Relationship of thyrotropin-releasing hormone-induced spike and plateau phases in cytosolic free Ca2+ concentrations to hormone secretion. Selective blockade using ionomycin and nifedipine

TRH and BAY K 8644 synergistically stimulate prolactin release but not 45Ca2+ uptake

Thyrotropin-releasing hormone (TRH) (1 microM) and the Ca2+-channel agonist BAY K 8644 (1 microM) each induced transient increases in prolactin secretion from primary cultures of rat anterior pituitary cells in perifusion. When BAY K 8644 was added after a TRH-induced secretory peak, the additional effect of BAY K 8644 on prolactin release was approximately twofold greater over a 30-min period than the effect of BAY K 8644 on previously untreated cells. TRH and BAY K 8644 were also synergistic when added in the opposite order or simultaneously. Substitution of other agents for BAY K 8644 revealed that only high K+ (40 mM) was at least additive with TRH in stimulating prolactin secretion; treatment with TRH inhibited, rather than facilitated, subsequent stimulation of prolactin secretion by angiotensin II (100 nM) or the ionophore A23187 (20 microM). The cooperative effect was not specific for TRH because BAY K 8644 also acted synergistically with angiotensin II or 40 mM K+. In GH4C1 cells, in which TRH and BAY K 8644 were also synergistic in releasing prolactin, measurements with the fluorescent indicator indo-1 showed that TRH and BAY K 8644 could each elevate cytosolic Ca2+ above the level stimulated by the other. Unexpectedly, TRH was found to inhibit BAY K 8644-stimulated 45Ca2+ uptake in both GH4C1 and primary cultured cells. These results indicate that BAY K 8644 and TRH synergistically stimulate prolactin secretion by a mechanism other than a cooperative effect on the activity of dihydropyridine-sensitive Ca2+ channels.

Biphasic activation of cytosolic free calcium and LH responses by gonadotropin-releasing hormone

Gonadotropin-releasing hormone (GnRH) stimulates rapid peak increases in [Ca2+]i and LH release, followed by lower but sustained elevations of both [Ca2+]i and hormone secretion. Omission of extracellular Ca2+ only slightly decreased the peak of [Ca2+]i, but reduced the peak LH response by 40% and prevented the prolonged increases in [Ca2+]i and LH release. Dihydropyridine calcium antagonists did not affect the peak [Ca2+]i and LH responses, but reduced the sustained increases by up to 50%. Whereas GnRH-induced mobilization of intracellular calcium initiates the LH peak, and Ca2+ entry through dihydropyridine-insensitive channels contributes to the peak and plateau phases of LH release, dihydropyridine-sensitive L-type Ca2+ channels participate only in the sustained phase of gonadotropin secretion.

Early signal transduction by the antigen receptor without commitment to T cell activation

The T lymphocyte antigen-receptor complex mediates antigen-specific cell activation, at least in part, through the production of inositolphospholipid-derived second messengers. Little is known about how second messenger events, typically measured within minutes of ligand binding, eventually lead to distal biologic responses such as expression of lymphokine genes. Several monoclonal antibodies directed against the receptor complex were tested for their ability to elicit transmembrane signaling in the parental Jurkat line and in a somatic mutant (J.CaM1) with a deficient receptor function. One antibody elicited substantial early Ca2+ mobilization responses in both cells but was unable to promote expression of the interleukin-2 gene in J.CaM1. In J.CaM1 there was a diminished production of phosphatidylinositol second messengers, and the elevation in intracellular free Ca2+ was transient. Thus, short-term Ca2+ mobilization does not always indicate complete signal transmission and lead to a full cellular response.

