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N Davidson

Publications and source records attributed to N Davidson.

At least 73 records · Page 4Linked to original sources
PubMed1996

Enterocolitis and colon cancer in interleukin-10-deficient mice are associated with aberrant cytokine production and CD4(+) TH1-like responses.

We have characterized the progressive stages of chronic intestinal inflammation that develops spontaneously in specific pathogen-free (SPF) mice with a targeted disruption in the IL-10 gene (IL-10-/-). Our longitudinal studies showed that inflammatory changes first appear in the cecum, ascending and transverse colon of 3-wk-old mutants. As the disease progressed, lesions appeared in the remainder of the colon and in the rectum. Some aged IL-10-/- mice also developed inflammation in the small intestine. Prolonged disease with transmural lesions and a high incidence of colorectal adenocarcinomas (60%) was observed in 6-mo-old mutants. Mechanistic studies have associated uncontrolled cytokine production by activated macrophages and CD4+ Th1-like T cells with the enterocolitis exhibited by IL-10-/- mice. A major role for a pathogenic Th1 response was further suggested by showing that anti-IFNgamma antibody (Ab) treatment significantly attenuated intestinal inflammation in young IL-10-/- mice. When weanlings were treated with IL-10, they failed to develop any signs of intestinal inflammation. Interestingly, IL-10 treatment of adults was not curative but did ameliorate disease progression. Our studies have also shown that inheritable factors strongly influence the disease susceptibility of IL-10-/- mice. In 3-mo-old mutants, intestinal lesions were most severe in IL-10-/- 129/SvEv and IL-10-/- BALB/c strains, of intermediate severity in the IL-10-/- 129 x C57BL/6J outbreds, and least severe in the IL-10-/- C57BL/6J strain.

Aging↗
PubMed1996

Inhibition of an inwardly rectifying K+ channel by G-protein alpha-subunits.

Cholinergic muscarinic, serotonergic, opioid and several other G-protein-coupled neurotransmitter receptors activate inwardly rectifying K+ channels of the GIRK family, slowing the heartbeat and decreasing the excitability of neuronal cells. Inhibitory modulation of GIRKs by G-protein-coupled receptors may have important implications in cardiac and brain physiology. Previously G alpha and G beta gamma subunits of heterotrimeric G proteins have both been implicated in channel opening, but recent studies attribute this role primarily to the G beta gamma dimer that activates GIRKs in a membrane-delimited fashion, probably by direct binding to the channel protein. We report here that free GTP gamma S-activated G alpha i 1, but not G alpha i 2 or G alpha i 3, potently inhibits G beta 1 gamma 2-induced GIRK activity in excised membrane patches of Xenopus oocytes expressing GIRK1. High-affinity but partial inhibition is produced by G alpha s-GTP gamma S. G alpha i 1-GTP gamma S also inhibits G beta 1 gamma 2-activated GIRK in atrial myocytes. Antagonistic interactions between G alpha and G beta gamma may be among the mechanisms determining specificity of G protein coupling to GIRKs.

Animals↗
PubMed1996

Sensitivity and specificity of QTc dispersion for identification of risk of cardiac death in patients with peripheral vascular disease.

OBJECTIVE: To determine whether QTc dispersion, which is easily obtained from a standard electrocardiogram, can predict those patients with peripheral vascular disease who will subsequently suffer a cardiac death, despite having no cardiac symptoms or signs. DESIGN: Patients with peripheral vascular disease were followed up for five years after they had had coronary angiography, radionuclide ventriculography, and their QTc dispersion calculated from their 12 lead electrocardiogram. SUBJECTS: 49 such patients were then divided into three groups: survivors (34), cardiac death (12), and non-cardiac death (3). MAIN OUTCOME MEASURE: Survival. RESULTS: The mean (SD; range) ejection fractions were similar in all three groups: survivors 45.9 (11.0; 27.0-52.0), cardiac death 44.0 (7.90; 28.5-59.0), and non-cardiac death 45.3 (4.55; 39.0-50.0). QTc dispersion was significantly prolonged in the cardiac death group compared with in the survivors (86.3(23.9; 41.0-139) v 56.5 (25.4; 25.0-164); P = 0.002). A QTc dispersion > or = 60 ms had a 92% sensitivity and 81% specificity in predicting cardiac death, QTc dispersion in patients with diffuse coronary artery disease was significantly (P < 0.05) greater than in those with no disease or disease affecting one, two, or three vessels. CONCLUSIONS: There is a strong link between QTc dispersion and cardiac death in patients with peripheral vascular disease. QTc dispersion may therefore be a cheap and non-invasive way of assessing the risk of cardiac death in patients with peripheral vascular disease.

