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Biomedical subjects

M V Thomas

Publications and source records attributed to M V Thomas.

At least 19 recordsLinked to original sources
PubMed2006

Mechanical and degradation behavior of polymer-calcium sulfate composites.

Calcium sulfate (CS) is one of the oldest bone graft materials still in use. Its main limitations are poor handling characteristics, poor mechanical properties, and a resorption rate that is too fast for some applications. The present study investigated the effect of viscous polymers, such as carboxymethylcellulose (CMC) and hyaluronan (HY), on the handling characteristics, mechanical properties, and degradation behavior of CS. CMC and HY were added to CS at concentrations from 1-10 wt%. Addition of CMC to CS at more than 4 wt% produced a putty-like material and decreased the density of the composite, while also increasing flexural and compressive strength at higher loadings. Incorporation of CMC produced a concentration-dependent increase in water absorption and degradation rate. At an equivalent loading, HY-containing CS composites showed better compressive strength than CS with CMC. Overall, addition of CMC or HY to CS resulted in composite materials with better handling characteristics and improved mechanical properties after set, however the degradation rate of the augmented materials was increased. These properties suggest that the enhanced CS materials may be useful in certain clinical situations, such as filling non-uniform bone defects and situations that require mechanical integrity of the bone graft substitute during implantation.

Biocompatible Materials↗
PubMed2000

Leprosy affected beggars as a hidden source for transmission of leprosy.

BACKGROUND & OBJECTIVES: Despite the large scale implementation of multidrug therapy (MDT), the incidence rates of leprosy have not declined in several hyperendemic countries. Before searching for non-human reservoirs of leprosy it would be necessary to look for hidden human sources. This would include destitute leprosy affected persons who resort to begging and operate in congested areas. Hence this study was undertaken. METHODS: One major town and three semi-urban areas in Vellore district of Tamil Nadu and Chittoor town in Andhra Pradesh were purposefully selected for the study. All beggars in these towns were systematically identified and examined by allopathic doctors. Skin smears were examined for bacteriological index. RESULTS: Among the 193 beggars screened, 58 had leprosy. Of these 10 were smear positive. Several beggars, although living separately, were in touch with their relatives. Most beggars were pavement dwellers and regularly begged at places of worship, bus stands and shopping centres. INTERPRETATION & CONCLUSION: The fact that nearly 20 per cent of the leprosy affected beggars were skin smear positive highlights the need for regular screening and treatment of such beggars. Those positive should be actively treated and their close contacts frequently screened. This hidden reservoir should be completely eliminated.

Adolescent↗
PubMed1984

Voltage-clamp analysis of a calcium-mediated potassium conductance in cockroach (Periplaneta americana) central neurones.

The electrical properties of motoneurone cell bodies in the metathoracic ganglion of the cockroach, Periplaneta americana, have been studied by the voltage-clamp technique. Most experiments were carried out on a single identified cell (cell 28), the cell body of which, as for most other insect motoneurones, is electrically inexcitable. For comparison, some experiments were also carried out on dorsal unpaired median (d.u.m.) cells, the cell bodies of which are excitable. The two cell types differed only in that the d.u.m. cells developed a transient net inward current when depolarized towards zero membrane potential. This current was reduced but not abolished by Ca-free saline. Both cell types had an N-shaped current-voltage relation, as typically seen for molluscan and other neurones, but the location of the falling phase of the relation showed an unusually strong time dependence. The N-shape was abolished by prolonged exposure to Ca-free saline, suggesting it to be due to a K conductance that was activated by the entry of Ca ions through voltage-dependent channels. An outward current was also elicited by ionophoretic injection of Ca ions. The reversal potential of this current varied with the saline K concentration, in the manner expected if the current was carried by K ions. The Ca-mediated K current was blocked by La ions and by the organic Ca antagonist D-600. A series of double-pulse experiments on voltage-clamped cell bodies of cell 28 suggested that very short periods of Ca entry were sufficient to activate the K conductance fully. These experiments also suggested that the K conductance activation was due to intracellular Ca accumulation rather than to its being directly linked to the inward Ca current. The activation of the K conductance by intracellular Ca ions was made more effective by cell membrane depolarization. The Ca-mediated conductance did not inactivate significantly under conditions in which substantial inactivation has been observed in other neurones. The physiological significance of the electrical properties of cell 28 is discussed.

Action Potentials↗
PubMed1984

Light and electron microscopic examinations of methane-producing biofilms from anaerobic fixed-bed reactors.

