San Diego State University

About: San Diego State University is a education organization based out in San Diego, California, United States. It is known for research contribution in the topics: Population & Poison control. The organization has 12418 authors who have published 27950 publications receiving 1192375 citations. The organization is also known as: SDSU & San Diego State College.

An improved model of C3 photosynthesis at high CO2: Reversed O2 sensitivity explained by lack of glycerate reentry into the chloroplast

Abstract: Current models of C3 photosynthesis incorporate a phosphate limitation to carboxylation which arises when the capacity for starch and sucrose synthesis fails to match the capacity for the production of triose phosphates in the Calvin cycle. As a result, the release of inorganic phosphate in the chloroplast stroma fails to keep pace with its rate of sequestration into triose phosphate, and phosphate becomes limiting to photosynthesis. Such a model predicts that when phosphate is limiting, assimilation becomes insensitive to both CO2 and O2, and is thus incapable of explaining the experimental observation that assimilation, under phosphate-limited conditions, frequently exhibits reversed sensitivity to both CO2 and O2, i.e., increasing O2 stimulates assimilation and increasing CO2 inhibits assimilation. We propose a model which explains reversed sensitivity to CO2 and O2 by invoking the net release of phosphate in the photorespiratory oxidation cycle. In order for this to occur, some fraction of the glycollate carbon which leaves the stroma and which is recycled to the chloroplast by the photorespiratory pathway as glycerate must remain in the cytosol, perhaps in the form of amino acids. In that case, phosphate normally used in the stromal glycerate kinase reaction to generate PGA from glycerate is made available for photophosphorylation, stimulating RuBP regeneration and assimilation. The model is parameterized for data obtained on soybean and cotton, and model behavior in response to CO2, O2, and light is demonstrated.

Comparison of finite difference and boundary integral solutions to three-dimensional spontaneous rupture

Abstract: The spontaneously propagating shear crack on a frictional interface has proven to be a useful idealization of a natural earthquake. The corresponding boundary value problems are nonlinear and usually require computationally intensive numerical methods for their solution. Assessing the convergence and accuracy of the numerical methods is challenging, as we lack appropriate analytical solutions for comparison. As a complement to other methods of assessment, we compare solutions obtained by two independent numerical methods, a finite difference method and a boundary integral (BI) method. The finite difference implementation, called DFM, uses a traction-at-split-node formulation of the fault discontinuity. The BI implementation employs spectral representation of the stress transfer functional. The three-dimensional (3-D) test problem involves spontaneous rupture spreading on a planar interface governed by linear slip-weakening friction that essentially defines a cohesive law. To get a priori understanding of the spatial resolution that would be required in this and similar problems, we review and combine some simple estimates of the cohesive zone sizes which correspond quite well to the sizes observed in simulations. We have assessed agreement between the methods in terms of the RMS differences in rupture time, final slip, and peak slip rate and related these to median and minimum measures of the cohesive zone resolution observed in the numerical solutions. The BI and DFM methods give virtually indistinguishable solutions to the 3-D spontaneous rupture test problem when their grid spacing Δx is small enough so that the solutions adequately resolve the cohesive zone, with at least three points for BI and at least five node points for DFM. Furthermore, grid-dependent differences in the results, for each of the two methods taken separately, decay as a power law in Δx, with the same convergence rate for each method, the calculations apparently converging to a common, grid interval invariant solution. This result provides strong evidence for the accuracy of both methods. In addition, the specific solution presented here, by virtue of being demonstrably grid-independent and consistent between two very different numerical methods, may prove useful for testing new numerical methods for spontaneous rupture problems.

Prefrontal Cortex Volumes in Adolescents With Alcohol Use Disorders: Unique Gender Effects