Aged↗
PubMed1996

A unique P-region residue is required for slow voltage-dependent gating of a G protein-activated inward rectifier K+ channel expressed in Xenopus oocytes.

1. The structural determinants of a G protein-activated inwardly rectifying potassium channel, GIRK1 (KIR3.1), involved in voltage- and time-dependent gating properties were investigated by heterologous expression of chimeric constructs and point mutants in Xenopus oocytes. 2. Chimeras between GIRK1 and the weakly rectifying potassium channel, ROMK1 (KIR1.1), indicate that residues in the putative transmembrane segments TM1 and TM2 affect the steep inward rectification of GIRK1, while residues in the main pore-forming domain, the P-region segment, are critical for the manifestation of GIRK1 time-dependent activation. 3. Phenylalanine 137 in the P-region of GIRK1 is unique; in ROMK1, as in other inward rectifiers, there is a serine residue at this position. Mutation of the phenylalanine 137 to serine leads to expression of currents with nearly time-independent activation. 4. An acidic residue (aspartate) in TM2 partially controls the time- and voltage-dependent gating in IRK1 (KIR2.1). Mutation of the equivalent aspartate 173 to glutamine in GIRK1 did not abolish the time-dependent activation but did decrease the degree of inward rectification. 5. These results reveal an important role for the P-region in controlling the time-dependent gating of an inwardly rectifying potassium channel and suggest a close relationship between permeation and gating in this family of K+ channels.

Animals↗
PubMed1996

Serotonin and protein kinase C modulation of a rat brain inwardly rectifying K+ channel expressed in xenopus oocytes.

In Xenopus laevis oocytes injected with rat brain poly(A)+ RNA, perfusion with a high-K+ solution (96 mM KCl) generated an inward current (IHK) which was absent in water-injected oocytes. Part of IHK was blocked by low concentrations of Ba2+ (half-maximal inhibitory concentration, IC50: 4.2 +/- 0.5 microM). When serotonin (5-HT) was applied to these oocytes a transient inward oscillating Cl- current arising from activation of Ca2+ -dependent Cl- channels, ICl (Ca), was observed. When this response decayed, a 30% reduction of IHK could be detected. Electrophysiological characterization of the K+ channel down-modulated by 5-HT revealed that it is an inward rectifier. Anti-sense suppression experiments revealed that the 5-HT2C receptor mediates the down-modulatory effect of 5-HT. The nature of the modulatory pathway was investigated by application of phorbol esters and intracellular injection of protein kinase C (PKC) inhibitors, ethylenebis (oxonitrilo)tetraacetate (EGTA) and inositol 1,4,5-trisphosphate. The results demonstrate that PKC is responsible for the down-modulatory effect.

Animals↗
PubMed1996

Time resolved kinetics of direct G beta 1 gamma 2 interactions with the carboxyl terminus of Kir3.4 inward rectifier K+ channel subunits.

The direct interaction of recombinant G beta 1 gamma 2 proteins with the carboxyl terminal domain of a G protein-gated inward rectifier K channel subunit, Kir3.4 (GIRK4), was measured in real time using biosensor chip technology. The carboxyl terminus of Kir3.4 (a.a. 186-419) was expressed in bacteria as a glutathione-S-transferase (GST) fusion protein, GST-Kir3. 4ct. GST-Kir3.4ct was immobilized to the surface of a biosensor chip by high affinity binding of the GST domain to a covalently attached anti-GST antibody. The association and dissociation rates of G beta 1 gamma 2 dimers with the immobilized Kir3.4ct domain were temporally resolved as a change in refractive index detected by surface plasmon resonance. Specific binding of G beta 1 gamma 2 dimers to Kir3.4ct was characterized by a dissociation rate (kd) of approximately 0.003 s-1. Association kinetics were dominated by a concentration-independent component (time constant approximately 50 s) which complicates models of binding and may indicate conformational changes during binding of G beta 1 gamma 2 to Kir3.4ct. The estimated equilibrium dissociation binding constant (Kd) was approximately 800 nM. These studies demonstrate that G beta gamma dimers interact directly with the Kir3.4 channel subunit, and suggest interesting details in the interaction with the major cytosolic carboxyl terminal domain. The slow G beta 1 gamma 2 dissociation rate measured on the sensor chip is similar in magnitude to a slow component of channel deactivation measured electrophysiologically in Xenopus oocytes expressing Kir3.1/3.4 multimeric channels and a G protein-coupled receptor. Biosensor-based experiments such as those described here will complement electrophysiological studies on the molecular basis of G protein interactions with Kir channels and other ion channel proteins.

