Influence of arginine vasopressin on the ultradian dynamics of Hypothalamic-Pituitary-Adrenal axis

Numerous studies on humans and animals have indicated that the corticotrophin-releasing hormone (CRH) and arginine vasopressin (AVP) stimulate both individually and synergistically secretion of adrenocorticotropic hormone (ACTH) by corticotropic cells in anterior pituitary. With aim to characterize and better comprehend the mechanisms underlying the effects of AVP on Hypothalamic-Pituitary-Adrenal (HPA) axis ultradian dynamics, AVP is here incorporated into our previously proposed stoichiometric model of HPA axis in humans. This extended nonlinear network reaction model took into account AVP by: reaction steps associated with two separate inflows of AVP into pituitary portal system, that is synthesized and released from hypothalamic parvocellular and magnocellular neuronal populations, as well as summarized reaction steps related to its individual and synergistic action with CRH on corticotropic cells. To explore the properties of extended model and its capacity to emulate the effects of AVP, nonlinear dynamical systems theory and bifurcation analyses based on numerical simulations were utilized to determine the dependence of ultradian oscillations on rate constants of the inflows of CRH and AVP from parvocellular neuronal populations, the conditions under which dynamical transitions occur due to their synergistic action and, moreover, the types of these transitions. The results show that under certain conditions, HPA system could enter into oscillatory dynamic states from stable steady state and vice versa under the influence of synergy reaction rate constant. Transitions between these dynamical states were always through supercritical Andronov-Hopf bifurcation point. Also, results revealed the conditions under which amplitudes of ultradian oscillations could increase several-fold due to CRH and AVP synergistic stimulation of ACTH secretion in accordance with results reported in the literature. Moreover, results showed experimentally observed superiority of CRH as a stimulator of ACTH secretion compared to AVP in humans. The proposed model can be very useful in studies related to the role of AVP and its synergistic action with CRH in life-threatening circumstances such as acute homeostasis dynamic crisis, autoimmune inflammations or severe hypovolemia requiring instant or several-days sustained corticosteroid excess levels. Moreover, the model can be helpful for investigations of indirect AVP-induced HPA activity by exogenously administered AVP used in therapeutic treatment

A Monte Carlo simulation of ion transport at finite temperatures

We have developed a Monte Carlo simulation for ion transport in hot background gases, which is an alternative way of solving the corresponding Boltzmann equation that determines the distribution function of ions. We consider the limit of low ion densities when the distribution function of the background gas remains unchanged due to collision with ions. A special attention has been paid to properly treat the thermal motion of the host gas particles and their influence on ions, which is very important at low electric fields, when the mean ion energy is comparable to the thermal energy of the host gas. We found the conditional probability distribution of gas velocities that correspond to an ion of specific velocity which collides with a gas particle. Also, we have derived exact analytical formulas for piecewise calculation of the collision frequency integrals. We address the cases when the background gas is monocomponent and when it is a mixture of different gases. The developed techniques described here are required for Monte Carlo simulations of ion transport and for hybrid models of non-equilibrium plasmas. The range of energies where it is necessary to apply the technique has been defined. The results we obtained are in excellent agreement with the existing ones obtained by complementary methods. Having verified our algorithm, we were able to produce calculations for Ar$^+$ ions in Ar and propose them as a new benchmark for thermal effects. The developed method is widely applicable for solving the Boltzmann equation that appears in many different contexts in physics.Comment: 14 page

