GABA(B) receptors function as a heteromeric assembly of the subunits GABA(B)R1 and GABA(B)R2

The principal inhibitory neurotransmitter GABA (gamma-aminobutyric acid) exerts its effects through two ligand-gated channels, GABA(A) and GABA(C) receptors, and a third receptor, GABA(B) , which acts through G proteins to regulate potassium and calcium channels. Cells heterologously expressing the cloned DNA encoding the GABA(B)R1 protein exhibit high-affinity antagonist-binding sites, but they produce little of the functional activity expected from studies of endogenous GABA(B) receptors in the brain. Here we describe a new member of the GABA(B) polypeptide family, GABA(B)R2, that shows sequence homology to GABA(B)R1. Neither GABA(B)R1 nor GABA(B)R2, when expressed individually, activates GIRK-type potassium channels; however, the combination of GABA(B)R1 and GABA(B)R2 confers robust stimulation of channel activity. Both genes are co-expressed in individual neurons, and both proteins co-localize in transfected cells. Moreover, immunoprecipitation experiments indicate that the two polypeptides associate with each other, probably as heterodimers. Several G-protein-coupled receptors (GPCRs) exist as high-molecular-weight species, consistent with the formation of dimers by these receptors, but the relevance of these species for the functioning of GPCRs has not been established. We have now shown that co-expression of two GPCR structures, GABA(B)R1 and GABA(B)R2, belonging to the same subfamily is essential for signal transduction by GABA(B) receptors.

Tat-associated kinase, TAK, activity is regulated by distinct mechanisms in peripheral blood lymphocytes and promonocytic cell lines

TAK, a multisubunit cellular protein kinase that specifically associates with the human immunodeficiency virus Tat proteins and hyperphosphorylates the carboxyl-terminal domain of RNA polymerase II, is a cofactor for Tat and mediates its transactivation function. The catalytic subunit of TAK has been identified as cyclin-dependent kinase Cdk9, and its regulatory partner has been identified as cyclin T1; these proteins are also components of positive transcription elongation factor P-TEFb. TAK activity is up-regulated upon activation of peripheral blood lymphocytes and following macrophage differentiation of promonocytic cell lines. We have found that activation of peripheral blood lymphocytes results in increased mRNA and protein levels of both Cdk9 and cyclin T1. Cdk9 and cyclin T1 induction occurred in purified CD4(+) primary T cells activated by a variety of stimuli. In contrast, phorbol ester-induced differentiation of promonocytic cell lines into macrophage-like cells produced a large induction of cyclin T1 protein expression from nearly undetectable levels, while Cdk9 protein levels remained at a constant high level. Measurements of cyclin T1 mRNA levels in a promonocytic cell line suggested that regulation of cyclin T1 occurs at a posttranscriptional level. These results suggest that cyclin T1 and TAK function may be required in differentiated monocytes and further show that TAK activity can be regulated by distinct mechanisms in different cell types.

Halothane attenuates calcium sensitization in airway smooth muscle by inhibiting G-proteins

BACKGROUND

Halothane directly relaxes airway smooth muscle partly by decreasing the Ca2+ sensitivity. In smooth muscle, receptor stimulation is thought to increase Ca2+ sensitivity via a cascade of heterotrimeric and small monomeric guanine nucleotide-binding proteins (G-proteins). Whether this model is applicable in the airway and where halothane acts in this pathway were investigated.

METHODS

A beta-escin-permeabilized canine tracheal smooth muscle preparation was used. Exoenzyme C3 of Clostridium botulinum, which inactivates Rho monomeric G-proteins, was used to evaluate the involvement of this protein in the Ca2+ sensitization pathway. The effects of halothane on different stimulants acting at different levels of signal transduction were compared: acetylcholine on the muscarinic receptor, aluminum fluoride (AIF4-) on heterotrimeric G-proteins, and guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) on all G-proteins.

