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Biomedical subjects

F Andrietti

Publications and source records attributed to F Andrietti.

13 recordsLinked to original sources
PubMed1994

The movement of spermatozoa with helical head: theoretical analysis and experimental results.

The present work is concerned with the study of the swimming of flagellated microscopic organisms with a helical head and a helical pattern of flagellar beating, such as Xenopus sperms. The theoretical approach is similar to that taken by Chang and Wu (1971) in the study of helical flagellar movement. The model used in the present study allows us to determine the velocity of propulsion (U) and the frequency of rotation of the sperm head (fh) as a function of the frequency of the wave of motion (ft) traveling along the tail. The results relative to the case of helical and planar flagellar waves are compared. Our main finding is that the helical shape of the head seems to increase the efficiency of propulsion of the spermatozoon when compared with the more commonly shaped spherical head. Experimentally measured values of fh versus U may be fitted by a linear plot whose slope is much higher than that corresponding to the case of planar flagellar beating. This fact is consistent with an effectively three-dimensional (nonplanar) movement of the flagellar tail. However, the results do not fit those predicted from a circular helix, suggesting that a different shape of the flagellar beating should be considered.

Animals↗
PubMed1993

Theoretical analysis of cotransport: its use in alanine uptake in plasma membrane vesicles.

A theoretical model of a cotransport system in plasma membrane vesicles has been utilized for the analysis of the Na(+)-dependent L-alanine transport into plasma membrane vesicles purified from Yoshida ascites hepatoma (AH 130) cells in the exponential and stationary phases of growth. The analysis was performed by comparing the experimental curves with computer simulations. In particular we considered the differences in alanine uptake observed in the two preparations and we tried to ascribe them to changes of some parameters of the transport model. The simulations indicate that sodium, alanine or water passive permeability changes cannot explain the experimental data which are consistent, on the contrary, with a relevant enhancement of the Vmax of the transport agency. The involvement of the membrane electrical potential difference is also discussed.

Alanine↗
PubMed1991

Membrane vesicles in the study of transport processes: a critical analysis of the experimental procedure.

A critical analysis of the use of membrane vesicles in the study of cotransport processes is presented. Transport experiments were simulated according to two different models, stressing those conditions that seemed more relevant in affecting the measurements. In particular, we observed that the experimental Vmax values were underestimated. This underevaluation depended on the incubation time employed to measure the initial uptake rate and on the time necessary to wash the vesicles. Also the temperature and the composition of the washing solution, together with the Q10 of the transport process taken into consideration, had a consistent influence on the uptake. All the above mentioned effects were affected by the vesicle volume: the smaller the volume, the greater the underestimate of the uptake. This theoretical analysis underlines, on the one side, that the experimental data should be interpreted with some caution, on the other, that the examined procedure allows an internal check of its validity by adopting suitable simulations of the experiments. The use of the presented models as a tool for the planning and the critical analysis of the experimental results is suggested.

Animals↗
PubMed1990

Time course analysis of cotransport in membrane vesicles: solutes and tracers.

A theoretical analysis of the time course of a ternary cotransport system in membrane vesicles has been developed by extending previous work (Weiss, S.D. et al. (1981) J. Theor. Biol. 93, 597-608; Heinz, E. and Weinstein, M. (1984) Biochim. Biophys. Acta 776, 83-91). It has been assumed that the translocation of the carrier is the rate-limiting step of the transport process. Our approach includes, in particular, the presence of isotope tracer fluxes and the generalization to the case when many solutes share the same carrier. The situation when the tracer and the solute behave differently, as in the countertransport case, is stressed. Also, the interaction of two different solutes, internal and external to vesicles, is considered. Other points regard the analysis of the solute binding to the membrane vesicles, the influence of water permeability and the possible asymmetry of the transport system. In the Appendix, the assumption of no net translocation of all carrier species is discussed.

Biological Transport↗
PubMed1988

Xenopus spermatozoon: correlation between shape and motility.

