Differential effects of endothelin-1 on basal and isoprenaline-enhanced Ca2+ current in guinea-pig ventricular myocytes

1. We examined the effect of endothelin-1 (ET-1) on basal and isoprenaline-enhanced L-type Ca2+ current (ICa,L) in guinea-pig ventricular myocytes under nystatin-perforated patch configuration. 2. ET-1 at concentrations of 1, 5 and 10 nM had little effect on basal ICa,L. However, ICa,L enhanced by isoprenaline (500 nM) was significantly attenuated by 5 nM ET-1 by more than 50%. This effect was reversed upon washout. ICa,L enhanced by forskolin was also decreased by ET-1. 3. The inhibitory effect of ET-1 against isoprenaline was completely blocked by the ETA receptor antagonist BQ-123 (1 microM). In myocytes incubated with pertussis toxin (PTX, 2 micrograms ml-1) for 5 h, ET-1 did not inhibit isoprenaline-enhanced ICa,L. 4. Although ET-1 has been shown to activate specific protein kinase C (PKC) isoforms, a significant inhibitory effect of ET-1 was maintained in the presence of the PKC inhibitor bisindolylmaleimide (20 nM). The nitric oxide (NO) donor SIN-1 (10 microM) attenuated but failed to prevent the ET-1 effect. 5. In summary, our results demonstrate that ET-1 is devoid of any significant effects on basal ICa,L. However, it exerts a potent inhibitory effect against isoprenaline-enhanced ICa,L. This effect is mediated through ETA receptors coupled to PTX-sensitive G-proteins and occurs in the presence of PKC inhibition and NO generation.

Muscarinic stimulation of tracheal smooth muscle cells activates large-conductance Ca2+-dependent K+ channel

Pages C658-C665: G. R. Wade and S. M. Sims. “Muscarinic stimulation of tracheal smooth muscle cells activates large-conductance Ca2+-dependent K+ channel.” Page C662, Fig. 6: the second half of the trace in A was inadvertently duplicated from B. The revised Fig. 6 below shows the correct channel traces. We wish to emphasize that the scientific point of the figure, the reversible antagonism of the cholinergic response by atropine, as well as the quantification in D, remains sound. The data were filtered at 400 Hz and sampled off-line from digital videotape at 2 kHz.

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Intracellular calcium stores in isolated tracheal smooth muscle cells

We investigated the effects of acetylcholine (ACh) and histamine on intracellular calcium concentration ([Ca2+]i) and contraction of freshly isolated guinea pig tracheal smooth muscle cells. Previous electrophysiological studies revealed that agonists elicit cation and Cl- currents, but a role for Ca2+ in mediating these effects remains unresolved. Here we characterize agonist-induced changes of [Ca2+]i, using fura 2, and examine the contribution of the sarcoplasmic reticulum (SR) to regulation of [Ca2+]i. We provide evidence that the rise of [Ca2+]i and the contraction elicited by ACh or histamine are largely due to release of Ca2+ from stores. Agonists elicited Ca2+ transients in Ca(2+)-free solution with 0.5 mM ethylene glycol-bis (beta-aminoethyl ether)-N, N, N',N'-tetraacetic acid (EGTA), whereas prolonged exposure to Ca(2+)-free solution diminished the rise of [Ca2+]i. In addition, blockade of SR Ca(2+)-adenosinetriphosphatase (ATPase) by cyclopiazonic acid (CPA) or thapsigargin caused elevation of [Ca2+]i and reduction of ACh-evoked increase of [Ca2+]i. In many cells, [Ca2+]i fell below baseline (undershoot) after ACh or caffeine. CPA abolished this undershoot and reduced the rate of recovery of [Ca2+]i to basal levels. Furthermore, oscillations of [Ca2+]i were elicited in the presence or absence of extracellular Ca2+, and these too were reversibly abolished by CPA. Our results provide evidence that Ca2+ stores play a significant role in agonist-mediated increase of [Ca2+]i in tracheal muscle and that the SR contributes to the restoration of basal Ca2+ levels.

Osteoclast activation in inflammatory periodontal diseases

OBJECTIVE

In this paper, we review the mechanisms thought to be involved in the activation of osteoclasts in periodontitis.

SUMMARY

Osteoclasts are regulated by both microbial and host factors. Some factors act directly on cells of the osteoclast lineage, whereas others act indirectly through other cell types in the bone environment. The proinflammatory cytokines (interleukins 1 and 6, tumor necrosis factors) have been implicated in the stimulation of osteoclastic resorption. The roles of the immunoregulatory cytoknes (interleukins 2 and 4, interferon gamma) are less clear, but decreased levels of these factors may contribute to periodontitis. A number of lipid mediators may be involved in stimulation of bone resorption. These include bacterial lipopolysaccharide and host-derived platelet-activating factor and prostaglandins. More recently, reactive oxygen intermediates and extracellular nucleotides, both present at sites of inflammation, have been investigated as possible modulators of osteoclast activity. The potential use of antiresorptive therapies in periodontitis is reviewed.

