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Biomedical subjects

D H Thomas

Publications and source records attributed to D H Thomas.

At least 37 records · Page 2Linked to original sources
PubMed1989

Function and regulation of the avian caecal bulb: influence of dietary NaCl and aldosterone on water and electrolyte fluxes in the hen (Gallus domesticus) perfused in vivo.

The function of the caecal bulb, and its adaptation to chronic high- or low-Na+ intake, was investigated by in vivo perfusion of anaesthetised birds. Effects of acute aldosterone injection (125 micrograms.kg-1 body mass) were also measured. Evidence was found for primary active net absorption of Na+, inducing parallel Na-linked absorption of water and Cl- and secretion of K+. Around 20-35% of total Cl- absorption and K+ secretion were independent of Na+ fluxes, and these components appear to be driven by passive processes with apparent conductances of 6.3 X 10(-3) (GCl) and 1.1 X 10(-3) (GK) S.cm-2. Acetate (40 mM) stimulated Na+ fluxes (8.5-9.9 microEq.cm-2.h-1) and Na-linked water fluxes (27-44 microliters.cm-2.h-1). Increased coupling ratios (2.9-4.6 microliters.microEq-1) and other data indicate that these effects may be due to increased osmotic permeabilities of barriers involved in the Na-linked water transfer pathway. Low-Na+ maintenance enhanced EPD (49-69 mV, serosa positive) and all net fluxes: JNa (6.8-11.6); JK (-3.2--4.3); JCl (4.3-5.6 microEq.cm serosal area-2.h-1); Jv (28-43 microliters.cm-2.h-1) (mucosal-serosal fluxes positive). Acute aldosterone enhanced JNa (10.8-14.0 microEq.cm-2.h-1) and EPD (54-66 mV) by 3 h after injection, but had no effect on the Na-linked components of JK or JCl.

Aldosterone↗
PubMed1989

A novel dialysis procedure for the crystallization of proteins.

Various dialysis methods are commonly employed for the crystallization of proteins. Typical procedures include the use of dialysis bags, dialysis buttons or Zeppezauer microdiffusion cells. The general principle involved is that the protein solution is gradually brought to a point of supersaturation by imposing a gradient of ionic strength or organic solvent concentration across the wall of the dialysis membrane. However, in some cases, the imposition of this gradient across the dialysis membrane can result in the formation of a large number of crystal nucleation sites, thereby giving rise to a reduction in the maximum size of the crystals which can be obtained. A novel 'double-dialysis' procedure which incorporates a second dialysis membrane, thus reducing the rate of equilibration in the crystallization experiment, has been developed in our laboratory. The system has been employed successfully on the delta toxin of Staphylococcus aureus resulting in a useful increase in crystal size. A more quantitative analysis of the technique has been carried out on rat liver malic enzyme. The results of a limited series of crystallization trials with this protein have shown that employment of the 'double-dialysis' technique allows a fine control of the rate of crystal nucleation and therefore provides a mechanism for the controlled growth of large crystals.

Animals↗
PubMed1988

Properties of ion channels formed by Staphylococcus aureus delta-toxin.

The delta-toxin of Staphylococcus aureus has been investigated in terms of its potential to form ion channels in planar lipid bilayers formed at the tip of patch electrodes. Channel formation has been shown to occur for delta-toxin concentrations in the range 0.1 to 2.0 microM. In 0.5 M KCl, two major classes of channels were seen--'small' with conductances of 70-100 pS, and 'large' with a conductance of approx. 450 pS. Current-voltage relationships for lipid bilayers containing several delta-toxin channels revealed both voltage-dependent and independent components to channel gating. Reversal potential measurements showed the channels to be cation selective. In the presence of 3.0 M KCl, the channel gating kinetics were complex, with multiple open and closed states. The results are interpreted in terms of a model for the channel consisting of a hexameric cluster of alpha-helical delta-toxin molecules.

Bacterial Toxins↗
PubMed1988

Transport function and control in bird caeca.

