Calcium Agonist Research Articles

Pharmacology polyene — The compound made on the basis of polynonsaturated fatty acids of omega-3 type

The influence of a calcium agonist (BAYK 8644) and several calcium channel blockers on nicotine-induced antinociception was investigated in mice. The effect of nicotine was sharply increased by BAYK 8644. This potentitation by BAYK 8644 was blocked by pretreating the animals with nifedipine at 2 and 10 mg/kg. The calcium channel antagonists, nifedipine and nimodipine at doses that had no effect on tail-flick time, reduced significantly the antinociception induced by nicotine (1.5 mg/kg s.c.). However, the effect of verapamil on nicotine was not significant. These results indicate that DHP calcium channels (type L-channel) play a role in some of the pharmacological effects of nicotine, particularly, locomotor activity and antinociception.

Role of L-type calcium channel modulation in nonconvulsive epilepsy in rats.

Old male Wistar rats spontaneously showing hundreds of spike-wave discharges daily were used to investigate the role of calcium ions in nonconvulsive epilepsy. The effects of the L-type calcium channel blocker nimodipine and the L-type channel opener BAY K 8644 on number and duration of these spike-wave discharges were investigated. In rats aged 84-94 weeks standard EEG electrodes were chronically implanted; animals were allowed to recover for 10 days. After a baseline registration, nimodipine 2.2, 8.8, and 35.2 mg/kg or BAY K 8644 in dosages of 0.12, 0.47, and 1.88 mg/kg was administered. A control group received the solvent. EEG recordings were made to evaluate drug effects. The highest dose of nimodipine increased the number of spike-wave discharges, whereas BAY K 8644 reduced the number of spike-wave discharges dose dependently. The highest dose of BAY K 8644 also induced fatal convulsions in 3 animals. Our results demonstrate that the L-type calcium antagonist nimodipine facilitates spike-wave discharges and that the L-type calcium agonist BAY K 8644 protects against these discharges, in contrast to previous results suggesting that calcium channel blockers act as antiepileptic drugs (AEDs) and that calcium channel openers act as convulsants. Our results are a further example of the different pharmacologic profile of convulsive and nonconvulsive epilepsy and are also in contrast to what has been described for T-type calcium channel modulation. We therefore propose that modulation of L-type and T-type calcium channels have opposite effects in nonconvulsive epilepsy.

Leukotriene B4, an arachidonic acid metabolite, regulates intracellular free calcium release in eggs and mitotic cells of the sand dollar (Echinaracnius parma).

Previous articleNext article No AccessCELL DIVISION, CELL MOTILITY, AND DEVELOPMENTLeukotriene B4, an Arachidonic Acid Metabolite, Regulates Intracellular Free Calcium Release in Eggs and Mitotic Cells of the Sand Dollar (Echinaracnius parma)R. B. Silver, J. B. Oblak, G. S. Jeun, J. J. Sung, and T. C. DuttaR. B. Silver Search for more articles by this author , J. B. Oblak Search for more articles by this author , G. S. Jeun Search for more articles by this author , J. J. Sung Search for more articles by this author , and T. C. Dutta Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Biological Bulletin Volume 187, Number 2October 1994 Published in association with the Marine Biological Laboratory Article DOIhttps://doi.org/10.1086/BBLv187n2p242 Views: 10Total views on this site Copyright © 1994 by Marine Biological LaboratoryPDF download Crossref reports the following articles citing this article:Robert A. Knudson, Shinya Inoué, and Makoto Goda Centrifuge Polarizing Microscope with Dual Specimen Chambers and Injection Ports, The Biological Bulletin 201, no.22 (Sep 2016): 234–234.https://doi.org/10.2307/1543339Robert B. Silver LtB4 Evokes the Calcium Signal That Initiates Nuclear Envelope Breakdown through a Multi-enzyme Network in Sand Dollar (Echinaracnius parma) Cells, The Biological Bulletin 201, no.22 (Sep 2016): 248–250.https://doi.org/10.2307/1543348R. B. Silver and N. M. Deming Leukotriene B4 as Calcium Agonist for Nuclear Envelope Breakdown: An Enzymological Survey of Endomembranes of Mitotic Cells, The Biological Bulletin 197, no.22 (Sep 2016): 268–270.https://doi.org/10.2307/1542644R. B. Silver, D. E. Strongin, L. R. Hurwitz, and A. P. Reeves Identification of Phospholipase A2 and Phospholipase C Activities in Calcium Regulatory Endomembranes of Sand Dollar Cells, The Biological Bulletin 193, no.22 (Sep 2016): 236–237.https://doi.org/10.1086/BBLv193n2p236 R. B. Silver Leukotriene B4 Induces Release of Calcium From Endomembrane Stores In Vivo in Eggs and Second Cell Cycle Blastomeres of the Sand Dollar Echinaracnius parma, The Biological Bulletin 189, no.22 (Sep 2016): 203–204.https://doi.org/10.1086/BBLv189n2p203 M. Krendel and S. Inoué Anaphase Spindle Dynamics Under D20-enhanced Microtubule Polymerization, The Biological Bulletin 189, no.22 (Sep 2016): 204–205.https://doi.org/10.1086/BBLv189n2p204