RESULTS

Exoenzyme C3 equally attenuated acetylcholine- and AIF4--induced Ca2+ sensitization, suggesting that these pathways are both mediated by Rho. Halothane applied before stimulation equally attenuated acetylcholine- and AIF4--induced Ca2+ sensitization. However, when added after Ca2+ sensitization was established, the effect of halothane was greater during Ca2+ sensitization induced by acetylcholine compared with AIF4-, which, along with the previous result, suggests that halothane may interfere with dissociation of heterotrimeric G-proteins. Halothane applied during GTPgammaS-induced Ca2+ sensitization had no significant effect on force, suggesting that halothane has no effect downstream from monomeric G-proteins.

CONCLUSION

Halothane inhibits increases in Ca2+ sensitivity of canine tracheal smooth muscle primarily by interfering with the activation of heterotrimeric G-proteins, probably by inhibiting their dissociation.

The interaction between HIV-1 Tat and human cyclin T1 requires zinc and a critical cysteine residue that is not conserved in the murine CycT1 protein

HIV-1 Tat activates transcription through binding to human cyclin T1, a regulatory subunit of the TAK/P-TEFb CTD kinase complex. Here we show that the cyclin domain of hCycT1 is necessary and sufficient to interact with Tat and promote cooperative binding to TAR RNA in vitro, as well as mediate Tat transactivation in vivo. A Tat:TAR recognition motif (TRM) was identified at the carboxy-terminal edge of the cyclin domain, and we show that hCycT1 can interact simultaneously with Tat and CDK9 on TAR RNA in vitro. Alanine-scanning mutagenesis of the hCycT1 TRM identified residues that are critical for the interaction with Tat and others that are required specifically for binding of the complex to TAR RNA. Interestingly, we find that the interaction between Tat and hCycT1 requires zinc as well as essential cysteine residues in both proteins. Cloning and characterization of the murine CycT1 protein revealed that it lacks a critical cysteine residue (C261) and forms a weak, zinc-independent complex with HIV-1 Tat that greatly reduces binding to TAR RNA. A point mutation in mCycT1 (Y261C) restores high-affinity, zinc-dependent binding to Tat and TAR in vitro, and rescues Tat transactivation in vivo. Although overexpression of hCycT1 in NIH3T3 cells strongly enhances transcription from an integrated proviral promoter, we find that this fails to overcome all blocks to productive HIV-1 infection in murine cells.

Cloned human and rat galanin GALR3 receptors. Pharmacology and activation of G-protein inwardly rectifying K+ channels

The neuropeptide galanin has been implicated in the regulation of processes such as nociception, cognition, feeding behavior, and hormone secretion. Multiple galanin receptors are predicted to mediate its effects, but only two functionally coupled receptors have been reported. We now report the cloning of a third galanin receptor distinct from GALR1 and GALR2. The receptor, termed GALR3, was isolated from a rat hypothalamus cDNA library by both expression and homology cloning approaches. The rat GALR3 receptor cDNA can encode a protein of 370 amino acids with 35% and 52% identity to GALR1 and GALR2, respectively. Localization of mRNA by solution hybridization/RNase protection demonstrates that the GALR3 transcript is widely distributed, but expressed at low abundance, with the highest levels in the hypothalamus and pituitary. We also isolated the gene encoding the human homologue of GALR3. The human GALR3 receptor is 90% identical to rat GALR3 and contains 368 amino acids. Binding of porcine 125I-galanin to stably expressed rat and human GALR3 receptors is saturable (rat KD = 0.98 nM and human KD = 2.23 nM) and displaceable by galanin peptides and analogues in the following rank order: rat galanin, porcine galanin approximately M32, M35 approximately porcine galanin-(-7 to +29), galantide, human galanin > M40, galanin-(1-16) > [D-Trp2]galanin-(1-29), galanin-(3-29). This profile resembles that of the rat GALR1 and GALR2 receptors with the notable exception that human galanin, galanin-(1-16), and M40 show lower affinity at GALR3. In Xenopus oocytes, activation of rat and human GALR3 receptors co-expressed with potassium channel subunits GIRK1 and GIRK4 resulted in inward K+ currents characteristic of Gi/Go-coupled receptors. These data confirm the functional efficacy of GALR3 receptors and further suggest that GALR3 signaling pathways resemble those of GALR1 in that both can activate potassium channels linked to the regulation of neurotransmitter release.