1. The paired caeca at the junction of the ileum and rectum help determine the ionic composition of the voided excreta. 2. The caeca play a role in the transport of water, sodium, potassium, and chloride, the dominant effect being that of sodium transport.

Aldosterone↗
PubMed1987

Crystallization of an NADP+-dependent malic enzyme from rat liver.

Crystals of a tetrameric NADP+-dependent malic enzyme from rat liver have been grown in the presence of NADP+ using the hanging-drop method of vapour diffusion with ammonium sulphate as the precipitant. Measurement of the crystal density and calculation of the values of Vm for different numbers of polypeptide chains in the unit cell indicate that the asymmetric unit of the crystal contains a complete tetramer, allowing the application of non-crystallographic symmetry to the determination of the molecular structure of this enzyme. This structure would provide only the second example for an enzyme involved in oxidative decarboxylation, the other being 6-phosphogluconate dehydrogenase. In addition, then, to providing an insight into the structure-function relationship in malic enzyme, the successful structure determination would permit valuable comparisons to be made between these two and other enzymes with this catalytic activity.

Animals↗
PubMed1987

Recovery from steroid-induced osteoporosis.

The reversibility of steroid-induced osteoporosis, a major complication of Cushing syndrome and long-term use of exogenous corticosteroids, is not well documented. We measured the bone mineral density of lumbar vertebras and the femoral neck by dual-photon absorptiometry and determined the biochemical variables of bone turnover in two patients successfully treated for Cushing syndrome who were followed for the next 24 months. Our data are the first to show marked increases in bone density (up to 20%) during the recovery period. The accompanying biochemical changes, particularly the marked increase in serum osteocalcin levels, confirm that enhanced bone formation occurred during the recovery phase. These findings suggest that steroid-induced osteoporosis can be reversed at least in young persons.

Adrenalectomy↗
PubMed1986

Crystallization of the delta toxin of Staphylococcus aureus.

Delta toxin is a small cytolytic polypeptide produced and secreted by the organism Staphylococcus aureus and belongs to a family of surface-active toxins that exhibit pronounced effects on a wide variety of cellular membranes. Although this class of proteins has been much studied by a wide variety of physical techniques, no consensus has been reached on their mode of action. Therefore, in order to investigate their role in causing membrane damage, a structural analysis of the delta toxin has been initiated. Crystals of this protein have been grown by dialysis against mixtures of 2-methylpentan-2,4-diol and water. These crystals are relatively insensitive to radiation damage and diffract to high resolution. The results of this study should provide a valuable insight into the cytolytic properties of these molecules.

Bacterial Toxins↗
PubMed1982

Time course of adaptation to low and high NaCl diets in the domestic fowl. Effects on electrical behaviour of isolated epithelia from the lower intestine.

Short circuit currents (Isc), resting electrical potential differences (p.d.) and resistances were measured in vitro for stripped rectal ("colonic") and coprodeal epithelia from fowls transferred to low from high NaCl diet for 1, 2, 4 or 8 days (NaCl depletion), and from birds adapted to low NaCl diet and sacrificed 8, 16 and 24 h after NaCl-loading (resalination). NaCl-depletion enhanced Isc and p.d. fully by 4 days in rectum but took 8 days in coprodeum (half times 1.2 days and 4 days respectively); rectal amino acid-sensitive Isc was abolished and amiloride-sensitive Isc was established by 4 days (half-time 1 day). Resistance changes were transient and relatively small in rectum, but coprodeal resistance decreased. Resalination reversed these changes very rapidly in both tissues (within 25-50 h; half times 10-20 h). It is concluded that responses to NaCl-depletion are mediated by aldosterone (and possibly by some other unknown agonist); responses to resalination may be due in part to suppressed aldosterone secretion and possibly to the evocation of some other controller (perhaps prolactin or corticosterone) antagonising aldosterone.

Adaptation, Physiological↗
PubMed1982

Salt and water excretion by birds: the lower intestine as an integrator of renal and intestinal excretion.

