Modulation of calcium channel function by drugs

Urinary Paraben Concentrations and Associations with the Periconceptional Urinary Metabolome: Untargeted and Targeted Metabolomics Analyses of Participants from the Early Pregnancy Study

BACKGROUND

Parabens, found in everyday items from personal care products to foods, are chemicals with endocrine-disrupting activity, which has been shown to influence reproductive function.

OBJECTIVES

This study investigated whether urinary concentrations of methylparaben, propylparaben, or butylparaben were associated with the urinary metabolome during the periconceptional period, a critical window for female reproductive function. Changes to the periconceptional urinary metabolome could provide insights into the mechanisms by which parabens could impact fertility.

METHODS

Urinary paraben concentrations were measured in paired pre- and postconception urine samples from 42 participants in the Early Pregnancy Study, a prospective cohort of 221 women attempting to conceive. We performed untargeted and targeted metabolomics analyses using ultrahigh-performance liquid chromatography quadrupole time-of-flight mass spectrometry. We used principal component analysis, orthogonal partial least-squares discriminant analysis, and permutation testing, coupled with univariate statistical analyses, to find metabolites associated with paraben concentration at the two time points. Potential confounders were identified with a directed acyclic graph and used to adjust results with multivariable linear regression. Metabolites were identified using fragmentation data.

RESULTS

Seven metabolites were associated with paraben concentration (variable importance to projection score > 1 , false discovery rate-corrected q -value < 0.1 ). We identified four diet-related metabolites to the Metabolomics Standards Initiative (MSI) certainty of identification level 2, including metabolites from smoke flavoring, grapes, and olive oil. One metabolite was identified to the class level only (MSI level 3). Two metabolites were unidentified (MSI level 4). After adjustment, three metabolites remained associated with methylparaben and propylparaben, two of which were diet-related. No metabolomic markers of endocrine disruption were associated with paraben concentrations.

DISCUSSION

This study identified novel relationships between urinary paraben concentrations and diet-related metabolites but not with metabolites on endocrine-disrupting pathways, as hypothesized. It demonstrates the feasibility of integrating untargeted metabolomics data with environmental exposure information and epidemiological adjustment for confounders. The findings underscore a potentially important connection between diet and paraben exposure, with applications to nutritional epidemiology and dietary exposure assessment. https://doi.org/10.1289/EHP12125.

Synthesis and X-ray structure conformation of novel unsymmetrical 1,4-dihydropyridine

Methyl 2,6-dimethyl-5-(2-methylphenylcarbamoyl)-4-(3-nitrophenyl)-1,4-dihydropyridine-2-carboxylate (3) was prepared by known Hantzch method and its structure was elucidated by X-ray diffraction method.

Ca²⁺ channel activators reveal differential L-type Ca²⁺ channel pharmacology between native and stem cell-derived cardiomyocytes

Human stem cell-derived cardiomyocytes provide new models for studying the ion channel pharmacology of human cardiac cells for both drug discovery and safety pharmacology purposes. However, detailed pharmacological characterization of ion channels in stem cell-derived cardiomyocytes is lacking. Therefore, we used patch-clamp electrophysiology to perform a pharmacological survey of the L-type Ca²⁺ channel in induced pluripotent and embryonic stem cell-derived cardiomyocytes and compared the results with native guinea pig ventricular cells. Six structurally distinct antagonists [nifedipine, verapamil, diltiazem, lidoflazine, bepridil, and 2-[(cis-2-phenylcyclopentyl)imino]-azacyclotridecane hydrochloride (MDL 12330)] and two structurally distinct activators [methyl 2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)phenyl]-1,4-dihydropyridine-3-carboxylate (Bay K8644) and 2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methyl ester (FPL 64176)] were used. The IC₅₀ values for the six antagonists showed little variability between the three cell types. However, whereas Bay K8644 produced robust increases in Ca²⁺ channel current in guinea pig myocytes, it failed to enhance current in the two stem cell lines. Furthermore, Ca²⁺ channel current kinetics after addition of Bay K8644 differed in the stem cell-derived cardiomyocytes compared with native cells. FPL 64176 produced consistently large increases in Ca²⁺ channel current in guinea pig myocytes but had a variable effect on current amplitude in the stem cell-derived myocytes. The effects of FPL 64176 on current kinetics were similar in all three cell types. We conclude that, in the stem cell-derived myocytes tested, L-type Ca²⁺ channel antagonist pharmacology is preserved, but the pharmacology of activators is altered. The results highlight the need for extensive pharmacological characterization of ion channels in stem cell-derived cardiomyocytes because these complex proteins contain multiple sites of drug action.

Calcium Antagonism by Cobalt Ions on Contraction of Guinea-pig Taenia Coli

In guinea-pig taenia coli, cobalt ions (Co2+) inhibited the tonic response to a highly concentrated K+ solution (high-K+, 40 Mm) more strongly than the phasic response. Co2+ displaced Ca2+ concentration-response curves to the right, inhibited the increase in tissue calcium content caused by high-K +, and inhibited Ca2+ binding at low affinity sites more than at high affinity sites. After treatment with Co2+, the tonic tension caused by high-K+ was not restored by a wash with normal medium, but it was restored by a wash with EDTA. The cobalt remaining in muscles was almost completely eliminated after a 20–30 min wash with EDTA. The results suggest that Co2+ binds chiefly to the surface membrane of taenia coli. Co2+ probably reduced the tension in response to high-K+ mainly by inhibiting Ca2+ influx rather than by affecting Ca2+ release.

Influence of centrally administered diltiazem on behavioural responses, clinical symptoms, reticulo-ruminal contractions and plasma catecholamine level after experimentally induced duodenal distension in sheep

A different role of L-type antagonists for voltage-gated calcium channels (VGCC) has been previously identified in different types of experimental and clinical pain in man and animals. Present study examined the role of VGCC blocker - diltiazem administered icv (0.25, 0.5, 1.0 and/or 2.0mg in toto) on the development of pain related symptoms, clinical signs, plasma catecholamine level and the inhibition of reticulo-rumen motility caused by 5min lasting mechanical duodenum distension (DD) in the sheep. Experimental DD was conducted by insertion (during surgery) of rubber balloon into the duodenum and the distension by 40ml of warm water. Duodenal distension resulted in a significant increase of behavioural pain responses, tachycardia, hyperventilation, inhibition of reticulo-rumen contractions rate (from 85% to 45% during 15-20min), an increase of plasma catecholamine concentration (over sevenfold increase of epinephrine during 2h following DD, two-times norepinephrine and 84% increase of dopamine). Diltiazem infusion given 10min before DD decreased intensity of visceral nocifensive responses such as: behavioural changes, tachycardia, hyperventilation, reticulo-rumen motility and efficiently prevented appearance of catecholamine release. These data demonstrated that the development and persistence of acute duodenal pain depends on the activation of Ca(2+) ion flux leading to neurotransmitters release and modulation of membrane excitability. It seems that diltiazem given icv 10min prior to DD (as a source of acute visceral pain), inhibited specific receptors α(1) subunits of VGCCs in target tissues, prevent depolarization of cell membranes and release of neurotransmitters responsible for pain sensitivity in sheep. The observed antinociceptive action of VGCCs type-L blockers suggests that these channels play a crucial role in the modulation of acute visceral pain in sheep.