Xenopus spermatozoa have a characteristic corkscrew-shaped head and a flagellum with a conventional "9 + 2" structure (Bernardini et al.: J Ultrastruct Md Struct Res: 94:188-194. They are motile in water and media of low osmolarities, but not in media of osmolarities higher than 200 mosm/liter, regardless of the ionic composition. External calcium and pH are not involved in the inhibition of sperm movement at high osmolarities. The duration of sperm motion was less than 10 min, and both flagellar beat frequencies and sperm velocities declined progressively. However, in demembranated and reactivated tails, flagellar beating was sustained for longer times. Flagella propagated three-dimensional waves that induced a spinning motion of the whole spermatozoon. This pattern of movement is not dependent on the shape of the sperm head, since isolated, reactivated flagella exhibited three-dimensional waves.

Animals↗
PubMed1986

Computer reconstruction of the spread of excitation in nerve terminals with inhomogeneous channel distribution.

A direct numerical integration method, as modified by Du Fort and Frankel (1953), has been used to solve the partial differential equation system which describes the spread of action potential in a mammalian nerve terminal. Branching of the terminal as well as inhomogeneous distributions of Na+ and K+ voltage-dependent channels (Brigant and Mallart 1982) have been incorporated in the model. Using the channel densities and the kinetic parameters measured in the node of Ranvier, the depolarization in the terminal branches has an amplitude of only 60% of the action potential in the node. Furthermore, the time courses of the calculated membrane currents differ considerably from the ones measured by Brigant and Mallart (1982) and by Konishi and Sears (1984). Increasing the Na+ and K+ channel densities may considerably increase the terminal depolarization and also reproduce qualitatively the current wave-forms observed experimentally. The model can also reproduce some of the effects of pharmacological channel blocks. The simulation allows a new interpretation of the different components of membrane current along the terminal.

Action Potentials↗
PubMed1984

Segmented and "equivalent" representation of the cable equation.

The linear cable theory has been applied to a modular structure consisting of n repeating units each composed of two subunits with different values of resistance and capacitance. For n going to infinity, i.e., for infinite cables, we have derived analytically the Laplace transform of the solution by making use of a difference method and we have inverted it by means of a numerical procedure. The results have been compared with those obtained by the direct application of the cable equation to a simplified nonmodular model with "equivalent" electrical parameters. The implication of our work in the analysis of the time and space course of the potential of real fibers has been discussed. In particular, we have shown that the simplified ("equivalent") model is a very good representation of the segmented model for the nodal regions of myelinated fibers in a steady situation and in every condition for muscle fibers. An approximate solution for the steady potential of myelinated fibers has been derived for both nodal and internodal regions. The applications of our work to other cases dealing with repeating structures, such as earthworm giant fibers, have been discussed and our results have been compared with other attempts to solve similar problems.

Action Potentials↗
PubMed1984

Analysis of lumped and distributed elements models of cut muscle fibers in vaseline or sucrose gap preparations.

A general method of finding the time course and the steady state distribution of potential in Vaseline or sucrose gap preparations is given by making use of the linear cable equation. The general solution has been found analytically in terms of its Laplace transform and then numerically inverted. Two particular experimental situations, namely the single gap and the double gap preparations, have been analyzed. The results have been compared with the solutions of the commonly used lumped elements models. While for the double gap no large errors are introduced by the lumped model, for the single gap there are significant differences. The dependence of the voltage distribution on various electrical and geometrical parameters has been examined. It is suggested that the proposed mathematical treatment might be used by experimenters as a reference to assess the validity of simplified lumped models.

Animals↗
PubMed1976

Exact solution of the unidimensional Poisson-Boltzmann equation for a 1:2 (2:1) electrolyte.

The unidimensional Poisson-Boltzmann equation for a 1:2 (2:1) electrolyte has been solved analytically. The results have been compared with those obtained from the linearized equation. It is shown that in physiological conditions the difference may be greater than 10%. The value of the derivative of the potential in x=0, (dpsi/dx)x=0, has been used by many authors in the evaluation of the superficial charges of biological membranes. The value of (dpsi/dx)x-0 have also been compared with the ones derived from the linearized equation. The difference may be greater than 25%. Our results suggest that the linearization of the Poisson-Boltzmann equation for a 1:2(2:1) electrolyte may be greatly misleading.

Cell Membrane↗