Role of L-Type Calcium Channels on Stimulated Calcium Influx and on Proliferative Activity of Human Coronary Smooth Muscle Cells

Summary: Dihydropyridine (DHP) calcium channel blockers are widely used in treatment of coronary artery disease. To evaluate the specific role of L-type calcium channels in the antianginal and possibly antiatherosclerotic properties of DHP inhibitors, we examined the effects of a 1,4-DHP agonist and antagonist on angiotensin II (ANG II)- and serum-stimulated calcium influx and proliferation of human coronary smooth muscle cells (cSMC). Fluorometry of fura-2 was used to measure changes in free cytosolic Ca2+ concentration ([Ca2+]i) in cSMC after short- and long-term pretreatment with the calcium agonist Bay K 8644 or the antagonist nitrendipine, respectively. Proliferative activity was quantified during exponential growth in serum-supplemented medium with or without both DHPs. Short- and long-term pretreatment with Bay K 8644 increased basal [Ca2+]i significantly in resting cells and augmented ANG II- and serum-induced sustained [Ca2+]i responses. Concordantly, proliferation rate was increased. In contrast, nitrendipine had no significant effect on basal or stimulated [Ca2+]i after short-term treatment, but decreased [Ca2+]i after 24-h incubation, attenuated the plateau phase of ANG II- and serum-evoked [Ca2+]i transients, and reduced proliferative activity of these cells. The results indicate that 1,4-DHPs modulate ANG II- and seruminduced Ca2+ influx in cSMC. Thus, L-type calcium channels may contribute to [Ca2+]i transients evoked by ANG II and serum. Moreover, the modulating effects of both DHPs on proliferative activity suggest involvement of DHP-sensitive calcium channels. Calcium influx through L-type channels may be one of the mechanisms that determine responsiveness to vasoconstrictors and proliferative activity of human cSMC.

Role of L-Type Calcium Channels on Stimulated Calcium Influx and on Proliferative Activity of Human Coronary Smooth Muscle Cells

