Biomedical subjects

C Y Kwan

Publications and source records attributed to C Y Kwan.

At least 73 records · Page 4Linked to original sources

PubMed1996

Dual effects of SK&F 96365 in human leukemic HL-60 cells. Inhibition of calcium entry and activation of a novel cation influx pathway.

The effects of the receptor-mediated Ca2+ entry blocker, SK&F 96365 on thapsigargin (TSG)-induced Ca2+ entry in fura-2-loaded HL-60 cells were studied. After Ca2+ release induced by 30 nM TSG, readmission of Ca2+ resulted in a sustained Ca2+ entry, which could be partially inhibited by 1-3 microM SK&F 96365. Surprisingly, SK&F 96365 at 30-100 microM, instead of causing a stronger inhibition, actually promoted Ca2+ entry. Furthermore, at 16-100 microM, this drug released intracellular Ca2+ on its own and induced Ca2+ entry upon readmission of Ca2+. This SK&F 96365-activated Ca2+ entry pathway was insensitive to nifedipine and, interestingly, accessible to Ni2+ and La3+. However, SK&F 96365 (30 microM) almost completely blocked (basal) Mn2+ entry and only caused 4.4% of the cells to be stained with trypan blue, strongly suggesting that the SK&F 96365-activated cation entry was not due to damage nor to a very nonselective permeabilization of the plasma membrane. These data indicate that low concentrations of SK&F 96365 inhibited Ca2+ entry and higher concentrations activated a novel cation entry pathway. Because these 2 opposing effects overlapped at an intermediate concentration (16 microM), which is within the range commonly used to block Ca2+ entry, cautious use of this Ca2+ antagonist appears to be warranted.

Calcium↗

PubMed1996

Plant alkaloids, tetrandrine and hernandezine, inhibit calcium-depletion stimulated calcium entry in human and bovine endothelial cells.

Depletion of internal Ca2+ stores causes capacitative Ca2+ entry which occurs through non-selective cation channels sensitive to blockade by SK&F 96365. Recently, alkaloids of Chinese herbal medicinal origin, tetrandrine and hernandezine, have been shown to possess actions including inhibition of Ca2+ channels in non-excitable cell types. In this study, we compared the actions of these novel inhibitors to those of SK&F 96365 in fura-2-loaded endothelial cells from human umbilical vein and bovine pulmonary artery. Depletion of Ca2+ from the internal stores was accomplished in Ca(2+)-free medium using an endoplasmic reticulum Ca2+ pump inhibitor, cyclopiazonic acid (CPA) or receptor agonists, histamine and bradykinin. Stimulation with histamine or bradykinin caused a marked and rapid transient increase in Ca2+ signal whereas CPA caused a smaller amplitude increase of longer duration. Restoring Ca2+ to the medium caused marked and sustained increases in the fluorescence indicating movement of Ca2+ into the cytosol presumably stimulated by the emptied Ca2+ stores. SK&F 96365 as well as tetrandrine and hernandezine antagonized depletion-induced Ca2+ entry. The results suggest that these putative inhibitors interact with Ca2+ entry triggered by depletion of the internal Ca2+ stores and their action is presumed to be on the non-selective cation channels. Their effectiveness may be enhanced by the mechanisms which lead to the opening of the Ca2+ influx channel.

Alkaloids↗

PubMed1996

Inhibition by multivalent cations of contraction induced by Chinese cobra venom cardiotoxin in guinea pig papillary muscle.

The effects of cardiotoxin (CTX), purified from the venom of Chinese Cobra (Naja naja atra) by a three-step chromatography, on the contractile responses of isolated guinea pig papillary muscle preparation and its antagonism by lanthanum ion (La3+) and divalent cations were examined. CTX induced tonic contraction following a transient augmentation of electrically evoked rhythmic contractions, which is similar to that seen in perfused heart preparation. Multivalent cations, La3+, Ca2+, Mn2+ and Mg2+, concentration-dependently blocked CTX-induced contraction. In Ca(2+)-free medium, CTX did not induce contraction and CTX-induced contraction was not modified in Na(+)-free medium. Nifedipine (1 mumol/L), effectively blocked KCl-induced contracture, but only partially inhibited CTX-induced contraction; thus suggesting that Ca2+ influx induced by CTX utilizes channels other than L-type Ca2+ channels. These cations may compete with CTX for the negatively charged membrane binding site which is responsible for the modulation of Ca2+ movement.