Abstract: In the United States, alcohol is the most widely consumed intoxicant among adolescents, with 75% of 12th graders having drank in their lifetime. More startling is that 30% of seniors reported getting drunk in the previous month (Johnston et al., 2005). Given the prevalence of alcohol use during adolescence, its effects on neuromaturation are of great interest. In adults, the prefrontal cortex (PFC) appears to be particularly vulnerable to the neurotoxic effects of alcohol use (Desmond et al., 2003; George et al., 2004; Pfefferbaum et al., 1997, 2001; Schweinsburg et al., 2001; Sullivan et al., 2003). Postmortem studies have found reduced glial and neuronal cell densities in the PFC of adults with alcohol use disorders (AUD) (Miguel-Hidalgo et al., 2006). Morphometric studies have revealed smaller PFC total volume (Kubota et al., 2001; Liu et al., 1998), gray matter (Chanraud et al., 2007; Liu et al., 1998; Pfefferbaum et al., 1997), and white matter volume (Kril et al., 1997; Pfefferbaum et al., 1997) among adults with AUD compared with controls. The more anterior portions of the PFC appear most vulnerable to alcohol-induced neurotoxic effects (Kril et al., 1997). Gender may moderate the effects of alcohol on PFC morphometry. It appears that women with AUD may be more sensitive to the neurotoxic effects of alcohol than men (Agartz et al., 1999; Hommer et al., 2001; Mann et al., 2005; Schweinsburg et al., 2003), although some studies have pointed to relative vulnerabilities in males (Pfefferbaum et al., 2001) and others found no evidence of brain abnormalities among alcoholic females (Kroft et al., 1991; Pfefferbaum et al., 2002). Because neuromaturation continues into late adolescence (Giedd et al., 1996a; Lenroot and Giedd, 2006; Sowell et al., 2004) studies based on adults cannot necessarily generalize to adolescents with AUD, particularly findings in the PFC, a late region to undergo gray matter pruning (Gogtay et al., 2004; Lenroot and Giedd, 2006; Sowell et al., 2004). Maturation of white matter appears to continue into the twenties, with white matter volume typically increasing (Giedd et al., 1999; Jernigan and Gamst, 2005; Paus et al., 2001) and PFC white matter integrity improving (Ashtari et al., 2007; Barnea-Goraly et al., 2005; Watts et al., 2003) from childhood to adulthood, although 1 study found decreased white matter density from ages 16 to 17 (Paus et al., 1999). Gender differences in neuromaturation also exist. Females' frontal gray matter volumes peaks at age 11.0 while males' peak at age 12.1 years on average, and male brains are about 9% larger than females' (Giedd et al., 1996b). Furthermore, in contrast with most prior studies of white matter neuromaturation (e.g., Giedd et al., 1999; Reiss et al., 1996), our laboratory found that among healthy adolescent females ages 12 to 18, PFC white matter volume actually decreased from age 15 to 18, while males' PFC white matter remained relatively stable (Nagel et al., 2006). Thus, as male and female brains mature with somewhat different time courses during adolescence, neurotoxic exposures may have disparate effects. Animal models indicate that adolescence is a period of particular vulnerability to the neurotoxic effects of alcohol (Barron et al., 2005; Carpenter-Hyland and Chandler, 2007; Crews et al., 2000; Spear, 2000). Compared with adults, adolescent animals exposed to alcohol are more likely to demonstrate cognitive, social, and neuronal damage (Crews et al., 2000; Little et al., 1996; Pyapali et al., 1999; Silveri and Spear, 1998; Swartzwelder et al., 1995, 1998; Varlinskaya and Spear, 2006; White et al., 2002; Yttri et al., 2004). Despite that many of these abnormalities are subserved by frontal systems, few studies have examined the PFC morphometric consequences of heavy alcohol use during adolescence in humans. Thus far, human adolescent studies examining the neural effects of heavy drinking have suggested alcohol-related structural (De Bellis et al., 2000, 2005; Medina et al., 2007a; Nagel et al., 2005), and functional (Tapert et al., 2004) abnormalities. Specifically, despite similar performance during a spatial working memory task, adolescents with AUD showed less PFC and cerebellar activation to complete the task with controls, with a greater degree of abnormality linked to lifetime alcohol withdrawal symptoms (Tapert et al., 2004). These results may indicate that the brain compensates for subtle alcohol-induced neuronal insult by relying on other areas (e.g., parietal cortex). A follow-up study revealed that gender moderated the relationship between alcohol use and brain response to this spatial working memory task (Caldwell et al., 2005). That is, after controlling for typical blood alcohol concentrations, girls with AUD demonstrated less frontal cortex response than female controls, while the males showed the opposite pattern. Overall, females demonstrated more alcohol-related abnormalities than males. The only study to date examining PFC morphometry (defined as anterior to the genu) in adolescents with AUD compared a treatment sample of 13- to 21-year olds with AUD and other Axis I disorders (including conduct, depressive, attention deficit hyperactivity, post-traumatic stress, generalized anxiety, bipolar, and cannabis and hallucinogen use disorders) to controls without a history of AUD (De Bellis et al., 2005). Even after controlling for comorbid disorders, males and females with AUD demonstrated smaller PFC total and white matter volumes than controls, although psychiatric contributions cannot entirely be ruled out, and gender-by-group interactions were not found. Therefore, no studies to date have examined PFC volume among adolescents with AUD without comorbid psychiatric disorders. As comorbid psychiatric and other premorbid risk factors for substance use disorders (e.g., familial AUD) are also linked to neurocognitive abnormalities (Kamarajan et al., 2006; Kim et al., 2001; Kruesi et al., 2004; Meyerhoff et al., 2004; Tapert and Brown, 2000), the current study sought to examine the relationship between AUD and PFC structure among adolescents without comorbid mood, attention, or anxiety disorders. Conduct disorder, which is highly prevalent among adolescents with AUD, was statistically controlled for, and family history of substance use disorders was equivalent between the groups. A secondary goal of the study was to examine whether gender moderated the relationship between alcohol use and PFC morphometry.

Organizational climate partially mediates the effect of culture on work attitudes and staff turnover in mental health services.

Abstract: Staff turnover in mental health service organizations is an ongoing problem with implications for staff morale, productivity, organizational effectiveness, and implementation of innovation. Recent studies in public sector services have examined the impact of organizational culture and climate on work attitudes (i.e., job satisfaction and organizational commitment) and, ultimately, staff turnover. However, mediational models of the impact of culture and climate on work attitudes have not been examined. The present study examined full and partial mediation models of the effects of culture and climate on work attitudes and the subsequent impact of work attitudes on staff turnover. Multilevel structural equation models supported a partial mediation model in which organizational culture had both direct influence on work attitudes and indirect influence through organizational climate. Work attitudes significantly predicted one-year staff turnover rates. These findings support the contention that both culture and climate impact work attitudes and subsequent staff turnover.