CONCLUSIONS

A wide range of host and bacterial factors contribute to the loss of alveolar bone in periodontitis. However, much remains to be understood about the complex mechanisms through which these factors regulate osteoclast activity. Further studies at the cellular and molecular level will lead to a better understanding of these processes and perhaps suggest new approaches for periodontal therapy.

Cholinergic inhibition of Ca2+ current in guinea-pig gastric and tracheal smooth muscle cells

1. Cholinergic regulation of L-type Ca2+ channels was investigated in freshly dissociated guinea-pig gastric and tracheal smooth muscle cells. Acetylcholine (ACh, 50 microM) decreased Ca2+ channel current (ICa) by 37 +/- 3% (mean +/- S.E.M., 46 cells). 2. ACh reduced ICa at all voltages, with no shift in the current-voltage relationship. Effects of ACh were rapid (within 5 s) and repeatable, with multiple applications reproducibly inhibiting ICa in the continued presence of extracellular Ca2+ and in the presence of protein kinase C inhibitors. 3. The involvement of Ca2+ stores in this inhibition was investigated using Ca(2+)-free solution or cyclopiazonic acid (CPA) to deplete the stores. ACh initially inhibited ICa in the Ca(2+)-free solution (Na+ as charge carrier, 53 +/- 4% decrease, 18 cells) with subsequent responses significantly attenuated (n = 9). CPA (1 microM) reduced, then abolished, the effects of ACh on ICa (n = 5). 4. When studied in cell-attached patches (Ba2+ as charge carrier), ACh reduced Ca2+ channel open probability in twenty-two of thirty-six cells, consistent with the involvement of a diffusible cytosolic messenger. 5. ACh also inhibited ICa in tracheal muscle cells (reduction of 38 +/- 6% in 1 mM Ca2+, 4 cells; 77 +/- 3% in Ca(2+)-free solution, 7 cells). Furthermore, in cells where ACh elicited oscillating Ca(2+)-activated Cl- current, oscillatory inhibition of ICa was also observed (3 cells). 6. In summary, ACh causes rapid and reversible inhibition of ICa in gastric and tracheal muscles. Ca2+ stores were required to initiate this effect, with the rapid onset and oscillatory inhibition consistent with Ca2+ inhibition of the channel. Suppression of ICa would reduce Ca2+ entry during cholinergic excitation.

Influence of organic acid excretion on cuticle pH and drug absorption by Haemonchus contortus

To determine if a cuticle microenvironment pH is maintained by adult Haemonchus contortus, organic acid excretion kinetics and absorption kinetics of selected model weak acids and a weak base were measured in incubation media that varied in buffer capacity (0.25-20 mM HEPES or 5 mM glycine) and initial pH (7.5 or 3.5). To evaluate the importance of the cuticle as a pathway for organic acid excretion and drug absorption the pharynx was paralyzed with 1 nM ivermectin. H. contortus changed the media pH from initial values of 7.5 or 3.25 to an asymptotic value of approximately 5.6. The rate of pH change depended on the buffer capacity, but was not affected by chemical ligation with ivermectin. The intrinsic rate of excretion of organic acids (0.045 +/- 0.016 micromol/cm2 x h) was constant during the first 8-12 h of incubation and was independent of initial pH, buffer capacity or ivermectin ligation. The rates of absorption of the model weak acids, benzoic acid and p-nitrophenol, and the model weak base, aniline, were not affected by initial pH, buffer capacity or ivermectin ligation. These results suggest that H. contortus excretes organic acid endproducts of carbohydrate metabolism across its cuticle, and that these acids maintain a microenvironment pH within the water-filled pores of the cuticle that controls the rate of adsorption of weakly acidic or basic drugs.

