Computing the stochastic dynamics of phosphorylation networks.

Cells of all organisms share the ability to respond to various extracellular signals. Depending on the cell type and the organism, these signals may include hormones secreted by other cells or changes in nutrient concentrations. The signals are processed by an intricate network of protein-protein interactions, including phosphorylation and de-phosphorylation events. As some signaling proteins are only present in low concentrations, random fluctuations may affect the dynamics of the network. The mathematical modeling of networks with significant random fluctuations requires the use of stochastic methods. The stochastic dynamics of a chemical reaction system are described by the Chemical Master Equation. Often the numerical evaluation of this equation is problematic. The first problem is that many systems have an infinite number of possible states; leaving simulations of individual trajectories as the only alternative. To circumvent this problem, we focus on a class of systems that have a finite state space. More specifically, we focus on networks of phosphorylation cycles without taking into account the synthesis and degradation of proteins. The second problem is that memory requirements cause a practical limit to the size of systems that can be evaluated. In this paper, we discuss how these limitations can be overcome using parallel computation and methods dealing efficiently with the available memory. These methods were implemented in a parallel C++ program. We discuss two networks for which the stochastic dynamics were evaluated using this program: a single phosphorylation cycle and an oscillating MAP-kinase cascade.

South African psychiatrists' attitudes to the present implementation of the Abortion and Sterilisation Act of 1975.

In 1980 and 1990 reports were published in the SAMJ on the attitudes of South African gynaecologists to the present Abortion and Sterilisation Act of 1975. The proportion of respondents favouring abortion on demand in the first trimester rose from 32% to 40% over this period, while that of respondents requesting a review of the Act increased from 71% to 85%. The Society of Psychiatrists of South Africa decided to conduct a similar survey of its members using the same questionnaire with some additional questions on current psychiatric practice. Of the 50% of psychiatrists who responded, 78% were personally involved in the implementation of the Act. Some 51% of psychiatrists supported termination on request before the 12th week of pregnancy, an equal number felt that the role of psychiatrists in the implementation of the Act should be reduced, and 89% considered that the present Abortion and Sterilisation Act requires review. Thus, many psychiatrists and gynaecologists are seeking a revision of existing legislation on the medical termination of pregnancy.

Impression order effects as a function of the personal relevance of the object of description.

The influence of different target stimuli on impression order effects was tested. Sixty Ss evaluated bivalent sets of personality trait adjectives which purportedly described a hypothetical stranger, the E, or the Ss themselves. Under unwarned recall conditions, variations in the object of description significantly influenced impression order effects (p < .01). It was suggested that this effect may have been a function of attention decrements and implicit evaluations associated with various levels of personal relevance of the impression formation task. Analyses of adjective recall supported the verbal memory hypothesis of impression formation for only the LH sets in the forewarned recall condition. Recall analyses, however, suggested the importance of recall measurement criteria used in tests of the verbal memory hypothesis.