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Biomedical subjects

I Cohen

Publications and source records attributed to I Cohen.

At least 415 records · Page 23Linked to original sources
PubMed1975

Actin-activated ATPase from human erythrocytes.

A fibrillar protein complex, possessing ouabain-insensitive Ca2+-ATPase activity was isolated from human erythrocyte membranes by using a low ionic strength extraction procedure. Mg2+-ATPase activity was revealed upon addition of rabbit skeletal muscle actin, thus demonstrating the presence of a myosin-like protein in the crude extract of the erythrocyte membrane. Upon sodium dodecylsulfate gel electrophoresis, the extract showed mainly the doublet of subunit molecular weight bands of 230 000 and 210 000, and more than 10 faster moving bands. Gel filtration of the erythrocyte membrane extract on Sepharose 4B furnished 4 fractions. Fraction I, containing the doublet and 80 000, 60 000 and 46 000 subunit molecular weight bands was 5-fold purified with respect to Ca2+-ATPase activity, but was devoid of actin-activated Mg2+-ATPase activity. Fraction II, containing only the doublet, was devoid of Ca2+ and actin-activated Mg2+-ATPase activity. The 210 000 subunit molecular weight protein could be phosphorylated in the presence of Mg2+ in the crude extract and Fraction I but not in Fraction II.

Actins↗
PubMed1975

Fibrin-blood platelet interaction in a contracting clot.

The interaction of fibrin and blood platelets was studied by measuring tension development in clots containing different ratios of these two components. Isometric tension attained a saturation value (MT) when plotted as a function of the concentration of either component at a constant concentration of the other. The value of MT increases linearly with the minimal platelet concentration (Ps) required in order to reach saturation at various different fibrin concentrations. A linear relationship also holds in a plot of Ps versus the logarithm of the corresponding fibrin concentration. When the cylindrical clots contracted isotonically, the velocity, normalized with respect to the momentary length, was found to be constant. This velocity decreased in an hyperbolic manner with increasing fibrin concentration at a constant concentration of blood platelets. The results are interpreted in terms of a three-dimensional network in which the blood platelets serve as contractile cross-bridges connecting fibrin fibres. Tension development is assumed to be accompanied by the reduction of the angle between fibrin fibres. An attempt was made to calculate the isometric tension developed by a platelet.

Blood Coagulation↗
PubMed1975

Effects of ADP and ATP on bovine fibrinogen- and ristocetin-induced platelet aggregation in Glanzmann's thrombasthenia.

With thrombasthenic platelets, aggregation induced by ristocetin (R) and bovine fibrinogen (BF) is reversible, provided ATP is not added to the system. ADP inhibits BF and R induced aggregation, and when added after the onset of aggregation causes rapid disaggregation. ATP prevents these inhibitory and disaggregation effects of ADP. A mechanism is suggested whereby co-operative effects occur between receptors in the membranes of thrombasthenic platelets.

Adenosine Diphosphate↗
PubMed1975

Right to treatment.

Explore the source record for details and available documents.

Compensation and Redress↗
PubMed1974

The intervals between miniature end-plate potentials in the frog are unlikely to be independently or exponentially distributed.

1. It has been suggested that spontaneous quantal release of transmitter at the neuromuscular junction is a Poisson process. One logical argument against accepting the Poisson hypothesis is that so far relatively few intervals between miniature end-plate potentials (min.e.p.p.s) have been studied in any single experiment. Release is known to occur from many sites on the nerve terminal, so many intervals must be studied before drawing any conclusions about the timing of release from the individual sites. Moreover, the statistical methods that have been used are relatively insensitive to deviations from Poisson predictions.2. The Poisson hypothesis is evaluated with respect to three major criteria:(a) The fit to the exponential distribution is analysed by five goodness of fit tests which were applied to eleven sets of data, showing that it is unlikely that the data sets were generated by an exponential distribution.(b) The independence of intervals is assessed in two ways. First, the autocorrelogram of intervals is constructed. This shows an excess of significant positive correlations beyond the 5% limits of the Poisson expectation. Secondly, the unsmoothed power spectrum is calculated, and compared to the Poisson prediction by means of the modified mean test. Again, most sets deviate significantly from the Poisson expectation. It is unlikely that the intervals are independent.(c) The possibility of simultaneous occurrences is evaluated by construction of the amplitude histogram of min.e.p.p.s. In all sets the Poisson prediction for the frequency of multiples of the unit height was exceeded by the empirical data sets. The over-all conclusion is that the process which generates spontaneous releases is unlikely to be Poisson.