Transport of gatifloxacin involves Na+/Ca2+ exchange and excludes P-glycoprotein and multidrug resistance associated-proteins in primary cultured rat brain endothelial cells

The characteristics of gatifloxacin transport across blood brain barrier were investigated using primary cultured rat brain microvessel endothelial cells (rBMECs) as an in vitro model. Gatifloxacin uptake by rBMECs was time-, temperature- and energy-dependent. Gatifloxacin uptake by rBMECs was not influenced by P-glycoprotein (P-GP) inhibitor cyclosporine A or multidrug resistance associated-proteins (MRPs) inhibitor probenecid. However, verapamil inhibited the uptake in a concentration-dependent manner. Transendothelial transport study showed that transport of gatifloxacin across rBMEC monolayer was bidirectional, verapamil concentration-dependently inhibited transport from the apical to basolateral side, but did not significantly affect transport from basolateral to apical side. Gatifloxacin uptake was decreased in Ca(2+)-deprived medium but increased in Mg(2+)-deprived medium significantly. Furthermore, organic Ca(2+) channel blockers nifedipine and diltiazem had no effect on gatifloxacin uptake, but inorganic Ca(2+) channel blockers Ni(2+) and Mg(2+) inhibited the gatifloxacin uptake. The present study suggests that gatifloxacin transport across rBMECs involves a Na(+)/Ca(2+) exchange mechanism and extracellular Ca(2+) but not P-GP and MRPs.

High-Throughput Screening for N-Type Calcium Channel Blockers Using a Scintillation Proximity Assay

N-type calcium channels located on presynaptic nerve terminals regulate neurotransmitter release, including that from the spinal terminations of primary afferent nociceptors. Accordingly, N-type calcium channel blockers may have clinical utility as analgesic drugs. A selective N-type calcium channel inhibitor, ziconotide (Prialt), is a neuroactive peptide recently marketed as a novel nonopioid treatment for severe chronic pain. To develop a small-molecule N-type calcium channel blocker, the authors developed a 96-well plate high-throughput screening scintillation proximity assay (SPA) for N-type calcium channel blockers using [125I]-labeled ω-conotoxin GVIA as a channel-specific ligand. Assay reagents were handled using Caliper’s Allegro automation system, and bound ligands were detected using a PerkinElmer TopCount. Using this assay, more than 150,000 compounds were screened at 10 μM and approximately 340 compounds were identified as hits, exhibiting at least 40% inhibition of [125I]GVIA binding. This is the 1st demonstration of the use of [125I]-labeled peptides with SPA beads to provide a binding assay for the evaluation of ligand binding to calcium channels. This assay could be a useful tool for drug discovery.

Calcium antagonists improve cardiac mechanical performance after thermal trauma

Burn trauma initiates a pathophysiologic cascade, which includes cardiac dysfunction and intramyocyte calcium accumulation. This study examined the hypothesis that therapeutic interventions which limit intracellular cardiac Ca(2+) accumulation after burn trauma will improve cardiac function. Guinea pigs were anesthetized (methoxyflurane), burned over 43% of total body surface area, and fluid resuscitated (FR) for 24 h. Burn guinea pigs were randomly divided into three groups: Group 1, FR alone, Group 2, FR plus dantrolene (10 mg/kg body wt, IV, 30 min, 8 and 22 h postburn), a drug which inhibits the Ca(2+) release channel (ryanodine receptor) of the cardiac sarcoplasmic reticulum, and Group 3, FR plus diltiazem (0.20-0.22 mg/kg given IV as a slow infusion over 6 h postburn), a drug which specifically blocks Ca(2+) slow channels; sham burn guinea pigs were given vehicle (Group 4), dantrolene (Group 5), or diltiazem (Group 6) as described above (respective controls). Cardiac dysfunction was impaired in fluid-treated burns (Group 1) compared to sham burns (Group 4) as indicated by reduced developed left ventricular pressure (LVP) (86 +/- 2 vs 52 +/- 3 mm Hg, P < 0.05), rate of LVP rise, (+dP/dt max, 1379 +/- 64 vs 909 +/- 44 mm Hg/s, P < 0.05), and LVP fall (-dP/dt max, 1184 +/- 31 vs 881 +/- 40 mm Hg/s, P < 0.05), and time to peak pressure (110 +/- 2 vs 102 +/- 2 ms, P < 0.05). In addition, [Ca(2+)](i) rose in cardiomyocytes harvested from fluid-treated burns (Group 1, 307 +/- 29 nM) compared to vehicle-treated controls (Group 4, 152 +/- 6 nM, P < 0.05). Neither calcium antagonist altered ventricular function or [Ca(2+)](i) in sham burns (Groups 5 and 6). In contrast, antagonists given after burn injury reduced cardiomyocyte [Ca(2+)](i) (Group 2, dantrolene-treated burns: 196 +/- 8 nM, and Group 3, diltiazem treated burns: 216 +/- 8 nM) and improved cardiac performance compared to that measured in burns given FR alone. Our data suggest that calcium antagonists given after burn trauma restored intracellular Ca(2+) homeostasis, decreased cardiac cell injury, and improved cardiac contractile function.

Effects of low temperature on the chronotropic and inotropic responses to zatebradine, E-4031 and verapamil in isolated perfused dog atria

We investigated the effects of hypothermia (25 degrees C) on the chronotropic and inotropic effects of zatebradine (a blocker of hyperpolarization-activated inward current, I(f)), E-4031 (a blocker of the rapid type of the delayed rectifier K+ current, I(Kr)) and verapamil, and on the positive cardiac responses to isoproterenol after treatment with zatebradine and E-4031 in isolated, blood-perfused dog atria. Hypothermia shifted the dose-response curves to the right for the negative chronotropic and inotropic effects of verapamil and for the negative chronotropic and positive inotropic effects of zatebradine, but not for the negative chronotropic and positive inotropic effects of E-4031. Hypothermia attenuated the positive chronotropic response to isoproterenol or Bay k 8644 (an L type Ca2+ channel agonist) and was attenuated more than the inotropic one. Zatebradine selectively inhibited the positive chronotropic response to isoproterenol at a normal temperature, but in hypothermia, it inhibited neither the chronotropic nor inotropic responses. E-4031 did not affect the positive responses to isoproterenol. These results suggest that verapamil and zatebradine but not E-4031 influence the atrial rate and contractile force much less in hypothermia than in normothermia and that the I(f) and inward Ca2+ current are sensitive to hypothermia in the heart.

Effects of aluminum ions on K(+)-induced contraction in ileal longitudinal smooth muscle

The mechanism of the inhibition of K(+)-induced contraction caused by aluminum ions (Al3+) was analyzed in ileal longitudinal muscle and taenia coli of guinea-pig. Al3+ (2-5 mM) inhibited dose-dependently the high-K+ (60 mM, hypertonic)-induced contraction in ileal muscle. Al3+ reduced the size of the maximal response to Ca2+ without shifting the concentration-response curves to Ca2+ in taenia coli. Al3+ caused a significant decrease in Ca uptake, measured by the lanthanum method, during the K(+)-induced ileal first phase of the response. Following treatment with 5 mM Al3+ for 0.5-20 min, the ileal K(+)-induced contraction returned to a greater extent, after washing with medium containing the chelator deferoxamine, compared to normal medium. However, with increasing duration, > 30 min of Al3+ treatment, the K(+)-induced contraction did not recover, despite washing with deferoxamine. The aluminum in the cells was even more eliminated by washing with deferoxamine when the duration of Al3+ treatment was < 20 min. The present findings indicate that the inhibitory action by Al3+ on K(+)-induced contraction at the shorter treatment (< 20 min) may result from the interference of calcium permeability at the cell membrane in ileal muscle. However, with increasing duration of Al3+ treatment, Al3+ ions accumulate in intracellular compartments where deferoxamine cannot reach, and it may exert an inhibitory action on internal sites in ileal cells.

Identification of L-type calcium channels associated with kappa opioid receptors in human placenta

Transduction pathways of kappa receptor activation are not fully understood. Human placenta at term expresses only this type of opioid receptors and therefore offers a unique advantage for such investigations. It has previously been postulated that kappa receptors-mediated modulation of acetylcholine and placental lactogen release from human placentas require the influx of extracellular calcium and into the cells, possibly via voltage-dependent channels. We report here that another opioid-regulated placental function, the release of human chorionic gonadotropin (hCG), depends on extracellular calcium and the modality of its influx via L-type channels. Data presented demonstrated that the stimulation of hCG secretion by the kappa-selective agonist U69,593 was abolished in presence of either EGTA or the calcium channel blocker nifedipine. Results obtained on the combined effect of opioids and dihydropyridines indicated that placental kappa opioid receptors could be directly coupled to L-type calcium channels. The identification of the latter in villus membrane preparations, reported here for the first time, further contributes to the hypothesis that, in human placenta, kappa receptors-linked transduction mechanisms involve calcium and its conductance across villus membranes.