Animals↗

PubMed1996

Tetrandrine, a calcium antagonist of Chinese herbal origin, interacts with vascular muscle alpha 1-adrenoceptor.

The effects of tetrandrine (TET) on the contractile responses of rat aortic rings and perfused rat mesenteric arteries to phenylephrine (PE) were investigated. TET inhibited the maximal contraction to PE in a concentration-dependent manner. TET significantly inhibited the transient contraction in Ca(2+)-free medium presumably due to release of intracellular Ca2+ after activation of alpha 1-adrenoceptors. However, it caused a stronger inhibition of the sustained contraction in Ca(2+)-containing medium presumably the result of Ca2+ influx. TET has no inhibitory effect on caffeine-induced transient contraction. Radioligand receptor binding study using isolated dog aortic muscle membranes indicated that TET inhibited the binding of 3H-prazosin in a competitive manner, hence showing that TET interacted directly with the alpha 1-adrenoceptors. Thus, TET affected PE-induced aortic contractions by multiple mechanisms, inhibiting interaction of PE with alpha 1-adrenoceptors and interfering with PE-induced responses involving both Ca2+ entry and release.

Alkaloids↗

PubMed1996

Intraoral juvenile xanthogranuloma. A case report and literature review.

Juvenile xanthogranuloma rarely occurs in the oral cavity and has received little attention. A case of histologically documented juvenile xanthogranuloma of the oral cavity is described. This is the first intraoral case reported in the Oriental race and in the vestibule. Pertinent literature regarding intraoral lesions of this condition is also reviewed.

Adolescent↗

PubMed1996

Effects of tetrandrine and closely related bis-benzylisoquinoline derivatives on cytosolic Ca2+ in human leukaemic HL-60 cells: a structure-activity relationship study.

1. Previously it has been shown that tetrandrine (TET), a bis-benzylisoquinoline alkaloid, isolated from a Chinese herb Stephania tetrandra, can block non-voltage-operated Ca2+ entry activated by intracellular Ca2+ store depletion induced by thapsigargin (TSG) and can release intracellular Ca2+ in HL-60 cells. The present study attempted to identify the chemical group(s) of the TET molecule responsible for these dual effects. The effects of TET and its closely related analogues, hernandezine (HER) and berbamine (BER), on Ca2+ entry and Ca2+ release were compared in fura-2-loaded HL-60 cells. 2. Berbamine was much less potent (IC50 = 200 mumol/L) than TET and HER (both IC50 values = 25 mumol/L) in inhibiting Ca2+ entry activated by TSG. Furthermore, at 100 mumol/L, BER was much less effective than TET and HER in suppressing TSG-induced Mn2+ entry. At 30-100 mumol/L, BER was significantly less effective than both TET and HER in causing Ca2+ release from internal stores. However, only BER was able to cause store depletion-activated Ca2+ entry (or the so-called 'capacitative Ca2+ entry') upon Ca2+ readmission. 3. Taken together, the data from this structure-activity relationship study reveal that the -OCH3 group of one particular benzene ring of TET, which distinguishes TET from BER, in part produces the dual pharmacological actions of TET.

Alkaloids↗

PubMed1996

Probing excitation-contraction coupling in trachealis smooth muscle with the mycotoxin cyclopiazonic acid.

1. Muscarinic stimulation-induced tonic contraction of airway smooth muscle is independent of membrane potential. This contraction is not sensitive to inhibition by voltage-operated Ca2+ channel blockers or by K+ channel openers. 2. Cyclopiazonic acid (CPA) inhibits Ca2+ loading of internal stores but does not affect maximal tonic contraction induced by acetylcholine (ACh) in steady state conditions. 3. After depletion of internal Ca2+ stores with CPA, ACh-induced tonic contraction becomes dependent upon values of membrane potential. The contraction is then sensitive to voltage-operated Ca2+ channel blockers and to K+ channel openers. 4. Treatment of trachealis muscle with CPA potentiates the M2-mediated component of ACh stimulation, but this potentiation is not entirely responsible for the switch in excitation-contraction (E-C) coupling. 5. It is proposed that depletion of internal Ca2+ stores with CPA and promotion of M2-stimulation can lead to a switch in E-C coupling in trachealis smooth muscle from pharmaco- to electromechanical mode, perhaps by targeting a plasma membrane K+ channel.