Journal Article↗
PubMed1974

The stochastic properties of spontaneous quantal release of transmitter at the frog neuromuscular junction.

1. Earlier results showed that it is unlikely that spontaneous quantal release of transmitter at the frog neuromuscular junction is produced by a Poisson process.2. Data sets were tested, by using the u statistic, to see whether if they are assumed to be generated by a Poisson process, the mean interval is changing monotonically with time. By this critieria, some of the data sets are stationary, others are not.3. A variety of mathematical transforms are employed on empirical data sets to characterize the properties of the spontaneous quantal release.(a) The intensity function, which calculates the frequency distribution of all possible combinations of intervals, shows an excess of short intervals, without any sign of periodicity.(b) The variance-time curve, which estimates the accumulated variance of the series as a function of time into the series, lies significantly above the Poisson prediction.(c) The power spectrum, whether calculated on the intervals or on the number of intervals in time bins, deviates significantly from the Poisson prediction at the low frequencies.(d) The ln-survivor curve has two phases: a concave section for the short intervals, and a roughly linear section for the intervals of greater length.These transforms indicate that the min.e.p.p.s are clustered.4. A series of models for spontaneous quantal release were considered.(a) A Poisson model. Rejected because of consistent failure to fit the data.(b) A periodic model. Rejected because the intervals should be ordered rather than clustered.(c) A time-dependent model, in which quantal release is governed by a Poisson process with a mean interval that is oscillating in time. This model will generate clustering; by the transforms the model can be shown to closely fit the data. However, an autocorrelation of min.e.p.p. amplitudes shows that there is a relationship between the amplitudes and their position in the series. This is not predicted by the time-dependent oscillating model.(d) A branching Poisson model, in which a primary release, generated by a Poisson process, is likely to be followed by one or more subsidiary releases from the same site. The parameters of the branching model can be determined from ln-survivor curves. Theoretical curves, created with these parameters, give power spectra, variance-time curves, and ln-survivor curves that strongly resemble those calculated from the data. The model also predicts a significant autocorrelation of amplitudes.5. Min.e.p.p.s recorded with an extracellular electrode also fit well to a branching Poisson model.6. The effects of raised [Ca(2+)](o) on the intervals between min.e.p.p.s were studied. In our experiments the change in extracellular solution did not produce any notable change in release statistics.7. The effects of elevated [K(+)](o) on the intervals between spontaneous releases were studied. Depolarization of the nerve terminal increases the frequency of primary releases and decreases the chance of having subsidiary releases.8. Possible physical mechanisms by which quantal release of transmitter from a nerve terminal would fit a branching Poisson model are described.

Journal Article↗
PubMed1974

Stochastic properties of spontaneous transmitter release at the crayfish neuromuscular junction.

1. Miniature excitatory junctional potentials (min.e.j.p.s) were recorded with an intracellular electrode from the adductor muscle of the dactyl of the first or second walking leg of the crayfish, Orconectes virilis.2. The intervals between the min.e.j.p.s were compared to the exponential prediction by five goodness of fit tests. The results indicate that the intervals are not exponentially distributed.3. The autocorrelogram of intervals shows that the intervals are unlikely to be independent.4. A stochastic analysis that includes the power spectrum of intervals, the variance-time curve, and the ln-survivor curve suggest that there is a clustering of min.e.j.p.s. The results are similar to those on the frog neuromuscular junction.5. An autocorrelogram of the min.e.j.p. amplitudes suggests that sizes are not independently distributed.6. These results, which are similar to those previously reported from the frog neuromuscular junction, support the use of the branching Poisson process as a theoretical model for the stochastic properties of spontaneous quantal release of transmitter.

Journal Article↗