Aluminium perturbs oscillatory phosphoinositide-mediated calcium signalling in hormone-stimulated hepatocytes

Aluminium is known to be toxic to cells from bone, brain and bone marrow but the molecular target(s) affected by Al3+ are not known. We show here that Al3+ disrupts the oscillatory free Ca2+ responses of hepatocytes exposed to the Ca2(+)-mobilizing agonist phenylephrine. Al3+ initially increases the frequency of the oscillations and later induces broad Ca2+ spikes lasting several minutes. These broad spikes persist after removal of both agonist and Al3+ from the medium. In the absence of agonist, Al3+ has no effect on free Ca2+. The data suggest that some component(s) of the receptor-phosphoinositide-Ca2+ signalling pathway might be the site at which Al3+ exerts toxic effects.

cAMP- and diacylglycerol-mediated pathways elevate cytosolic free calcium concentration via dihydropyridine-sensitive, omega-conotoxin-insensitive calcium channels in normal rat anterior pituitary cells

The effect of 8-bromocyclic AMP (8-Br-cAMP) and phorbol 12-myristate 13-acetate (PMA), a protein kinase C activator, on cytosolic free calcium concentration ([Ca2+]i) in normal rat anterior pituitary cells was examined. [Ca2+]i was monitored directly by the fluorescent indicator fura-2. 8-Br-cAMP as well as PMA elevated [Ca2+]i in a concentration-dependent manner. Forskolin (10 mumol/l), which activates adenylate cyclase, and 1-oleoyl-2-acetyl-glycerol (10 mumol/l), another activator of protein kinase C, also increased [Ca2+]i. Both the 8-Br-cAMP (2 mmol/l)- and the PMA (100 nmol/l)-induced increase in [Ca2+]i was dependent on the presence of extracellular calcium and could be inhibited by the calcium channel blockers Mg2+ and nifedipine, but not by omega-conotoxin (100 nmol/l). The half-maximally inhibitory concentrations of Mg2+ and nifedipine were about 12 mmol/l and 160 nmol/l, respectively, for the [Ca2+]i response to 8-Br-cAMP (2 mmol/l), and were about 6 mmol/l and 400 nmol/l, respectively, for the PMA (100 nmol/l)-induced increase in [Ca2+]i. The sodium channel blocker tetrodotoxin (5 mumol/l) or PMA (100 nmol/l) on [Ca2+]i. After pretreatment for 3 min with PMA (100 nmol/l), the subsequent K+ (100 mmol/l)- or arachidonic acid (3 mumol/l)-induced increase in [Ca2+]i was decreased by about 50%. By contrast, pretreatment (3 min) with 8-Br-cAMP (2-10 mmol/l) markedly enhanced the subsequent [Ca2+]i response to K+ (100 mmol/l), and left the effect of arachidonic acid (3 mumol/l) on [Ca2+]i unimpaired.(ABSTRACT TRUNCATED AT 250 WORDS)

Opioids and cytosolic calcium in rat anterior pituitary: dynorphin preparation showed LHRH-like action due to contamination

The effect of dynorphin A-(1-13) (Dyn A-(1-13] and other opioids on the cytosolic free calcium concentration [(Ca2+]i) in rat anterior pituitary cells was examined using the fluorescent indicator fura-2. A commercial synthetic Dyn A-(1-13) preparation elevated [Ca2+]i. Results, which were obtained with receptor antagonists, and in LHRH receptor radioligand binding studies as well as by HPLC combined with LHRH radioimmunoassay, strongly suggest that this effect of the dynorphin preparation was due to contamination with a LHRH-like compound. Dyn A-(1-13), purified by HPLC, as well as Dyn A-(2-13), [Leu5]enkephalin, beta-endorphin, morphine, or U50,488H had no effect on [Ca2+]i. LHRH caused a rapid increase in [Ca2+]i by about 50 nM which was blocked by the LHRH antagonist, [D-pGlu1,D-Phe2,D-Trp3,6] LHRH.