Pharmacological characterization of two calcium currents in GH3 cells

Whole cell patch-clamp techniques were used to investigate the pharmacological properties of calcium currents in the clonal rat pituitary cell line GH3. Current traces induced by a 100-ms pulse to 0 mV from a holding potential of -80 mV consisted of a component that rapidly inactivated during the pulse and a component that slowly inactivated during the pulse. When the holding potential was reduced to -32 mV, the rapidly inactivating component of the trace disappeared. The dihydropyridine calcium channel blocker nitrendipine affected only the slowly inactivating component of the trace. At a holding potential of -80 mV, nitrendipine blocked the slowly inactivating current with an IC50 of 1 microM. The IC50 for nitrendipine was found to be dependent on the holding potential, decreasing to 10 nM when the holding potential was -32 mV. The dihydropyridine agonist Bay-K 8644, like nitrendipine, affected only the slowly inactivating component. The inorganic blocker Cd2+ blocked both components but the slowly inactivating current was three- to fourfold more sensitive. These results are best explained by the existence of two types of calcium channels in these cells, one sensitive to dihydropyridines and one insensitive to dihydropyridines. These channels appear analogous to the T-type channel (inactivating current) and L-type channel (slowly inactivating current) described in other preparations.

Mobilization of cellular Ca2+ by lysophospholipids in rat islets of Langerhans

To determine whether lysophospholipids mobilize cellular Ca2+, intact rat islets were prelabelled with 45Ca2+ and subjected to three maneuvers designed to simulate the physiologic accumulation of lysophospholipids: (1) exogenous provision; (2) addition of porcine pancreatic phospholipase A2; and (3) provision of p-hydroxymercuribenzoic acid, which impedes both the reacylation and hydrolysis of endogenous lysophospholipids, leading to their accumulation in islets. Each maneuver provoked 45Ca2+ efflux at concentrations nearly identical to those previously reported to induce insulin release in the absence of toxic effects on the islets. Lysophosphatidylcholine (lysoPC) and lysophosphatidylinositol were active, whereas the ethanolamine and serine derivatives, and lysophosphatidic acid, were much less effective. The effects of lysoPC were reversible; they also were reduced by lanthanum or gentamicin (which are probes of superficial, plasma membrane-bound stores of Ca2+) or by prior depletion of membrane-bound cellular Ca2+ stores using ionomycin, but not by removal of extracellular Ca2+ or Na+. The effects of lysoPC, phospholipase A2 and p-hydroxymercuribenzoic acid were largely independent of any hydrolysis to, or accumulation of, free fatty acids as assessed by resistance to dantrolene or trifluoperazine (which selectively reduce arachidonic acid-induced 45Ca2+ efflux and insulin release). Thus, lysophospholipids are a newly recognized class of lipid mediators which may promote insulin release at least in part via mobilization of a pool(s) of Ca2+ ('trigger Ca2+') bound in the plasma membrane and possibly in other cellular membranes.

Inositol 1,4,5-trisphosphate-induced calcium release from platelet plasma membrane vesicles

A platelet membrane preparation, enriched in plasma membrane markers, took up 45Ca2+ in exchange for intravesicular Na+ and released it after the addition of inositol 1,4,5-trisphosphate (IP3). The possibility that contaminating dense tubular membrane (DTS) vesicles contributed the Ca2+ released by IP3 was eliminated by the addition of vanadate to inhibit Ca+-ATPase-mediated DTS Ca2+ sequestration and by the finding that only plasma membrane vesicles exhibit Na+-dependent Ca2+ uptake. Ca2+ released by IP3 was dependent on low extravesicular Ca2+ concentrations. IP3-induced Ca2+ release was additive to that released by Na+ addition while GTP or polyethylene glycol (PEG) had no effect. These results strongly suggest that IP3 facilitates extracellular Ca2+ influx in addition to release from DTS membranes.