Ca(2+)-dependent Cl- current in canine tracheal smooth muscle cells

Our goal was to investigate the role of Ca2+ entry in regulating Cl- current (ICl) in smooth muscle cells from canine trachealis. When studies were done using the perforated patch configuration, depolarization elicited a dihydropyridine-sensitive Ca2+ current (ICa), followed in many cells by a sustained current. This sustained current reversed direction close to the Cl- equilibrium potential, consistent with its representing ICl. The ICl was also apparent as slowly deactivating tail currents seen upon repolarization to negative potentials. The Cl- channel blocker niflumic acid abolished both the sustained and tail currents, without affecting ICa. Several observations indicated that the ICl was dependent on Ca2+ entry. ICl was increased in magnitude when Ca2+ influx was augmented [by prolonging the depolarization or using BAY K 8644 or acetylcholine (ACh)] and decreased in magnitude when Ca2+ influx was reduced (using nifedipine). Based on these findings, we conclude that depolarization causes Ca2+ entry, with resultant elevation of cytosolic free Ca2+ concentration leading to activation of ICl (ICl(Ca)). We investigated whether Ca(2+)-induced Ca2+ release from the sarcoplasmic reticulum was involved in activation of ICl(Ca), by depleting intracellular stores of Ca2+ using cyclopiazonic acid to block the sarcoplasmic Ca(2+)-adenosinetriphosphatase and repeated stimulation with ACh. In such Ca(2+)-depleted cells, depolarization-mediated Ca2+ entry continued to activate ICl(Ca), suggesting that Ca(2+)-induced Ca2+ release was not required for its activation. We conclude that Ca2+ entry can activate Cl- channels in tracheal smooth muscle. This represents a positive-feedback system, which would promote excitation and contraction of airway muscle.

Platelet-activating factor induces pseudopod formation in calcitonin-treated rabbit osteoclasts

We demonstrated previously that platelet-activating factor (PAF), a potent inflammatory mediator, acts on osteoclasts to elevate cytosolic [Ca2+] and stimulate resorption. However, it is not clear whether the effects of PAF on resorptive activity are direct or indirect. In the present study, we investigated the effects of PAF on osteoclast motility. Osteoclasts were isolated from the long bones of neonatal rabbits, and cell motility and morphology were monitored using time-lapse video microscopy. Calcitonin, a hormone known to induce retraction of pseudopods and inhibit resorptive activity, was used to render osteoclasts quiescent. Within 10 minutes of calcitonin treatment (100 ng/ml, final), pronounced retraction of pseudopods was observed in 68 of 112 cells tested. When PAF (200 nM, final) was added 10 minutes after calcitonin treatment, pseudopods were evident 1 h later in 15 of 37 calcitonin-responsive cells tested. In contrast, pseudopods were evident in only 4 of 31 calcitonin-responsive cells treated with control solutions (PAF-vehicle or S-PAF, the biologically inactive stereoisomer of PAF). Pseudopod formation was quantified by measuring the planar area of pseudopods with a computer-based video analysis system. When assessed 60 minutes following PAF treatment, the pseudopod area was significantly greater in PAF-treated cells than in control cells. In some calcitonin-treated osteoclasts, PAF induced pseudopod formation when applied focally using an extracellular micropipette, consistent with a direct action of PAF. We conclude that PAF directly induces pseudopod formation in calcitonin-inhibited osteoclasts, a morphologic response indicative of osteoclast activation.

Spontaneous transient inward currents and rhythmicity in canine and guinea-pig tracheal smooth muscle cells

Spontaneous transient inward currents (STICs) were recorded in canine and guinea-pig tracheal myocytes held at negative membrane potentials. STICs were Cl- selective since their reversal potential was dependent on the Cl- gradient and they were blocked by the Cl- channel blocker niflumic acid. STICs were insensitive to Cs+, charybdotoxin, and nifedipine. Ca(2+)-activated K+ currents often preceded STICs, suggesting that the STICs are Ca2+ dependent. In support of this suggestion, we found the Cl- currents were: (1) abolished by depleting intracellular Ca2+ stores using caffeine, acetylcholine, histamine, or substance P; (2) enhanced by increasing external concentrations of Ca2+; (3) evoked by voltage-dependent Ca2+ influx. The channels responsible for this Cl- current are of small unitary conductance (< 20 pS). Decay of the STICs was described by a single exponential with a time constant of 94 +/- 9 ms at -70 mV; the time constant increased considerably at more positive potentials. Using Ca(2+)-dependent Cl- currents and contractions as indices of internal levels of Ca2+, we found that isolated tracheal cells are capable of exhibiting rhythmic behaviour: bursts of currents and contractions with a periodicity of less than 0.1 Hz and which continued for more than 20 min. These rhythmic events were recorded at negative membrane potentials, suggesting that cyclical release of internally sequestered Ca2+ is responsible. We conclude that spontaneous release of Ca2+ from intracellular stores in tracheal muscle cells leads to transient currents in some cases accompanied by rhythmic contractions. Our studies provide evidence for a cellular mechanism that could underly myogenic oscillations of membrane potential in smooth muscle.

