Biomedical subjects
Publications and source records attributed to S A Ward.
The synergistic potential of a range of biguanides, their triazine metabolites, tetracyclines, and pyrimethamine in combination with atovaquone has been assessed. All five biguanides tested interacted synergistically with atovaquone against Plasmodium falciparum in vitro. All of the other compounds tested were either additive or antagonistic.
BACKGROUND: Passive training of specific locomotor muscle groups by means of neuromuscular electrical stimulation (NMES) might be better tolerated than whole body exercise in patients with severe chronic obstructive pulmonary disease (COPD). It was hypothesised that this novel strategy would be particularly effective in improving functional impairment and the consequent disability which characterises patients with end stage COPD. METHODS: Fifteen patients with advanced COPD (nine men) were randomly assigned to either a home based 6 week quadriceps femoris NMES training programme (group 1, n=9, FEV(1)=38.0 (9.6)% of predicted) or a 6 week control period before receiving NMES (group 2, n=6, FEV(1)=39.5 (13.3)% of predicted). Knee extensor strength and endurance, whole body exercise capacity, and health related quality of life (Chronic Respiratory Disease Questionnaire, CRDQ) were assessed. RESULTS: All patients were able to complete the NMES training programme successfully, even in the presence of exacerbations (n=4). Training was associated with significant improvements in muscle function, maximal and endurance exercise tolerance, and the dyspnoea domain of the CRDQ (p<0.05). Improvements in muscle performance and exercise capacity after NMES correlated well with a reduction in perception of leg effort corrected for exercise intensity (p<0.01). CONCLUSIONS: For severely disabled COPD patients with incapacitating dyspnoea, short term electrical stimulation of selected lower limb muscles involved in ambulation can improve muscle strength and endurance, whole body exercise tolerance, and breathlessness during activities of daily living.
1. A prior bout of high-intensity square-wave exercise can increase the temporal adaptation of pulmonary oxygen uptake (.V(O2)) to a subsequent bout of high-intensity exercise. The mechanisms controlling this adaptation, however, are poorly understood. 2. We therefore determined the dynamics of intramuscular [phosphocreatine] ([PCr]) simultaneously with those of .V(O2) in seven males who performed two consecutive bouts of high-intensity square-wave, knee-extensor exercise in the prone position for 6 min with a 6 min rest interval. A magnetic resonance spectroscopy (MRS) transmit-receive surface coil under the quadriceps muscle allowed estimation of [PCr]; .V(O2) was measured breath-by-breath using a custom-designed turbine and a mass spectrometer system. 3. The .V(O2) kinetics of the second exercise bout were altered compared with the first such that (a) not only was the instantaneous rate of .V(O2) change (at a given level of .V(O2)) greater but the phase II tau was also reduced - averaging 46.6 +/- 6.0 s (bout 1) and 40.7 +/- 8.4 s (bout 2) (mean +/- S.D.) and (b) the magnitude of the later slow component was reduced. 4. This was associated with a reduction of, on average, 16.1% in the total exercise-induced [PCr] decrement over the 6 min of the exercise, of which 4.0% was due to a reduction in the slow component of [PCr]. There was no discernable alteration in the initial rate of [PCr] change. The prior exercise, therefore, changed the multi-compartment behaviour towards that of functionally first-order dynamics. 5. These observations demonstrate that the .V(O2) responses relative to the work rate input for high-intensity exercise are non-linear, as are, it appears, the putative phosphate-linked controllers for which [PCr] serves as a surrogate.
