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Biomedical subjects

M J Walker

Publications and source records attributed to M J Walker.

At least 91 records · Page 5Linked to original sources
PubMed1994

A novel Escherichia coli expression-export vector containing alkaline phosphatase as an insertional inactivation screening system.

An Escherichia coli expression-export vector was constructed (pCGV1, 6.3 kb) containing the alkaline phosphatase structural gene (phoA) located downstream from the phage lambda pR and pL promoters positioned in tandem and the cIts857 gene encoding lambda thermosensitive repressor. The phoA gene is fused to DNA encoding a hybrid signal sequence that contains the N-terminal portion of the beta-lactamase (Bla) signal sequence and the C-terminal region of the PhoA signal sequence. Within the DNA encoding hybrid signal sequence, a unique NheI restriction site is present where polymerase chain reaction (PCR)-amplified genes may be cloned. The 5' PCR primers reconstitute the C-terminal portion of either the PhoA or Bla signal sequences to restore an intact signal peptide. Recombinant phoA- clones are selected on indicator plates containing 5-bromo-4-chloro-3-indolyl phosphate.

Alkaline Phosphatase↗
PubMed1994

An electrophysiological basis for the antiarrhythmic actions of the kappa-opioid receptor agonist U-50,488H.

This study examined the actions of the selective kappa-opioid receptor agonist, U-50,488H, on voltage activated Na+ and K+ currents in isolated rat cardiac myocytes. U-50,488H produced a concentration-dependent block of the transient Na+ current with an ED50 of about 15 microM, and, at higher concentrations (40-50 microM), a block of the plateau K+ current and an increase in the rate of decay of the transient K+ current. In addition U-50,488H produced a hyperpolarising shift in the inactivation curve for the transient Na+ current without altering the voltage dependence for activation and without an effect on the voltage dependence of inactivation or activation of K+ currents. The block of Na+ currents by U-50,488H showed pronounced use dependence. The kappa-opioid receptor antagonist MR2266 did not itself produce any change in the Na+ or K+ currents and did not change the channel blocking properties of U-50,488H. Thus, since the antiarrhythmic actions of U-50,488H are not blocked by MR2266 or naloxone, the effects of U-50,488H to block Na+ and K+ currents are the most likely reasons for its antiarrhythmic actions, rather than an action at kappa-opioid receptors.

3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cycloh↗
PubMed1994

The regional treatment of liver metastases from breast cancer.

To determine the effect of aggressive regional therapy for liver metastasis from breast cancer, we retrospectively reviewed data on 74 patients identified with liver metastases. Forty had only liver metastases. In this group of 40 patients, 18 were treated with regional therapy only, i.e., surgical resection and/or regional chemotherapy via hepatic artery or portal vein catheters whereas 22 patients had systemic chemotherapy. The two groups were comparable. The regional chemotherapy regimen was 5-FU, Adriamycin, methotrexate, and cytoxan. Median survival (27 months) for those patients treated with regional therapy (N = 18) was significantly longer than for those (N = 22) treated with systemic therapy (5 months) (P = 0.001). Only 45% of the regional treatment group failed in the liver. Our data, although retrospective and selective, suggest that certain subgroups of breast cancer patients with metastatic liver disease may benefit from aggressive regional therapy.

Adult↗
PubMed1994

Lipopolysaccharide O-antigen biosynthesis in Shigella dysenteriae serotype 1: analysis of the plasmid-carried rfp determinant.

The O-antigen polysaccharide of the lipopolysaccharide of Shigella dysenteriae serotype 1 is encoded by determinants located on a 9 kb plasmid (rfp) and on the chromosome near the his locus (rfb). Molecular genetic and biochemical studies of the rfp determinant reported here show that the rfp region contains two genes, rfpA and rfpB, lying in an operon. rfpB was demonstrated to encode a membrane-bound galactosyl-transferase. The low G+C content of rfp DNA suggests that it did not originate in Shigella.

