The Carbon Assimilation Network in Escherichia coli Is Densely Connected and Largely Sign-Determined by Directions of Metabolic Fluxes

Gene regulatory networks consist of direct interactions but also include indirect interactions mediated by metabolites and signaling molecules. We describe how these indirect interactions can be derived from a model of the underlying biochemical reaction network, using weak time-scale assumptions in combination with sensitivity criteria from metabolic control analysis. We apply this approach to a model of the carbon assimilation network in Escherichia coli. Our results show that the derived gene regulatory network is densely connected, contrary to what is usually assumed. Moreover, the network is largely sign-determined, meaning that the signs of the indirect interactions are fixed by the flux directions of biochemical reactions, independently of specific parameter values and rate laws. An inversion of the fluxes following a change in growth conditions may affect the signs of the indirect interactions though. This leads to a feedback structure that is at the same time robust to changes in the kinetic properties of enzymes and that has the flexibility to accommodate radical changes in the environment

Dinâmica folicular ovariana de vacas leiteiras no pós-parto após tratamentos com buserelina (GnRH) e cloprostenol (PGF2alfa) Ovarian follicular dynamics of postpartum dairy cows after treatment with buserelin (GnRH) and cloprostenol (PGF2alpha)

O objetivo deste trabalho foi verificar os efeitos da administração de 10 mig de buserelina (GnRH), mediante a presença de um folículo ovariano com diâmetro ³ 10 mm, associada a uma dose de 500 mig de cloprostenol seis dias após, ou de duas doses de 500 mig de cloprostenol (PGF2alfa) no 12º 26º dia após o parto, sobre a dinâmica folicular e o restabelecimento da atividade ovariana cíclica em 15 vacas mestiças holandês-zebu, eqüitativamente e aleatoriamente distribuídas em três grupos de tratamento. Realizou-se a ultra-sonografia em dias alternados do 14º ao 26º dia pós-parto e em dias consecutivos até o final do segundo ciclo estral e coletaram-se amostras de sangue, duas vezes por semana, para determinação das concentrações de progesterona, pelo método de radioimunoensaio. Observou-se que, independente do tratamento hormonal, os segundos ciclos estrais pós-parto apresentam como padrão duas ondas de crescimento folicular, em que a emergência da primeira e da segunda onda ocorreu nos dias 0 e 10, respectivamente, e folículos dominantes da primeira onda podem persistir durante todo o intervalo inter-ovulatório, sem influenciar a dinâmica folicular. O tratamento com duas doses de PGF2a reduziu em 20 dias o período de serviço e tendeu a melhorar o índice de concepção ao primeiro serviço, o que sugere possível efeito deste hormônio no eixo hipotalâmico-hipofisário-ovariano de vacas pós-parto.<br>The objective of this work was to study the effects of the injection of 10 mug buserelin (GnRH) in the presence of an ovarian follicle with ³ 10 mm diameter associated to a dose of 500 mug of cloprostenol six days later, or with two doses of 500 mug of cloprostenol (PGF2alpha) at the 12th and 26th days postpartum about the follicular dynamic and the reestablishment of cyclic ovarian activity in crossbred Holstein-Zebu cows. Fifteen crossbred Holstein-Zebu cows were randomly and evenly allocated into three treatments. Ultrasonography was performed every other day from the 14th to the 26th day postpartum and every day, from 26th day until the end of the second estrous cycle. Blood samples were collected twice a week, in order to determine progesterone concentrations by radioimmuno assay. It was observed a pattern of two follicular growth waves during the second postpartum estrous cycles, independent of the hormonal treatment, with the emergence of the first and the second waves, at days 0 and 10, respectively. The dominant follicle from the first wave could persist throughout the interovulatory period without an effect on the follicular dynamic. The two doses of PGF2alpha reduced days open by 20 days and the conception rate at the first service was slightly improved, suggesting a possible effect of the treatment on the hypothalamic-pituitary-ovarian axis