BACKGROUND: The multidrug transporters P-glycoprotein (P-gp) and MRP1 are functionally expressed in several subclasses of lymphocytes. HIV-1 protease inhibitors interact with both; consequently the transporters could reduce the local concentration of HIV-1 protease inhibitors and, thus, influence the selection of viral mutants. OBJECTIVES: To study the effect of the expression of P-gp and MRP1 on the transport and accumulation of HIV-1 protease inhibitors in human lymphocytes and to study the effects of specific P-gp and MRP1 inhibitors. METHODS: The initial rate and the steady-state intracellular accumulation of radiolabelled ritonavir, indinavir, saquinavir and nelfinavir was measured in three human lymphocyte cell lines: control CEM cells, CEM-MDR cells, which express 30-fold more P-gp than CEM cells, and CEM-MRP cells, which express fivefold more MRP1 protein than CEM cells. The effect of specific inhibitors of P-gp (GF 120918) and MRP1 (MK 571) was also examined. RESULTS: Compared with CEM cells, the initial rates of uptake and the steady-state intracellular concentrations of all protease inhibitors are significantly reduced in CEM-MDR cells. The intracellular concentrations of the protease inhibitors are increased upon co-administration with GF 120918, in some cases to levels approaching those in CEM cells. The intracellular concentrations of the protease inhibitors are also significantly reduced in CEM-MRP cells. Co-administration with MK -571 can partially overcome these effects. CONCLUSIONS: The overexpression of multidrug transporters significantly reduces the accumulation of protease inhibitors at this major site of virus replication, which, potentially, could accelerate the acquisition of viral resistance. Targeted inhibition of P-gp may represent an important strategy by which this problem can be overcome.
1. The maximal oxygen uptake (V(O(2),peak)) during dynamic muscular exercise is commonly taken as a crucial determinant of the ability to sustain high-intensity exercise. Considerably less attention, however, has been given to the rate at which V(O(2)) increases to attain this maximum (or to its submaximal requirement), and even less to the kinetic features of the response following exercise. 2. Six, healthy, male volunteers (aged 22 to 58 years), each performed 13 exercise tests: initial ramp-incremental cycle ergometry to the limit of tolerance and subsequently, on different days, three bouts of square-wave exercise each at moderate, heavy, very heavy and severe intensities. Pulmonary gas exchange variables were determined breath by breath throughout exercise and recovery from the continuous monitoring of respired volumes (turbine) and gas concentrations (mass spectrometer). 3. For moderate exercise, the V(O(2)) kinetics were well described by a simple mono-exponential function, following a short cardiodynamic phase, with the on- and off-transients having similar time constants (tau(1)); i.e. tau(1,on) averaged 33 +/- 16 s (+/- S.D.) and tau(1,off) 29 +/- 6 s. 4. The on-transient V(O(2)) kinetics were more complex for heavy exercise. The inclusion of a second slow and delayed exponential component provided an adequate description of the response; i.e. tau(1,on) = 32 +/- 17 s and tau(2,on) = 170 +/- 49 s. The off-transient V(O(2)) kinetics, however, remained mono-exponential (tau(1,off) = 42 +/- 11 s). 5. For very heavy exercise, the on-transient V(O(2)) kinetics were also well described by a double exponential function (tau(1,on) = 34 +/- 11 s and tau(2,on) = 163 +/- 46 s). However, a double exponential, with no delay, was required to characterise the off-transient kinetics (i.e. tau(1,off) = 33 +/- 5 s and tau(2,off) = 460 +/- 123 s). 6. At the highest intensity (severe), the on-transient V(O(2)) kinetics reverted to a mono-exponential profile (tau(1,on) = 34 +/- 7 s), while the off-transient kinetics retained a two-component form (tau(1,off) = 35 +/- 11 s and tau(2,off) = 539 +/- 379 s). 7. We therefore conclude that the kinetics of V(O(2)) during dynamic muscular exercise are strikingly influenced by the exercise intensity, both with respect to model order and to dynamic asymmetries between the on- and off-transient responses.