Amino Acid Sequence↗
PubMed1994

Effects of acute selective 5-HT1, 5-HT2, 5-HT3 receptor and alpha 2 adrenoceptor blockade on naloxone-induced antinociception.

Several studies have demonstrated a paradoxical form of antinociception induced by the repeated administration of opioid antagonists accompanied by exposure to a painful stimulus. The underlying mechanism of this naloxone-induced antinociception (NIA) is still unknown, but the results of several studies suggest that it is a non-opioid response. This study was designed to investigate serotonergic and noradrenergic involvement in NIA. Rats were treated daily with systemic injections of 5 mg/kg naloxone, followed by a 45-s hot plate test of nociception (temperature = 51.5 +/- 0.5 degree C). After rats reached plateau levels of NIA, they received a test trial in which they were treated with various doses of different selective 5-HT or alpha 2 adrenoceptor antagonists in addition to naloxone before the hot plate test. Rats treated with 0.16, 0.32 and 0.63 mg/kg pirenperone or 2.5 mg/kg ritanserin showed significant reductions in paw lick latency with respect to rats treated with vehicle. In addition, high doses of yohimbine (7.5-10 mg/kg) also effectively reversed NIA. In contrast, NIA was not affected by acute blockade of 5-HT1 or 5-HT3 receptors by methiothepin or MDL 72222, respectively, or by the alpha 2 adrenoceptor blocker idazoxan. None of the 5-HT or alpha 2 adrenoceptor antagonists had any effect on the paw lick latencies of saline-treated rats. A possible role of 5-HT2 receptors in the antinociception induced by opioid receptor blockade is discussed.

Adrenergic alpha-2 Receptor Antagonists↗
PubMed1994

Wither, whether and whither pharmacology.

There has been considerable discussion over the last couple of years about the nature of pharmacology as a discipline and the training programmes for pharmacologists. Recently the British Pharmacological Society devoted a whole symposium to this important issue. In the UK in particular, pressure from government has nearly doubled the number of undergraduate students entering the university sector over the last five years, which has put considerable pressure on the teaching of essential practical courses for this experimental discipline. Similar pressures face the teaching of pharmacology in other countries and the situation, discussed here by Clive Page, Morley Sutter and Michael Walker, clearly has implications for academic pharmacology and the pharmaceutical industry. It is intended that the following article will stimulate both dialogue and positive action towards redressing this important problem.

Animals↗
PubMed1994

Relationship between QaT and RR intervals in rats, guinea pigs, rabbits, and primates.

The ECG is routinely used in many species to monitor effects of drugs. While it is relatively easy to measure both PR and QRS, measurement of QT is complicated by the fact that this interval can change with heart rate. In order to compensate for variations in QT due to variations in heart rate, various correction factors have been used, including those of Bazett and Hodges. Such corrections were devised for humans and may have limited applicability in other species. We have systematically varied heart rate in anesthetized rats, guinea pigs, rabbits, and primates using procedures such as vagal stimulation, direct atrial stimulation, injection of cold saline and drugs, including anesthetics, and measured the resulting QT (as QaT and related measures). Over a wide range of heart rates we tested various formulas for their value in correcting for the variation in QT interval associated with changes in heart rate. In rats the "QT" interval did not change appreciably with heart rate. In the other species QaT intervals varied in the expected manner with heart rate in that they decreased with tachycardia and increased with bradycardia. Various formulas were tested for their utility in correcting measures of the QaT interval (QaTc) for changes in heart rate in guinea pigs, rabbits, and primates. In species other than rats, there was little difference between the various formulas in their ability to increase the precision of QaTc and the normality of its distribution, although the best correction is that derived from the regression (either linear, square root, or polynomial) equation relating RR and QaT.

Animals↗
PubMed1993

Transduction of human melanoma cell lines with the human interleukin-7 gene using retroviral-mediated gene transfer: comparison of immunologic properties with interleukin-2.