Cellular accumulation of the new ketolide RU 64004 by human neutrophils: comparison with that of azithromycin and roxithromycin

We analyzed the uptake of RU 64004 by human neutrophils (polymorphonuclear leukocytes [PMNs]) relative to those of azithromycin and roxithromycin. RU 64004 was strongly and rapidly accumulated by PMNs, with a cellular concentration/extracellular concentration ratio (C/E) of greater than 200 in the first 5 min, and this was followed by a plateau at 120 to 180 min, with a C/E of 461 +/- 14.8 (10 experiments) at 180 min. RU 64004 uptake was moderately sensitive to external pH, and activation energy was also moderate (63 +/- 3.8 kJ/mol). RU 64004 was mainly located in PMN granules (about 70%) and egressed slowly from loaded cells, owing to avid reuptake. The possibility that PMN uptake of RU 64004 and other macrolides occurs through a carrier-mediated system was suggested by three key results. First, there existed a strong interindividual variability in uptake kinetics, suggesting variability in the numbers or activity of a transport protein. Second, macrolide uptake displayed saturation kinetics characteristic of that of a carrier-mediated transport system: RU 64004 had the highest Vmax value (3,846 ng/2.5 x 10(6) PMNs/5 min) and the lowest Km value (about 28 microM), indicating a high affinity for the transporter. Third, as observed previously with other erythromycin A derivatives, Ni2+ (a blocker of the Na+/Ca2+ exchanger which mediates Ca2+ influx in resting neutrophils) impaired RU 64004 uptake by PMNs, with a 50% inhibitory concentration of about 3.5 mM. In addition, we found that an active process is also involved in macrolide efflux, because verapamil significantly potentiated the release of all three macrolides tested. This effect of verapamil does not seem to be related to an inhibition of Ca2+ influx, because neither EGTA [ethylene glycol-bis (beta-aminoethyl ether)-N,N',N'-tetraacetic acid] nor Ni2+ modified macrolide efflux. The nature and characteristics of the entry- and efflux-mediating carrier systems are under investigation.

Different sympathetic-parasympathetic interactions on sinus rate and atrioventricular conduction in dog hearts

We investigated the sympathetic-parasympathetic interactions involved in SA nodal pacemaker activity and AV conductivity in the anesthetized dog heart. Stimulation of the intracardiac parasympathetic nerves to the SA nodal region (SAPS) and stimulation of the intracardiac parasympathetic nerves to the AV nodal region (AVPS) induced negative chronotropic and dromotropic responses, respectively. Cardiac sympathetic stimulation, aminophylline, 3-isobutyl-1-methylxanthine (IBMX, a relatively pure nonselective phosphodiesterase inhibitor) and methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-p iridine-5-carboxylate (Bay k 8644, a Ca2+ channel agonist) increased sinus rate and decreased AV conduction time. Sympathetic stimulation augmented the negative chronotropic response to SAPS but not the negative dromotropic response to AVPS, IBMX augmented both responses, Bay k 8644 augmented the chronotropic response and attenuated the dromotropic response, and aminophylline did not affect the chronotropic response to SAPS and inhibited the dromotropic response to AVPS. Additionally, when Bay k 8644 directly given via the AV node artery decreased AV conduction time, it attenuated the negative dromotropic response to AVPS and carbachol injected into the AV node artery. These results suggest that the differential sympathetic-parasympathetic interactions on sinus rate and AV conduction are at least partly induced by an interaction between changes in slow inward Ca2+ current or intracellular Ca2+ and the cardiac effects of acetylcholine in the heart in situ.

Beta-adrenergic-induced calcium efflux in rat parotid gland. Is mitochondrial Na+/Ca2+ exchange involved?

The effects of diltiazem were examined on 45Ca efflux in rat parotid glands. First, we showed that mitochondrial Na+/Ca2+ exchange occurs in rat parotid glands and that diltiazem inhibited the mitochondrial Na(+)-dependent calcium efflux. We also confirmed that in rat parotid gland, diltiazem did not modify calcium movements at plasma membrane level. Secondly, we tested the effects of diltiazem on pieces of parotid glands. Diltiazem alone induced 45Ca efflux from parotid lobules. When the effect of diltiazem was tested on isoproterenol-induced 45Ca efflux, the effects of the two drugs were less than additive. By comparison, diltiazem did not modify carbachol induced 45Ca efflux. Diltiazem was able to induce calcium efflux from an intracellular calcium pool, which is not the IP3 sensitive one. These data support the previous hypothesis that isoproterenol and carbachol do not mobilize the same calcium pool. Although we did not precisely determine the calcium pool sensitive to beta-adrenergic stimulation, we cannot rule out the hypothesis that mitochondria would be that store.

Volume regulation and the efflux of amino acids from cells in incubated slices of rat cerebral cortex. II. Dependence on Ca2+ ions

The efflux of gamma-aminoisobutyric acid (GABA) and L-glutamate from pre-loaded cells in rat cerebral cortical slices has been studied during interventions designed to affect the availability of intracellular Ca2+ during hyposmotic swelling and membrane depolarization due to raised extracellular K+. Calmodulin-dependent acceleration of amino acid efflux in hyposmotic media, with cell swelling less than would be predicted on the basis of perfect osmometric behaviour (see Ref. [1]), was unaffected by Ca-ionophore in the presence of external Ca2+ or by the omission of external Ca2+, but was suppressed by pre-exposure of slices to thapsigargin (2 microM), which is reported to deplete cytosolic Ca2+, and by TMB-8 (0.5 mM), which blocks release of Ca2+ from internal stores. TMB-8 also led to significant cell swelling. The effects of TMB-8 were reversed by Ca-ionophore. Raised external K+ (54 mM) led to accelerated amino acid efflux which required calmodulin activation and was blocked by (i) omission of external Ca2+, (ii) the voltage-sensitive Ca2+ channel blocker nifedipine (1 microM), (iii) the anion transport inhibitor DIDS (25 microM for GABA, 100 microM for L-glutamate, see Ref. [1]), and (iv) the -SH group acetylator N-ethylmaleimide. TMB-8 was without effect in high K+ media. These results suggest that while enhanced amino acids efflux probably occurs through the same population of Ca/calmodulin-dependent, DIDS-sensitive pathways following hyposmotic shock or membrane depolarization, the source of Ca2+ ions required for the activation of these pathways may depend upon which of these acceleratory stimuli is applied.

Role of calcium in photodynamically induced cell damage of human fibroblasts

Photodynamically induced changes in the cytoplasmic free calcium concentration ([Ca2+]i) and its role in cell damage were investigated in human skin fibroblasts using confocal laser microscopy. Fluorescence and absorbance spectrophotometry measurements indicate that the photosensitizer aluminum phthalocyanine tetrasulfonate (AIPcS4) binds to the plasma membrane and only after irradiation is able to enter the cells, causing massive morphologic alterations. Upon irradiation of sensitizer-treated cells, the increase in [Ca2+]i is related to the amount of light and extracellular [Ca2+]e. The increase in [Ca2+]i was substantially reduced in the absence of [Ca2+]a. Cell damage or death after photodynamic treatment was prevented and shifted toward higher fluence by increasing [Ca2+]i at high [Ca2+]e and was greater at low [Ca2+]e. Application of Ca2+ channel blockers, such as Co2+, Cd2+ or verapamil, could not prevent the increase of [Ca2+]i. Our results indicate that activation of the photosensitizer, AIPcS4, causes an influx of Ca2+, which protects cells from, photodamage. At low [Ca2+]e and high fluence values, release of Ca2+ from internal stores probably as a protective measure occurs in order to increase the [Ca2+]i.