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The role of antioxidants in the neurotoxicity of the antimalarial endoperoxides artemether and dihydroartemisinin was studied in vitro by quantitative image analysis of neurite outgrowth in the neuroblastoma cell line NB2a. Intracellular glutathione concentrations were measured by high performance liquid chromatography with fluorescence detection. Both dihydroartemisinin (1 microM) and a combination of artemether (0.3 microM) plus haemin (2 microM) significantly inhibited neurite outgrowth from differentiating NB2a cells to 11.5 +/- 11.0% (SD) and 19.6 +/- 15.2% of controls, respectively. The inhibition by artemether/haemin was prevented by the antioxidants superoxide dismutase (109.7 +/- 47.8% of control), catalase (107.0 +/- 29.3%) glutathione (123.8 +/- 12.4%), L-cysteine (88.0 +/- 6.3%), N-acetyl-L-cysteine (107.8 +/- 14.9%), and ascorbic acid (104.3 +/- 12.7%). Dihydroartemisinin-induced neurotoxicity was completely or partially prevented by L-cysteine (99.5 +/- 17.7% of control), glutathione (57.9 +/- 23.4% of control), and N-acetyl-L-cysteine (57.3 +/- 9.5%), but was not prevented by superoxide dismutase, catalase, or ascorbic acid. Buthionine sulphoximine, an inhibitor of gamma-glutamylcysteine synthetase, significantly increased the neurotoxic effect of non-toxic concentrations of artemether/haemin (0.1 microM/2 microM) and dihydroartemisinin (0.2 microM), suggesting that endogenous glutathione participates in the prevention of the neurotoxicity of artemether/haemin and dihydroartemisinin. Artemether/haemin completely depleted intracellular glutathione levels, whereas dihydroartemisinin had no effect. We conclude that although glutathione status is an important determinant in the neurotoxicity of endoperoxides, depletion of glutathione is not a prerequisite for their toxicity. This is consistent with their mechanisms of toxicity being free radical-mediated damage to redox-sensitive proteins essential for neurite outgrowth, or alteration of a redox-sensitive signalling system which regulates neurite outgrowth.
The combination of TMSOTf and AgClO(4) promotes the efficient C-10-phenoxylation of dihydroartemisinin (3) in good chemical yield and excellent stereoselectivity. All of the new phenoxy derivatives have potent in vitro antimalarial activity. On the basis of the excellent yield and stereoselectivity obtained for the p-trifluoromethyl derivative 7b, this compound and the parent phenyl-substituted derivative 5b were selected for in vivo biological evaluation against Plasmodium berghei in the mouse model and for metabolism studies in rats. Compound 7b demonstrated excellent in vivo antimalarial potency with an ED(50) of 2.12 mg/kg (cf. artemether = 6 mg/kg) versus P. berghei. Furthermore, from preliminary metabolism studies, this compound was not metabolized to dihydroartemisinin; suggesting it should have a longer half-life and potentially lower toxicity than the first-generation derivatives artemether and arteether. From biomimetic Fe(II)-catalyzed decomposition studies and ESR spectroscopy, the mechanism of action of these new lead antimalarials is proposed to involve the formation of both primary and secondary C-centered cytotoxic radicals which presumably react with vital parasite thiol-containing cellular macromolecules.
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Since dyspnoeic sensation (delta) increases progressively with work rate (WR) and the duration of a volitional breath-hold (tBH) shortens, we wished to explore whether tBH might correlate sufficiently closely with delta to provide a quantitative and descriptor-free index of respiratory sensation during dynamic exercise. Nine healthy males exercised on a cycle ergometer at a series of constant WRs, above and below the lactate threshold. Ventilatory and gas exchange variables were measured breath-by-breath. At each WR, breath-holds to the limit of tolerance were taken; delta was recorded (visual-analog scale) immediately prior to and throughout each breath-hold. During breath-holds, delta increased with time as a "break-away" monoexponential characteristic, reaching the maximum (100%) at the break-point. Despite end-tidal partial pressure of carbon dioxide at the break-point being higher and end-tidal partial pressure of oxygen being lower with increasing WR, the relationship between WR and tBH declined curvilinearly (i.e. with large falls in tBH occurring in the low WR range, but far smaller reductions at higher WRs). The tBH/minute ventilation relationship had a similar form. The relationship between pre-breath-hold delta and tBH was also complex: the large reductions in tBH in the low WR range were associated with only modest increases in pre-BH delta while, at higher WRs, the progressively smaller decrements in tBH were associated with progressively larger increases in delta. We therefore conclude that breath-hold duration is unlikely to provide a useful correlate of exertional dyspnoea during dynamic exercise. Furthermore, the relative prolongation of tBH at high WRs (accounting for the more-extreme levels of end-tidal gas tensions) may reflect the attention-diverting influence of the exercise per se.