Two human melanoma cell lines were transduced with the human interleukin (IL)-7 and IL-2 genes using retroviral-mediated gene transfer. Stable, high-level cytokine expression was achieved. The in vitro growth of transduced tumors was unaltered. Neither of the IL-2-transduced melanoma cell lines grew in athymic mice, whereas one IL-7-transduced melanoma line showed retarded in vivo growth. This is consistent with animal studies suggesting a predominantly T-cell response to IL-7-transduced tumors and a more nonspecific response to IL-2-transduced tumors. Both IL-7- and IL-2-transduced melanoma cell lines could induce cytotoxic lymphocytes in mixed lymphocyte-tumor cultures. The expression of putative melanoma antigens (MAGE)-1 and MAGE-3 was unaltered by cytokine transduction. In one cell line, IL-7 transduction resulted in a marked inhibition of the immunosuppressive peptide transforming growth factor (TGF)beta 1. The results allow a comparison of immunobiologic properties of IL-7- and IL-2-transduced human melanoma cell lines in consideration of their use in genetically engineered tumor vaccines. IL-7 transduction results in stable cytokine expression and phenotypic alterations that appear to be favorable for enhanced immunogenicity and it deserves clinical testing.

Animals↗
PubMed1993

Arrhythmia models in the rat.

The rat has been used extensively for arrhythmia studies, and a large number of models have been developed, which include chemical, electrical, and pathological procedures for including arrhythmias. Such models have been used to investigate mechanisms for inducing arrhythmias as well as the actions of antiarrhythmic drugs. Arrhythmia models in rats are of particular importance in examination of the antiarrhythmic action of various drugs.

Animals↗
PubMed1993

Endogenous chemical mediators of ventricular arrhythmias in ischaemic heart disease.

The causes of ventricular arrhythmias in the acute setting of coronary artery disease (myocardial ischaemia and reperfusion) may be approached using two paradigms. One, the electrophysiological paradigm (disturbance of ionic homeostasis, electrogenesis, and conduction) has not been addressed in detail here. Instead, we have focused on the concept of a chemical paradigm of arrhythmogenesis. Many endogenous chemical substances (derived from the myocardium, nerves, blood plasma, platelets, leucocytes, and endothelium) accumulate in the ischaemic tissue or are produced during reperfusion and many of these have been suggested to modulate ventricular arrhythmias. Some substances may be arrhythmogenic and others may be antiarrhythmic. Together they determine whether or not arrhythmias occur. Potentially arrhythmogenic substances include potassium, catecholamines, cAMP, histamine, 5-HT, lysophosphatidylcholine, palmitylcarnitine, platelet activating factor, prostaglandins, leukotrienes, thromboxane A2, angiotensin II, endothelin, opioids, protons, calcium, and free radicals. We have considered each of these, with the objective of evaluating which are important in arrhythmogenesis in acute ischaemia and reperfusion. Two alternative models of arrhythmogenesis are possible in the context of the chemical paradigm: a series model (where one substance or its effects determines the arrhythmogenicity of another) and a parallel model (where numerous substances operate independently to cause ventricular arrhythmias). It is not yet clear which model is most appropriate; a combination of the two is possible, so a working prototype has been constructed which accommodates both. A set of criteria (hitherto lacking) for establishing whether a substance is sufficient and necessary for arrhythmogenesis is proposed. Some generalisations are given on approaches to establishment of these criteria for putative arrhythmogenic substances. Finally, we have considered how arrhythmogenic drug development may be influenced by using the chemical paradigm as an alternative to the electrophysiological paradigm of arrhythmogenesis.

Acute Disease↗
PubMed1993

Electrophysiological and antiarrhythmic actions of the kappa agonist PD 129290, and its R,R (+)-enantiomer, PD 129289.