1. In the fowl, the small intestine is important for net absorption of Ca2+ and K+, but not for Na+ nor water (in this and several other species). 2. Net water absorption in birds with large saccate caeca occurs in caeca greater than rectum greater than coprodeum, but net Na+ absorption (an active process motivating other absorptive functions) occurs in rectum less than caeca and coprodeum. 3. Interspecific variability and the scarcity of comparative studies militate against broad, well-founded generalisations in this subject.

Animals↗
PubMed1979

Dietary Na+ effects on transepithelial transport of NaCl by hen (Gallus domesticus) lower intestine (colon and coprodeum) perfused luminally in vivo.

The colon and coprodeum of anaesthetised laying hens was perfused luminally with solutions of varying compositions, to elucidate the influence of three levels of dietary NaCl and of luminal NaCl concentrations on coprodeal and colonic transport of NaCl. Net Na+ and Cl- absorption rates were increased in response to low dietary Na+ levels and higher luminal NaCl concentrations. Net Na+ absorption was always against the prevailing electrochemical gradient, saturably dependent on luminal Na+ concentrations in low Na+ diet birds, and linearly dependent on luminal concentrations in medium and high Na+ diet birds. Net Cl- absorption was usually down its electrochemical gradient. The Cl- flux rates were strongly linearly dependent on Na+ flux rates, and statistically this relationship could account for all significant variation of Cl- fluxes associated with dietary and perfusate treatments. Transmural electrical potential differences were invariably serosa positive, and partly correlated with Na+ fluxes.

Animals↗
PubMed1979

Transepithelial transport of K+, NH4+, inorganic phosphate and water by hen (Gallus domesticus) lower intestine (colon and coprodeum) perfused luminally in vivo.

The colon and coprodeum of anaesthetised laying hens was perfused luminally with solutions of varying compositions to elucidate the influence of three levels of dietary Na+ and of luminal fluid composition on transport of electrolytes and water. Net rates of secretion of K+ and absorption of NH4+ were increased in response to dietary Na+ depletion and increased luminal Na+ concentrations, but these fluxes were linked to Na+ transport and no dietary or perfusion treatment effects could be detected on them independently of effects on Na+ flux. Similar results were found for Na+-linked water absorption from isosmotic perfusates. Serosal-mucosal net osmotic flow with Na+-free perfusates was significantly reduced by Na+-depletion, but no significant dietary effect could be detected on the serosal-mucosal osmotic permeability coefficient. The reflexion coefficient was effectively unity. Mucosal-serosal inorganic phosphate flux was small. The flux was independent of Na+ flux and dietary Na+ levels, and apparently facilitated by serosalmucosal osmotic flow. The data allowed determination of the absorption/secretion in coprodeum and colon of the ions investigated, as compared to the renal excretion rate.

Ammonium Chloride↗
PubMed1979

Acute effects of aldosterone on water and electrolyte transport in the colon and coprodeum of the domestic fowl (Gallus domesticus) in vivo.

White Leghorn laying hens were maintained on commercial poultry food (medium-Na+ diet) and fresh water. High-Na+ diet birds received, in addition, 10 ml 9% (w/v) NaCl/kg by stomach load for 2 days before perfusion experiments. The lumen of the coprodeum and colon of anaesthetized birds was perfused with a hyperosmotic solution resembling ureteral urine. Transmural solute and water fluxes and potential difference (p.d.) values were measured for 2.5 h before and for 8 h after i.v. injection of aldosterone (120 microgram/kg). After administration of aldosterone, the lag, increasing and stabilized plateau phases were identified for Na+, K+ and Cl- fluxes (which together formed an electroneutral ion-exchange system); plateau flux values were significantly greater than preinjection values and were comparable to values for birds maintained on low-Na+ diets in parallel experiments. Ammonium, phosphate and water fluxes were unresponsive to aldosterone and p.d. values showed a transient increase in medium-Na+ diet birds only. In parallel experiments on birds on low sodium diets the ammonium flux and p.d. increased but the osmotic flow and phosphate transfer did not respond. Therefore acute injection of aldosterone reproduced some but not all of the responses to dietary Na+ restriction in fowls.

Aldosterone↗