Theoretical perspectives on anthelmintic drug discovery: interplay of transport kinetics, physicochemical properties, and in vitro activity of anthelmintic drugs

This multidisciplinary study demonstrates the utility of the biophysical model approach to assess biological activity of anthelmintics in light of drug-delivery principles. The relationships between drug absorption and efficacy for a set of structurally disparate anthelmintics were determined in cultures of Haemonchus contortus, a nematode that parasitizes the ruminant gastrointestinal tract. Uptake, parameterized by the permeability coefficient, Pe, was shown to occur by absorption across the cuticle. Rates of drug appearance in nematode carcasses paralleled rates of drug disappearance from the medium, and absorption reached an apparent equilibrium within a few hours. The parasite/medium partition coefficient, K, was derived from the ratio of drug concentration in the parasite vs the medium at equilibrium. Pe and K values for each anthelmintic were correlated with lipophilicity (as measured by the partition coefficient (PC) in n-octanol/water) and both parameters plateaued at log PC approximately 2.5, with maximum Pe approximately 8 x 10(-4) cm/min and log K < or = 2.0. Absorption kinetics were related to in vitro potency by monitoring motility of H. contortus. The time required to reduce motility by 50% (t* 50) and Pe were used to calculate Cn*, the drug concentration in the parasite at t* 50, as an indicator of intrinsic potency. The quantitative interplay of apparent biological activity expressed as t* 50, dose, and intrinsic potency highlights the important contribution of drug-uptake kinetics.

Substance P activates Cl- and K+ conductances in guinea-pig tracheal smooth muscle cells

Substance P (SP) causes bronchoconstriction, but its effects on airway smooth muscle ion conductances are unknown. We investigated the effects of SP on single smooth muscle cells dissociated from guinea-pig trachealis. Under voltage clamp at -60 mV, SP evoked reversible contractions and inward current (ISP). ISP had a latency of approximately 1 s, reached a peak of 1039 +/- 147 pA (n = 19) about 2 s after onset of application, and declined to baseline levels over the next 5-10 s. At more positive holding potentials (-25 and 0 mV), the inward current was decreased in magnitude and preceded by outward current. With 140 mM K+ in the electrode and Cl- equilibrium potential (ECl) of about 0 mV, ISP was outwardly rectifying and reversed at -11 +/- 2 mV. When K+ currents were blocked using Cs+, the current-voltage relationship for ISP was linear and reversed at 3 +/- 1 mV. The reversal potential was dependent on the Cl- gradient across the membrane. These results suggest that SP caused a transient activation of Cl- and K+ conductances. Following the initial transient inward current, SP caused a prolonged suppression of spontaneously active K+ currents. The findings that SP evoked contractions during voltage clamp at potentials at which voltage-dependent Ca2+ channels are not active, and that current oscillations were also evoked by SP, suggest that SP is acting through release of Ca2+ from internal stores. Furthermore, SP occluded the inward current evoked by acetylcholine, suggesting that the peptidergic and cholinergic signalling pathways converge.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of extracellular calcium and protons on osteoclast potassium currents

During resorption of mineralized tissues, osteoclasts are exposed to marked changes in the concentration of extracellular Ca2+ and H+. We examined the effects of these cations on two types of K+ currents previously described in these cells. Whole-cell patch clamp recordings of membrane currents were made from osteoclasts freshly isolated from neonatal rats. In control saline (1 mM Ca2+, pH 7.4), the voltage-gated, outwardly rectifying K+ current activates at approximately -45 mV and the conductance is half-maximally activated at -29 mV (V0.5). Increasing [Ca2+]out rapidly and reversibly shifted the current-voltage (I-V) relation to more positive potentials. Current at -29 mV decreased to 28 and 9% of control current at 5 and 10 mM [Ca2+]out, respectively. This effect of elevating [Ca2+]out was due to a positive shift of the K+ channel voltage activation range. Zn2+ or Ni2+ (5 to 500 microM) also shifted the I-V relation to more positive potentials and had additional effects consistent with blockade of the K+ channel. Based on the extent to which these divalent cations affected the voltage activation range of the outwardly rectifying K+ current, the potency sequence was Zn2+ > Ni2+ > Ca2+. Lowering or raising extracellular pH also caused shifts of the voltage activation range to more positive or negative potentials, respectively. In contrast to their effects on the outwardly rectifying K+ current, changes in the concentration of extracellular H+ or Ca2+ did not shift the voltage activation range of the inwardly rectifying K+ current.(ABSTRACT TRUNCATED AT 250 WORDS)

Murine osteoclasts and spleen cell polykaryons are distinguished by mRNA phenotyping