Arachidonic acid elevates cytosolic free calcium concentration in rat anterior pituitary cells

Arachidonic acid is liberated from phospholipids by various hypothalamic releasing hormones and may be involved in stimulus-secretion coupling in rat adenohypophysis. In the present study, the effect of exogenous arachidonic acid on calcium homeostasis in rat anterior pituitary cells was investigated in vitro. Arachidonic acid markedly stimulated the release of various anterior pituitary hormones (beta-endorphin, luteinizing hormone, growth hormone). Arachidonic acid (10 mumol/l) decreased the initial rate of 45Ca2+ uptake. In cells prelabelled with 45Ca2+, arachidonic acid (10 mumol/l) decreased the exchangeable cell calcium content and increased the rate of 45Ca2+ extrusion. Cytosolic free calcium concentration [( Ca2+]i) was measured with the fluorescent indicator fura-2. Arachidonic acid markedly elevated [Ca2+]i. The concentration dependency of this effect (1 mumol/l and above) was similar to that on hormone secretion. Arachidonic acid (6 mumol/l) elevated [Ca2+]i by about 300 nmol/l, and arachidonic acid (10 mumol/l) raised [Ca2+]i into the micromolar range. The effect of arachidonic acid (3 mumol/l) on [Ca2+]i was not influenced by inhibitors of arachidonic acid metabolism (nordihydroguaiaretic acid, BW755C). In Ca2+-free media (Ca2+ omitted, EGTA 2 mmol/l), the effect of arachidonic acid (3 mumol/l) on [Ca2+]i was almost unimpaired, whereas the effect of arachidonic acid (10 mumol/l) was reduced. Thus, the secretagogue arachidonic acid induces calcium mobilization and an increase in cytosolic free calcium concentration. These actions further qualify arachidonic acid as a potential intracellular mediator of stimulus-induced hormone secretion from rat adenohypophysis.

Potassium depolarization elevates cytosolic free calcium concentration in rat anterior pituitary cells through 1,4-dihydropyridine-sensitive, omega-conotoxin-insensitive calcium channels

Changes in membrane potential may influence Ca2+-dependent functions through changes in cytosolic free calcium concentration [( Ca2+]i). This study characterized pharmacologically those voltage-dependent Ca2+ channels in normal rat anterior pituitary cells that are involved in the elevation of [Ca2+]i upon high potassium-induced membrane depolarization. The [Ca2+]i was monitored directly by means of the intracellularly trapped fluorescent indicator fura-2. The addition of K+ (6-100 mM) increased [Ca2+]i in a concentration-dependent manner. The fluorescent signal reached a peak within seconds and then decayed to form a new elevated plateau. K+ at the highest concentration used (100 mM) raised [Ca2+]i by about 450 nM. The K+-induced increase in [Ca2+]i was absent in a Ca2+-free medium. BAY K 8644, a 1,4-dihydropyridine Ca2+ channel agonist, also caused an increase in [Ca2+]i. The maximum response in [Ca2+]i upon stimulation with BAY K 8644 (100 nM) was about 40 nM. The half-maximally effective concentration of BAY K 8644 (100 nM) was about 20 nM. The response in [Ca2+]i upon BAY K 8644-stimulation was abolished in a Ca2+-free medium. Predepolarization with various K+ concentrations enhanced the effect of BAY K 8644 (1 microM) on [Ca2+]i. Pretreatment with BAY K 8644 (1 microM) enhanced the response in [Ca2+]i induced by K+ (25 mM). The addition of Mg2+ (30 mM) and nifedipine (1 microM) lowered the resting [Ca2+]i by about 40 and 20 nM, respectively. Mg2+, nifedipine, nimodipine, Gö 5438, verapamil, and diltiazem inhibited the K+ (25 mM)-induced increase in [Ca2+]i; the order of potency (and half-maximally inhibitory concentrations) were nimodipine = Gö 5438 = nifedipine (approximately 100 nM) greater than verapamil (900 nM) greater than diltiazem (greater than 10 microM) greater than Mg2+ (6 mM). Omega-Conotoxin (100 nM) did not inhibit the K+ (25 mM)-induced increase in [Ca2+]i. These data demonstrate that, over a wide range, membrane depolarization induced by high potassium concentration is indeed associated with increases in [Ca2+]i in normal rat anterior pituitary cells. This elevation of [Ca2+]i is mainly due to an influx of Ca2+ through 1,4-dihydropyridine-sensitive, omega-conotoxin-insensitive calcium channels (L-type).