Dihydropyridine (DHP) calcium channel blockers are widely used in treatment of coronary artery disease. To evaluate the specific role of L-type calcium channels in the antianginal and possibly antiatherosclerotic properties of DHP inhibitors, we examined the effects of a 1,4-DHP agonist and antagonist on angiotensin II (ANG II)- and serum-stimulated calcium influx and proliferation of human coronary smooth muscle cells (cSMC). Fluorometry of fura-2 was used to measure changes in free cytosolic Ca2+ concentration ([Ca2+]i) in cSMC after short- and long-term pretreatment with the calcium agonist Bay K 8644 or the antagonist nitrendipine, respectively. Proliferative activity was quantified during exponential growth in serum-supplemented medium with or without both DHPs. Short- and long-term pretreatment with Bay K 8644 increased basal [Ca2+]i significantly in resting cells and augmented ANG II- and serum-induced sustained [Ca2+]i responses. Concordantly, proliferation rate was increased. In contrast, nitrendipine had no significant effect on basal or stimulated [Ca2+]i after short-term treatment, but decreased [Ca2+]i after 24-h incubation, attenuated the plateau phase of ANG II- and serum-evoked [Ca2+]i transients, and reduced proliferative activity of these cells. The results indicate that 1,4-DHPs modulate ANG II- and serum-induced Ca2+ influx in cSMC. Thus, L-type calcium channels may contribute to [Ca2+]i transients evoked by ANG II and serum. Moreover, the modulating effects of both DHPs on proliferative activity suggest involvement of DHP-sensitive calcium channels. Calcium influx through L-type channels may be one of the mechanisms that determine responsiveness to vasoconstrictors and proliferative activity of human cSMC.

Analgesic and reinforcing effects of morphine in mice. Influence of Bay K-8644 and nimodipine

The aim of the present work was to clarify the role of calcium influx through L-type calcium channels in the rewarding and analgesic effects of morphine. Therefore the effects of Bay K-8644 and nimodipine, dihydropyridine calcium agonist and antagonist, respectively, on the analgesic and rewarding effects of morphine in mice were studied. Morphine-induced analgesia was measured with the aid of writhing test, hot plate test and tail clip test. The rewarding properties of morphine were studied using i.v. self-administration in drug-naive mice. Bay K-8644 potentiated morphine-induced analgesia in all the tests. The influence of nimodipine on morphine analgesia was more complicated and depended on those of morphine and test used. In self-administration experiments morphine exhibited the bell-shaped concentration-response curve. Bay K-8644 produced a shift of the curve to the left, while nimodiine had the oposite action indicating, respectively, facilitating and inhibitory influence on morphine rewarding effect. It is concluded that nimodipine exhibites partial antagonistic properties towards the rewarding action of morphine and slightly potentiates morphine-induced analgesia while Bay K-8644 increases either the rewarding or the analgesic effects of morphine.

Rhythmic contractions of isolated small arteries from rat: role of calcium.

In order to investigate the mechanisms behind rhythmic contractions in small arteries of the mesenteric arcade from Wistar rats, the calcium dependency of the oscillations in response to noradrenaline activation was tested on isolated vessels. Application of 1 microM ryanodine or 30 microM TMB-8 (procedures known to inhibit Ca2+ release from intracellular stores) totally abolished the rhythmic activity, even though the antagonists had opposite effects on the amplitude of the contractile response to noradrenaline. Verapamil (1 microM) or felodipine (1 nM) (agents known to inhibit influx of extracellular Ca2+) also abolished the oscillations and reduced the maximal noradrenaline response by about 40%. Reducing the extracellular Ca2+ concentration to 0.1 mM reduced the amplitude of the noradrenaline response to a similar extent as 1 nM felodipine, but did not eliminate the oscillations. This may indicate that the effect of calcium entry blockers was to eliminate the voltage-dependency of Ca2+ inflow rather than just reducing the Ca2+ level. Manoeuvres that would increase the cytosolic Ca2+ concentration (exposure to caffeine or to the calcium agonist BAY-K 8644) increased the frequency of the oscillations. These observations indicate an important role, not only for voltage-operating channels, but also for intracellular calcium stores in the generation of rhythmic contractions in these small arteries. Oscillations appear to be generated by an interplay between membrane activation and intracellular calcium stores.

Influences of the calcium antagonists nicardipine and nifedipine, and the calcium agonist BAY-K-8644, on the pharmacokinetics of propranolol in rats.