Extensive drug resistance in Plasmodium falciparum emphasizes the urgent requirement for novel antimalarial agents. Here we report potent antimalarial activity of a number of diamidine compounds. The lead compound pentamidine is concentrated 500-fold by erythrocytes infected with P. falciparum. Pentamidine accumulation can be blocked by inhibitors of hemoglobin digestion, suggesting that the drug binds to ferriprotoporphyrin IX (FPIX). All of the compounds bound to FPIX in vitro and inhibited the formation of hemozoin. Furthermore, inhibitors of hemoglobin digestion markedly antagonized the antimalarial activity of the diamidines, indicating that binding to FPIX is crucial for the activity of diamidine drugs. Pentamidine was not accumulated into uninfected erythrocytes. Pentamidine transport into infected cells exhibits an initial rapid phase, nonsaturable in the micromolar range and sensitive to inhibition by furosemide and glibenclamide. Changing the counter-ion in the order Cl(-) < Br(-) < NO(2)(-) < I(-) <SCN(-) markedly stimulated pentamidine transport. These data suggest that pentamidine is transported although a pore or ion channel with properties similar to those of the recently characterized 'induced permeability pathway' on the infected red cell membrane. In summary, the diamidines exhibit two levels of selectivity against P. falciparum. The route of entry and molecular target are both specific to malaria-infected cells and are distinct from targets in other protozoa. Drugs that target the hemoglobin degradation pathway of malaria parasites have a proven record of accomplishment. The employment of induced permeability pathways to access this target represents a novel approach to antiparasite chemotherapy and offers an additional level of selectivity.
The prediction that chronic obstructive pulmonary disease (COPD) will be the third leading cause of death worldwide by 2020 has enormous economic repercussions. Yet many issues and questions remain unresolved. For example, how can population studies of morbidity and mortality be viewed as comparable, without a worldwide consensus on the definition of COPD? How can the early diagnosis of COPD be improved? Why is it that only a minority of smokers develop COPD, despite tobacco smoking being the primary risk factor for chronic bronchitis and emphysema? How can the efficacy of smoking cessation interventions be improved? To what extent are the pathologic changes in the lungs reversible - and, if so, at what stage? And to what degree is it appropriate to emphasize the similar features of COPD and asthma? It is to be hoped that the emerging post-genomic and proteomic climate will facilitate the unlocking of the genetic substrate for COPD, and thus promote greater therapeutic specificity and efficacy.
The ATP turnover rate during constant-load exercise is often estimated from the initial rate of change of phosphocreatine concentration ([PCr]) using 31P-magnetic resonance spectroscopy (MRS). However, the phase and amplitude characteristics of the sample-to-sample fluctuations can markedly influence this estimation (as well as that for the time constant (tau) of the [PCr] change) and confound its physiological interpretation especially for small amplitude responses. This influence was investigated in six healthy males who performed repeated constant-load quadriceps exercise of a moderate intensity in a whole-body MRS system. A transmit- receive surface coil was placed under the right quadriceps, allowing determination of intramuscular [PCr]; pulmonary oxygen uptake (VO2) was simultaneously determined, breath-by-breath, using a mass spectrometer and a turbine volume measuring module. The probability density functions (PDF) of [PCr] and VO2 fluctuations were determined for each test during the steady states of rest and exercise and the PDF was then fitted to a Gaussian function. The standard deviation of the [PCr] and VO2 fluctuations at rest and during exercise (sr and sw, respectively) and the peak centres of the distributions (xc(r) and xc(w)) were determined, as were the skewness (gamma1) and kurtosis (gamma2) coefficients. There was no difference between sr and sw for [PCr] relative to the resting control baseline (s(r) = 1.554 %delta (s.d. = 0.44), s(w) = 1.514 %delta (s.d. = 0.35)) or the PDF peak centres (xc(r) = -0.013 %delta (s.d. = 0.09), xc(w) -0.197 %delta (s.d. = 0.18)). The standard deviation and peak centre of the 'noise' in VO2 also did not vary between rest and exercise (sr = 0.0427 l min(-1) (s.d. = 0.0104), s(w) = 0.0640 l min(-1) (s.d. = 0.0292); xc(r) = -0.0051 l min(-1) (s.d. = 0.0069), xc(w) 0.0022 l min(-1) (s.d. = 0.0034)). Our results demonstrate that the intersample 'noise' associated with [PCr] determination by 31P-MRS may be characterised as a stochastic Gaussian process that is uncorrelated with work rate, as previously described for VO2. This 'noise' can significantly affect the estimation of tau[PCr] and especially the initial rate of change of [PCr], i.e. the fluctuations can lead to variations in estimation of the initial rate of change of [PCr] of more than twofold, if the inherent 'noise' is not accounted for. This 'error' may be significantly reduced in such cases if the initial rate of change is estimated from the time constant and amplitude of the response.