Animals↗

PubMed1996

In vitro inhibitory effects of chebulinic acid on the contractile responses of cardiovascular muscles.

1. The effects of chebulinic acid, which has been shown to elicit blood pressure lowering effect in rats, on aortic vascular contraction as well as cardiac contraction were studied in rats. 2. Chebulinic acid had no effect on KCl-induced aortic contraction, but irreversibly inhibited the contractile responses to phenylephrine in an apparently non-competitive manner. Chebulinic acid also inhibited contractile responses of rat aorta to 5-hydroxytryptamine and angiotensin II. 3. Chebulinic acid inhibited the binding of [3H]-prazosin to dog aortic microsomal membranes in a concentration-dependent manner with an IC50 value of 0.34 mmol/L. Results of saturation binding experiments suggest a mixed mode of inhibition by chebulinic acid (i.e. a decrease in both the maximal number of binding sites and the affinity for prazosin). 4. Chebulinic acid concentration-dependently and reversibly inhibited the maximal left ventricular pressure of rat heart in a Langendorff preparation with 50% inhibition occurring at a concentration of 0.3 nmol/L. 5. We conclude that chebulinic acid exerts non-specific inhibitory actions in vascular preparations. Its inhibitory effect on cardiac contraction was reversible and three orders of magnitude more potent than that on vascular contraction. We suggest that the hypotensive effect of chebulinic acid is probably mediated via the decrease in cardiac output resulting from reduced left ventricular contraction.

Adrenergic alpha-Antagonists↗

PubMed1996

Tetrandrine as a calcium antagonist.

1. The Ca(2+)-antagonism of tetrandrine (TET) on the Ca2+ mobilization in various types of cells were reviewed. Inositol trisphosphate (IP3)-generating drugs were used as Ca(2+)-mobilizing agonists and the effects were compared with those produced by using the microsomal Ca(2+)-ATPase inhibitor thapsigargin (TG), which is a tool for analysing Ca2+ store-regulated Ca2+ entry (capacitative Ca2+ entry). 2. In rat phaeochromocytoma PC12 cells, 100 mumol/L TET abolished high K+ (30 mmol/L)-induced sustained increases in cytoplasmic Ca2+ concentrations ([Ca2+]i) and partially inhibited bradykinin (1 mumol/L)- or TG (100 nmol/L)-induced Ca2+ entry. 3. In NIH/3T3 fibroblasts and rat parotid acinar cells, 100 mumol/L TET abolished Ca2+ entry induced by bombesin (1 mumol/L) and carbachol (100 mumol/L), respectively, or TG (100 nmol/L). However, in the human leukaemia T cell line Jurkat, 100 mumol/L TET did not inhibit Ca2+ entry evoked by either the anti-CD3 antibody OKT3 (10 mg/L) or TG (100 nmol/L). 4. In rat glioma C6 cells, the effects of TET on Ca2+ mobilization were further examined. At a high concentration, TET (300 mumol/L) alone did not affect [Ca2+]i in C6 cells. Tetrandrine inhibited the peak and sustained increases in [Ca2+]i induced by bombesin and TG in a dose-dependent manner. Although TET or TG did not produce increases in IP3, TET did inhibit increases in IP3 produced by bombesin. 5. Our results suggest that the action of TET on Ca2+ entry is dependent on cell types and that TET inhibits both Ca2+ entry from the extracellular medium and Ca2+ release from intracellular stores in rat glioma C6 cells.

Alkaloids↗

PubMed1996

Relative contributions of extracellular Ca2+ and Ca2+ stores to smooth muscle contraction in arteries and arterioles of rat, guinea-pig, dog and rabbit.

