Qualitative differences in [Ca2+]i increases and InsP3 generation following stimulation of N1E-115 cells with micromolar and millimolar ATP

Neuronal and glial cell lines as model systems for studying P2Y receptor pharmacology

Investigation of the role of extracellular nucleotides in nervous system has been one of the main topics of the P2Y receptor research throughout the years. In parallel to numerous studies on primary culture systems, various neuronal and non-neuronal cell lines have been used to model in vitro the processes mediated by extracellular nucleotides. In this review article, a survey of expression profiles of G protein-coupled P2Y receptor subtypes in nervous-system-derived cell lines is presented, by analysing the receptor expression at the mRNA, protein, and functional level. The variability of receptor expression profiles in established cell lines is further discussed, bringing forward some general properties for neuronal and glial malignant cell lines.

ATP receptor activation potentiates a voltage-dependent Ca channel in hippocampal neurons

Activation of a purinergic P2 receptor by adenosine 5'-triphosphate (ATP) has previously been shown to open a non-selective cation channel with a reversal potential of approximately 0 mV. We examined the effect of P2 receptor activation on voltage-gated ionic currents in acutely isolated CA3 pyramidal neurons from guinea pig hippocampus using the whole-cell-patch technique. Under conditions designed to isolate current through voltage-dependent Ca channels (ICa), ATP (50 microM) potentiated ICa by 36%. This increase in ICa desensitized back to control levels within 4 min. In contrast to the non-selective cation channel, ICa elicited from a holding potential (HP) of -100 mV showed significant potentiation in response to ATP when depolarized to a test potential (TP) of -10 mV but showed no effect on ICa when the same neuron was alternately depolarized to TP = -70 mV. No change in holding current at HP = -100 mV occurred. Tail currents were unaffected by ATP exposure suggesting that ICa potentiation was not due to modulation of L-type Ca channels. This potentiation was also observed either with ATP-gamma-s, the slowly hydrolyzable ATP analog, or with ATP in the presence of alpha, beta-methylene-ADP, an ectonucleotidase inhibitor, indicating that the effects observed were not due to activation of an adenosine receptor that required ATP hydrolysis. The potentiation of ICa was not observed with the P2X agonist, beta, gamma-methylene-ATP. These results suggest that ATP receptors can modulate voltage- as well as ligand-gated channels permeable to calcium and may play an important role in the dynamics of intracellular Ca2+ in these neurons.

Release of arachidonic acid via Ca2+ increase stimulated by pyrophosphonucleotides and bradykinin in mammary tumour cells

The relationship between the increase of intracellular Ca2+ and the release of arachidonic acid by bradykinin and pyrophosphonucleotides was studied in cultured mammary tumour cells, MMT060562. Bradykinin, ATP, UTP and UDP induced an increase of intracellular Ca2+ and the release of arachidonic acid from phospholipids into the extracellular fluid. Release of arachidonic acid was also induced by the application of the Ca2+ ionophore, A23187. Liberation of arachidonic acid by bradykinin and ATP was reduced by mepacrine, a blocker of phospholipase A2 and W-7, a calmodulin antagonist. It is suggested that the increase in cytosolic Ca(2+)-induced release of arachidonic acid occurs through activation of calmodulin-dependent phospholipase A2.

Changes of intracellular free calcium concentration in human polymorphonuclear leukocytes after repeated stimulations with N-formyl-methionyl-leucyl-phenylalanine

A rapid transient rise in the intracellular free calcium concentration ( Ca2+]i) is an important step in human polymorphonuclear leukocytes (PMNL) activation. This can be caused by many inflammatory mediators and has been implicated in the regulation of various cellular reactions. In this study we investigated the changes of [Ca2+]i in human PMNL activated three times with 10(-7)M n-formyl-methionyl-leucyl-phenylalanine (FMLP). PMNL in the presence of 1 mM Ca2+ were able to respond to three consecutive stimulations with FMLP. The first Ca2+ response was the highest one and was a result of Ca2+ release from internal stores (which was responsible for about 30% of maximal increment in [Ca2+]i) and the extracellular Ca2+ influx. Experiments with PMNL suspended in a medium containing 100 nM Ca2+ and pretreated with 1 nM Ni2+ (an inorganic calcium channel blocker) revealed that the second and third response is completely dependent on the extracellular Ca2+ influx. Changes of the time interval between stimulations had no influence on the occurrence of extracellular Ca2+ influx related to second addition of FMLP. Elongation of the time interval up to 30 min did not restore the release of Ca2+ from internal stores. It indicates the occurrence of dissociation of Ca2+ release from intracellular stores and extracellular Ca2+ influx during the second and third PMNL response to FMLP.