Actions of rat growth hormone-releasing factor and norepinephrine on cytosolic free calcium and inositol trisphosphate in rat C-cells

Rat growth hormone-releasing factor (rGRF) and norepinephrine (NE) stimulate secretion of calcitonin (CT) and neurotensin (NT) from cultured C-cells. The mechanism by which these agents cause secretion has not been well studied. We have examined the actions of the CT and NT secretagogues rGRF and NE on cytosolic free calcium concentrations ([Ca2+]i) in the rat C-cell line rMTC 44-2. Because inositol trisphosphate (IP3) has been shown to cause release of intracellular calcium stores in several cell types, we have also examined the effects of rat GRF, NE, and increases in extracellular calcium on IP3 accumulation in rMTC 44-2 cells. Stimulation by 10(-6) M rGRF caused a biphasic response in [Ca2+]i consisting of a rapid spike to 136 +/- 4% (mean +/- SE) of basal [Ca2+]i. This increase in [Ca2+]i decayed to base line and then gradually increased to 173 +/- 13% of basal [Ca2+]i. Stimulation by 10(-6) M NE gave a similar biphasic increase in [Ca2+]i. The increases in [Ca2+]i induced by both rGRF and NE were inhibited by pretreatment with EGTA or verapamil. rGRF, NE, and increasing concentrations of extracellular calcium, which all caused rapid increases in [Ca2+]i, failed to increase IP3 accumulation in rMTC 44-2 cells. These results suggest that rGRF- and NE-induced secretion in C-cells are mediated by changes in [Ca2+]i. These increases in [Ca2+]i appear to be generated by extracellular calcium influx rather than by release of intracellular calcium stores.

Calcium mobilization by muscarinic receptors in human astrocytoma cells: measurements with quin 2

Activation of muscarinic cholinergic receptors on 1321N1 human astrocytoma cells leads to Ca2+ mobilization as measured by quin 2 fluorescence. Acetylcholine and methacholine were full and potent agonists, while carbachol and muscarine, were fully efficacious but 6- and 10-fold less potent than acetylcholine. The carbachol-induced Ca2+ response was also observed in absence of extracellular Ca2+ and was blocked by muscarinic receptor antagonists but not by organic Ca2+ channel blockers, tetrodotoxin (TTX), tetraethylammonium (TEA) or metal cations, suggesting that Ca2+ is mobilized from intracellular storage sites rather than through plasma membrane ion channels. Muscarinic receptor-mediated Ca2+ release was also detected in kidney epithelial cells but not in rat fibroblasts, glial cells or differentiated neuroblastoma x glioma hybrid cells.

Intracellular free calcium in rat anterior pituitary cells monitored by fura-2

Rat anterior pituitary cells, loaded with the calcium indicator dye fura-2 after primary culture, were challenged with prolactin and growth hormone secretagogues and inhibitory hormones. To initially validate the technique, the calcium channel activator maitotoxin effectively increased intracellular free calcium [( Ca++]i). Various concentrations of the secretagogues thyrotropin releasing hormone or angiotensin II induced peak increases in [Ca++]i within 15 sec, followed by a lower and prolonged plateau phase. The inhibitory hormones dopamine and somatostatin maximally reduced [Ca++]i by 15-20 sec, followed by a spontaneous return to baseline over 5-10 min. The receptor antagonists saralacin and spiperone blocked the angiotensin II and dopamine effects, respectively. Thus, fura-2 appears to be an adequate probe for resolving second-to-second changes in [Ca++]i induced by hormone receptor activation in anterior pituitary cells.

Multiple coupling of neurohormone receptors with cyclic AMP and inositol phosphate production in anterior pituitary cells