1. These studies describe the functional effects of modulation of the sarcoplasmic reticulum (SR) Ca2+ stores at three levels of the vasculature: (i) large arteries (rat and guinea-pig aorta); (ii) small resistance arteries (rat tail artery, rabbit mesenteric artery, dog mesenteric artery); and (iii) arterioles (guinea-pig submucosal arterioles of the small intestine). 2. All tissues responded to phenylephrine (PE; 10 mumol/L) with a transient contraction in Ca(2+)-free Krebs', reflecting Ca2+ release from PE-sensitive Ca2+ stores. After pretreatment with cyclopiazonic acid (CPA; 30 mumol/L) or thapsigargin (TSG; 1 mumol/L), putative SR Ca2+ pump inhibitors, the PE-induced contraction in a Ca(2+)-free medium was significantly inhibited in arterial tissues at all levels of the vasculature. Similarly, ryanodine (RYA; 30 mumol/L), an agonist that enhances Ca2+ release from the SR, also reduced the PE contraction in a Ca(2+)-free solution. 3. CPA or TSG alone in the presence of extracellular Ca2+, caused marked and sustained contraction in the rat and guinea-pig aorta and marked but transient or no contraction in the resistance arteries. In the rat and guinea-pig aorta, RYA caused a slowly developing tension. Little increase in basal tension was produced by RYA in resistance arteries and arterioles. 4. The findings show that an agonist-releasable Ca2+ pool is present at all levels of the vasculature that is independent of the size of the vessels and suggest that under normal physiological conditions there is an intimate balance between the roles of the plasma membrane and of the SR in the maintenance of vascular contractility. It appears that the role of the SR diminishes as the arteries become smaller, while Ca2+ fluxes across the plasma membrane predominates.

Animals↗

PubMed1996

Unusual alpha-adrenoceptor subtype in canine saphenous vein: comparison to mesenteric vein.

1. We investigated the nature of the adrenoceptors in the dog saphenous vein (DSV) and dog mesenteric vein (DMV) to determine the nature of the unexpected interactions of phenylephrine and methoxamine with rauwolscine in the DSV, i.e. the ability of the putative alpha 2-adrenoceptor antagonist to inhibit competitively contractions to these alpha 1-agonists. Radioligand binding studies were performed in parallel with contractility studies. 2. Functionally, in the DSV, phenylephrine and methoxamine-induced, contractions were antagonized by rauwolscine with Schild slopes of -0.52 and -0.46, respectively and apparent pA2 values of 8.5 and 9.2, respectively. Such antagonism was not observed in the DMV. In the DSV, prazosin competes for [3H]-rauwolscine binding sites with a high and a low affinity binding site (Ki of 1.49 +/- 0.65 and 94.7 +/- 51 microM, n = 6, respectively). 3. Pretreatment with 100 microM chloroethylclonidine (CEC) for 15 min abolished [3H]-prazosin binding in microsomes from both veins and reduced binding (Bmax) of [3H]-rauwolscine in microsomes by 55.1 +/- 0.8% (n = 3) in the DSV but did not affect the Bmax in the DMV. CEC pretreatment in the venular rings denuded of endothelium caused persistent contraction in the DSV but not in the DMV. In the DSV, CEC appeared to interact with a single [3H]-rauwolscine binding site. In both the DSV and the DMV, CEC (100 microM) caused a significant shift in the EC50 values for phenylephrine and methoxamine. Maximum responses in the DMV were significantly attenuated while those in the DSV were unaffected when total tension was considered. 4. Studies of the functional interactions of the DSV and the DMV with WB 4101 or 5-methylurapidil (5-MU) suggested the presence of alpha 1D-adrenoceptors in the DSV and alpha 1A-adrenoceptors in the DMV. The receptors inactivated by CEC in the DMV and DSV may represent some or all of the receptors with properties of alpha 1D and alpha 1A-receptors present in the two veins. Studies of radioligand binding interactions of these two antagonists with [3H]-prazosin, were consistent with the presence of some alpha 1D-receptors in DSV and alpha 1A-receptors in DMV. These findings raise questions about the selectivity of CEC in differentiating alpha 1-adrenoceptor subtypes. 5. B-HT 920 caused contractions in the DSV smaller than those to the alpha 1-agonists but the maximum was not affected by CEC pretreatment. The EC50 values were shifted to the left after CEC. In radioligand binding studies, B-HT 920 competition for [3H]-rauwolscine binding was not significantly affected by CEC pretreatment. 6. These results suggest the presence of unusual alpha-adrenoceptors in the DSV. In addition to alpha 2-adrenoceptors, receptors recognizing rauwolscine as well as prazosin, WB 4101, phenylephrine and methoxamine and susceptible to inactivation by CEC are present. They appear to be, in part, unusual alpha 1D-adrenoceptors.