To determine the effect of dihydropyridines on the metabolism of propranolol, we studied the effects of a single oral dose of nicardipine, nifedipine, and BAY-K-8644 on the pharmacokinetics of propranolol in male Wistar rats fitted with a catheter in the jugularis vein. Oral propranolol (15 mg/kg and 1.5 mg/kg) and intravenous propranolol (1.5 mg/kg) were administered either alone or together with oral nicardipine (2.5 mg/kg). Oral propranolol (15 mg/kg) was administered with oral nifedipine (1.5 mg/kg) and with oral BAY-K-8644 (1.5 mg/kg). Nicardipine increased significantly the AUC and Cmax of oral propranolol (1.5 mg/kg and 15 mg/kg). However, the plasma concentration time curve of intravenous propranolol (1.5 mg/kg) was unaffected. Nifedipine also significantly increased the AUC and Cmax of oral propranolol (15 mg/kg), whereas with BAY-K-8644 there was only a slight increase in the bioavailability of oral propranolol (15 mg/kg). The results indicate that the dihydropyridine calcium antagonists decrease the metabolism of propranolol as a result of a decrease in first-pass clearance. Although an interaction at the level of cytochrome P450 may also be involved, the results of the present study suggest that the inhibitory effect can be largely attributed to changes in liver blood flow.

Multiple G-protein-dependent pathways mediate the antisecretory effects of somatostatin and clonidine in the HT29-19A colonic cell line.

Using the functionally differentiated colonic cell line, HT29-19A, we have examined sites at which inhibitory G-proteins mediate the antisecretory actions of somatostatin (SST) and the alpha 2-adrenergic agonist, clonidine (CLON) at the epithelial level. Both agents caused a dose-dependent inhibition (EC50:SST 35 nM; CLON 225 nM) of Cl- secretion (assessed by changes in short circuit current) activated by cAMP-mediated agonists, PGE2 and cholera toxin. Inhibition was accompanied by a reduction in intracellular cAMP accumulation and could be blocked by pretreatment with pertussis toxin at a concentration (200 ng/ml) which activated ADP-ribosylation of a 41-kD inhibitory G protein in HT29-19A membranes. Secretion stimulated by the permeant cAMP analogue, dibutyryl cAMP, was also inhibited by SST and CLON (30-50%; P < 0.005), indicating additional inhibitory sites located distal to cAMP production. Both agents were effective inhibitors of secretion mediated through the Ca2+ signaling pathway. SST (1 microM) and CLON (10 microM) reduced the Isc response to the muscarinic agonist, carbachol, by 60-70%; inhibition was reversed in pertussis toxin-treated cells. These effects did not, however, involve inhibition of the carbachol-induced increase in cellular inositol 1,4,5-trisphosphate levels or the rise in cytosolic calcium, [Ca]i. Inhibition by SST of secretion induced by phorbol 12,13 dibutyrate but not by the calcium agonist, thapsigargin, suggests that SST may act at a distal inhibitory site in the Ca(2+)-dependent secretory process activated by protein kinase C. We conclude that SST and alpha 2-adrenergic agonists can act directly on intestinal epithelial cells to exert a comprehensive inhibition of Cl- secretion mediated through both cAMP and Ca2+/protein kinase C signaling pathways. Inhibition is mediated via pertussis toxin-sensitive G-proteins at sites located both proximal and distal to the production of second messengers.

ChemInform Abstract: Synthesis and Chemical Characterization of the Calcium Agonist Bay‐K‐ 8644 and the By‐Products of the Hantzsch Synthesis.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Nicotinic receptor-mediated release of noradrenaline in the human neuroblastoma SH-SY5Y.