Regulation of adenohypophyseal hormone secretions has been shown to involve cyclic AMP production, modulation of phosphatidyl inositol diphosphate breakdown and Ca2+ mobilization. Various neurohormone receptors are positively or negatively coupled to adenylate cyclase activity in anterior pituitary cells. The effects of these neurohormones on adenylate cyclase activity are consistent with the effect on hormone secretions, suggesting that modulation of the enzyme activity is actually involved in the regulation of adenohypophyseal secretions. Thus DA inhibits, whereas VIP stimulates adenylate cyclase activity of the same cell type, which, according to the effect of these neurohormones on prolactin secretion, appear to be lactotrophs. On the other hand, SRIF inhibits, whereas GRF stimulates the adenylate cyclase activity of another cell type, namely somatotrophs, whereas CRF appears to act on a third cell type, corticotrophs. Peripheral hormones have been shown to modulate the sensitivity of anterior pituitary cells to these neurohormones. Estradiol long-term treatment has an anti-dopaminergic effect on prolactin secretion. The steroid also suppresses the dopamine inhibition of adenylate cyclase. This effect appears selective to the DA inhibition, since AII inhibition of the enzyme is only partially reduced, whereas the somatostatin inhibition is markedly increased. Peripheral hormones seem to affect the sensitivity of adenohypophyseal cells not only by modulating the number of receptors for a given neurohormone but also by interfering with the coupling mechanisms of these receptors. AII and DA inhibit the adenylate cyclase activity of lactotroph cells. The prolactin stimulation induced by angiotensin is not consistent with the effect of the peptide on adenylate cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

Ion channels activated by inositol 1,4,5-trisphosphate in plasma membrane of human T-lymphocytes

Hydrolysis of membrane-associated phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)-P2) to water soluble inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) is a common response by many different kinds of cells to a wide variety of external stimuli (see refs 1 and 2 for review). Ins (1,4,5)P3 is a putative second messenger which increases intracellular Ca2+ by mobilizing internal Ca2+ stores, a hypothesis which has been substantiated by studies with chemically permeabilized cells and with isolated microsomal membrane fractions. But the possibility that Ins(1,4,5)P3 could induce in intact cells an influx of external Ca2+ through transmembrane channels, originally hypothesized by Michell in 1975, has never been directly tested. We report here single-channel recordings of an Ins(1,4,5)P3-activated conductance in excised patches of T-lymphocyte plasma membrane. The Ins(1,4,5)P3-activated transmembrane channel appears to be identical to the recently described mitogen-regulated, voltage-insensitive Ca2+ permeable channel involved in T-cell activation. We suggest that Ins(1,4,5)P3 acts as the second messenger mediating transmembrane Ca2+ influx through specific Ca2+-permeable channels in mitogen-stimulated T-cell activation.

On the involvement of cyclic AMP and extracellular Ca2+ in the regulation of hormone release from rat pituitary tumour (GH3) cells in culture

Thyroliberin (TRH), dibutyryl cyclic AMP (db-cAMP), and 3-isobutyl-1-methylxanthine (MIX) had a stimulatory effect on prolactin (PRL) and growth hormone (GH) release from GH3 cells. Half-maximal and maximal effects were observed for TRH at 2.5 nM and 10 nM; for db-cAMP at 0.6 mM and 5 mM, respectively. MIX (0.1 mM-1 mM) induced a dose-dependent accumulation of cellular cyclic AMP, while the hormone release was already maximally stimulated at 0.1 mM MIX. The maximal effects on hormone release of TRH and db-cAMP, but not of TRH and MIX, were additive. The Ca2+ channel blockers Co2+ (5 mM) and verapamil (100 microM) and the Ca2+ chelator EGTA (4 mM) abolished the stimulatory effect of TRH (1 microM) on hormone release. Co2+ and verapamil, but not EGTA, inhibited the stimulatory effect of db-cAMP (5 mM) on hormone release. The inhibitory effects of Co2+ and verapamil on GH release were counteracted by the combination of TRH and db-cAMP. For PRL release Co2+, but not verapamil, was able to inhibit the combined action of TRH and db-cAMP. Co2+, verapamil, and EGTA eliminated the stimulatory effect of MIX (1 mM) on PRL release while only Co2+ and EGTA affected the GH release. Hormone release in the presence of MIX plus verapamil or EGTA, but not Co2+, was increased by TRH. The calmodulin antagonist trifluoperazine (TFP) at 30 microM inhibited basal hormone release and hormone release stimulated by TRH (1 microM), db-cAMP (5 mM), and MIX (1 mM). The Ca2+ ionophore A23187 (5 microM) had a stimulatory effect on basal hormone release which was abolished by 30 microM TFP.