cGMP-independent mechanism of airway smooth muscle relaxation induced by S-nitrosoglutathione

This study tested the hypothesis that the NO donor S-nitrosoglutathione (GSNO) relaxes canine tracheal smooth muscle (CTSM) in part by a cGMP-independent process that involves reversible oxidation of intracellular thiols. GSNO caused a concentration-dependent relaxation in ACh-contracted strips (EC50 approximately 1.2 microM) accompanied by a concentration-dependent increase in cytosolic cGMP concentration ([cGMP]i). The soluble guanylate cyclase inhibitor methylene blue prevented the increase in [cGMP]i induced by 1 and 10 microM GSNO, but isometric force decreased by 10 +/- 4 and 55 +/- 3%, respectively. After recovery of [cGMP]i to baseline, GSNO-induced relaxation persisted during continuous ACh stimulation. Dithiothreitol caused a rapid recovery of isometric force to values similar to those obtained with ACh alone in these strips. We conclude that GSNO relaxes CTSM contracted by ACh in part by oxidation of intracellular protein thiols.

Drug-specific effects of volatile anesthetics on Ca2+ sensitization in airway smooth muscle

Halothane directly relaxes airway smooth muscle, partly by decreasing the Ca2+ sensitivity during membrane receptor stimulation. The effects of other volatile anesthetics on Ca2+ sensitivity are unclear. In the current study, we compared the ability of halothane, sevoflurane, and isoflurane to inhibit increases in Ca2+ sensitivity during muscarinic receptor stimulation. Beta-escin-permeabilized canine tracheal smooth muscle strips were used. Anesthetics were applied during contractions induced by 3 microM acetylcholine and 10 microM guanosine 5'-triphosphate at a constant cytosolic Ca2+ concentration of 0.3 microM. Effects were evaluated as a percent relaxation from initial force corrected for time. Halothane significantly decreased force at both low (0.76 minimum alveolar anesthetic concentration [MAC]) and high (1.8 MAC) concentrations in a concentration-dependent manner. Sevoflurane also decreased force, significantly so at a high concentration (1.7 MAC). Isoflurane did not significantly affect force even at a high concentration (1.7 MAC). Halothane's relaxing effect was significantly greater than that of the other two anesthetics at each corresponding MAC concentration. Among these three volatile anesthetics compared at equipotent anesthetic concentrations, halothane was the most potent in reducing Ca2+ sensitivity during muscarinic receptor stimulation in canine tracheal smooth muscle. This may contribute to halothane's greater relaxing effect compared with isoflurane at the same MAC concentrations in intact airway smooth muscle.

IMPLICATIONS

In this study, we showed that three volatile anesthetics (halothane, sevoflurane, and isoflurane) compared at equipotent anesthetic concentrations differed in their ability to inhibit Ca2+ sensitivity during muscarinic receptor stimulation in airway smooth muscle. The potency order was halothane > sevoflurane > or = isoflurane.

Effect of phorbol esters on Ca2+ sensitivity and myosin light-chain phosphorylation in airway smooth muscle

We studied in beta-escin-permeabilized canine tracheal smooth muscle (CTSM) the effect of the protein kinase C (PKC) agonist phorbol 12,13-dibutyrate (PDBu) on isometric force at a constant submaximal Ca2+ concentration (i.e., the effect on Ca2+ sensitivity) and regulatory myosin light-chain (rMLC) phosphorylation. PDBu increased Ca2+ sensitivity, an increase associated with a concentration-dependent, sustained increase in rMLC phosphorylation. PDBu altered the relationship between rMLC phosphorylation and isometric force such that the increase in isometric force was less than that expected for the increase in rMLC phosphorylation observed. The effect of four PKC inhibitors [calphostin C, chelerythrine chloride, a pseudosubstrate inhibitor for PKC, PKC peptide-(19-31) (PSSI), and staurosporine] on PDBu-induced Ca2+ sensitization as well as the effect of calphostin C and PSSI on rMLC phosphorylation were determined. Whereas none of these compounds prevented or reversed the PDBu-induced increase in Ca2+ sensitivity, the PDBu-induced increase in rMLC phosphorylation was inhibited. We conclude that PDBu increases rMLC phosphorylation by activation of PKC but that the associated PDBu-induced increases in Ca2+ sensitivity are mediated by mechanisms other than activation of PKC in permeabilized airway smooth muscle.