Adrenergic alpha-Agonists↗

PubMed1996

Selective inhibition of oxalate-stimulated Ca2+ transport by cyclopiazonic acid and thapsigargin in smooth muscle microsomes.

45Ca2+ uptake and efflux studies were performed on membranes prepared from dog mesenteric artery and rat vas deferens. Oxalate-stimulated, ATP-dependent Ca2+ uptake in microsomal vesicles, a property characteristic of sarcoplasmic reticulum, was completely inhibited in a concentration-dependent manner by cyclopiazonic acid (0.1-30 microM) and thapsigargin (10 nM-10 microM). Using discontinuous sucrose gradient centrifugation, rat vas deferens microsomes were separated into two fractions, one enriched in plasma membrane (F2), the other enriched in sarcoplasmic reticulum (F3). The F3 fraction had a major increase in Ca2+ uptake in the presence of oxalate, which was completely inhibited by either cyclopiazonic acid or thapsigargin. In the F2 fraction Ca2+ uptake in the presence of oxalate was lower than in F3 and was not completely inhibited by thapsigargin and cyclopiazonic acid. Instead, the F2 fraction had a thapsigargin-insensitive and cyclopiazonic acid insensitive, saponin-sensitive component of uptake, which probably represents Ca2+ uptake by plasma membrane. In the absence of oxalate, the inhibition of Ca2+ uptake by saponin and cyclopiazonic acid or thapsigargin was additive in the F2 and F3 fractions, suggesting that cyclopiazonic acid and thapsigargin selectively inhibited sarcoplasmic reticulum derived Ca2+ uptake and did not affect plasma membrane derived Ca2+ uptake. Measurement of the initial rate of Ca2+ uptake in the presence and absence of oxalate by rat vas deferens microsomes demonstrated selective inhibition of oxalate-stimulated Ca2+ uptake by cyclopiazonic acid and thapsigargin. Ca2+ efflux from rat vas deferens microsomes actively loaded with 45Ca2+ either in presence or the absence of oxalate was not increased by cyclopiazonic acid or thapsigargin, showing that the inhibition of Ca2+ accumulation was not due to an increase in Ca2+ efflux. In both rat vas deferens and dog mesenteric artery, the maximal inhibitory effects of cyclopiazonic acid developed rapidly, whereas for maximal inhibition thapsigargin required pretreatment of microsomes prior to measurement of Ca2+ uptake. In rat vas deferens microsomes the inhibitory effects of cyclopiazonic acid could be quickly and completely reversed, whereas the effects of thapsigargin were not easily reversed. Collectively, these results suggest selectivity of cyclopiazonic acid and thapsigargin for the sarcoplasmic reticulum Ca2+ pump. Their selective inhibitory properties and differences in onset and offset of inhibition make cyclopiazonic acid thapsigargin useful pharmacological tools in the study of the physiological and pathophysiological roles of the sarcoplasmic reticulum Ca2+ pump in regulating smooth muscle Ca2+.

Aminoisobutyric Acids↗

PubMed1996

Antitumor effects of a refined polysaccharide peptide fraction isolated from Coriolus versicolor: in vitro and in vivo studies.

RPSP, a refined polysaccharide peptide fraction isolated by fast performance liquid chromatography (FPLC) from the crude powder of total peptide-bound polysaccharides of cultivated Coriolus versicolor Cov-1 dose-dependently inhibited the proliferation of a human hepatoma cell line (HEPG2). The effective dose causing 50% inhibition following 3-day exposure to RPSP was 243 +/- 36 micrograms/ml for HEPG2. However, little or no inhibitory effects were detected in normal human foetal hepatocytes. On the other hand, in the pretreatment group, in which RPSP was administered i.p. for two weeks before sarcoma 180 inoculation in nude mice, the incidence of tumor growth was less (2 out of 5 mice) than that of the control group (all 5 mice). The tumor size of the control group was about 3-5 times bigger than that of the pretreatment group. In tumor-bearing nude mice, 5 days after sarcoma 180 inoculation, i.v. administration of RPSP significantly suppressed the growth of tumor mass. The inhibition rate was 93.6% on day 13. Furthermore, administration of RPSP did not cause any pathological lesions in vital organs of rabbits such as heart, liver, spleen, lung and kidney. In conclusion, these results indicate that RPSP acts by directly suppressing tumor cell growth in vitro and the prevention of in vivo growth of tumor mass is probably mediated also via its immunomodulating effects.