Dimethylphenylpiperazinium iodide (a nicotinic agonist) evokes noradrenaline release from human neuroblastoma SH-SY5Y cells that have been pretreated with 12-O-tetradecanoylphorbol 13-acetate for 8 min. This effect of dimethylphenylpiperazinium iodide was inhibited by 1 microM mecamylamine but not by 1 microM atropine, which suggests that SH-SY5Y cells express nicotinic receptors coupled to the release of noradrenaline. Dimethylphenylpiperazinium iodide-evoked release was enhanced by 5 microM Bay K 8644 (an L-type calcium agonist) and inhibited by 1 microM nifedipine. Dimethylphenylpiperazinium iodide depolarised SH-SY5Y cells and enhanced the level of intracellular calcium in cells loaded with fura 2. The effects of dimethylphenylpiperazinium iodide on noradrenaline release, depolarisation, and intracellular calcium levels were all inhibited by 1 microM desmethylimipramine. The results of this study show that nicotinic receptors in SH-SY5Y cells stimulate noradrenaline release by activation of L-type calcium channels.

Specific bindings of [3H](+)PN200-110 and [125I]omega-conotoxin to crude membranes from differentiated NG108-15 cells.

The characteristics of the specific bindings of [3H](+)PN200-110 (PN: L-type Ca channel antagonist) and [125I]omega-conotoxin G VI A (omega-CgTX: neuronal L- or N-type Ca channel antagonist) to crude membranes from undifferentiated neuroblastoma X glioma hybrid NG108-15 (NG108-15) cells and differentiated cells induced with dibutyryl cAMP (Bt2cAMP) were examined, because we have already observed that the magnitude and rate of KCl-stimulated 45Ca uptake by NG108-15 cells increased progressively during differentiation of the cells induced with Bt2-cAMP (unpublished results). The specific binding of [3H](+)PN to these crude membranes was saturable at various concentrations of 2.5-5.0 nM [3H](+)PN. Scatchard analysis showed that the specific binding of [3H](+)PN at equilibrium was significantly increased after differentiation of the NG108-15 cells with Bt2cAMP, but that the apparent Kd value for the specific binding of [3H](+)PN was not influenced by treatment with Bt2cAMP. The specific binding of [3H](+)PN to crude membranes from Bt2cAMP-treated NG108-15 cells was inhibited by a calcium agonist and antagonists, the order of their inhibitory potencies being (+)PN > nitrendipine > (-)PN > or = Bay K 8644 > > diltiazem = verapamil. Thus, PNs showed significant stereoselective inhibition of the specific binding of [3H](+)PN. On the other hand, [125I]omega-CgTX at concentrations of 0.075-0.6 nM showed scarcely any specific binding to these crude membranes, although at 0.6 nM it showed specific binding to crude membranes from rat brain in the same experimental conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of indapamide and hydrochlorothiazide on spontaneous contraction of cardiomyocytes in culture: The effect on alterations of extracellular calcium or potassium

1. 1. The purpose of this study was to determine whether indapamide or hydrochlorothiazide had a direct action on the heart, specifically to determine whether either diuretic affected spontaneous contractile function of cardiomyocytes in culture or altered the response to increases in extracellular potassium or calcium. 2. 2. Chick embryonic ventricular cells were cultured from 7-day-old chick embryo and myocardial cell aggregates were prepared. Spontaneous cardiac contractile frequency of ventricular myocyte aggregates was monitored. Indapamide [chloro-4- N-(methyl-2-indolinyl-1)-sulfamoyal-3-benzamide] over a concentration range of 10 −9−10 −6 M did not alter cardiac contractile frequency. 3. 3. Indapamide, in a dose dependent manner, significantly ( P < 0.05) antagonized the effect of increases in extracellular potassium [K +] 0 that produced a concentration dependent reduction in cardiac contractile frequency. In contrast, hydrochlorothiazide accentuated the effect of increased [K +] 0 while hydrochlorothiazide did not alter spontaneous contractile frequency at the usual [K +] 0 of 2.0 mM. 4. 4. Indapamide produced a significant ( P < 0.05) reduction in the effect of increases in extracellular calcium [Ca 2+] 0 on cardiac contractility while hydrochlorothiazide was associated with a slight accentuation of the effect of increased [Ca 2+] 0. Indapamide also slightly reduced the effect of the calcium agonist Bay K 8644, which increases intracellular calcium through voltage operated calcium channels. 5. 5. These data indicate that diuretics modulate a cardiac response to changes in extracellular potassium and calcium. Indapamide antagonizes the effects of increases in extracellular potassium and calcium while hydrochlorothiazide has the opposite effect on the cardiac response to increases in [K +] 0 or [Ca 2+] 0.