Synthesis and calcium antagonistic activity of 8-[N-[2-(3,4-dimethoxy- phenyl)ethyl]-beta-alanyl]-5,6,7,8-tetrahydrothieno[3,2-b][1,4]thiazepi ne fumarate

KT-362 is an antiarrhythmic and antihypertensive agent with vasodilating activity. Since it carries a homoveratryl group in the side chain, an obvious relation exists to the verapamil-type calcium antagonists. Replacement of the fused aromatic moiety in KT-362 with thiophene provided 8-[N-[2-(3,4-dimethoxyphenyl) ethyl]-beta-alanyl]-5,6,7,8-tetrahydrothieno[3,2-b][1,4] thiazepine (1). Compound 1 shows a negative chronotropic activity in spontaneously beating right atria (IC50 = 23 microM, n = 7), and a negative inotropic effect in papillary muscles (IC50 = 2.7 microM, n = 7) and left atria (IC50 = 4 microM, n = 6) of the guinea-pig heart. The decrease of contractility in papillary muscles could be antagonized by increasing the extracellular calcium concentration. Compound 1 was found to affect high (IC50: 70 +/- 5 microM) and low (IC50: 129 +/- 34 microM) voltage-activated calcium channel currents as well as voltage-activated sodium channel currents (IC50: 80 +/- 13 microM) in chick dorsal root ganglion neurons. In addition nicotine-induced currents were potently inhibited (IC50: 6 +/- 0.7 microM) in bovine chromaffin cells.

Stimulation of ecdysteroidogenesis by small prothoracicotropic hormone: role of calcium

Insect prothoracic glands are regulated by neuropeptide prothoracicotropic hormones (PTTH). In Manduca sexta PTTH exists as two size variants, big PTTH (approximately 25.5 kDa) and small PTTH (approximately 7 kDa). Previous studies indicate that both size variants employ cAMP as a second messenger and that stimulation of ecdysteroid secretion by big PTTH is Ca(2+)-dependent. In the present study, experiments were performed to assess the role of Ca2+ in small PTTH-stimulated ecdysteroid secretion by prothoracic glands from fifth instar larvae. Basal ecdysteroid secretion was not affected by Ca2+ channel blockers (verapamil or lanthanum) or by omission of Ca2+ from the incubation medium. Treatment of glands with a Ca2+ ionophore (A23187 or ionomycin) produced a concentration-dependent stimulation of ecdysteroid secretion. Stimulation of ecdysteroid secretion by small PTTH was suppressed (1) by Ca2+ channel blockers and (2) in Ca(2+)-free medium. A cAMP analog (Sp-cAMPS) stimulated ecdysteroid secretion in the presence of a Ca2+ channel blocker (verapamil) and in Ca(2+)-free incubation medium, and ionophore-induced ecdysteroid secretion appeared to be suppressed by a cAMP antagonist (Rp-cAMPS). The combined results indicate that basal ecdysteroid secretion is not dependent on external Ca2+, and suggest that small PTTH-stimulated ecdysteroid secretion is mediated by an influx of Ca2+ that precedes cAMP formation.

Role of extracellular calcium in in vitro uptake and intraphagocytic location of macrolides

We compared the uptakes and intracellular locations of four 14-membered-ring macrolides (roxithromycin, dirithromycin, erythromycin, and erythromycylamine) in human polymorphonuclear neutrophils (PMNs) in vitro. Intracellular location was assessed by cell fractionation and uptake kinetics in cytoplasts (granule-poor PMNs). Trapping of dirithromycin within PMN granules (up to 80% at 30 min) was significantly more marked than the intracellular trapping of the other drugs (erythromycylamine, 45% +/- 5.1%; erythromycin, 42% +/- 3.7%; roxithromycin, 35% +/- 3.0%). A new finding was that, in the absence of extracellular calcium, the uptakes of all of the macrolides by PMNs and cytoplasts were significantly impaired, by about 50% (PMN) and 90% (cytoplasts). Furthermore, inorganic Ca2+ channel blockers inhibited macrolide uptake in a concentration-dependent manner, with 50% inhibitory concentrations of 1.6 to 2.0 mM and 29 to 35 microM, respectively, for Ni2+ and La3+. The intracellular distributions of the drugs were unchanged in the presence of Ni2+ and La3+ and in Ca(2+)-free medium supplemented with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. The organic Ca2+ channel blocker nifedipine had no effect on macrolide uptake, whereas verapamil inhibited it in a time- and concentration-dependent manner. These data show the importance of extracellular Ca2+ in macrolide uptake by phagocytes and suggest a link with Ca2+ channels or a Ca2+ channel-operated mechanism.

Zatebradine attenuates cyclic AMP-related positive chronotropic but not inotropic responses in isolated, perfused right atria of the dog

1. Inhibition of I(f) or ICa by zatebradine has been reported in mammalian SA nodal cells. We thus investigated whether zatebradine differentially attenuates the positive chronotropic and inotropic responses to norepinephrine, isoproterenol, NKH 477 (an adenylyl cyclase activator), 3-isobutyl-1-methylxanthine (IBMX) and Bay k 8644 (a calcium channel agonist) in the isolated, blood-perfused dog atrium. 2. When zatebradine (0.03-1 mumol) decreased sinus rate from 104 +/- 4.5 to 73 +/- 4.9 beats/min dose-dependently, it selectively attenuated the positive chronotropic but not inotropic responses to norepinephrine in a dose-related manner. Zatebradine decreased the norepinephrine-induced tachycardia (by approximately 80% from the control) more effectively than the spontaneous sinus rate (by approximately 30% from the control). 3. Zatebradine similarly attenuated the positive chronotropic but not inotropic responses to isoproterenol, NKH 477 and IBMX. Fifty per cent inhibition doses of zatebradine (0.10-0.18 mumol) for the chronotropic responses to each substance were not significantly different. 4. On the other hand, zatebradine attenuated neither positive chronotropic nor inotropic responses to Bay k 8644. 5. We therefore suggest that zatebradine selectively attenuates the positive chronotropic but not inotropic responses to cyclic AMP-related substances due to inhibition of I(f) but not ICa in the dog heart.

Gastric lipid peroxidation, glutathione and calcium channel blockers in the stress-induced ulcer model in rats

The antiulcer activity of verapamil and its analogues devapamil and gallopamil was studied. All three drugs reduced cold-restraint stress-induced ulcer development. Gallopamil almost abolished gastric ulcers. Verapamil prevented the increase in gastric lipid peroxidation (LP) due to stress. On the other hand, devapamil and gallopamil increased gastric lipid peroxidation and decreased glutathione levels. This effect may be attributed to the increase in oxygen supply due to possible effective vasodilation at gastric mucosa. The second part of this study revealed that stress-induced gastric ulcers in rats rapidly and spontaneously heal and disappear within 24 h. During recovery, gastric LP decreased and glutathione levels increased within 12 h after the withdrawal of stress, preceded by an initial reduction in glutathione. After 72 h, an unexplained increase in gastric LP and a decrease in glutathione were observed. Treatment with verapamil, devapamil and gallopamil promoted healing, gallopamil being again the most effective. Their effects on gastric LP and glutathione levels are in accordance with the results of pretreatment experiments. In conclusion, devapamil and gallopamil are effective antiulcer agents against stress-induced ulcers, but unlike verapamil, antioxidant activity does not seem likely to be among their mechanisms of action.

Ca2+ entry through L-type voltage-sensitive Ca2+ channels stimulates the release of human chorionic gonadotrophin and placental lactogen by placental explants

The release of human chorionic gonadotrophin (hCG) and placental lactogen (hPL) by human placental explants can be stimulated by Ca2+ entry. The aim of the present study was to characterize the modality of Ca2+ entry in the presence of high extracellular K+ concentration ([K+]o). A rise in [K+]o from 5 to > or = 50 mM induced a rapid and marked increase in the release of hCG and hPL from human term placental explants. The stimulatory effects of an excess [K+]o on the release of hCG and hPL were blocked in the absence of extracellular Ca2+ or in the presence of 0.5 mM Co2+. The presence of 50 microM methoxyverapamil, 20 microM nifedipine or 40 microM Cd2+ in the medium inhibited the stimulatory effects of [K+]o addition. Lastly, 40 microM Ni2+ failed to affect the increases in hCG and hPL releases elicited by [K+]o addition. Our data clearly show that a rise in [K+]o stimulates the release of hCG and hPL from placental explants. These secretory effects can be viewed as resulting from a Ca2+ entry through voltage-sensitive Ca2+ channels of the L-type.