Animals↗

PubMed1996

Hyporesponsiveness to Ca2+ of aortic smooth muscle in endotoxin-treated rats: no-dependent and -independent in vitro mechanisms.

The aim of this study was to assess the nature of vascular hyporeactivity to vasopressor agents in rats with endotoxemia. Endotoxemia was induced in rats by bacterial endotoxin (E. Coli lipopolysaccaharide, LPS). In LPS-treated rats, the reactivity of endothelium-denuded aortic rings to phenylephrine (PE) and potassium chloride (KCl) was characterized by a decreased magnitude of contraction, a slower onset of contraction and a faster rate of relaxation when compared to the control aortic rings. Addition of L-arginine (L-arg), the substrate of nitric oxide synthase (NOS), but not D-arginine (D-arg), reduced further PE-induced contraction in rings from LPS-treated rats. Inhibition of contraction in rings of LPS-treated rats was partially antagonized by the inhibitor of NOS, N omega-nitro-L-arginine methyl ester (L-NAME). Thus, production of non-endothelial nitric oxide (NO) was in part responsible for the hyporesponsiveness to PE. Rings from LPS-treated rats also displayed hyporeactivity and decreased sensitivity to Ca2+ in depolarizing medium (60 mM K+). Hyporeactivity and hyposensitivity to Ca2+ could only be partially reversed by L-NAME. The inhibitory effects of LPS-treatment on both PE-and KCl-induced aortic responses and the reversal effects of L-NAME confirm the contention that NO formation is involved in vascular hyporesponsiveness in endotoxic shock. The partial reversal by L-NAME of the hyporesponsiveness to KCl- and PE-induced contraction, and hyposensitivity to Ca2+ in depolarized aorta suggest that factors other than the action of nonendothelial source of NO formation in vitro from L-arg also contribute to endotoxin-induced vascular hyporesponsiveness to vasopressor agents.

Acetylcholine↗

PubMed1996

Cobra venom cardiotoxin induces aggregation of washed rat platelets: dual effects of Ca2+.

The effects of cardiotoxin isolated and purified from the venom of Naja naja atra on the platelet aggregation were investigated using washed rat platelets prepared chromatographically. In the presence of 1 mM Ca2+, cardiotoxin concentration-dependently induced aggregation of platelets. While extracellular Ca2+ was necessary for the induction of aggregation by cardiotoxin, high concentration of Ca2+ (e.g., 5 mM) prevented cardiotoxin-induced aggregation. Cardiotoxin-induced platelet aggregation was also blocked by 1 mM Ni2+ but not by 100 microM indomethacin or 1 microM nifedipine, indicating that cardiotoxin-induced platelet aggregation involved Ca(2+)-entry pathway other than L-type Ca2+ channels. High concentration of Ca2+ may prevent cardiotoxin-induced aggregation by competing with cardiotoxin for binding site on the platelet membranes.

Animals↗

PubMed1996

Capacitative Ca2+ entry in HL-60 cells: tetrandrine and SK & F 96365 as probes.

Agonist-activated Ca2+ entry is important in many biological responses such as secretion and cell growth(1,2). In nonexcitable cells which have no voltage-operated Ca2+ channels (VOCC), agonist-receptor interaction can trigger Ca2+ entry across the plasmalemma via several entry pathways(1-3) (Fig 1): (A) channels which are intrinsic structures of the receptor (receptor-operated channels), (B) channels which are coupled to receptors via a G-protein (G-protein-operated channels), (C) channels which are activated by some second messengers (second-messenger-operated channels), and (D) channels which open upon intracellular nonmitochondrial Ca2+ store depletion (Ca2+ release-activated channels) resulting from inositol 1, 4, 5-trisphosphate-induced Ca2+ release or inhibition of Ca2+ re-uptake (see next section). Ca2+ entry via the 4th type of channel, also known as "capacitative Ca2+ entry" (CCE)[4], has aroused much interest in the past decade because of its intriguing nature as retrograde signalling. In this brief review, we present the evidence for and the possible biochemical processes involved in CCE. We also discuss the use of 2 novel Ca2+ entry blockers: tetrandrine and SK&F 96365. Emphasis will be put on the human leukemic HL-60 cell line, a popular cell system for intracellular Ca2+ homeostasis studies and also a model the signal transduction of which we have been investigating during the past few years.