Angiotensin II responses after protein kinase C activation in vascular smooth muscle cells of spontaneously hypertensive rats.

To examine the interaction of protein kinase C (PKC) with agonist-induced calcium fluxes in hypertension, cytosolic free calcium ([Ca2+]i) was measured in vascular smooth muscle cells (vSMC) of normotensive and spontaneously hypertensive rats (SHR) after incubation with phorbol,-12 myristate,-13 acetate (PMA) and application of angiotensin II (AII). To distinguish between calcium influx through voltage-dependent calcium channels and calcium mobilization from intracellular stores, the calcium agonist BayK 8644 was used. Resting [Ca2+]i was 108.0 +/- 10.6 nM (mean +/- SEM, n = 25) in normotensive and 102.0 +/- 11.4 nM (n = 21) in hypertensive cells. After pretreatment with PMA 10(-7) M for 60 min, resting [Ca2+]i of normotensive vSMC increased to 145.0 +/- 13.8 nM (n = 17) while the resting level of the hypertensive cells decreased to 68.0 +/- 2.4 nM (n = 14, p < 0.05 as compared with normotensive cells) in hypertensive vSMC. Maximum increase in [Ca2+]i induced with 10 M AII for normotensive and hypertensive vSMC was similar: 230.5 +/- 34.4 nM (n = 14) and 212.5 +/- 26.7 nM (n = 17). After pretreatment with PMA 10(-7) M, the maximum increase in [Ca2+]i induced by AII in hypertensive cells was limited to 108.0 +/- 6.2 nM (p < 0.05 as compared with normotensive cells), whereas the increase in [Ca2+]i in normotensive vSMC remained the same as before: 211.5 +/- 23.4 nM. After administration of 10(-5) M BayK 8644, [Ca2+]i increased by 54.3 +/- 12.2 nM (n = 4) and 43.4 +/- 17.4 nM (n = 5) in normotensive and hypertensive vSMC, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium agonists and antagonists of the dihydropyridine type: effect on nicotine-induced antinociception and hypomotility

The influence of a calcium agonist (BAYK 8644) and several calcium channel blockers on nicotine-induced antinociception was investigated in mice. The effect of nicotine was sharply increased by BAYK 8644. This potentitation by BAYK 8644 was blocked by pretreating the animals with nifedipine at 2 and 10 mg/kg. The calcium channel antagonists, nifedipine and nimodipine at doses that had no effect on tail-flick time, reduced significantly the antinociception induced by nicotine (1.5 mg/kg s.c.). However, the effect of verapamil on nicotine was not significant. These results indicate that DHP calcium channels (type L-channel) play a role in some of the pharmacological effects of nicotine, particularly, locomotor activity and antinociception.

Modulation of nicotine-induced analgesia by calcium agonist and antagonist in adult rats.

The involvement of calcium in nicotine-induced analgesia in male rats was explored using the tail-flick test. A single dose of nicotine (1 mg/kg SC) produced a maximal effect on tail-flick latency (15 s) within 8-10 min, which lasted for 4 min. Pretreatment with the calcium chelator, EDTA (250 microM/kg SC four injections at 15 min intervals), before the single dose of nicotine accelerated the onset and prolonged the duration of the nicotine-induced analgesia. The maximal effect on tail-flick latency occurred within 2 min and lasted for 10-20 min. Conversely, pretreatment with calcium chloride (1.5 mM/kg IP) attenuated nicotine-induced analgesia. It is suggested that nicotine may exert its antinociceptive effects via modulation of calcium fluxes across the neural membrane.