Ultrastructural examinations were performed on biofilms from eight anaerobic fixed-bed reactors filled with various packing materials and operated on fresh swine waste. By using light, UV, scanning, and transmission electron microscopy, the distribution of a diverse microbial population composed of bacteria and a few yeasts was determined. This is the first time that the ultrastructure of in situ anaerobic digestor biofilms has been reported. A large number of methanogenic bacteria were identified by their fluorescence under 420 nm of radiation. Of these, two morphologically distinct types were most prevalent in the films. Methanothrix spp. was present in high numbers at the film surface, whereas Methanosarcina spp. were commonly embedded in the lower regions of the film. Inhabitants of the film were surrounded by an exopolysaccharide matrix that was very dense toward the base. An extensive network of channels was observed throughout the matrix that may facilitate gas and nutrient exchange to the lower regions of the film.

Journal Article↗
PubMed1983

Isolation and Characterization of Methanomicrobium paynteri sp. nov., a Mesophilic Methanogen Isolated from Marine Sediments.

A new mesophilic methanogenic bacterial species isolated from marine sediments collected in the Cayman Islands is described. Cells are small rods occuring singly without filaments, are not motile, and do not possess flagella. Colonies are semitransparent and off-white in color. After 2 weeks of incubation at 37 degrees C colonies are 1 to 2 mm in size, circular, and have entire edges. Only hydrogen-carbon dioxide is a substrate for growth and methane formation. Cells can tolerate a variety of organic secondary buffers (bicarbonate-CO(2) being the primary buffer). Cells do not require yeast extract or Trypticase, but do require acetate, for growth. The optimum growth temperature is 40 degrees C. The optimum sodium concentration is 0.15 M. The optimum pH for growth is 7.0. The minimum generation time is 4.8 h. The DNA base composition is 44.9 mol% guanine plus cytosine. The name Methanomicrobium paynteri is proposed in honor of M. J. B. Paynter. The type strain is G-2000 (=ATCC 33997, =DSM 2545).

Journal Article↗
PubMed1982

Ionic requirements for membrane oscillations and their dependence on the calcium concentration in a molluscan pace-maker neurone.

1. Membrane currents from the bursting pace-maker neurone R-15 of Aplysia were measured under conditions designed to simulate membrane oscillations. Changes in the absorbance of the Ca(2+)-sensitive dye arsenazo III were used to monitor changes in the free intracellular Ca(2+) concentration, [Ca](i), under these conditions. In addition, changes in the extracellular K(+), concentration [K](o) were measured with K(+)-sensitive electrodes.2. In normal external ionic conditions the depolarizing phase of pace-maker activity was associated with a slow inward current and the hyperpolarizing phase with a slow outward current.3. In cells where the early inward Na(+) current was blocked by tetrodotoxin and outward K(+) currents were suppressed by intracellular EGTA and extracellular tetraethylammonium and 4-aminopyridine, the slow inward current was significantly larger in amplitude and was suppressed by removal of external Ca(2+) or the addition of external La(3+), but not by the removal of external Na(+).4. The slow inward current was increased when [Ca](o) was raised and decreased when it was reduced in the manner expected for current flow through a Ca(2+) channel. The selectivity of the slow inward current for divalent cations was [Formula: see text].5. The slow inward current was only slightly reduced by a 10 degrees C reduction in temperature.6. In normal external and internal ionic conditions changes in dye absorbance occurred when the membrane was depolarized with slow triangular voltage ramps or long depolarizing steps within the pace-maker oscillation range. The obsorbance change, and thus the increase in Ca(2+), [Ca](i), was well correlated with the appearance of the slow inward current. Moreover, the magnitude of the slow outward current was dependent upon the change in [Ca](i).7. The slow inward current and a substantial fraction of the outward current, as well as the change in [Ca](i), were reduced appreciably by the addition of La(3+) ions (3 mM) to the external medium.8. The increase in [Ca](i) during prolonged depolarization was not affected by external tetrodotoxin or by the removal of external Na(+), but was abolished by a Ca(2+)-free external medium containing EGTA. Nevertheless, significant changes occurred in [Ca](i) during depolarization in 0.1 mM-external Ca(2+).9. In normal external and internal ionic conditions extracellular K(+), [K](o), increased during the depolarizing phase of the pace-maker cycle and decayed during the hyperpolarizing phase.10. There was a measurable increase in [K](o) during small prolonged depolarizing steps which produced a net inward current, indicating that inward and outward currents overlap under normal conditions.11. In the absence of action potential discharge, [Ca](i) increased during the depolarizing phase and decreased during the hyperpolarizing phase of the membrane oscillation.12. It is proposed that pace-maker oscillations depend upon three separate but linked systems which include a voltage-dependent Ca(2+) current, the free intracellular Ca(2+) concentration and the Ca(2+)-activated K(+) current.