Animals↗
PubMed1996

Functional analysis of the weaver mutant GIRK2 K+ channel and rescue of weaver granule cells.

In the neurological mutant mouse weaver, granule cell precursors proliferate normally in the external germinal layer of the cerebellar cortex, but fail to differentiate. Granule neurons purified from weaver cerebella have greatly reduced G protein-activated inwardly rectifying K+ currents; instead, they display a constitutive Na+ conductance. Expression of the weaver GIRK2 channel in oocytes confirms that the mutation leads to constitutive activation, loss of monovalent cation selectivity, and increased sensitivity to three channel blockers. Pharmacological blockade of the Na+ influx in weaver granule cells restores their ability to differentiate normally. Thus, Na+ flux through the weaver GIRK2 channel underlies the failure of granule cell development in situ.

Animals↗
PubMed1996

Voltage-jump relaxation kinetics for wild-type and chimeric beta subunits of neuronal nicotinic receptors.

We have studied the voltage-jump relaxation currents for a series of neuronal nicotinic acetylcholine receptors resulting from the coexpression of wild-type and chimeric beta 4/beta 2 subunits with alpha 3 subunits in Xenopus oocytes. With acetylcholine as the agonist, the wild-type alpha 3 beta 4 receptors displayed five- to eightfold slower voltage-jump relaxations than did the wild-type alpha 3 beta 2 receptors. In both cases, the relaxations could best be described by two exponential components of approximately equal amplitudes over a wide range of [ACh]'s. Relaxation rate constants increased with [ACh] and saturated at 20- to 30-fold lower concentrations for the alpha 3 beta 2 receptor than for the alpha 3 beta 4 receptor, as observed previously for the peak steady state conductance. Furthermore, the chimeric beta 4/beta 2 subunits showed a transition in the concentration dependence of the rate constants in the region between residues 94 and 109, analogous to our previous observation with steady state conductances. However, our experiments with a series of beta-subunit chimeras did not localize residues that govern the absolute value of the kinetic parameters. Hill coefficients for the relaxations also differed from those previously measured for steady state responses. The data reinforce previous conclusions that the region between residues 94 and 109 on the beta subunit plays a role in binding agonist but also show that other regions of the receptor control gating kinetics subsequent to the binding step.

Acetylcholine↗
PubMed1996

beta-Alanine protects against taurine and NaCl--induced hypernatremia in the rat.

Rats drinking a combination of taurine and hypertonic saline solution rapidly develop hypernatremia, but rats drinking either solution alone do not. The mechanism by which taurine disrupts the ability to deal with a salt load is not clear. Rats housed in metabolism cages were studied. Food intake, fluid intake, plasma sodium concentration, urine output, sodium balance, visible water balance, and urine osmolality were determined over a period of 8 days. Rats drinking 0.1 M taurine plus 1.8% NaCl developed a mean plasma sodium concentration of 160 +/- 18 mM by Day 6, compared with 137 +/- 1.6 mM in water drinking controls. Ingestion of 1.8% saline alone produced only a mild, transient rise in plasma sodium (<150 mM), which returned to control levels by Day 8. Ingestion by neither 0.1 M taurine alone nor 0.1 M beta-alanine, a taurine transport antagonist, produced any evidence of hypernatremia throughout the experiment. When beta-alanine was added to the taurine + saline regimen, mean plasma sodium reached only 149 +/- 16 mM (Day 6). Inspection of the ratio of cumulative sodium balance to cumulative water balance revealed a rapid increase until Day 2, followed by a virtual plateau thereafter in the taurine + saline group. Rats drinking saline alone showed an equally rapid rise in the ratio, but to a lower plateau level, suggesting that taurine exerts a much more pronounced disturbance of sodium balance than of water balance. The addition of beta-alanine to the regimens of taurine + saline or saline alone produced ratios of cumulative sodium to cumulative water balance significantly lower than that of either regimen without beta-alanine. These findings suggest that taurine induces hypernatremia by interfering with normal homeostatic control mechanisms and that beta-alanine counteracts that action of taurine. The effect of beta-alanine in rats drinking saline alone is consistent with a role for endogenous taurine in normal electrolyte homeostasis.