The role of intracellular free calcium mobilization in the mechanism of action of antitumour ether lipids SRI 62-834 and ET18-OMe

Membrane-active antitumour ether lipids such as ET18-OMe (1- O-octadecyl-2- O-methyl- rac-glycero-3-phosphocholine) and SRI 62–834 ((±)-2-{Hydroxy[tetrahydro-2-(octadecyloxy) methylfuran-2-yl] methoxyl phosphinyloxy}- N, N, N-trimethylethaniminium hydroxide) are selectively cytotoxic to tumour cells in vitro. Their precise mechanisms of action are unclear, but they are known to have effects on cell membranes and cell signalling. A previous report suggested that ether lipids cause a biphasic sustained rise in intracellular free calcium [Lazenby et al., Cancer Res 50: 3327–3330, 1990]. We show here that the second phase is an experimental artefact due to cell membrane permeabilization by ether lipids in serum-free buffers. In serum-free medium, the membrane toxicity of antitumour ether lipids was increased 50–60 fold, when compared to medium containing 10% serum. Membrane disruption was neither dependent on extracellular calcium, nor modulated by preloading cells with the calcium chelators bis( o-aminophenoxy)ethane- N, N, N′, N′-tetraacetic acid or 2-[[2-[bis (carboxymethyl) amino ] — 5 -methylphenoxy] methyl] — 6 -methoxy — 8 -[bis(carboxymethyl) amino] quinoline. This indicates that the mechanism of membrane damage by ether lipids does not involve changes in calcium homeostasis. Using indo-1 and fura-2 as calcium probes, we established that lower concentrations of antitumour ether lipids do elicit a genuine monophasic and transient rise in intracellular free calcium, predominantly mobilized from internal stores. This acute calcium agonist activity of ether lipids is distinct from the inhibitory effects on cell signalling reported previously after more prolonged exposure. It appears that the calcium elevation induced by antitumour ether lipids is unlikely to be instrumental in their selective and potent antitumour activity.

Mechanism of cytoplasmic calcium changes in platelets in contact with polystyrene and poly(acrylamide-co-methacrylic acid) surfaces

Changes in cytoplasmic free calcium levels ([Ca2+]i) in platelets in contact with polystyrene (PSt) and poly(acrylamide-co-methacrylic acid) (PAAmMAc) particles were evaluated and results were compared with those from two representative biological calcium agonists; thrombin and calcium ionophore A23187. PSt particles stimulated a steep increase in cytoplasmic calcium levels in platelets as much as thrombin and A23187. Serratia protease-treated platelets showed a steep increase in [Ca2+]i by PSt particles, suggesting that PSt surfaces can initiate platelet activation independent of a glycoprotein Ib (GPIb)-mediated pathway. By contrast, dibucaine-treated platelets showed little increase in [Ca2+]i by PSt particles, indicating that microfilament assembly, including binding of GPIb with actin binding protein, should be required for platelet activation in contact with PSt surfaces. PAAmMAc particles induced little increase in cytoplasmic calcium levels in platelets. However, PAAmMAc particle-treated platelets demonstrated little response to thrombin in terms of an increase in [Ca2+]i and ATP release, suggesting the possibility that PAAmMAc surfaces may regulate [Ca2+]i by influencing platelet metabolism. Furthermore, sodium azide-treated platelets showed an increase in [Ca2+]i in platelets when contacting PAAmMAc particles, supporting the suggestion that PAAmMAc surfaces could regulate platelet functions. Fluorescence polarization measurements using 1,6-diphenyl-1,3,5-hexatriene-loaded platelets revealed that PAAmMAc particles increased membrane fluidity in platelets, which may be due to physicochemical interaction with PAAmMAc surfaces.