To probe osteoclast gene expression, we combined the techniques of cell microisolation and RT-PCR to develop a novel and sensitive method for the isolation and mRNA phenotyping of small numbers of authentic osteoclasts and spleen cell polykaryons. Using this method we report (1) direct evidence for the presence of calcitonin receptor mRNA in osteoclasts, (2) confirmation of the recent finding of osteopontin mRNA in osteoclasts, and (3) demonstration that the specific expression of mRNA for tartrate-resistant acid phosphatase, carbonic anhydrase II, calcitonin receptor, and osteopontin enable one to distinguish the osteoclast from the morphologically similar and developmentally related spleen cell polykaryon. We also show that mRNA associated with the osteoblast phenotype, such as alkaline phosphatase, osteocalcin, and type I collagen, are absent in osteoclasts. This is the first report in which such an approach has been used successfully to distinguish the mRNA expression pattern of an authentic osteoclast from a macrophage polykaryon, and as such it should provide an important new tool for evaluating the results of various cell culture model systems designed to examine the origin and ontogeny of osteoclasts. Our results also indicate that these procedures can be used as an alternative to in situ hybridization methods for the cell-specific localization of specific mRNA in a mixed cell preparation and for colocalization of multiple mRNA species to a single cell type.

Outwardly rectifying chloride current in rabbit osteoclasts is activated by hyposmotic stimulation

1. We characterized chloride currents in freshly isolated rabbit osteoclasts using whole-cell and single channel patch-clamp recording configurations. Depolarization activated an outwardly rectifying current in 40-50% of cells, distinct from the inwardly rectifying K+ current we have previously reported in osteoclasts. 2. The outwardly rectifying current persisted under conditions where all K+ currents were blocked. Furthermore, the outward current was reversibly inhibited by Cl- transport blockers 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS); 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS); 4,4'-dinitrostilbene-2,2'-disulphonic acid (DNDS); and niflumic acid. The blocked current had a reversal potential close to the predicted chloride equilibrium potential and was dependent on the chloride concentration gradient. 3. In those osteoclasts in which outwardly rectifying current was not initially apparent, exposure to hyposmotic extracellular solution resulted in its reversible activation. The induced current was due to Cl-, based on its reversal close to the chloride equilibrium potential and sensitivity to blockade by Cl- channel inhibitors. The hyposmotically induced current could be activated in Ca(2+)-free solutions containing 0.2 mM EGTA. 4. When studied in the current-clamp configuration, hyposmotic stimulation caused depolarization from -76 +/- 5 to -5 +/- 6 mV (mean +/- S.D., n = 7). 5. Unitary Cl- currents were recorded in the cell-attached patch configuration at positive potentials. Single channels had a slope conductance of 19 +/- 3 pS (n = 5). Reduction of the external [Cl-] shifted the current-voltage relationship in the positive direction, supporting the conclusion that these were Cl- currents. Like the whole-cell currents, single channel Cl- currents were activated by exposure of cells to hyposmotic bathing solution. 6. We conclude that rabbit osteoclasts express an outwardly rectifying Cl- current that can be activated by osmotic stress. Cl- channels may play a role in cell volume regulation and may also provide conductive pathways for dissipating the potential difference that arises from electrogenic proton transport during bone resorption.

Lamellipod extension and K+ current in osteoclasts are regulated by different types of G proteins

Osteoclasts are the cells responsible for the resorption of bone and other mineralized tissues. GTP-binding proteins (G proteins) play important roles in regulating the activity of many cell types; however, there is limited knowledge of their functions in osteoclasts. We used the patch-clamp technique in the whole-cell configuration to introduce either hydrolysis-resistant guanosine triphosphate analogues or fluoroaluminate into single rat osteoclasts, and examined the effects of G protein activation on cell morphology and ionic conductances. Guanosine 5'-O-(3-thiotriphosphate) or 5'-guanylyl-imidodiphosphate, but not the control compounds adenosine 5'-O-(3-thiotriphosphate) or guanosine 5'-O-(2-thiodiphosphate), induced: (1) prompt spreading due to extension of lamellipodia; and (2) after a latency of several minutes, complete suppression of the inwardly rectifying K+ current. Pertussis toxin did not alter either spreading or suppression of K+ current induced by guanosine 5'-O-(3-thiotriphosphate). Cytochalasin D, but not colchicine, prevented guanosine 5'-O-(3-thiotriphosphate)-induced spreading, consistent with actin polymerization underlying lamellipod extension. Whole-cell capacitance did not change during guanosine 5'-O-(3-thiotriphosphate)-induced spreading, which is consistent with a lack of change in total plasma membrane area. Fluoroaluminate did not induce spreading, but it did suppress the K+ current. The differential effects of fluoroaluminate and guanosine 5'-O-(3-thiotriphosphate) suggest that lamellipod extension is regulated by a small molecular mass, monomeric G protein, whereas the inwardly rectifying K+ current is regulated by a large molecular mass, heterotrimeric G protein. Thus, osteoclast motility and ion transport are regulated by separate G protein-coupled pathways.