Effects of RHC 80267, a diglyceride lipase inhibitor, on prolactin secretion and calcium uptake in GH3 pituitary cells

The effect of the diglyceride lipase inhibitor RHC 80267 on the prolactin secretory process was examined in clonal anterior pituitary GH3 cells. This compound reduced basal prolactin secretion as well as secretion induced by TRH and phospholipase C but not that induced by phorbol myristate acetate. Although exogenous phospholipase C increased diglyceride, no increase in the products of diglyceride lipase was detected. Moreover, low doses of RHC 80267 were observed to effectively block potassium-stimulated 45calcium influx. It is unlikely that RHC 80267 inhibits prolactin release solely by inhibiting diglyceride lipase. These data suggest blockade of plasma membrane calcium channels as an alternate mechanism for the inhibitory actions of RHC 80267 on intact GH3 cells. These observations may have implications for RHC 80267 action in other cell types.

Ionomycin acts as an ionophore to release TRH-regulated Ca2+ stores from GH4C1 cells

In the GH4C1 strain of rat pituitary cells, ionomycin, a divalent cation ionophore, induces a rapid and transient spike in cytosolic free Ca2+ concentrations [( Ca2+]i) similar to that induced by the Ca2+-mobilizing hormone thyrotropin-releasing hormone (TRH). To test directly the hypothesis that ionomycin causes the spike in [Ca2+]i by altering cellular Ca2+ stores, we have measured ionomycin-induced changes in 45Ca2+ fluxes and have compared these to previously characterized changes induced by TRH. Ionomycin (half-maximal concentration = 30 nM) rapidly (within 1 min) induced a release into the medium of 50-60% of cell-associated 45Ca2+, paralleling the spike in [Ca2+]i. The ionomycin-induced 45Ca2+ efflux was greater than with TRH, and TRH did not induce further 45Ca2+ efflux in the presence of ionomycin. Ionomycin pretreatment blocked induction of the spike in [Ca2+]i elicited by TRH but did not alter basal or TRH-induced enhancement of inositol phosphate levels. These results provide evidence that the spike in [Ca2+]i induced by ionomycin or TRH is produced largely by release of Ca2+ into the cytosol from the same intracellular pool, followed by rapid extrusion of the released Ca2+ into the extracellular space. However, unlike TRH, ionomycin appears to release cellular Ca2+ directly, acting as an ionophore, without the generation of known second messengers.

Bradykinin-induced intracellular Ca2+ elevation in neuroblastoma X glioma hybrid NG108-15 cells; relationship to the action of inositol phospholipids metabolites

The effect of bradykinin on the intracellular Ca2+ concentration ([Ca2+]i) in NG108-15 cells was studied using a Ca2+ indicator quin 2. Bradykinin induced two phases of change in [Ca2+]i. Bradykinin induced a spike phase of [Ca2+]i increase which was detectable within 15 s and decayed to near-basal concentration in 3 min and then a prolonged plateau phase of [Ca2+]i increase which continued for 15 min. The bradykinin-induced spike phase was not diminished by decreasing extracellular Ca2+ concentration ([Ca2+]o) to 1 microM. On the contrary, the plateau phase was dependent on [Ca2+]o and inhibited by Ca2+ blockers, verapamil (50 microM), nifedipine (1 microM). The iontophoretic injection of inositol-trisphosphate (IP3) into the single cell induced the increase of [Ca2+]i, which was independent of [Ca2+]o. These results indicate that the bradykinin-induced spike phase is mediated by the release of intracellular Ca2+ stores induced by IP3, while the plateau phase is mediated by influx of extracellular Ca2+ probably through voltage-sensitive Ca2+ channels.