Canine left ventricular hypertrophy predisposes to ventricular tachycardia induction by phase 2 early afterdepolarizations after administration of BAY K 8644

Objectives. The purpose of this study was to test the hypothesis that the longer duration of ventricular action potentials in hypertrophied hearts predisposes to the development of early after-depolarizations and triggered ventricular tachyarrhythmias.Background. For unknown reasons, the incidence of sudden death is greater in patients with myocardial hypertrophy.Methods. We measured left ventricular monophasic action potentials in normal dogs and dogs with left ventricular hypertrophy before and after administration of the calcium agonist BAY K 8644 and the potassium channel blocker cesium.Results. We demonstraieo longer action potential durations in dogs with than in those without left ventricular hypertrophy. Also, BAY K 8644 produced phase 2 early afterdepolarizations and ventricular tachyarrhythmias more frequently in the dogs with than in those without left ventricular hypertrophy. Phenylephrine, an alpha agonist, further increased the action potential duration in hypertrophied hearts and the propensity to develop early afterdepolarizations and ventricular tachyarrhythmia after administration of BAY K 8644. Control and hypertrophied hearts developed early afterdepolarizations and ventricular tachyarrhythmia equally when exposed to cesium.Conclusions. Although in vitro studies have shown that fibers of hypertrophied ventricular myocardium can develop triggered activity as a result of both early and late afterdepolarizations, the present study is the first to show in vivo that the hypertrophied ventricular myocardium compared with the normal ventricle is predisposed to develop phase 2 early afterdepolarizations that appear to trigger ventricular tachyarrhythmia. It is possible that such a mechanism contributes to the development of ventricular tachyarrhythmia and sudden cardiac death in patients with left ventricular hypertrophy. If this is shown to be true, specific pharmacologic interventions can be suggested.

Maitotoxin-induced intracellular calcium rise in PC12 cells: involvement of dihydropyridine-sensitive and omega-conotoxin-sensitive calcium channels and phosphoinositide breakdown.

The biological activities of maitotoxin are strictly dependent on the extracellular calcium concentration and are always associated with an increase of the free cytosolic calcium level. We tested the effects of voltage-sensitive calcium channel blockers (nicardipine and omega-conotoxin) on maitotoxin-induced intracellular calcium increase, membrane depolarization, and inositol phosphate production in PC12 cells. Maitotoxin dose dependently increased the cytosolic calcium level, as measured by the fluorescent probe fura 2. This effect disappeared in a calcium-free medium; it was still observed in the absence of extracellular sodium and was enhanced by the dihydropyridine calcium agonist Bay K 8644. Nicardipine inhibited the effect of maitotoxin on intracellular calcium concentration in a dose-dependent manner. The maitotoxin-induced calcium rise was also reduced by pretreating cells with omega-conotoxin. Pretreatment of cells with maitotoxin did not modify 125I-omega-conotoxin and [3H]PN 200-110 binding to PC12 membranes. Nicardipine and omega-conotoxin inhibition of maitotoxin-evoked calcium increase was reduced by pertussis toxin pretreatment. Maitotoxin caused a substantial membrane depolarization of PC12 cells as assessed by the fluorescent dye bisoxonol. This effect was reduced by pretreating the cells with either nicardipine or omega-conotoxin and was almost completely abolished by the simultaneous pretreatment with both calcium antagonists. Maitotoxin stimulated inositol phosphate production in a dose-dependent manner. This effect was reduced by pretreating the cells with 1 microM nicardipine and was completely abolished in a calcium-free EGTA-containing medium. The findings on maitotoxin-induced cytosolic calcium rise and membrane depolarization suggest that maitotoxin exerts its action primarily through the activation of voltage-sensitive calcium channels, the increase of inositol phosphate production likely being an effect dependent on calcium influx. The ability of nicardipine and omega-conotoxin to inhibit the effect of maitotoxin on both calcium homeostasis and membrane potential suggests that L- and N-type calcium channel activation is responsible for the influx of calcium following exposure to maitotoxin, and not that a depolarization of unknown nature causes the opening of calcium channels.