Animals↗
PubMed1995

Inhibition of function in Xenopus oocytes of the inwardly rectifying G-protein-activated atrial K channel (GIRK1) by overexpression of a membrane-attached form of the C-terminal tail.

Coexpression in Xenopus oocytes of the inwardly rectifying guanine nucleotide binding (G)-protein-gated K channel GIRK1 with a myristoylated modification of the (putative) cytosolic C-terminal tail [GIRK1 aa 183-501 fused in-frame to aa 1-15 of p60src and denoted src+ (183-501)] leads to a high degree of inhibition of the inward G-protein-gated K+ current. The nonmyristoylated segment, src- (183-501), is not active. Although some interference with assembly is not precluded, the evidence indicates that the main mechanism of inhibition is interference with functional activation of the channel by G proteins. In part, the tail functions as a blocking particle similar to a "Shaker ball"; it may also function by competing for the available supply of free G beta gamma liberated by hormone activation of a seven-helix receptor. The non-G-protein-gated weak inward rectifier ROMK1 is less effectively inhibited, and a Shaker K channel was not inhibited. Immunological assays show the presence of a high concentration of src+ (183-501) in the plasma membrane and the absence of any membrane forms for the nonmyristoylated segment.

Amino Acid Sequence↗
PubMed1995

Evidence that neuronal G-protein-gated inwardly rectifying K+ channels are activated by G beta gamma subunits and function as heteromultimers.

Guanine nucleotide-binding proteins (G proteins) activate K+ conductances in cardiac atrial cells to slow heart rate and in neurons to decrease excitability. cDNAs encoding three isoforms of a G-protein-coupled, inwardly rectifying K+ channel (GIRK) have recently been cloned from cardiac (GIRK1/Kir 3.1) and brain cDNA libraries (GIRK2/Kir 3.2 and GIRK3/Kir 3.3). Here we report that GIRK2 but not GIRK3 can be activated by G protein subunits G beta 1 and G gamma 2 in Xenopus oocytes. Furthermore, when either GIRK3 or GIRK2 was coexpressed with GIRK1 and activated either by muscarinic receptors or by G beta gamma subunits, G-protein-mediated inward currents were increased by 5- to 40-fold. The single-channel conductance for GIRK1 plus GIRK2 coexpression was intermediate between those for GIRK1 alone and for GIRK2 alone, and voltage-jump kinetics for the coexpressed channels displayed new kinetic properties. On the other hand, coexpression of GIRK3 with GIRK2 suppressed the GIRK2 alone response. These studies suggest that formation of heteromultimers involving the several GIRKs is an important mechanism for generating diversity in expression level and function of neurotransmitter-coupled, inward rectifier K+ channels.

Acetylcholine↗
PubMed1995

Nicotinic receptor binding site probed with unnatural amino acid incorporation in intact cells.

The nonsense codon suppression method for unnatural amino acid incorporation has been applied to intact cells and combined with electrophysiological analysis to probe structure-function relations in the nicotinic acetylcholine receptor. Functional receptors were expressed in Xenopus oocytes when tyrosine and phenylalanine derivatives were incorporated at positions 93, 190, and 198 in the binding site of the alpha subunit. Subtle changes in the structure of an individual side chain produced readily detectable changes in the function of this large channel protein. At each position, distinct features of side chain structure dominated the dose-response relation, probably by governing the agonist-receptor binding.

Animals↗
PubMed1995

The inward rectifier potassium channel family.

Recent cloning of a family of genes encoding inwardly rectifying K+ channels has provided the opportunity to explain some venerable problems in membrane biology. An expanding number of novel inwardly rectifying K+ channel clones has revealed multiple channel subfamilies that have specialized roles in cell function. The molecular determinants of inward rectification have been largely elucidated with the discovery of endogenous polyamines that act as voltage-dependent intracellular channel blockers, and with the identification of a critical site in the channel that mediates high-affinity block by both polyamines and Mg2+.

Animals↗