Role of inositol lipid breakdown in the generation of intracellular signals. State of the art lecture

Many hormones, neurotransmitters, and secretagogues act by increasing the intracellular free Ca2+ concentration in target cells. The initial event following binding of agonists to specific receptors in the plasma membrane involves a receptor-mediated activation of a guanosine nucleotide-binding protein (G protein), which induces a Ca2+-independent activation of phospholipase C. This novel, presently uncharacterized G protein is inactivated by pertussis toxin-catalyzed adenosine 5'-diphosphate ribosylation in some but not all cell types. Phospholipase C catalyzes the breakdown of inositol lipids, notably phosphatidylinositol 4,5-bisphosphate, with the production of inositol phosphates and 1,2-diacylglycerol. Inositol 1,4,5-trisphosphate (IP3) is responsible for a rapid mobilization of intracellular Ca2+ by activating Ca2+ efflux from a subpopulation of the endoplasmic reticulum. The properties of this process are consistent with its being a ligand-activated ion channel with electrogenic Ca2+ efflux being charge-compensated by K+ influx. Sustained hormonal responses require extracellular Ca2+ and a prolonged elevation of the cytosolic free Ca2+. This is brought about by hormone-mediated changes of Ca2+ flux across the plasma membrane involving both an inhibition of Ca2+ efflux and an activation of Ca2+ influx. This review summarizes recent findings concerning the role of G proteins in receptor coupling to phospholipase C; the regulation of enzymes of phosphoinositide metabolism; the evidence for IP3 being a Ca2+-mobilizing second messenger and its mechanism of action; the formation of new inositol phosphates and their possible significance; the relation of intracellular Ca2+ mobilization and plasma membrane Ca2+ fluxes to the kinetics of the hormone-induced cytosolic free Ca2+ transient; and the possible roles of protein kinase C in influencing the hormone-mediated functional response.

Agonist-induced calcium transients in cultured smooth muscle cells: measurements with fura-2 loaded monolayers

Elevation of cytosolic Ca2+ in response to depolarization and various receptor agonists was measured in several types of cultured smooth muscle cells (DDT1, A10, rabbit aorta) loaded with the either quin-2 or fura-2, and assayed either in suspension or in monolayer cultures attached to plastic cover slips. Agonists (norepinephrine, vasopressin) induced both the release of intracellular Ca2+ and the influx of extracellular Ca2+. Agonist-induced Ca2+ influx was not blocked by dihydropyridines, and depolarization did not induce Ca2+ influx. However, in fura-2 loaded monolayers of PC12 cells, depolarization did induce dihydropyridine-sensitive Ca2+ influx. Thus cultured smooth muscle cells appear to express receptor-operated Ca2+ channels, but not functional voltage-operated Ca2+ channels.

Comparison of patterns of prolactin release in GH4C1 cells and primary pituitary cultures

The effects of 12-O-tetradecanoylphorbol-13-acetate (TPA, an activator of C-kinase), the cation ionophore A23187, forskolin (an activator of adenylate cyclase) and thyrotropin-releasing hormone (TRH) on prolactin release from anterior pituitary cells in primary culture were investigated and compared to the effects of these same agents on prolactin release from GH4C1 cells. In both GH4C1 cells and primary pituitary cultures, 100 nM TRH increased prolactin release 3- to 5-fold within 4 min after the stimulation started. This peak response was followed by a fall to a sustained increased rate of release approximately 1.5-fold above the basal rate. The decline after the early peak was slower in primary cultures than in GH4C1 cells. Addition of 20 microM A23187 to primary cultures caused a rapid 2- to 4-fold increase in release that fell to basal values within 12 min after the stimulation started. In GH4C1 cells, A23187 caused a rise in prolactin release of less than 2-fold that was sustained longer than the rise seen in primary cultures. Perifusion of either type of cells with 50 nM TPA caused a rapid 2- to 2.5-fold increase in release that also was sustained for 30 min or more in both types of cells. Perifusion with combined TPA and A23187 caused a 3- to 5-fold increase in rate of release from each cell type that declined rapidly to a 2-fold sustained release in primary cultures, and that declined more slowly in GH4C1 cells. Forskolin, 1 microM, had only a small effect by itself, but potentiated the effect of TPA or combined TPA and A23187 in both types of cells.(ABSTRACT TRUNCATED AT 250 WORDS)