Animals↗
PubMed1981

Glass microelectrode tip capacitance: its measurement and a method for its reduction.

The frequency response of input amplifiers used for measurement of bioelectric signals from small cells is severely limited by the resistance and capacitance of the fine glass microelectrodes which are required for these measurements. A significant improvement in the frequency response can be realized by employing the technique of input capacitance neutralization. This capacitance neutralization, however, is incomplete since: (1) the bandwidth of the input amplifier is finite; and (2) a fraction of the electrode capacitance is isolated from the amplifier input by a part of the electrode resistance and cannot be compensated. It is therefore desirable to ensure that the electrode capacitance is as low as possible before neutralization. A method is discussed for measuring and predicting the distributed capacitance of the microelectrode and a technique is described for coating the outside of the electrode near the tip with a substance which can lower the electrode capacitance per unit length by as much as 7-fold. The significance of the improved frequency response of the input amplifier that this technique provides is discussed in light of recent advances in intracellular single electrode voltage clamp technique.

Animals↗
PubMed1981

Intracellular calcium and the control of neuronal pacemaker activity.

Pacemaker activity of the Aplysia bursting pacemaker neuron R-15 was analyzed. It was shown that the free intracellular Ca2+ concentration, as measured by arsenazo III, increases during the depolarizing phase of the pacemaker cycle and declines throughout the hyperpolarizing phase that follows. This increase in Ca2+ results from the activation of voltage-dependent Ca2+ channels that open during the depolarizing phase of the cycle. The extracellular K+ concentration also increases during the depolarizing phase of the cycle and is correlated with an outward K+ current that opposes the inward current carried by Ca2+ ions. The increase in internal Ca2+ is sufficient to activate a K+ conductance that depends on the magnitude of the change in internal Ca2+ and on membrane potential, which is responsible for the hyperpolarizing phase of the cycle. It is proposed that the membrane oscillation depends on three separate but linked systems, which include a voltage-dependent Ca2+ channel, the internal Ca2+ concentration, and a Ca2+-activated K+ channel.

Action Potentials↗
PubMed1980

Intracellular calcium accumulation during depolarization in a molluscan neurone.

1. The bursting pacemaker neurone R-15 of Aplysia was injected with the Ca2+ sensitive dye arsenzo III. Changes in absorbance were measured with a differential spectrophotometer to monitor changes in free intracellular Ca2+ during membrane depolarization under voltage clamp conditions. 2. Dye absorbance increased linearly for depolarizing pulse durations up to 100 msec and approximately linearly between 100 and 300 msec, but for longer durations the absorbance change decreased. 3. The absorbance change vs. voltage relation increased steeply between -20 and 0 mV (e-fold per 8.5 mV), peaked at +36 mV and declined non-linearly to an estimated null or suppression potential of about +139 mV. 4. TTX (5 x 10(-5 M) had no effect on the change in dye absorbance produced by brief or long duration stimuli whereas Ca2+ free ASW abolished all changes in dye absorbance. 5. The absorbance change saturated with increasing external Ca2+ concentrations. The relation between dye absorbance and external Ca2+ concentration was hyperbolic and for a small range of external Ca2+ concentration and membrane potentials could be fitted by a Michaelis--Menten expression where the dissociation constant and the maximum absorbance change are voltage dependent. 6. The absorbance change was reduced by external divalent ions which block the Ca2+ channel (e.g. Cd2+ and Ni2+). The suppression of dye absorbance was increased by membrane depolarization and suggests that there is a voltage dependent site within the Ca2+ channel which binds divalent ions. 7. The decline of the absorbance--voltage relation from its peak to the suppression potential showed a greater nonlinearity when longer duration voltage clamp pulses were used. The non-linearity can be explained if the accumulation of Ca2+ ions next to the inner surface of the membrane during depolarization reduces the driving force on Ca2+ ions and thus decreases Ca2+ ion influx. 8. The suppression potential estimated from the absorbance--voltage relation increased 29 mV per tenfold change in the external Ca2+ concentration and thus can be used to estimate the Ca2+ equilibrium potential. 9. The change in dye absorbance produced by brief depolarizing voltage clamp steps was inactivated at positive holding potentials (50% inactivation at about -14 mV). Our results suggest that the slow decrease in dye absorbance during prolonged depolarization is caused by inactivation of the Ca2+ channel.

Animals↗
PubMed1980

Potassium conductance and internal calcium accumulation in a molluscan neurone.