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A new exercise protocol has been proposed, with respect to cardiopulmonary exercise testing, which starts at a low work rate (WR) and increases exponentially by a standard percentage of the previous work rate every minute: the test is termed STEEP (standardised exponential exercise protocol). The potential advantage of this protocol is that it can accommodate a wide range of subjects, since it allows a maximum to be attained with a relatively narrow variation of tolerance time, regardless of subjects exercise capacity. To date, only the VO2max has been compared with that from the current standard ramp protocol. The ramp, however, also allows other important parameters of aerobic function to be estimated: the anaerobic threshold (AT); the response time constant; and delta VO2/delta WR. The aim of this study was, therefore, to clarify whether these aerobic parameters can be readily discerned from the responses to the STEEP protocol both from a theoretical and practical viewpoint. As a result of theoretical considerations, we demonstrated that the VO2 time constant and delta VO2/delta WR may not both be estimated uniquely. As a practical expedient, a procedure was proposed for estimating the parameter analogues. The preliminary results for six subjects between the STEEP and ramp protocols showed consistent positive correlation for VO2max (r = 0.997) and AT-VO2 (r = 0.980), whereas the correlation for the VO2 time constant and delta VO2/delta WR were not significant. Further study is needed to clarify the reason(s) for the discrepancies both from a theoretical and practical viewpoint.
A novel gene was cloned from Plasmodium falciparum. Database searches indicated this gene to be a member of the 20S proteasome beta-subunit family. Comparison of the gene's genomic DNA sequence with cDNA sequence revealed a 156-bp intron 85 bp downstream from the start codon. The nucleotide sequence of the gene contains one open reading frame encoding 265 amino acids with a predicted molecular mass of 30.9 kDa and a pI of 6.2. Northern blot analysis showed the transcript size to be approximately 1.6 kb indicating that some 800 bp of the transcript is non-coding.
Models of the exercise hyperpnoea have classically incorporated elements of proportional feedback (carotid and medullary chemosensory) and feedforward (central and/or peripheral neurogenic) control. However, the precise details of the control process remain unresolved, reflecting in part both technical and interpretational limitations inherent in isolating putative control mechanisms in the intact human, and also the challenges to linear control theory presented by multiple-input integration, especially with regard to the ventilatory and gas-exchange complexities encountered at work rates which engender a metabolic acidosis. While some combination of neurogenic, chemoreflex and circulatory-coupled processes are likely to contribute to the control, the system appears to evidence considerable redundancy. This, coupled with the lack of appreciable error signals in the mean levels of arterial blood gas tensions and pH over a wide range of work rates, has motivated the formulation of innovative control models that reflect not only spatial interactions but also temporal interactions (i.e. memory). The challenge is to discriminate between robust competing control models that: (a) integrate such processes within plausible physiological equivalents; and (b) account for both the dynamic and steady-state system response over a range of exercise intensities. Such models are not yet available.