The in vivo production of Spodoptera littoralis nuclear polyhedrosis virus

The in vivo production of the nucleopolyhedrovirus (NPV) of the Egyptian cotton leafworm Spodoptera littoralis was studied experimentally. Larvae (7 days old) of 30-50 mg were experimentally infected with a range of NPV doses then harvested alive at various times after dosing to determine the effect of dose and incubation time on NPV productivity. Maximum NPV production achieved after 7 days incubation was 1.86 x 10(9) polyhedral inclusion bodies (PIBs) per larvae using an inoculum of 1 x 10(4) PIBs. Adjusting the inoculum dose had limited impact on NPV productivity but the correct selection of harvesting time was crucial in maximising the yield, both to achieve peak NPV production in individual larvae and to avoid losses from the death and disintegration of larvae if harvesting was delayed too long.

A novel CDK9-associated C-type cyclin interacts directly with HIV-1 Tat and mediates its high-affinity, loop-specific binding to TAR RNA

The HIV-1 Tat protein regulates transcription elongation through binding to the viral TAR RNA stem-loop structure. We have isolated a novel 87 kDa cyclin C-related protein (cyclin T) that interacts specifically with the transactivation domain of Tat. Cyclin T is a partner for CDK9, an RNAPII transcription elongation factor. Remarkably, the interaction of Tat with cyclin T strongly enhances the affinity and specificity of the Tat:TAR RNA interaction, and confers a requirement for sequences in the loop of TAR that are not recognized by Tat alone. Moreover, overexpression of human cyclin T rescues Tat activity in nonpermissive rodent cells. We propose that Tat directs cyclin T-CDK9 to RNAPII through cooperative binding to TAR RNA.

Prediction of femoral fracture load using automated finite element modeling

Hip fracture is an important cause of morbidity and mortality among the elderly. Current methods of assessing a patient's risk of hip fracture involve local estimates of bone density (densitometry), and are limited by their inability to account for the complex structural features of the femur. In an effort to improve clinical and research tools for assessing hip fracture risk, this study investigated whether automatically generated, computed tomographic (CT) scan-based finite element (FE) models can be used to estimate femoral fracture load in vitro. Eighteen pairs of femora were examined under two loading conditions one similar to loading during the stance phase of gait, and one simulating impact from a fall. The femora were then mechanically tested to failure and regression analyses between measured fracture load and FE-predicted fracture load were performed. For comparison, densitometry measures were also examined. Significant relationships were found between measured fracture load and FE-predicted fracture load (r = 0.87, stance; r = 0.95, fall; r = 0.97, stance and fall data pooled) and between measured fracture load and densitometry data (r = 0.78, stance; r = 0.91, fall). These results indicate that this sophisticated technique, which is still early in its development, can achieve precision comparable to that of densitometry and can predict femoral fracture load to within -40% to +60% with 95% confidence. Therefore, clinical use of this approach, which would require additional X-ray exposure and expenditure for a CT scan, is not justified at this point. Even so, the potential advantages of this CT/FE technique support further research in this area.