Selective inhibition by E4080, a novel bradycardic agent, of positive chronotropic responses to norepinephrine in isolated dog hearts

E4080, a novel bradycardic agent acts on various ionic currents including the hyperpolarization-activated inward current (I(f)), L-type Ca2+ current (ICa) and ATP-sensitive K+ (K+ATP) current in mammalian heart and vascular tissues. We thus investigated the chronotropic and inotropic effects of E4080 and its interaction with the positive cardiac responses to norepinephrine, 3-isobutyl-1-methyl-xanthine (IBMX) and Bay k 8644 in the isolated, blood-perfused dog right atria and left ventricles. E4080 (0.01-1 mumol) decreased the sinus rate and atrial and ventricular contractile forces in a dose-related manner. Glibenclamide (3 mumol) partly blocked the decrease in atrial force but not the decreases in sinus rate and ventricular force induced by E4080. Atropine (10 nmol) did not affect the negative cardiac responses to E4080. E4080 (0.01-1 mumol) inhibited the positive chronotropic responses to norepinephrine and IBMX dose dependently, but did not inhibit the positive inotropic ones in isolated atria. E4080 affected neither positive chronotropic nor inotropic responses to Bay k 8644. These results suggest that (1) the activation of K+ATP channels by E4080 is partly related to the decrease in atrial force but not the decreases in sinus rate and ventricular force, and (2) the selective inhibition of E4080 of the cyclic AMP-dependent positive chronotropic responses but not inotropic ones is probably due to the inhibition of I(f) rather than other properties, e.g., activation of K+ATP channels and inhibition of ICa in the dog heart.

Protective effect of various calcium antagonists against an experimentally induced calcium overload in isolated hepatocytes

The effect of the hepatotoxic substance diamidinothionaphthene (98/202) on cytosolic, mitochondrial and extra-mitochondrial calcium distribution was measured in isolated rat hepatocytes. The drastic disturbance of the intracellular calcium homeostasis caused by this substance (increase of the cytosolic and mitochondrial calcium contents and depletion of extra-mitochondrial calcium stores, which at last lead to cell death) gave rise to an investigation of the possible cytoprotective effect of calcium antagonists of various chemical classes: verapamil, diltiazem, and nifedipine on isolated hepatocytes. Our results show that all three calcium antagonists prevented cell death caused by 98/202. The 98/202-induced increase of cytosolic and mitochondrial calcium content was inhibited by all three calcium antagonists. However, only verapamil was able to inhibit the depletion of extra-mitochondrial calcium stores. Since 98/202-induced cell death occurs only in the presence of extracellular calcium, it is concluded that calcium antagonists are also able to inhibit the influx of extracellular calcium in liver cells, which leads to a calcium overload of the cytosol and mitochondria. The various ways of interfering with the calcium homeostasis of liver cells qualifies the hepatotoxic substance 98/202 as a suitable in vitro hepatotoxicity model for testing the hepatoprotective effect of different calcium antagonists.

Role of L-type calcium channels on yohimbine-precipitated clonidine withdrawal in vivo and in vitro

This study was designed to elucidate the possible participation of L-type calcium channels in the expression of clonidine-withdrawal precipitated by yohimbine in clonidine-dependent animals. Mice implanted for 5 days with osmotic minipumps containing the alpha 2-adrenoceptor agonist clonidine showed symptoms of a withdrawal syndrome (jerks, headshakes, defecations and weight loss) when yohimbine, an alpha 2-adrenoceptor antagonist, was injected. Similarly, isolated rat ilea incubated with clonidine in vitro showed a withdrawal contracture when yohimbine was added to the organ bath. The effects of L-type calcium channel blockers (verapamil and diltiazem) and the stimulant Bay K 8644 on these two different types of withdrawal responses were evaluated. A dose-dependent decrease in yohimbine-precipitated clonidine withdrawal in vivo was observed when verapamil (10-40 mg/kg, s.c. and 120 micrograms/mouse, i.c.v.) or diltiazem (5-20 mg/kg, s.c. and 160 micrograms/mouse, i.c.v.) were administered to mice dependent on clonidine. No effect was found after Bay K 8644 (0.5-5 mg/kg, s.c. and 1-5 micrograms/mouse) was injected under these conditions. In vitro, both verapamil (0.1-5 microM) and D-cis-diltiazem (1-50 microM) concentration-dependently reduced the height of the yohimbine-precipitated withdrawal contracture in rat ileum incubated with clonidine. Furthermore, the effect of diltiazem was stereospecific, as D-cis-diltiazem 10 microM markedly inhibited clonidine withdrawal, whereas the same concentration of L-cis-diltiazem had no effect. In contrast, the calcium channel stimulant Bay K 8644 (0.1-1 microM) increased the height of the ileum withdrawal contracture.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effects of hexavalent chromium ions on the contraction in ileal longitudinal smooth muscle of guinea-pig

1. The high-K (60 mM)-induced ileal tonic tensions were not affected after 5 min with (0.01-1 mM) Cr6+. However, they were inhibited dose-dependently by Cr6+ after 30 min application. 2. Cr6+ inhibited the tonic response to K more strongly than the phasic response. 3. Cr6+ did not displace Ca(2+)-concentration-response curves to the right, although it reduced the Ca2+ response size. 4. The low affinity binding sites of Ca2+ and the tissue ATP concentration during K contraction was not entirely inhibited after 5 min with 0.5 mM Cr6+. However, they were fairly inhibited after 30 min with the Cr6+. 5. We suggest that Cr6+ probably reduced tension by the interference of Ca2+ influx at the cell membrane resulting from the inhibitory action on mitochondria.

Calcium channel blockers in vivo inhibit serotonin N-acetyltransferase (NAT) activity in chicken retina stimulated by darkness and not by agents elevating intracellular cyclic AMP level

The molecular mechanism underlying the role of calcium influx in the regulation of retinal serotonin N-acetyltransferase (NAT) activity was studied in vivo in chickens. Systemic administration of organic antagonists of voltage-sensitive calcium channels (VSCC), i.e., nimodipine and nifedipine, resulted in a marked suppression of the nocturnal increase of NAT activity in chicken retina. In contrast, NAT activity stimulated by nonhydrolysable analogs of cyclic AMP (dibutyryl-cyclic AMP and 8-bromo-cyclic AMP), forskolin, a direct activator of adenylate cyclase, and by phosphodiesterase inhibitors (aminophylline and 3-isobutyl-1-methylxanthine), was not significantly affected by various tested VSCC antagonists. The inhibitory effect of nimodipine on the dark-dependent increase in NAT activity of chicken retina was abolished by Bay K 8644, a selective VSCC agonist. The results presented in this paper indicate an important role of calcium influx through L-type of VSCC in the induction of NAT activity in chicken retina, and suggest that a requirement of calcium ions in the process of NAT induction in the retina may be primarily at the level of cyclic AMP production.

Two-state model of Paramecium bursaria thigmotaxis

A theoretical framework has been developed for analysis of the interaction between Paramecium bursaria and a glass surface. Adhesion to and detachment from a solid substrate were considered in the model as transitions between alternative states in cell behavior: (a) swimming, and (b) motionless (positive thigmotactic) state. According to the model and experimental data, a change in the fraction of swimming cells is described by a negative exponential course. The proposed model allows positive thigmotaxis, generally referred to in the literature simply as thigmotaxis, to be considered as the rate-constant of transition into the motionless state. This approach permits quantitative determination of thigmotaxis, and reveals its dependence on the phase of culture growth and the type of medium surrounding the cells. In the mineral maintenance solution, paramecia from the stationary phase of growth swim more slowly than those in the logarithmic phase of growth, and show enhanced thigmotaxis. However, a general relationship between thigmotaxis and swimming speed was not established.