The increase in the intracellular Ca2+ concentration induced by mechanical stimulation is propagated via release of pyrophosphorylated nucleotides in mammary epithelial cells

Mechanical stimulation of one mammary tumor cell in culture induced an increase in its intracellular calcium concentration which spread to surrounding cells. The increase in calcium can also be induced by addition of a solution in which cultured mammary tumor cells were stimulated by repeated pipetting (solution after pipetting cells, SAPC). The activity of the SAPC was completely abolished by treatment with snake venom phosphodiesterase or pyrophosphatase. Uridine triphosphate (UTP), uridine diphosphate (UDP) and ATP (1 microM each) were detected in the SAPC, whereas 5'-UMP and 5'-AMP were produced by phosphodiesterase digestion. A mixture of UTP, UDP and ATP (1 microM each) elicited a calcium response which was comparable to that induced by SAPC, while UTP, UDP or ATP alone at 1 microM elicited a small increase in calcium concentration in mammary tumor cells. Suramin, a competitive antagonist of P2 purinoceptors, diminished the spreading of the calcium wave induced by mechanical stimulation. It also blocked the responses to SAPC, UTP, UDP and ATP. These findings suggest that the mechanical stimulation results in the release of UTP, UDP and ATP into the extracellular space which mediates induction of the spreading calcium response via P2U-type purinoceptors.

Activation of specific ATP receptors induces a rapid increase in intracellular calcium ions in rat hypothalamic neurons

We have used real-time dynamic video imaging of Fura-2 fluorescence to study the acute effects of external ATP on [Ca2+]i in cultured rat hypothalamic neurons. The addition of ATP at microM concentrations, but not adenosine, AMP, ADP or GTP, produced a rapid, dose-dependent increase in cytosolic Ca2+. The hydrolysis-resistant ATP analogues 3-thio-ATP and beta,gamma-imido-ATP produced a similar response but alpha,beta-methylene ATP had much lower efficacy. The ATP response was inhibited by 10 microM nifedipine, abolished by 50 microM cadmium and by the absence of extracellular Ca2+, but was unaffected by ryanodine or omega-conotoxin GVIA. The P2-purinoceptor antagonist suramin reversibly and selectively inhibited the ATP response but had no effect on other neurotransmitter-induced Cai2+ responses. Antagonists to muscarinic, nicotinic, NMDA, non-NMDA, GABA, 5-HT and adenosine receptors had no effect on the ATP response. Thus the Ca2+ response of hypothalamic neurons to ATP is mediated by specific suramin-sensitive ATP-receptors, activation of which is independent of ATP hydrolysis and results in an influx of extracellular Ca2+ largely through high voltage-gated Ca2+ channels. These findings support the assertion that ATP acts in the CNS as an excitatory neurotransmitter.

The effects of beta-phorbol-12,13 dibutyrate on agonist-induced increases in [Ca2+]i in N1E-115 cells. Differential modulation of responses to angiotensin II and bradykinin

Addition of angiotensin II (A2; 500 nM) to populations of fura-2-loaded N1E-115 cells resulted in a transient increase in intracellular calcium which was abolished by pre-treatment with the phorbol ester, beta-phorbol-12,13 dibutyrate (PDBu) (1.5 microM). The inhibitory effects were reversed by the protein kinase C inhibitor, staurosporine (150 nM), and down-regulation of protein kinase C was observed over 48 hr. Responses to maximally effective concentrations of histamine (300 microM), ATP (100 microM), UTP (100 microM) and carbachol (100 microM) were similarly inhibited by phorbol pre-treatment but the response to bradykinin (BK) (100 nM) was unaffected. When the concentrations of BK and A2 were adjusted to produce the same-sized calcium signals, PDBu pre-treatment abolished the response to A2 but only partially inhibited the response to BK. From the data presented here we can conclude that the calcium response to BK in N1E-115 cells is less susceptible to the inhibitory effects of protein kinase C activation than the response produced by A2.