The role of cGMP in the relaxation to nitric oxide donors in airway smooth muscle

The aim of this study was to determine the effect of the soluble guanylyl cyclase inhibitors methylene blue and LY83583 (6-anilino-5,8-quinolinedione) on relaxation and increases in intracellular guanosine 3',5'-cyclic monophosphate (cGMP) concentration ([cGMP]i) induced by sodium nitroprusside, 3-morpholinosydnonimine (SIN-1) and diethylamine-nitric oxide (NO) in porcine tracheal smooth muscle in vitro. We measured (1) the effect of NO donors on isometric force and [cGMP]i and (2) the ability of methylene blue and LY83583 to antagonize these effects. In muscle strips contracted with carbachol (0.1-0.3 microM), both sodium nitroprusside and diethylamine-NO caused relaxation and an increase in [cGMP]i. By contrast, SIN-1 caused a relaxation which was not associated with a concomitant increase in [cGMP]i. Methylene blue (10 microM) and LY83583 (10 microM) completely blocked the increase in [cGMP]i induced by sodium nitroprusside and diethylamine-NO; however substantial relaxation remained. It is concluded that in porcine airway smooth muscle, (1) relaxation induced by some NO donors may occur without a concomitant increase in [cGMP]i; and (2) whereas relaxation induced by some NO donors may be associated with increases in [cGMP]i, the relaxation is not completely dependent upon it.

Cloning and characterization of the human galanin GALR2 receptor

We present the molecular cloning and characterization of the human galanin receptor, hGALR2. hGALR2 shares 85%, 39%, and 57% amino acid identities to rGALR2, hGALR1, and hGALR3, respectively. hGALR2, along with rGALR2, can be distinguished from the other cloned galanin receptors by a tolerance for both N-terminal extension and C-terminal deletion of galanin, as well as by a primary signaling mechanism involving phosphatidyl inositol hydrolysis and calcium mobilization. By RT-PCR, GALR2 mRNA was abundant in human hippocampus, hypothalamus, heart, kidney, liver, and small intestine. A weak GALR2 mRNA signal was detected in human retina, and no signal was detected in cerebral cortex, lung, spleen, stomach, or pituitary.

Histone acetyltransferases regulate HIV-1 enhancer activity in vitro

Specific inhibitors of histone deacetylase, such as trichostatin A (TSA) and trapoxin (TPX), are potent inducers of HIV-1 transcription in latently infected T-cell lines. Activation of the integrated HIV-1 promoter is accompanied by the loss or rearrangement of a positioned nucleosome (nuc-1) near the viral RNA start site. Here we show that TSA strongly induces HIV-1 transcription on chromatin in vitro, concomitant with an enhancer factor-assisted increase in the level of acetylated histone H4. TSA treatment, however, did not detectably alter enhancer factor binding or the positioning of nuc-1 on the majority of the chromatin templates indicating that protein acetylation and chromatin remodeling may be limiting steps that occur only on transcriptionally competent templates, or that remodeling of nuc-1 requires additional factors. To assess the number of active chromatin templates in vitro, transcription was limited to a single round with low levels of the detergent Sarkosyl. Remarkably, HIV-1 transcription on chromatin was found to arise from a small number of active templates that can each support nearly 100 rounds of transcription, and TSA increased the number of active templates in each round. In contrast, transcription on naked DNA was limited to only a few rounds and was not responsive to TSA. We conclude that HIV-1 enhancer complexes greatly facilitate transcription reinitiation on chromatin in vitro, and act at a limiting step to promote the acetylation of histones or other transcription factors required for HIV-1 enhancer activity.

Role of protein kinase C in calcium sensitization during muscarinic stimulation in airway smooth muscle

Muscarinic receptor stimulation increases Ca2+ sensitivity, i.e., the amount of force produced at a constant submaximal cytosolic Ca2+ concentration ([Ca2+]i), in permeabilized smooth muscle preparations. It is controversial whether this increase in Ca2+ sensitivity is in part mediated by protein kinase C (PKC). With the use of a beta-escin permeabilized canine tracheal smooth muscle (CTSM) preparation, the effect of four putative PKC inhibitors [calphostin C, chelerythrine chloride, a pseudosubstrate inhibitor for PKC [PKC peptide-(19-31)], and staurosporine] on Ca2+ sensitization induced by acetylcholine (ACh) plus GTP was determined. Preincubation with each of the inhibitors did not affect subsequent Ca2+ sensitization induced by muscarinic receptor stimulation in the presence of a constant submaximal [Ca2+]i, neither did any of these compounds reverse the increase in Ca2+ sensitivity induced by ACh plus GTP. Administration of a 1,2-diacylglycerol analog, 1-oleoyl-2-acetyl-sn-glycerol, did not induce Ca2+ sensitization at a constant submaximal [Ca2+]i. Thus we found no evidence that PKC mediates increases in Ca2+ sensitivity produced by muscarinic receptor stimulation in permeabilized CTSM.