GRF elevates cytosolic free calcium concentration in rat anterior pituitary cells

The effect of human growth hormone-releasing factor (GRF) on intracellular free calcium concentration ([Ca2+]i) was examined in rat anterior pituitary cells. The [Ca2+]i was monitored directly by means of the intracellularly trapped fluorescent indicator, fura-2. GRF rapidly elevated [Ca2+]i, reaching a new plateau within approximately 30 s. The half-maximally effective concentration of GRF was approximately 130 pM. GRF produced a maximal increase in [Ca2+]i by approximately 120 nM. The GRF (2 nM)-induced elevation of [Ca2+]i was abolished by removal of extracellular calcium (Ca2+ omitted, 2 mM EGTA). The GRF (2 nM)-caused rise in [Ca2+]i was largely reduced in the presence of the calcium channel blockers Mg2+ (31.2 mM) or nifedipine (1 microM). An increase in [Ca2+]i by approximately 60 nM was elicited by the addition of prostaglandin E2 (1 microM), which can stimulate growth hormone secretion independent of GRF receptors. These data indicate that GRF elevates the [Ca2+]i, possibly in somatotrophs; this GRF-induced increase in [Ca2+]i may depend on an influx of extracellular Ca2+, largely through Mg2+- and nifedipine-sensitive calcium channels.

Intracellular free calcium concentration in rat anterior pituitary cells as indicated by fura-2: effect of arginine-vasopressin

Arginine-vasopressin (AVP) stimulates adrenocorticotropin and beta-endorphin release from corticotrophs of the anterior pituitary gland through mechanisms which are not initiated by an elevation of the cellular levels of adenosine-3',5'-cyclic-monophosphate. In the present study the effect of AVP on the cytoplasmic concentrations of free calcium ions in rat anterior pituitary cells was examined. Cytosolic free Ca2+ concentrations were monitored directly using the new, intracellularly trapped fluorescent indicator fura-2. In cells incubated in medium containing 1.3 mmol/l Ca2+, AVP (100 nmol/l) caused an immediate elevation of the cytoplasmic Ca2+ concentration by about 50 nmol/l (P less than 0.001). The intracellular Ca2+ levels remained elevated during the observation period of 2-3 min. This effect of AVP was blocked by a specific vasopressin antagonist. By contrast, the glucocorticoid dexamethasone did not affect the AVP-induced elevation of cytosolic Ca2+ concentration. When the cells were incubated in Ca2+-free medium (Ca2+ omitted, EGTA 2 mmol/l), the AVP-induced as well as the K+ depolarization-induced increase in free cytoplasmic Ca2+ were abolished, whereas the ionophore ionomycin evoked a rapid transient elevation of free Ca2+. The increase in cytoplasmic Ca2+ concentration induced by AVP was preserved in medium containing the calcium channel blockers Mg2+ (Mg2+ 31.2 mmol/l; Ca2+ 1.3 mmol/l) or nifedipine (1 mumol/l). The potassium-evoked calcium signal was blocked by Mg2+ (31.2 mmol/l). We conclude that vasopressin induces a rapid rise in the cytoplasmic concentration of free calcium ions in corticotrophs. Vasopressin may mobilize calcium through mechanisms that neither are glucocorticoid-sensitive nor involve the influx of extracellular calcium through voltage-dependent calcium channels.(ABSTRACT TRUNCATED AT 250 WORDS)