trisphosphate production (Km = 30 nM and 40 nM, respectively), TRH was significantly more efficacious than bombesin. Maximal stimulation of inositol-1,4,5-trisphosphate formation by TRH was not further increased by addition of a maximally effective dose of bombesin, suggesting that the two hormones act through stimulation of a common pool of phospholipase C, and this enzyme pool can be fully stimulated by TRH, alone.

Calcium homeostasis in digitonin-permeabilized bovine chromaffin cells

The regulation of cytosolic calcium was studied in digitonin-permeabilized chromaffin cells. Accumulation of 45Ca2+ by permeabilized cells was measured at various Ca2+ concentrations in the incubation solutions. In the absence of ATP, there was a small (10-15% of total uptake) but significant increase in accumulation of Ca2+ into both the vesicular and nonvesicular pools. In the presence of ATP, the permeabilized cells accumulated Ca2+ into carbonyl cyanide m-chlorophenyl hydrazone (CCCP)-sensitive and -insensitive pools. The CCCP-sensitive pool--mainly mitochondria--was active when the calcium concentration was greater than 1 microM and was not saturated at 25 microM. The Ca2+ sequestered by the CCCP-insensitive pool could be inhibited by vanadate and released by inositol trisphosphate, a combination suggesting that this pool was the endoplasmic reticulum. The CCCP-insensitive pool had a high affinity for calcium, with an EC50 of approximately 1 microM. When the Ca2+ concentration was adjusted to the level in the cytoplasm of resting cells (0.1 microM), the presumed endoplasmic reticulum pool was responsible for approximately 90% of the ATP-stimulated calcium uptake. At a calcium level similar to the acetylcholine-stimulated level in intact cells (5-10 microM), most of the Ca2+ (greater than 95%) went into the CCCP-sensitive pool.

Role of inositol trisphosphate as a second messenger in signal transduction processes: an essay

This essay attempts to summarize some of the best evidence for the role of inositol trisphosphate as a second messenger in signal transduction processes. The following aspects are addressed in the essay: (a) The synthesis of inositol trisphosphate and other inositol lipids, (b) Receptor-phosphatidylinositol bisphosphate phospholipase C coupling and the N-ras protooncogene, (c) Inositol trisphosphate and intracellular calcium, (d) Cell growth and oncogenes, (e) Receptors linked to the phosphatidylinositol cycle, (f) Phototransduction and (g) Interactions between inositol trisphosphate and other second messengers.

Receptor-stimulated system mediated interactions of neuropeptides in GH3 cells

The interaction between two neuropeptides, VIP and TRH, was studied. The TRH receptor binding ability was examined using intact GH3 cells and its membrane fraction. The TRH binding ability decreased when intact cells were preincubated with VIP, forskolin or db-cAMP, but not when the membrane fraction was treated with these agents. The binding was reduced only when the membrane fraction was treated with catalytic subunit of cAMP dependent protein kinase (A-kinase). These results indicate that the binding ability of TRH receptors, which are linked to inositol phospholipid metabolism, is suppressed when A-kinase increases due to activation of the adenylate cyclase or its dependent system. This, in turn, suggests the presence of a communication via a cytoplasmic factor (probably A-kinase) between the two principal second messenger systems in the cell.

Bombesin stimulates prolactin secretion from cultured rat pituitary tumour cells (GH4C1) via activation of phospholipase C

Bombesin (BBS) stimulated prolactin (PRL) secretion from monolayer cultures of rat pituitary tumour cells (GH4C1) in a dose-dependent manner with half maximal and maximal effect at 2 nM and 100 nM, respectively. No additional stimulatory effect on PRL secretion was seen when BBS was combined with thyroliberin (TRH) used in concentrations known to give maximal effects, while the effects of BBS and vasoactive intestinal peptide (VIP) were additive. Using a parafusion system, BBS (1 microM) was found to increase PRL secretion within 4 s and the secretion profiles elicited by BBS and TRH (1 microM) were similar. Both BBS and TRH increased inositoltrisphosphate (IP3) as well as inositolbisphosphate (IP2) formation within 2 s. BBS also induced the same biphasic changes in the electrical membrane properties of GH4C1 cells as TRH, and both peptides caused a rapid and sustained increase in intracellular [Ca2+]. These results suggest that BBS stimulates PRL secretion from the GH4C1 cells via a mechanism involving the immediate formation of IP3 thus resembling the action of TRH.