Expression cloning of a rat hypothalamic galanin receptor coupled to phosphoinositide turnover

The neuropeptide galanin is widely distributed throughout the central and peripheral nervous systems and participates in the regulation of processes such as nociception, cognition, feeding behavior, and insulin secretion. Multiple galanin receptors are predicted to underlie its physiological effects. We now report the isolation by expression cloning of a rat galanin receptor cDNA distinct from GALR1. The receptor, termed GALR2, was isolated from a rat hypothalamus cDNA library using a 125I-porcine galanin (125I-pGAL) binding assay. The GALR2 cDNA encoded a protein of 372 amino acids exhibiting 38% amino acid identity with rat GALR1. Binding of 125I-pGAL to transiently expressed GALR2 receptors was saturable (KD = 0.15 nM) and displaceable by galanin peptides and analogues in rank order: porcine galanin approximately M32 approximately M35 approximately M40 >/= galanin-(1-16) approximately M15 approximately [D-Trp2]galanin-(1-29) > C7 >> galanin-(3-29). This profile resembles that of the rat GALR1 receptor with the notable exception that [D-Trp2]galanin exhibited significant selectivity for GALR2 over GALR1. Activation of GALR2 receptors with porcine galanin and other galanin analogues increased inositol phospholipid turnover and intracellular calcium levels in stably transfected Chinese hamster ovary cells and generated calcium-activated chloride currents in Xenopus oocytes, suggesting that the rat GALR2 receptor is primarily coupled to the activation of phospholipase C.

Muscarinic receptor stimulation modulates the effect of halothane on Mn2+ influx in airway smooth muscle

Prior studies suggest that the mechanism of action by which halothane relaxes airway smooth muscle depends on the contractile state of the cell. We hypothesized that halothane would inhibit the influx of Ca2+ into canine airway smooth muscle cells during submaximal, but not maximal, muscarinic stimulation. This hypothesis was tested by using the rate of quenching of fura 2 fluorescence by Mn2+ in strips of canine tracheal smooth muscle as an index of Ca2+ influx. Acetylcholine (ACh) produced a dose-dependent increase in Mn2+ influx. Halothane (0.64 +/- 0.05 microM) significantly decreased Mn2+ influx and intracellular Ca2+ concentration when added to strips stimulated with a submaximal concentration of ACh (0.3 microM) but had no effect on Mn2+ influx or intracellular Ca2+ concentration during maximal stimulation with ACh (100 microM). Similar results were observed when the strips were treated with verapamil. These results demonstrate that anesthetic effects on Ca2+ homeostasis in intact canine tracheal smooth muscle cells may be critically modulated by receptor-linked mechanisms.

Halothane inhibits agonist-induced potentiation of rMLC phosphorylation in permeabilized airway smooth muscle