Evidence for the presence of regional differences in the calcium antagonist receptors in lower urinary tract smooth muscle

(+)-[3H]PN 200-100 (a dihydropyridine calcium channel antagonist) was utilized to characterize calcium channel binding sites in rabbit bladder dome, bladder base, and urethra. Specific binding of (+)-[3H]PN 200-110 to membrane particulates was saturable, reversible, linear to protein concentration, and of high affinity. The density of (+)-[3H]PN 200-110 binding sites (Bmax values in fmol/mg of protein) and the affinity constants for (+)-[3H]PN 200-110 (KD value in pM) in urethra, bladder dome and bladder base were 64.1 +/- 7.8 and 179 +/- 31; 21.9 +/- 3.0 and 213 +/- 36; and 18.8 +/- 4.2 and 140 +/- 28, respectively. Agonists and antagonists inhibited (+)-[3H]PN 200-110 binding with Ki values in the following rank order: nitrendipine less than nifedipine less than niguldipine much less than Bay K 8644 much less than verapamil. Although carbachol-induced contractile responses were 20-30 times smaller in muscle strips from urethra than from bladder base or bladder dome, KCl-induced contractions were only 3-4 times smaller in urethra than in bladder tissues. Nifedipine inhibited carbachol-induced contractions in urethra, bladder dome, and bladder base by 76%, 64%, and 60%, respectively, and completely inhibited KCl-induced contractions in all three tissues. IC50 values for nifedipine inhibition of both carbachol- and KCl-induced contractions were significantly smaller in urethra than in bladder base or bladder dome. Nitrendipine, niguldipine and verapamil inhibited urethral contractions induced by carbachol and KCl to the same degree as did nifedipine. The IC50 values, obtained from functional studies, for calcium channel antagonists were in good agreement with Ki values obtained from binding studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of GABA and taurine from brain slices

In brain slices the mechanisms of release of GABA have been extensively studied, but those of taurine markedly less. The knowledge acquired from studies on GABA is, nevertheless, still fragmentary, not to speak of that obtained from the few studies on taurine, and firm conclusions are difficult, even impossible, to draw. This is mainly due to methodological matters, such as the diversity and pitfalls of the techniques applied. Brain slices are relatively easy to prepare and they represent a preparation that may most closely reflect relations prevailing in vivo, since the tissue structure and cellular integrity are largely preserved. In our opinion the most recommendable method at present is to superfuse freely floating agitated slices in continuously oxygenated medium. Taurine is metabolically rather inert in the brain, whereas the metabolism of GABA must be taken into account in all release studies. The use of inhibitors of GABA catabolism is discouraged, however, since a block in GABA metabolism may distort relations between different releasable pools of GABA in tissue. It is not known for sure how well, and homogeneously, incubation of slices with radioactive taurine labels the releasable pools but at least in the case of GABA there may prevail differences in the behavior of labeled and endogenous GABA. It is suggested therefore that the results obtained with radioactive GABA or taurine should be frequently checked and confirmed by analyzing the release of respective endogenous compounds. The spontaneous efflux of both GABA and taurine from brain slices is very slow. The magnitude of stimulation of GABA release by homoexchange is greater than that of taurine under the same experimental conditions. However, the release of both amino acids is generally enhanced by a great number of structural analogs, the most potent being those which are simultaneously the most potent inhibitors of uptake. This may result in part from inhibition of reuptake of amino acid molecules released from slices but the findings may also signify that the efflux of GABA and taurine is at least partially mediated by the membrane carriers operating in an outward direction. It is thus advisable not to interpret that stimulation of release in the presence of uptake inhibitors solely results from the block of reuptake of exocytotically released molecules, since changes in the carrier-mediated transport are also likely to occur upon stimulation. The electrical and K+ stimulation evoke the release of both GABA and taurine. The evoked release of GABA is several-fold greater than that of taurine in slices from the adult brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of melphalan uptake in murine L5178Y lymphoblasts in vitro by changes in ionic environment

The alkylating agent melphalan is actively transported in mammalian cells by two amino acid transport carriers: the sodium-dependent carrier with substrate preference for alanine-serine-cysteine (system ASC), and a sodium-independent carrier with preference for leucine (system L). The effect of altering the ionic environment of murine L5178Y lymphoblasts was investigated in order to determine not only the direct effects of hydrogen and calcium ions on these transport systems, but also the indirect effects of agents or modulators known to alter intracellular calcium. Melphalan transport followed a bell-shaped distribution curve over a pH range from 3 to 9 with a pH optimum of 4.3 and 4.6 for transport by systems ASC and L, respectively. Those agents that could cause a decrease in cytosolic calcium such as the calcium channel blockers verapamil, diltiazem and nitrendipine, the calcium chelator (ethyleneglycol-bis-(beta-aminoethylether) N,N,N',N'-tetraacetic acid (EGTA) and reduction of pH were found to augment melphalan uptake, whereas conditions that would elevate intracellular calcium such as the calcium ionophore A23187, the calcium channel agonist (-) Bay K 8644, elevation of extracellular calcium and the calcium pump inhibitor trifluoperazine were all found to decrease melphalan uptake. These findings suggest that modification of ionic environment directly or indirectly by agents known to alter intracellular calcium can modulate melphalan uptake.

Manipulation of intracellular calcium affects in vitro juvenile hormone synthesis by larval corpora allata of Manduca sexta

The effect of altering intracellular free Ca2+ on juvenile hormone (JH) and acid synthesis by larval and pupally-committed corpora allata (CA) of fifth stadium Manduca sexta was investigated. Larval CA required extracellular Ca2+ greater than or equal to 0.1 mM for maximal JH synthesis, while JH acid synthesis by glands after pupal commitment was independent of extracellular Ca2+. Free Ca2+ in the hemolymph ranged from 1.4 to 2.1 mM during the fifth stadium. Both calcium ionophores and caffeine, which releases Ca2+ from intracellular stores, inhibited JH synthesis by larval CA but stimulated JH acid synthesis by post-commitment CA. These results suggest that intracellular stores may be the principal source of Ca2+ for the biosynthetic activity of the post-commitment gland. Calcium channel blockers (La3+, Cd2+) and antagonists (verapamil, isradipine and nitrendipine) decreased both JH and JH acid synthesis, indicating the existence of Ca2+ channels in the CA cell membrane. Calmodulin (CaM) antagonists inhibited the activity of both larval and post-commitment CA, suggesting an integral relationship of CaM to the effects of Ca2+ on gland activity. One of these effects is the demonstrated requirement of 0.1 mM extracellular Ca2+ for allatostatin inhibition of JH I synthesis by larval CA.

Fibrin contact increases endothelial permeability to albumin

We studied the effects of contact of bovine pulmonary artery endothelial cell monolayers with fibrin on the endothelial barrier function. Fibrin formed by clotting purified fibrinogen (0.5 to 3.0 mg/ml) with alpha-thrombin (1 U/ml) was added to endothelial monolayers and permeability measurements were made after fibrin removal. Fibrin incubation for 3 hours resulted in 2- to 5-fold increases in transendothelial 125I-albumin permeability. Permeability returned to baseline value within 3 hours after fibrin removal. Direct contact with fibrin was necessary for the response, since fibrin separated from the endothelium did not increase permeability. Contact with agarose (2 mg/ml) or fibrinogen (0.5 to 3.0 mg/ml) also did not increase endothelial permeability. Transmission electron microscopic examination indicated normal appearance of interendothelial junctions at a time when albumin permeability was increased and no overt evidence of endothelial injury. Incubation of fibrin with endothelial monolayers at 4 degrees C prevented the increase in albumin permeability. We examined the possibility that increased albumin transcytosis was responsible for fibrin's effect using 14C-sucrose (Mr = 342D), a lipid insoluble tracer. Fibrin increased sucrose flux by 1.5-fold compared to 2- to 5-fold increases in albumin flux. The results indicate that fibrin contact with the endothelial cell increases endothelial permeability. The effect of fibrin may involve activation of temperature-sensitive bulk phase transcytosis of albumin.