Mammalian osteoclasts express a transient potassium channel with properties of Kv1.3

Previous studies have revealed that expression of K+ channels in osteoclasts correlates with cell morphology and is influenced by interaction with the extracellular matrix. In this study, we investigated the electrophysiological properties of an outwardly rectifying K+ channel in rat and mouse osteoclasts using patch-clamp techniques. Cell-attached patch recordings revealed a channel of approximately 14 pS conductance that opened upon depolarization, and had a reversal potential close to that predicted for a K+ channel. Channel activity was transient; inactivation of ensemble currents, like that of whole-cell currents, occurred as a single exponential process. Both single-channel and macroscopic currents exhibited use-dependent inactivation in response to repetitive depolarizations. Two scorpion toxins, margatoxin and charybdotoxin, blocked this transient K+ channel, with half-maximal inhibition at 200 pM and 5 nM, respectively. In contrast, dendrotoxin (500 nM) had little effect. In summary, the outwardly rectifying K+ channel in osteoclasts resembles the Shaker-related K+ channel, Kv1.3. When membrane potential was recorded in whole-cell configuration, charybdotoxin (50 nM) caused a depolarization of 5 to 10 mV from resting levels of -50 mV or more positive; therefore this K+ channel contributes to the membrane potential of osteoclasts under some conditions. To investigate the molecular nature of osteoclast K+ channels, we performed RT-PCR on osteoclast RNA using primers for Kv1.3 and the inward rectifier, IRK1. mRNA encoded by Kv1.3 and IRK1 was detected and message identity confirmed by restriction enzyme digestion and sequence analysis. We conclude that osteoclasts exhibit, in addition to the previously described inward rectifier, an outwardly rectifying K+ conductance with properties of the Kv1.3. channel.

Biophysical model of the transcuticular excretion of organic acids, cuticle pH and buffer capacity in gastrointestinal nematodes

A biophysical model was developed, using Ascaris suum as a model gastrointestinal nematode, to provide quantitative perspectives into the microenvironmental pH within the water-filled, porous, negatively charged cuticle matrix of gastrointestinal nematodes. The central features of the model include (a) the constant rate of excretion of organic acid metabolites across the cuticle, (b) the relationship between cuticle pH and pKa of the organic acids that determines the fraction of unionized and ionized species, and (c) the concentration gradient, mean concentration and buffer capacity within the cuticle that maintain the cuticle pH. The model may be used to predict the extent to which transcuticular absorption of weakly basic and acidic anthelmintics will be affected by transcuticular excretion of organic acid metabolites. Coupled with established models for drug absorption by nematodes and the host gastrointestinal tract, the cuticle pH model provides new insights to the design of drugs with physicochemical properties that favor absorption by nematodes.

Emptying and refilling of Ca2+ store in tracheal myocytes as indicated by ACh-evoked currents and contraction

Membrane currents and contractions evoked by acetylcholine (ACh) in freshly dissociated canine tracheal myocytes were investigated using the nystatin perforated-patch recording technique. In cells held at -60 mV in the presence of nifedipine, ACh evoked inward current (IACh) and contraction. Caffeine mimicked the effects of ACh. IACh and contractions could be evoked 3-4 min after removing external Ca2+ but were abolished by prolonged exposure to Ca(2+)-free media. Both responses were restored within minutes of reintroduction of Ca2+, even though the cells were held at -60 mV in the presence of nifedipine. IACh and ACh-evoked contractions were also reversibly abolished by continued exposure to caffeine. Cyclopiazonic acid (CPA), a blocker of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase, reduced IACh by > 95% within 15 min but had little or no effect on the contractile responses evoked by ACh. IACh was restored after washout of CPA even though cells were held at -60 mV. After depleting the Ca2+ store with the use of CPA, depolarization of the membrane to +10 mV immediately before application of ACh led to a partial restoration of IACh. This restorative effect of depolarization was potentiated by Bay K 8644 and antagonized by nifedipine. In conclusion, IACh and contractions in canine tracheal myocytes are mediated by Ca2+ released from an internal store that can be depleted by prolonged removal of extracellular Ca2+, prolonged exposure to caffeine, or by blockade of the SR Ca(2+)-ATPase. At least two Ca2+ influx pathways appear to contribute to refilling of the internal store: one pathway that is not activated by depolarization or ACh and a second involving dihydropyridine-sensitive voltage-activated Ca2+ channels that may be in direct contact with the SR (i.e., conduct extracellular Ca2+ directly into the SR, bypassing the cytosol).