Agonist-induced increases in CA2+ sensitivity are mediated in part by mechanisms that increase phosphorylation of the regulatory myosin light chain (rMLC) at constant cytosolic Ca2+ concentration ([Ca2+]i). The current study tested the hypothesis that halothane inhibits acetylcholine (ACh)-induced potentiation of rMLC phosphorylation in beta-escin-permeabilized canine tracheal smooth muscle. ACh plus GTP significantly potentiated the increase in isometric force and rMLC phosphorylation induced by 0.8 microM free Ca2+. However, whereas the potentiation of isometric force was sustained, the potentiation of rMLC phosphorylation was biphasic, peaking at 0.5 min and then declining by approximately 10 min to a steady-state level significantly above that induced by 0.8 microM free Ca2+ alone. This finding suggests that mechanisms in addition to changes in rMLC phosphorylation may mediate ACh-induced Ca2+ sensitization, as has been reported for vascular smooth muscle. Halothane (0.91 +/- 0.10 mM) significantly inhibited ACh plus GTP-induced potentiation of rMLC phosphorylation and isometric force after 2 (peak rMLC phosphorylation) and 15 (steady-state rMLC phosphorylation) min of stimulation. However, the effect of halothane on the potentiation of isometric force was significantly less than that expected from its effect on rMLC phosphorylation (i.e., halothane changed the relationship between rMLC phosphorylation and isometric force). These results demonstrate that halothane inhibits the ACh-induced increase in Ca2+ sensitivity by inhibiting the membrane receptor-coupled mechanisms that increase rMLC phosphorylation at constant submaximal [Ca2+]i. Possible additional effects of halothane on rMLC phosphorylation-independent mechanisms cannot be ruled out.

Halothane attenuation of calcium sensitivity in airway smooth muscle. Mechanisms of action during muscarinic receptor stimulation

BACKGROUND

In airway smooth muscle, muscarinic receptor stimulation is thought to increase calcium (Ca2+) sensitivity via a guanosine 5'-triphosphate (GTP)-binding protein/protein kinase C (PKC)-mediated mechanism. This study treated the hypothesis that halothane reduces Ca2+ sensitivity during muscarinic receptor stimulation by inhibiting these second messenger pathways.

METHODS

A beta-escin permeabilized canine tracheal smooth muscle preparation was used in which the cytosolic Ca2+ concentration ([Ca2+]i) is controlled and the GTP-binding protein/ PKC pathways remain intact and can be activated. The muscarinic receptor was activated with acetylcholine plus GTP; the GTP-binding proteins were directly activated with a nonhydrolyzable form of GTP, guanosine 5'-O-(3-thiotriphosphate; GTP gamma S); and PKC was directly activated with the PKC agonist phorbol 12,13-dibutyrate (PDBu).

RESULTS

Free Ca2+ caused a concentration-dependent increase in force. Acetylcholine plus GTP significantly decreased the median effective concentration for free Ca2+ from 0.52 +/- 0.06 microM to 0.21 +/- 0.02 microM, demonstrating an increase in Ca2+ sensitivity. Halothane (0.99 +/- 0.04 mM, equivalent to approximately 4 minimum alveolar concentration in dogs) significantly attenuated this increase in Ca2+ sensitivity induced by acetylcholine plus GTP, increasing the median effective concentration for free Ca2+ from 0.21 +/- 0.02 microM to 0.31 +/- 0.03 microM. However, halothane did not affect the increases in Ca2+ sensitivity induced by GTP gamma S or PDBu.

CONCLUSIONS

Halothane had no effect on increased Ca2+ sensitivity caused by direct activation of GTP-binding proteins with GTP gamma S or PKC with PDBu, suggesting that halothane attenuates acetylcholine-induced Ca2+ sensitization via a mechanism independent of these pathways in beta-escin-permeabilized canine tracheal smooth muscle.

Stereospecific effects of ketamine enantiomers on canine tracheal smooth muscle

1. Ketamine is a potent bronchodilator which relaxes airway smooth muscle (ASM). Clinically, ketamine is used as a 1:1 racemic mixture of enantiomers that differ in their analgesic and anaesthetic effects. The aim of this study was to determine whether there was a difference between the enantiomers in their ability to relax isolated ASM and to explore mechanisms responsible for any observed differences. 2. Canine tracheal smooth muscle strips were loaded with fura-2 and mounted in a photometric system to measure simultaneously force and [Ca2+]i. Calcium influx was estimated by use of a manganese quenching technique. 3. In strips stimulated with 0.1 microM ACh (EC50) R(-)-ketamine (1-100 microM) caused a significantly greater concentration-dependent decrease in force (P<0.0001) and [Ca2+]i than S(+)-ketamine (1-100 microM) (P<0.0005). In contrast, there was no significant difference between the enantiomers in their ability to inhibit calcium influx (45% decrease in influx rate for R(-)-ketamine and 44% for S(+)-ketamine, P =0.782). In strips contracted with 24 mM isotonic KCI (which activates voltage-operated calcium channels), the enantiomers modestly decreased force and [Ca2+]i; there was no significant difference between the enantiomers in their effects on force (P=0.425) or [Ca2+]i (P=0.604). 4. The R(-)-enantiomer of ketamine is a more potent relaxant of ACh-induced ASM contraction than the S(+)-enantiomer. This difference appears to be caused by differential actions on receptor-operated calcium channels.