Izvod oksidacija hidroksilnih jona na zeolitu modifikovanom platinom

NaX zeolite was modified by platinum via impregnation/thermal decomposition technique, using Pt(II)-acetyl acetonate in acetone as an impregnating solution. The samples were characterized by scanning electron microscopy (SEM) and cyclic voltammetry methods. The mixture of modified zeolite and 10 wt.% of carbon black, in a form of thin layer, was pasted to a glassy carbon surface. With this mixture as the electrode material, oxidation of hydroxide ions was studied by cyclic voltammetry. The response of modified zeolite electrodes was compared with bare platinum electrode and platinum electrode covered with Nafion film.Zeolit tipa NaX je modifikovan platinom tehnikom impregnacija/termalna degradacija. Kao impregnirajući rastvor je korišćen rastvor Pt (II) acetilacetonata u acetonu. Uzorci su okarakterisani metodama skenirajuće elektronske mikroskopije (SEM) i ciklične voltametrije. Modifikovani zeolit je homogeno dispergovan u rastvoru Nafiona (5 mas.%) u mešavini izopropil alkohola i vode, pomoću ultrazvučnog kupatila. Elektronska provodljivost uzorka je poboljšana dodatkom 10 mas.% ugljenika (carbon black Vulcan XC72 (Cabot Corp.)) u suspenziju. Kap suspenzije je postavljena na površinu elektrode o staklastog ugljenika. Nakon isparavanja rastvarača na elektrodi je formiran tanak film. Oksidacija hidroksilanih jona je ispitana na ovako formiranoj elektrodi metodom ciklične voltametrije. Na osnovu SEM mikrografija moglo se uočiti da ne postoji značajna razlika u obliku i veličini čestica polaznog zeolita i zeolita modifikovanog platinum. Rezultati EDX analize su značajno varirali u zavisnosti od mesta na uzorku na kome je izvršena analiza. Sadržaj platine se kretao u intervalu 1-10 mas.%. Reakcija oksidacija hidroksilnih jona je, poređenja radi, ispitana na čistoj platinskoj elektrodi i platinskoj elektrodi pokrivenoj filmom Nafiona. Na platinskoj elektrodi, kako čistoj tako i pokrivenoj Nafionom, dobijen je očekivani sigmoidalni talas sa platoom, koji odgovara graničnoj difuzionoj struji. Prisustvo Nafiona na platinskoj elektrodi dovelo je do nižih graničnih struja pri oksidaciji hidroksilnih jona. Međutim, reproduktivnost ciklovoltamograma je značajno poboljšana, a povećan je i opseg potencijala u kome se dobija jasno definisan plato. Ciklovoltamogrami zeolitske platinisane elektrode, snimljeni pri istim uslovima kao i platinska elektroda, ispoljili su pojavu pika usled velikog koncentracionog gradijenta. Tafelovi nagibi Nafionom pokrivene platinske elektrode i zeolitske platinisane elektrode pokazuju značajnu zavisnost od koncentracije hidroksilnih jona, dok Tafelovi nagibi čiste platinske elektrode nisu zavisili od koncentracije analita. Granična gustina struje pokazuje linearnu zavisnost od koncentracije hidroksilnih jona sa dobrim korelacinim faktorom, za sve tri ispitivane elektrode

The particle distribution functions and applications

The new generalized logarithmic equation defined by four parameters, called LG4, was formulated and proposed for describing particle size and shape distributions. For special choice of the parameters the LG4 was reduced to the logarithmic distribution LG2 defined by two parameters which permits the selection of mean value of the distribution as the size parameter appears explicitly in the distribution function. The LG2 has been suggested as a model for size and shape distributions of the particles (the "fourth state of matter" according to Heywood's definition) as, for example, the metallic and ceramic particles which attract a good deal of public attention as new promising high-performance materials for magnetic materials and then chemical catalysts, sintering promoting materials, sensors, etc. The shape and similarity of the particle for some purposes are very important. Total volume, total surface or any other useful property of the sample, may be related to shape and size of individual particles. If all particles are geometrically similar, all the subsequent treatment is simplified substantially in terms of shape. By introducing the shape parameter of the particle, the generalized similarity and the elliptic factor of the assembly of particles the application of the LG2 for the study of the shape distribution of the projected Nd2Fe14B particles is referred in this paper

A new approach to the velocity field investigation in case of the entry flow in curved pipes with circular cross section

In this paper an analysis of a laminar flow in a bend preceded by the straight section is presented. The flow is supposed to be unperturbed far upstream, while far downstream it becomes fully developed. In the vicinity of the sharp junction between the bended and the straight section, the solution is expanded in terms of integer powers of e(S) quantity (S is reduced distance from the curvature onset along the pipe axis). The corresponding expansion coefficients were determined in order to provide the velocity field continuity across the junction