1. The S,S (-)-enantiomer PD 129290, a kappa agonist, and its corresponding inactive R,R (+)-enantiomer (PD 129289) were studied in rat isolated hearts and in intact rats for cardiovascular and antiarrhythmic actions. The electrophysiological actions of PD 129290 were also studied in rat isolated cardiac myocytes using voltage clamp. 2. Ventricular pressure, heart rate and ECG were studied in isolated hearts while blood pressure, heart rate and ECG were studied in pentobarbitone-anaesthetized rats. In the latter, responses to electrical stimulation and coronary occlusion were also investigated. 3. In isolated hearts both enantiomers, over the concentration range 2-16 microM, dose-dependently reduced systolic ventricular pressure and heart rate while prolonging the P-R and QRS intervals of the ECG. 4. At doses of 1-32 mumol kg-1 both enantiomers reduced blood pressure and heart rate in anaesthetized rats. In addition, both enantiomers increased the size of the RSh and increased P-R, QRS, and Q-T intervals of the ECG. The thresholds for premature beats and ventricular fibrillation were dose-dependently increased by PD 129289. At lower doses PD 129290 decreased thresholds. These decreases were blocked by naloxone to reveal underlying increases similar to those seen with PD 129289. Both enantiomers increased refractory periods. 5. Naloxone (8 mumol kg-1) did not alter any of the actions of PD 129290, except to abolish the initial decreases in thresholds in intact rats seen with lower doses of PD 129290. 6. Both enantiomers (2 and 8 mumol kg-1) equally reduced arrhythmias in anaesthetized rats subject to occlusion of a coronary artery. 7. In rat isolated cardiac myocytes 20 microM PD 129290, in the presence and absence of naloxone decreased the amplitude of the transient sodium current by about 50% without affecting the voltage dependence of activation or inactivation of this current.8. The antiarrhythmic actions of both enantiomers appear to primarily result from their Class I(sodium channel blockade) properties which are independent of kappa agonism.

Animals↗
PubMed1993

Acute toxicity of tedisamil, a new potassium channel blocking drug.

The acute intravenous toxicity of tedisamil, a new potassium channel blocker, was assessed by infusing it at various rates (1.5 to 10 mg/kg/min.) to ventilated or spontaneously breathing rats subjected to blockade of their peripheral somatic and autonomic nervous systems. The lethal doses for ventilated rats (including pithed animals) were 2-3 times higher than those for spontaneously breathing animals. In spontaneously breathing rats death appeared to be due to respiratory depression which was possibly of central origin since the lethality of tedisamil was not markedly altered by vagotomy, bilateral carotid artery ligation, nor by autonomic nervous system blockade. On the other hand, in all of the artificially ventilated groups, death occurred at higher doses and was related to direct cardiac actions, possibly involving blockade of cardiac potassium and sodium channels. The cardiovascular responses to infusions of tedisamil included tachyarrhythmias, bradycardia, blood pressure changes, increased QRS width and Q-T interval duration. Arrhythmias occurred at sublethal doses and were eliminated by autonomic blockade and pithing. However, the bradycardia, ECG and blood pressure changes induced by tedisamil infusions were unaltered by the various treatments. Thus tedisamil may induce arrhythmias by actions on the autonomic system.

Animals↗
PubMed1993

Antibody responses in the serum and respiratory tract of mice following oral vaccination with liposomes coated with filamentous hemagglutinin and pertussis toxoid.

Mice were orally vaccinated with liposomes coated with filamentous hemagglutinin (FHA) and detoxified pertussis toxin (PT) of Bordetella pertussis. FHA- and PT-specific immunoglobulin G (IgG) was detected in serum, and both IgG and IgA were detected in lung washes following the immunization. Antibody responses in mice immunized with liposomes coated with FHA and PT were significantly higher than those in mice immunized with free FHA and PT, which demonstrated the adjuvanticity of the liposome carrier. The results indicate the potential usefulness of this approach for eliciting immune responses against FHA and PT (and perhaps other pertussis antigens) in humans and its possible utility in large-scale vaccination to protect against both B. pertussis infection and disease.

Adhesins, Bacterial↗