Desensitization of kainate receptors by kainate, glutamate and diastereomers of 4-methylglutamate

The potencies of kainate, glutamate and diastereomers of 4-methylglutamate were determined for activation and steady-state desensitization of GluR6 and dorsal root ganglion-type kainate receptors using whole-cell voltage clamp. In HEK293 cells expressing GluR6, all four diastereomers induced desensitizing inward currents at relatively high concentrations (> 50 microM), however, the 2S,4R diastereomer (2S,4R-4MG; SYM 2081) was approximately 100-fold more potent than the other three. The EC50 for receptor activation by 2S,4R-4MG (1.0 microM) was similar to that for kainic acid (1.8 microM), but 2S,4R-4MG was significantly more potent than kainate, glutamate or the other diastereomers of 4-methylglutamate at producing steady-state desensitization of GluR6 receptors. IC50s for desensitization quantified using a fixed concentration of kainate as a test agonist were 7.6, 31 and 667 nM for 2S,4R-4MG, kainate and glutamate, respectively. In addition, 2S,4R-4MG fully desensitized native kainate receptors (of the GluR5 subtype) in dorsal root ganglion neurons with an IC50 of 11 nM, compared to 3.4 microM for glutamate. For GluR6, recovery from desensitization displayed a similar time course for kainate and glutamate (tau = 3-4 s) but was roughly 20-fold slower for 2S,4R-4MG, which suggests that the rate of recovery is not entirely dependent on the affinity of ligand for the desensitized receptor. Following exposure to concanavalin A, application of kainate, glutamate and 2S,4R-4MG evoked very similar maximal currents that showed little or no desensitization. Lectin pretreatment produced a leftward shift in the concentration-response relationship for 2S,4R-4MG with an 11-fold reduction in the EC50; however, no significant change in the EC50 for kainate was observed. The characteristic of 2S,4R-4MG to potently and completely desensitize both recombinant GluR6 receptors and native receptors on dorsal root ganglion neurons suggests that this compound will be useful to study selective blockade of these receptors in the nervous system.

The effects of halothane pretreatment on manganese influx induced by muscarinic stimulation of airway smooth muscle

We hypothesized that halothane inhibits contraction of canine airway smooth muscle in part by depleting sarcoplasmic reticulum (SR) calcium stores, which affects subsequent force and calcium influx. This hypothesis was tested by using the rate of quenching of fura-2 fluorescence by manganese (Mn2+) as an index of calcium influx. When added 10 min before submaximum muscarinic stimulation (with 0.3 microM acetylcholine [ACh]), halothane (0.60 +/- 0.04 mM [mean +/- SE]) reduced subsequent isometric force and intracellular calcium concentration ([Ca2+]i) measured 10 min after contraction (to 55%) +/- 5% and 69% +/- 4% of control, respectively). The Mn2+ influx measured concurrently was significantly increased by halothane (by 57% +/- 22%). Depletion of SR calcium stores by ACh prior to contraction also increased Mn2+ influx (by 46% +/- 6%) but did not affect developed force or increase [Ca2+]i in response to submaximum muscarinic stimulation. Halothane did not affect [Ca2+]i or Mn2+ influx when added prior to maximum stimulation with 100 microM ACh but significantly reduced developed force. These findings are consistent with the hypothesis that halothane-induced SR depletion prior to contraction stimulates subsequent calcium influx, but they further suggest that halothane-induced SR depletion itself does not contribute significantly to the reduction in contractility produced by halothane in the canine airway smooth muscle.