Cholinergic Excitation of Smooth Muscle

Acetylcholine activates muscarinic receptors on smooth muscle to cause depolarization and contraction. Several mechanisms can contribute to depolarization, including activation of inward cation and Cl- currents, suppression of outward K+ currents, and augmentation of voltage-dependent Ca2+ current. Acetylcholine also releases Ca2+ from internal stores, directly contributing to contraction.

Muscarinic stimulation of tracheal smooth muscle cells activates large-conductance Ca(2+)-dependent K+ channel

We investigated the regulation of the large-conductance Ca(2+)-dependent K+ (KCa) channel by acetylcholine (ACh) in freshly dissociated tracheal smooth muscle cells. Channels were recorded in the cell-attached patch configuration, and cells were stimulated with ACh, muscarine, or caffeine. We identified KCa channel activity based on 1) the voltage dependence of channel opening; 2) the large unitary conductance (242 +/- 5 pS with symmetrical 135 mM K+); 3) dependence of the reversal potential on the [K+] gradient, shifting 56 +/- 3 mV/10-fold change in extracellular [K+]; and 4) opening of channels after elevation of cytosolic free Ca2+ concentration ([Ca2+]i) using the Ca2+ ionophore A23187. When cells were bathed either in a physiological saline solution or a solution containing 135 mM K+ (to clamp cell membrane potential near 0 mV), ACh caused contraction of cells and activation of voltage-dependent channels. With 135 mM extracellular K+, the channels activated by ACh had a unitary conductance of 247 +/- 10 pS, and currents reversed near the K+ equilibrium potential (-1 +/- 1 mV). The effects of ACh were reversible, blocked by atropine, and mimicked by muscarine. From these characteristics we conclude that muscarinic stimulation of canine tracheal smooth muscle cells leads to activation of the large-conductance KCa channel. Because the KCa channels were isolated from ACh by the patch pipette, the increased channel activity was probably mediated by a cytosolic second messenger. ACh shifted the threshold for KCa channel opening to less positive membrane potentials, similar to that seen with elevation of [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)

Haemonchus contortus: ivermectin-induced paralysis of the pharynx

How the avermectins cause the elimination of gastrointestinal nematodes from host animals has not yet been clearly identified. Using visual and radiometric parameters to measure oral ingestion in Haemonchus contortus, we showed that ivermectin (IVM) rapidly inhibited ingestion at concentrations > or = 10(-10) M. Motility, monitored quantitatively with an automated motility meter, was unaffected by IVM at concentrations < or = 10(-8) M, while ATP levels were unaffected at concentrations < or = 10(-6) M. Since motility and ATP levels, independent measures of short-term viability, are unaffected by concentrations of IVM that effectively block oral ingestion, the drug can be used as a chemical ligature. Although H. contortus was shown to be dependent upon an exogenous supply of glucose for survival in culture, IVM (10(-9) and 10(-7) M) altered neither the uptake of 3-O-[3H]methylglucose nor the metabolism of [13C]glucose by the parasite. These data suggest that H. contortus depends upon the transcuticular uptake of glucose in culture. If oral ingestion of other nutrients is essential for long-term survival in vivo, disruption of this process may represent the primary mechanism of IVM action.

Histamine activates Cl- and K+ currents in guinea-pig tracheal myocytes: convergence with muscarinic signalling pathway

1. We investigated the effects of histamine on membrane currents and contractile state of isolated guinea-pig tracheal myocytes using perforated patch and whole-cell recording techniques. The effects of histamine were compared to those of acetylcholine (ACh) and caffeine. 2. During voltage clamp (Vhold = -60 mV), histamine elicited contraction and an inward current (Ihist) which was often followed by current oscillations. Ihist had a reversal potential (Vrev) of -9 +/- 3 mV. 3. Ihist was dependent on the Cl- gradient and was antagonized by the Cl- channel blocker niflumic acid. Vrev was more positive (+2 +/- 1 mV) when K(+)-selective currents were blocked by Cs+ and TEA. When all external Na+ was replaced with N-methyl-D-glucamine, there was a small reduction in the amplitude of Ihist. 4. The histamine-induced current was similar to that elicited by ACh and by caffeine with respect to time course, amplitude, and current-voltage relationship. Responses to histamine and to ACh were non-additive, consistent with a convergence of histaminergic and cholinergic signalling pathways. Ihist was antagonized by the H1 histaminergic receptor antagonist astemizole, but not by atropine. 5. When recorded using the perforated patch configuration, Ihist could be elicited repeatedly for more than 30 min. When cells were studied in the whole-cell configuration using a pipette solution containing 0.025 mM EGTA, the amplitude of Ihist was initially the same as that obtained using perforated patch but then decreased; the time required for the responses to decrease to 50% (t1/2) was 8.2 +/- 1.0 min. When 1 mM EGTA was included in the pipette solution (whole-cell configuration), the initial response to histamine was significantly decreased in size and t1/2 was reduced to 3.3 +/- 0.7 min. 6. The characteristics of the signalling pathway were examined in cells studied using the whole-cell configuration with 0.025 mM EGTA in the recording pipette. Heparin significantly reduced t1/2 to 4.3 +/- 0.8 min. GTP gamma S elicited inward current and oscillations; both effects were enhanced by histamine. GTP gamma S also reduced t1/2 to 1.4 +/- 0.1 min. Pertussis toxin did not alter the amplitude or time course of Ihist. 7. We conclude that in guinea-pig tracheal myocytes, binding of histamine to H1 receptors leads to release of Ca2+ from intracellular stores and subsequent activation of Cl- and K+ conductances as well as contraction. Furthermore, we demonstrate that ACh elicits similar physiological responses due to a convergence of the histaminergic and muscarinic signalling pathways.