Calcium channel blockers prevent stress-induced ulcers in rats

Gastric mucosal damage induced by cold and restraint stress caused increase in gastric lipid peroxidation (LP) and decrease in gastric glutathione levels. Two calcium-channel blockers, verapamil and nicardipine, prevented stress-induced increase in gastric LP, as well as ulcer formation. Both calcium-channel blockers protected against stress-induced ulcers, and inhibition of LP may be among their mechanisms of action.

Calcium channel drugs affect nocturnal serotonin N-acetyltransferase (NAT) activity in rat pineal gland

The effects of different organic compounds that block and increase Ca2+ influx through the voltage-sensitive calcium channels (VSCC) on the nocturnal serotonin N-acetyltransferase (NAT) activity was investigated in vivo in rats. Systemic administration of VSCC antagonists, i.e. nimodipine, nifedipine, verapamil and diltiazem, resulted in a marked suppression of the nighttime pineal NAT activity. Bay K 8644, a VSCC agonist, injected to rats before the time of the light offset of the light-dark cycle significantly enhanced the nocturnal increase of the pineal NAT activity. Although Bay K 8644 given during the dark phase of an imposed illumination cycle had little effect on the nocturnal pineal NAT activity, it antagonized the nimodipine- and verapamil-induced decrease in the enzyme activity. These results support the role of Ca2+ influx through the VSCC in the nocturnal increase of NAT activity in the pineal gland of rat.

Comparison of adenosine receptor agonists with other vasodilators on noradrenaline-, potassium- and phorbol ester-contracted rabbit aorta

Isometric contractions of rabbit isolated aortic spirals were induced by noradrenaline (1 microM), potassium chloride (51 mM) or phorbol-12,13-dibutyrate (1 microM). A cumulative concentration-response curve for adenosine, N-ethylcarboxamidoadenosine (NECA), verapamil, sodium nitroprusside, isoprenaline or forskolin was then constructed. Sodium nitroprusside displayed selectivity towards noradrenaline- compared with potassium-contracted tissues. In contrast, verapamil selectively relaxed the potassium-contracted tissues. Adenosine preferentially relaxed the noradrenaline-contracted aorta. Adenosine, in the presence of the transport inhibitor dipyridamole (10 microM), and NECA were without effect upon potassium-contracted tissues. This absolute selectivity for noradrenaline-contracted aortas indicates that adenosine A2-receptor agonists do not interfere with calcium influx through voltage-operated channels but inhibit calcium influx via receptor-operated channels or its intracellular release. The inhibition by dipyridamole of a small relaxation by adenosine in potassium-contracted tissues indicates that this was due to stimulation of an additional intracellular site. The selectivity profile of adenosine and NECA was similar to that of forskolin and isoprenaline indicating that their relaxations were due to a common activation of adenylate cyclase and cAMP accumulation. Only sodium nitroprusside- and forskolin-relaxed aortas contracted with phorbol dibutyrate. Adenosine and NECA also failed to cause relaxation of noradrenaline-contracted aortas preincubated with the phorbol ester indicating that it may desensitize A2-receptors.

Effects of dihydropyridines and inorganic calcium blockers on aggregation and on intracellular free calcium in platelets

[Ca2+]i increase is necessary in physiological platelet activity, particularly aggregation and release. The increase of [Ca2+]i observed during platelet activation depends in part on Ca2+ influx from the extracellular medium. The participation of voltage-operated Ca2+ channels as a pathway for Ca2+ entry is controversial. In the present study we have attempted to reinvestigate this problem by measuring aggregation and [Ca2+]i changes in platelets activated by ADP or thrombin and incubated with organic or inorganic blockers of calcium channels. The main findings of the present paper can be summarized as follows: (i) Ni2+, Co2+ and Mn2+, well known inorganic blockers of Ca2+ channels, inhibited platelet aggregation induced by ADP or thrombin in a dose-dependent manner, Ni2+ being the most effective agent. (ii) Thrombin induced a rise in free [Ca2+]i in platelets incubated both in 1 mmol/l Ca(2+)-containing medium and in nominally Ca(2+)-free medium; the rise of free [Ca2+]i was in the first case up to 370 +/- 31 nmol/l and in the second case up to 242 +/- 26 nmol/l, indicating that this observed difference was due to Ca2+ entry from the extracellular medium. Co2+ and Ni2+ abolished that difference by inhibiting Ca2+ influx. (iii) Nisoldipine, nitrendipine and nimodipine (10-50 nmol/l) inhibited in a dose-dependent manner platelet aggregation induced by either ADP or thrombin in platelets incubated in normal-Ca2+ normal-K+ medium, also, aggregation was inhibited to a similar extent in platelets incubated in normal-Ca2+ high-K+ medium. (iv) Nisoldipine--the most effective dihydropyridine to inhibit platelet aggregation--also inhibited Ca2+ influx in platelets incubated in normal-Ca2+ medium, either in normal-K+ or high-K+ media. Our data support the existence of voltage-operated, dihydropyridine-sensitive calcium channels (L-type) and a physiological role for them in platelet function.

Quinolinic acid: effect on 45Ca content in perfused rat heart preparations and its calcium ion binding property in Krebs-Henseleit medium and blood serum

In the experiment it was found that quinolinic acid perfusion was accompanied with the increase in calcium 45Ca content in myocardium tissue. The increase in calcium content was associated with a decrease in heart contractility. Moreover, it was shown that quinolinic acid can form complexes with calcium ions in Krebs-Henseleit medium as well as in blood serum but only to a small degree.

Differential effects of L-type calcium channel blockers and stimulants on naloxone-precipitated withdrawal in mice acutely dependent on morphine

The effects of L-type calcium channel blockers and stimulants on naloxone-precipitated withdrawal in mice acutely dependent on morphine were evaluated. Verapamil (10-80 mg/kg), diltiazem (20-120 mg/kg) and nicardipine (20-160 mg/kg), when administered subcutaneously, produced a dose-dependent reduction in forepaw tremor and weight loss during the abstinence reaction; jumping was also reduced by all three drugs, although the effect was not statistically significant in the case of nicardipine. By contrast, the calcium agonist Bay K 8644 (0.5-2 mg/kg, SC) increased forepaw tremor and weight loss, although this latter effect did not reach statistical significance. The effects of the calcium channel active drugs on the rotarod test were also explored, no correlation appearing with the results observed in abstinence (except for the jumping response), which suggests that the withdrawal results are not influenced by motor incoordination or unspecific CNS depression. These findings suggest that L-type calcium channels probably play an important role in withdrawal after acute morphine dependence. Taken together with other observations in chronic models, these results show that calcium channels are similarly involved in morphine abstinence after acute and chronic dependence, in contrast to the differences in the content and uptake of neuronal calcium induced by morphine under both conditions.

Quinolinic acid: a modulator of the heart calcium channel in the rat and a binder of calcium ions

When rat heart preparations were perfused with quinolinic acid at a slow constant rate, a decrease in contractility was observed. A higher rate of perfusion resulted in a biphasic response, thus both a positive inotropic effect and then a decrease in heart contractility were visible. Using a polarographic method, the association constant of quinolinic acid with calcium ions (Ka) was found to be equal to 220. By combining the values from heart perfusion experiments with the calculated ones of free calcium ions, a linear correlation was obtained between the decreases of contractility and of calcium ions (r = 0.94).

