PubMed Health⌕ Search

Biomedical subjects

M V Thomas

Publications and source records attributed to M V Thomas.

14 recordsLinked to original sources
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↗
PubMed1978

Changes in the intracellular concentration of free calcium ions in a pace-maker neurone, measured with the metallochromic indicator dye arsenazo III.

1. The bursting pace-maker R-15 cell of Aplysia was injected with the Ca2+ sensitive dye arsenazo III. Changes in absorbance were measured with a differential spectrophotometer to monitor changes in free intracellular Ca2+, [Ca-a], during activity. 2. Dye absorbance increased during each pace-maker-induced burst of action potentials and decreased during the hyperpolarizing phase of the pace-maker cycle. 3. The increase in dye absorbance was, at least in part, dependent upon action potential discharge and was greater when action potential duration was prolonged by treatment with tetraethylammonium chloride. 4. Changes in dye absorbance occurred under voltage clamp conditions when the membrance was depolarized 5-15 mV from a holding potential near the resting potential and were larger with greater step depolarizations. 5. These changes were completely blocked by the addition of 3mM-La3+ to normal ASW. 6. The ratio of the absorbance change between two pairs of wave-lengths during the pace-maker cycle was compared with the ratio observed following injection of Ca2+, Mg2+ and H+ ions. The ratio for the pace-maker cycle was well matched by that for Ca2+ injection, but not by that for injection of Mg2+ or H+. 7. Intracellular Ca2+ injections which increased [Ca]1 to the same amount as occurred during the pace-maker cycle also produced an outward current of sufficient magnitude to account for the post-burst hyperpolarization. 8. Depolarization of the cell membrane by extrinsic current during the burst increased and prolonged the change in dye absorbance as well as the post-burst hyperpolarization. 9 It is suggested that Ca2+ enters during the pace-maker cycle, thereby increasing [Ca]i, and that this increase is sufficient to activate an outward current carried by K+ ions which causes or contributes to the post-burst hyperpolarization.

Action Potentials↗
PubMed1977

Internal calcium changes in a bursting pacemaker neuron measured with arsenazo III.

Arsenazo III was used to measure changes in the free intracellular calcium concentration during spontaneous bursting pacemaker activity in the Aplysia R1K neuron. Intracellular calcium increased during the burst, and this increase was sufficient to cause the hyperpolarization that followed. The results suggest that the interval between bursts is determined by the rate of subsequent decline of free intracellular calcium.

Action Potentials↗
PubMed1976

Insect blood-brain barrier: an electrophysiological investigation of its permeability to the aliphatic alcohols.

The anaesthetic effects of the aliphatic alcohols were used to measure their concentration at the neuronal surfaces of the cockroach central nervous system. The results were in most cases fairly closely described by first-order kinetics. The exchange half-times of the lower alcohols were only a few seconds, being little affected by the removal of the nerve sheath. The half-times for the higher alcohols were somewhat longer, and were more significantly reduced by desheathing; these observations were interpreted in terms of a reservoir effect resulting from their higher liposolubility. It was shown that the ionic diffusion barrier in intact nerve cords remained undamaged in the presence of the alcohols.

Action Potentials↗
PubMed1976

Insect blood-brain barrier: a radioisotope study of the kinetics of exchange of a liposoluble molecule (n-butanol).

About 90% of the butanol uptake by the cockroach abdominal nerve cord washed out with half-times of a few seconds, in good agreement with an electrophysiological estimate, and the temperature sensitivity suggested an activation energy of 3 Kcal mole-1. The remaining activity washed out far more slowly, with a similar time course to that observed in a previous investigation which had not detected the fast fraction. Its size was similar to the non-volatile uptake, and was considerably affected by the butanol concentration and incubation period. It apparently consisted of butanol metabolites, which could be detected by chromatography.

Animals↗
PubMed1975

An electrophysiological analysis of extra-axonal sodium and potassium concentrations in the central nervous system of the cockroach (Periplaneta americana L.).

Simultaneous intracellular and sucrose-gap recordings showed, in contrast to previous findings, that the electrical parameters of giant axons were similar to intact and desheathed connectives bathed with the 'extracellular Ringer' of Yamasaki & Narahashi. This implies that the extra-axonal sodium concentration, in situ, is likely to be lower than had been previously supposed. Axonal responses showed that, despite the high blood concentration of 24-2 mM-K+ measured by flame photometry, the effective concentration in the blood was 10-15 mM-K+ which corresponds to the measurements made with potassium-selective electrodes. The activity of the blood potassium ions caused a marked reduction in the amplitude of the action potentials following surgical desheathing or disruption of the blood-brain barrier with hypertonic urea. It is suggested that a regulatory mechanism exists in the central nervous system which counteracts the effects of the high blood potassium level.

Action Potentials↗