Mechanistic approaches to quantitate anthelmintic absorption by gastrointestinal nematodes

In this article, David Thompson, Norman Ho, Sandra Sims and Timothy Geary look at the problem o f quantitating drug absorption by gastrointestinal nematodes.

Platelet-activating factor stimulates resorption by rabbit osteoclasts in vitro

We have shown previously that platelet-activating factor (PAF), a potent inflammatory mediator, acts directly on isolated rat osteoclasts to elevate cytosolic free Ca2+ concentration ([Ca2+]i). The purpose of this study was to examine the effects of PAF on osteoclast function. Osteoclasts were isolated from the long bones of neonatal rabbits and studied in three ways. [Ca2+]i of fura-2-loaded osteoclasts was monitored by microspectrofluorimetry. In 9 out of 16 cells tested, PAF (10-100 nM) caused elevation of [Ca2+]i that peaked then returned to baseline. In contrast, the biologically inactive precursor and metabolite of PAF, lyso-PAF, was without effect. Using time-lapse videomicroscopy, we found that PAF elicited retraction of peripheral pseudopods. Although calcitonin induced sustained retraction and immobility, the response to PAF was transient and, within 30 min, pseudopods reformed. To assess effects of PAF on resorptive activity, osteoclasts were cultured on dentin slices for 48 h in the presence of vehicle, PAF (200 nM), or calcitonin (100 ng/ml). PAF increased the area of individual resorption pits (from control values of 1,660 +/- 110 to 2,240 +/- 200 microns2, P < 0.05) and the total planar area resorbed per unit area of substrate (from 7.6 +/- 1.6 to 14.5 +/- 3.1 x 10(4) microns2/cm2, P < 0.025). As expected, calcitonin significantly decreased resorptive activity. These data indicate that PAF activates osteoclastic resorption. PAF may play a role in mediating the resorption of bone and mineralized cartilage in inflammatory diseases such as rheumatoid arthritis and periodontitis.

Role for diacylglycerol in mediating the actions of ACh on M-current in gastric smooth muscle cells

The role of the second messenger diacylglycerol (DAG) in mediating muscarinic suppression of M-current, a type of a voltage-gated K+ current that is suppressed by acetylcholine (ACh), was examined in freshly isolated smooth muscle cells from toad stomach. Currents were recorded using a single electrode voltage clamp employing conventional microelectrodes. Extracellular application of 1,2-dioctanoyl-sn-glycerol (DiC8), a synthetic DAG that is a potent activator of protein kinase C (PKC), reversibly suppressed M-current. Current relaxations, representing the voltage-dependent closure of K+ channels underlying M-current, were also decreased by DiC8, although suppression was not always as complete as it was with ACh. In contrast, another DAG analogue, 1,2-dioctanoyl-3-thioglycerol, which has a structure closely related to DiC8 but does not activate PKC, failed to inhibit M-current. Furthermore, M-current induced by the beta-agonist isoproterenol, by a mechanism apparently mediated by adenosine 3',5'-cyclic monophosphate (S. M. Sims, L. H. Clapp, J. V. Walsh, Jr., and J. J. Singer. Pflugers Arch. 417: 291, 1990), was also suppressed by DiC8. Both ACh and DiC8 were found to suppress endogenous and isoproterenol-induced M-current without altering the time course of M-current deactivation, suggesting that these agents act by decreasing the number of channels available to be opened. These results provide evidence that muscarinic regulation of M-current is mediated by DAG.