Effect of manganese (II) bis(glycinate)dichloride on Ca2+ channel function in cultured chick atrial cells

Manganese (II) bis(glycinate)dichloride (Mn(glycinate)2) is a coordination complex of manganese with application as a contrast enhancement agent for magnetic resonance imaging in the heart. To determine the cardioactivity of the manganese ion in this chelation cage, the effects of Mn(glycinate)2 on Ca channel function in the cultured chick atrial cell was studied. Mn(glycinate)2 decreased amplitude of contraction in chick atrial cells from embryos 14 days in ovo with complete inhibition of beating at 1 mM and half-maximal effect at 0.1 mM. Under control conditions, Bay K 8644, a Ca channel activator increased amplitude of contraction by 86% with a half maximal effect at 3.2 x 10(-7) M. In the presence of 0.025 mM Mn(glycinate)2, a concentration which had no effect on the amplitude of contraction, the maximum response to Bay K 8644 was decreased to 31%. Mn(glycinate)2 had no effect on the EC50 for the response to Bay K 8644, 1.7 +/- 0.1 x 10(-9) M (S.E.M., n = 4) in control cells compared to 2.2 +/- 0.4 x 10(-9) M (S.E.M., n = 4) in cells incubated with Mn(glycinate)2. 45Ca2+ uptake over 5 min in cultured chick atrial cells decreased from 2.0 nmol/mg protein in control cells to 1.5 nmol/mg protein in the presence of 10(-5) M PN200-110, a Ca2+ channel blocker, a decrease of 28%. 45Ca2+ uptake decreased to 0.94 nmol/mg protein (53%) in the presence of 1 nmol Mn(glycinate)2. Effects of Mn(glycinate)2 and PN200 were not additive. These data demonstrate that Mn(glycinate)2 exerts its negative inotropic effect, at least partially, by interfering with the function of the L-type Ca channels at high concentrations.

Osteoclast cytosolic calcium, regulated by voltage-gated calcium channels and extracellular calcium, controls podosome assembly and bone resorption

The mechanisms of Ca2+ entry and their effects on cell function were investigated in cultured chicken osteoclasts and putative osteoclasts produced by fusion of mononuclear cell precursors. Voltage-gated Ca2+ channels (VGCC) were detected by the effects of membrane depolarization with K+, BAY K 8644, and dihydropyridine antagonists. K+ produced dose-dependent increases of cytosolic calcium ([Ca2+]i) in osteoclasts on glass coverslips. Half-maximal effects were achieved at 70 mM K+. The effects of K+ were completely inhibited by dihydropyridine derivative Ca2+ channel blocking agents. BAY K 8644 (5 X 10(-6) M), a VGCC agonist, stimulated Ca2+ entry which was inhibited by nicardipine. VGCCs were inactivated by the attachment of osteoclasts to bone, indicating a rapid phenotypic change in Ca2+ entry mechanisms associated with adhesion of osteoclasts to their resorption substrate. Increasing extracellular Ca2+ ([Ca2+]e) induced Ca2+ release from intracellular stores and Ca2+ influx. The Ca2+ release was blocked by dantrolene (10(-5) M), and the influx by La3+. The effects of [Ca2+]e on [Ca2+]i suggests the presence of a Ca2+ receptor on the osteoclast cell membrane that could be coupled to mechanisms regulating cell function. Expression of the [Ca2+]e effect on [Ca2+]i was similar in the presence or absence of bone matrix substrate. Each of the mechanisms producing increases in [Ca2+]i, (membrane depolarization, BAY K 8644, and [Ca2+]e) reduced expression of the osteoclast-specific adhesion structure, the podosome. The decrease in podosome expression was mirrored by a 50% decrease in bone resorptive activity. Thus, stimulated increases of osteoclast [Ca2+]i lead to cytoskeletal changes affecting cell adhesion and decreasing bone resorptive activity.

Characterization of 1,4, dihydropyridine calcium channel binding sites in the human prostate

The binding and functional properties of calcium channel receptors have not been previously characterized in the normal or hyperplastic prostate. Dihydropyridine (DHP) binding sites have been characterized in other tissues using the ligands 3H-nitrendipene and (+)3H-PN200-110. Saturation experiments were performed on homogenates obtained from five human prostate adenomas using these ligands. The binding of 3H-nitrendipine and (+)3H-PN200-110 in the prostate was saturable and of high affinity. The mean Kd of 3H-nitrendipine and (+)3H-PN200-110 was 0.92 +/- 0.11 nM and 0.14 +/- 0.02 nM, respectively. The mean Bmax of 3H-nitrendipine and (+)3H-PN200-110 was 0.57 +/- 0.06 and 0.19 +/- 0.02 fmol/mg. wet wt., respectively. The percent specific binding of 3H-nitrendipene and (+)3H-PN200-110 was 18 +/- 1% and 38 +/- 4%, respectively. The pharmacology of (+)3H-PN200-110 binding sites was further characterized using competition displacement experiments. The IC50 corrected values for Bay K 8644, nifedipine, verapamil, and diltiazem in the human prostate and other tissues are of the same order of magnitude. The prostate contains an abundance of high affinity DHP binding sites. The physiologic significance of the DHP binding sites in the prostate requires further investigation.

Inhibition by calcium channel blockers of the binding of platelet-activating factor to human neutrophil granulocytes

The inhibitory action of calcium channel blockers on platelet-activating factor (PAF)-induced activation of human polymorphonuclear granulocytes (PMNL) and on the binding of [3H]PAF to neutrophils was studied. Verapamil and diltiazem inhibited PAF (10(-8)-10(-15) M)-induced degranulation and superoxide production in a dose-dependent manner, with pA2 values of 5.6 and 6.1 for verapamil and 5.9 and 6.2 for diltiazem, respectively. Both channel blockers inhibited the specific binding of [3H]PAF to PMNL in dose-dependent fashion, with Ki values of 3.9 +/- 0.6 X 10(-5) M and 3.2 +/- 0.4 X 10(-5) M for verapamil and diltiazem, respectively. Scatchard analysis of the binding data revealed that both calcium channel blockers decreased the receptor binding affinity and slightly increased the number of high-affinity PAF receptors, whereas they did not affect the binding affinity and number of low-affinity receptors. These results indicate that calcium channel blockers can inhibit neutrophil responses elicited by PAF by a mechanism other than inhibition of calcium influx and suggest that the PAF receptor may be closely associated with calcium channels.

The calcium messenger system and the kinetics of elastase release from human neutrophils in patients with abdominal aortic aneurysms

Aortic elastase and neutrophil elastase is higher in patients with abdominal aortic aneurysms. The purpose of this study was to determine if these proteolytic elevations occur after abdominal aortic aneurysms have been repaired. Specifically, we studied the stimulation and inhibition of elastase degranulation from neutrophils in postoperative abdominal aortic aneurysm patients compared to aortic occlusive disease patients. Neutrophil elastase was determined in postoperative abdominal aortic aneurysm and aortic occlusive disease patients in response to calcium and the ionophore A23187. Inhibition of elastase release was determined with the calcium channel blocking agent Verapamil. Neutrophil elastase secretion was significantly higher in the abdominal aortic aneurysm patients (47%) versus aortic occlusive disease (20%) (p less than .05), while the effect of Verapamil in blocking this response was significantly lower in the abdominal aortic aneurysm patients (14%) compared to aortic occlusive disease patients (27%) (p less than .02). The time for degranulation to occur was longer in the abdominal aortic aneurysm patients (14.7 minutes) versus aortic occlusive disease patients (3.5 minutes), but the rate of secretion was not different between the two groups. These data indicate that, (1) neutrophils secrete more elastase in response to a calcium stimulus in abdominal aortic aneurysm patients; (2) it takes longer to secrete the increased amount of elastase in abdominal aortic aneurysm patients since the rate of secretion is similar between the two groups; and (3) Verapamil blocks elastase secretion ineffectively in abdominal aortic aneurysm patients. We conclude that the proteolytic alterations in abdominal aortic aneurysm patients are more likely a primary event and not a response to the abdominal aortic aneurysm and that Verapamil is a poor drug to use to medically manipulate the protease system in abdominal aortic aneurysm patients.