Alkaloids↗

PubMed1996

Tetrandrine differentially inhibits aggregation and ATP-release of rat platelets.

AIM: To examine the effects of tetrandrine (Tet) on the aggregation and ATP-release of rat washed platelets induced by several platelet activators. METHODS: Gel-filtration (Sepharose 2B) was used to isolate washed platelets from adult rats and the platelet aggragation and ATP-release were measured simultaneously. RESULTS: In the presence of Ca2+ 1 mmol.L-1, Tet 300 mumol.L-1 inhibited the aggregation induced by ADP (25 mumol.L-1), collagen (2.5 g.L-1), and thrombin (103 unit.L-1) by 62%, 60%, and 34%, respectively. It also inhibited arachidonic acid (1 mmol.L-1)-induced aggregation. Elevating intracellular Ca2+ concentration with the Ca2+ ionophore, calcimycin (30 mumol.L-1), or by blocking the intracellular calcium pump with cyclopiazonic acid (5 mumol.L-1) initiated platelet aggregation, which was also inhibited by Tet. In Ca(2+)-free medium, Tet still elicited an inhibitory effect on aggregation induced by ristocetin (2.5 g.L-1). Lower concentrations of Tet (30 nmol.L-1 to 3 mumol.L-1) failed to inhibit the aggregation (requiring Tet 10-300 mumol.L-1), but strongly suppressed ATP-release induced by ADP 10 mumol.L-1, both of which were measured simultaneously in a single sample. CONCLUSION: Tet elicits a nonselective inhibitory effect on platelet aggregation not solely due to its Ca2+ antagonism and may act on a final common pathway leading to platelet aggregation. Furthermore, Tet is a much potent inhibitor of the release of ATP in platelets.

Adenosine Triphosphate↗

PubMed1996

Protein kinase C plays no role in KCl-induced vascular contraction in Ca(2+)-free medium.

AIM: To examine the role of protein kinase C (PKC) on the sustained contractile responses of rat aorta to high KCl in isotonic Ca(2+)- and Mg(2+)-free solutions. METHODS: The effects of phorbol 12-myristate 13-acetate (PMA, a PKC activator) and Calphostin C (a selective PKC inhibitor) were observed on the sustained contraction of rat aorta induced by K+ 136 mmol.L-1. EGTA (100 mumol.L-1) was added to prepare the Ca(2+)-free medium and EDTA (100 mumol.L-1) was added to reduce or remove the Mg2+. RESULTS: Aortic contraction to KCl was prominent in low Mg2+ medium and was enhanced by EDTA (K-EDTA contraction). Such contraction was concentration-dependently inhibited by Mg2+, but was not affected by Calphostin C 1 mumol.L-1. Pretreatment of the aortic preparations with PMA (0.8 mumol.L-1) potentiated the contraction to KCl in Ca(2+)-free, low Mg2+ medium and higher concentration of Mg2+ was required to cause relaxation. Such a reduced sensitivity to Mg2+ in the presence of PMA was partially reversed by Calphostin C and was accompanied by an increased sensitivity to Ca2+, which concentration-dependently caused contraction following Mg(2+)-induced relaxation. However, in the presence of EDTA 100 mumol.L-1 (eg, Mg(2+)-free medium), the maximal contraction to KCl in Ca(2+)-free medium was not affected by PMA or Calphostin C. CONCLUSION: KCl-induced contraction in Ca(2+)-free and Mg(2+)-free + EDTA 100 mmol.L-1 medium was not affected by PMA or Calphostoin C, indicating that PKC plays no role in such contractile responses.

Animals↗