1. The Aplysia neurone R-15 was injected with the Ca(2+) sensitive dye arsenazo III. Changes in dye absorbance were measured with a differential spectrophotometer to monitor changes in the free internal Ca(2+) concentration, [Ca](i), during membrane depolarization and during intracellular Ca(2+) ion injection under voltage clamp conditions.2. The absorbance change, and thus [Ca](i), increases linearly with Ca(2+) injection intensity at constant duration. The absorbance change produced by a constant intensity Ca(2+) injection also increases with injection duration, but this increase is asymptotic.3. The Ca(2+) activated K(+) current, I(K, Ca), increases linearly with the increase in [Ca](i) and its rise and decay follows closely the time course of the absorbance change produced by internal Ca(2+) injection.4. The Ca(2+) activated K(+) conductance increases exponentially with membrane depolarization. The increase in K(+) conductance activated by a constant intensity and duration Ca(2+) injection is on average e-fold for a 25.3 mV change in membrane potential.5. The difference in net outward K(+) current measured during depolarizing pulses to different membrane potentials in normal and in Ca(2+) free ASW was used as an index of I(K, Ca). Its time course was approximately linear for the first 50-100 msec of depolarization, but for longer times the relation approached a maximum. Simultaneous measurements of the arsenazo III absorbance changes were broadly consistent with the activation of I(K, Ca) being brought about by the rise in [Ca](i) during a pulse.6. The relation between Ca(2+) activated K(+) conductance and membrane potential is bell shaped and resembles the absorbance vs. potential curve, but its maximum is displaced to more positive membrane potentials. The shift in the two curves on the voltage axis can be explained by the potential dependence of G(K, Ca).7. The net outward K(+) current measured with depolarizing voltage pulses in normal and in Ca(2+) free ASW is increased when [Ca](i) is elevated by internal Ca(2+) injection. With large and prolonged Ca(2+) injections the net outward current is depressed following the decline of [Ca](i).8. The time and frequency dependent depression of the net outward K(+) current which occurs during repetitive stimulation is shown to have no obvious temporal relation to the increase in [Ca](i). The depression is relieved by an increase in [Ca](i) caused by internal Ca(2+) injection.9. The net outward K(+) current measured with brief depolarizing pulses which approach the estimated Ca(2+) equilibrium potential and therefore do not cause Ca(2+) influx and accumulation is facilitated by a previous depolarizing pulse which causes a rise in [Ca](i)..10. The facilitation experiments also suggest that the activation of I(K, Ca) by [Ca](i) has a significant time constant. During a depolarizing pulse, the rise in [Ca](i) next to the membrane, and hence I(K, Ca) is expected to follow the square root of time, but a delay in the activation of I(K, Ca) by [Ca](i) could explain why the observed time course of I(K, Ca) is initially almost linear.11. The potential dependence of the Ca(2+) activated K(+) conductance can be explained if the internal Ca(2+) binding site is about half way through the membrane.

Animals↗
PubMed1979

Arsenazo III forms 2:1 complexes with Ca and 1:1 complexes with Mg under physiological conditions. Estimates of the apparent dissociation constants.

Experiments to determine the apparent dissociation constants of the Ca and Mg complexes of arsenazo III clearly indicated that the predominant Ca complex contains one Ca ion and two dye molecules, although previous reports have either claimed or assumed 1:1 complexing. The evidence is based on the effects of varying [dye] as well as [Ca] and [Mg], and clear evidence for the formation of 1:1 complexes with Ca was obtained only at submicromolar [dye], whereas Mg formed 1:1 complexes exclusively. The implications of these findings with regard to the use of arsenazo III as an indicator of intracellular free [Ca] are discussed, with particular reference to its selectivity for Ca and the interference effects of other ions.

Arsenazo III↗
PubMed1979

Stability of the arterial/alveolar oxygen partial pressure ratio. Effects of low ventilation/perfusion regions.

The alveolar-arterial oxygen partial pressure difference (AaDO2) and the arterial/alveolar oxygen partial pressure ratio (a/APO2) were compared for stability when inspired oxygen concentration (FIO2) changed. The analysis was based on a three-compartment lung model and experimental results in 10 patients with respiratory failure receiving assisted ventilation. It was found that a/APO2 was more stable than AaDO2 and more useful for: (1) comparing gas exchange in patients receiving different levels of FIO2, (2) following gas exchange in the same patient as FIO2 is changed, and (3) estimating the PaO2 expected at a given level of FIO2 if blood gas data are available at another level. However, areas with low ventilation/perfusion (V/Q) ratios may cause sudden changes in a/PO2 at certain critical values of PAO2. Most stable is a/APO2 and, therefore, most useful at FIO2 levels greater than 0.3, and PaO2 levels less than 100 torr.

Adult↗