Volume 63, Nº 4 (2018)

This paper presents a review focused on the study of the physicochemical properties of chitosans. The main attention is paid to the analysis of data on the conformation of the polymer molecules in aqueous solutions depending on the molecular weight and degree of N-acetylation of chitosan, as well as on external factors, such as the nature of the solvent, the pH, and the ionic strength of the solutions.

Composites of bacterial cellulose, which were synthesized in a culture of the strain of acetic acid bacteria Komagataeibacter xylinus VKPM B-12068, with silver nanoparticles were produced hydrothermally by varying the concentrations of AgNO₃ in the medium. The presence of silver in the composites was confirmed by elemental analysis. An increase in the number of silver nanoparticles in the composite from 1.08 to 9.1 wt % (from 0.044 to 0.370 mg/cm²) was shown under increasing AgNO₃ concentration in the medium from 0.0001 to 0.01 M. The structure, properties of the surface, and the physicochemical properties of the composites depending on the silver content were investigated using scanning electron microscopy, differential scanning calorimetry, X-ray diffraction, and a water contact-angle measurement system. Using the disk-diffusion method, it was shown that the resulting composites have a pronounced antibacterial activity against pathogenic microflora E. coli, Ps. eruginosa, and St. aureus.

The thermostability of acetylcholinesterase of rat erythrocyte membranes in the norm and moderate hypothermia was studied. It is shown that the kinetics of the thermal denaturation of acetylcholinesterase is nonlinear and corresponds to a model that involves two-step denaturation, fast and slow, of the enzyme’s native form. The rate constants of the fast phase, k₁, are much higher than those of the slow phase, k₂, while the energy of the fast phase activation is lower by only 19.4% compared to that of the slow one. Short-term moderate hypothermia is shown to increase k₁ and decrease the index of relative activity of the intermediate form of acetylcholinesterase (parameter β), leading to significant lowering of the activation energies of both stages; parameter β becomes more temperature dependent. The prolongation of hypothermia up to 3 h mainly contributes to a decrease in k₁ and k₂ relative to short-term hypothermia and the activation energy of denaturation increases. These data support the hypothesis according to which the structure of acetylcholinesterase is labilized at the initial stages of the development of the hypothermic state and stabilized during prolonged hypothermia.

A correlation between the transition of the oxidative phosphorylation system into a supercomplex state and a change of the functional state of mitochondrial energetics upon the transition from aerobic respiration to hypoxic conditions has been established for the first time. The effect was observed by two different methods in liver and heart mitochondria. The occurrence of highly ordered structures in the membranes of heart mitochondria under hypoxic conditions has been detected by small-angle neutron scattering and electron microscopy during the transition of oxidative phosphorylation into the supercomplex state. The structural parameters of cristae inferred from the data of small-angle neutron scattering and electron microscopy have been compared. The results of measurements using the two methods coincided. Successive exposure to hypoxia and weak osmotic stress signals showed a qualitative difference between these signals. Successive exposure to two signals, with hypoxia as the first one and weak osmotic stress as the second one, was shown to cause impairment of mitochondrial integrity. The effect was observed by small-angle neutron scattering and electron microscopy. Thus, we demonstrated the existence of two qualitatively different signals (hypoxia and osmotic stress), which altered the oxidative phosphorylation system and induced its transition into the supercomplex state.

In this paper, the changes in the frequency and amplitude of action potentials (AP) were investigated depending on the depolarization caused by the Ca²⁺ channels activity during the spontaneous synchronous activity (SSA) of hippocampal neurons in culture. It is known that the pacemaker neuronal electrical activity can be both tonic and bursting. Using the image analysis to measure [Са²⁺]_i and patch-clamp to register the membrane potential we show that depolarization caused by the GABA(A) receptor inhibitor results in a pattern of SSA in which tonic APs frequency of 2−3 Hz are generated by a neuron without any changes in cytosolic free Ca²⁺ concentration, ([Ca²⁺]_i). The tonic mode is interrupted by bursts activity, which is accompanied by slow depolarization and calcium pulses. The inhibitor of T-type calcium channels, ML218, suppresses this process. The frequency and amplitude of the AP are regulated by slow depolarization pulses as follows: on the depolarization front, the APs frequency increases. At the same time, the amplitude decreases due to Na⁺ channels inactivation. The higher is the depolarization rate, the higher is the APs frequency. If slow depolarization amplitude exceeds Na⁺ channels reactivation potential, the neuronal impulse activity stops. As the [Ca²⁺]_i increases and Ca²⁺-dependent K⁺ channels are activated, depolarization amplitude decreases slowly, and Na⁺ channels are reactivated, which leads to a gradual increase in the amplitude of APs against the background of depolarization decrease. The frequency of APs on the backside of depolarization pulse slows down to 3–4 Hz due to the occurrence of the faster Ca²⁺-potential oscillations (micro bursts of AP). Under these conditions, only one AP can be generated due to rapid depolarization at the leading edge. After that, APs generation is suppressed due to Na⁺ channel inactivation. The frequency of APs in this case coincides with the Ca²⁺ channel activation frequency (3−4 Hz). The burst firing is terminated due to [Ca²⁺]_i increase, Ca²⁺-dependent K⁺ channels activation, and voltage-gated Ca²⁺ channels (VGCC) inactivation. As a result, the membrane is hyperpolarized even more (10 mV lower than the critical potential), that suppress AP generation, activate HCN-like channels and reactivate Na⁺ and VGCC. The activity of HCN-like channels increases, the membrane slowly depolarizes and reaches the threshold potential. The generation of tonic APs begins, and then, the Ca²⁺ channels opens, and Ca²⁺ potential and Ca²⁺ signal induced again. Thus, the Ca²⁺-channels, is determining the pulse of slow depolarization, control the frequency and the amplitude of APs during SSA, regulating the activation and inactivation conditions of Na⁺ channels. The T-type channels inhibitors reduce the duration of burst firing and Ca²⁺ impulse in neurons in vitro, stimulating their survival during hyperexcitation and ischemia. Thus, reducing the Ca²⁺ pulse duration caused by the inhibitors of T-type Ca²⁺-channels, can be one of the reasons for the known neuroprotective effect of these compounds.

Keywords: spontaneous synchronous activity of neurons, calcium signal, T-type calcium channels, voltage-gated calcium channels, action potential, genesis of bursting activity, action potential bursts, depolarization shift, critical potential