The Influence of Ca2+ Buffers on Free [Ca2+] Fluctuations and the Effective Volume of Ca2+ Microdomains

Intracellular calcium (Ca2+) plays a significant role in many cell signaling pathways, some of which are localized to spatially restricted microdomains. Ca2+ binding proteins (Ca2+ buffers) play an important role in regulating Ca2+ concentration ([Ca2+]). Buffers typically slow [Ca2+] temporal dynamics and increase the effective volume of Ca2+ domains. Because fluctuations in [Oa(2+)] decrease in proportion to the square-root of a domain\u27s physical volume, one might conjecture that buffers decrease [Ca2+] fluctuations and, consequently, mitigate the significance of small domain volume concerning Ca2+ signaling. We test this hypothesis through mathematical and computational analysis of idealized buffer-containing domains and their stochastic dynamics during free Ca2+ influx with passive exchange of both Ca2+ and buffer with bulk concentrations. We derive Langevin equations for the fluctuating dynamics of Ca2+ and buffer and use these stochastic differential equations to determine the magnitude of [Ca2+] fluctuations for different buffer parameters (e.g., dissociation constant and concentration). In marked contrast to expectations based on a naive application of the principle of effective volume as employed in deterministic models of Ca2+ signaling, we find that mobile and rapid buffers typically increase the magnitude of domain [Ca2+] fluctuations during periods of Ca2+ influx, whereas stationary (immobile) Ca2+ buffers do not. Also contrary to expectations, we find that in the absence of Ca2+ influx, buffers influence the temporal characteristics, but not the magnitude, of [Ca2+] fluctuations. We derive an analytical formula describing the influence of rapid Ca2+ buffers on [Ca2+] fluctuations and, importantly, identify the stochastic analog of (deterministic) effective domain volume. Our results demonstrate that Ca2+ buffers alter the dynamics of [Ca2+] fluctuations in a nonintuitive manner. The finding that Ca2+ buffers do not suppress intrinsic domain [Ca2+] fluctuations raises the intriguing question of whether or not [Ca2+] fluctuations are a physiologically significant aspect of local Ca2+ signaling

A Population Density and Moment-Based Approach to Modeling Domain Calcium-Mediated Inactivation of L-Type Calcium Channels

We present a population density and moment-based description of stochastic domain calcium-mediated inactivation of L-type calcium channels. Our approach accounts for the effect of heterogeneity of local calcium signals on whole cell calcium currents; however, in contrast with prior work by Sherman et al. [Biophys J. 58(4):985, 1990], we do not assume that calcium domain formation and collapse are fast compared to channel gating. We demonstrate the population density and moment-based modeling approach using a12-state Markov chain model of an L-type calcium channel [Greenstein and Winslow, Biophys J. 83(6):2918, 2002]. Simulated whole cell voltage clamp responses yield an inactivation function for the whole cell calcium current that agrees or disagrees with the classic result of Sherman et al. when domains dynamics are fast or slow, respectively. We analyze the voltage-dependence of calcium inactivation that occurs via slow heterogeneous domains and find that when channel permeability is held constant, calcium inactivation increases as the domain time constant increases. However, when this parameter study is repeated for fixed maximum domain calcium concentration, inactivation decreases as the domain time constant increases. Comparison of simulation results using population densities and moment equations confirms the computational efficiency of the moment-based approach, and enables the validation of several distinct methods of truncating and closing the open system of moment equations. In general, a slow domain time constant requires higher order moment truncation for agreement between moment-based and population density simulations

Cloning, sequencing, and characterization of the hexahydro-1,3,5-trinitro-1,3,5-triazine degradation gene cluster from Rhodococcus rhodochrous

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is a high explosive which presents an environmental hazard as a major land and groundwater contaminant. Rhodococcus rhodochrous strain 11Y was isolated from explosive contaminated land and is capable of degrading RDX when provided as the sole source of nitrogen for growth. Products of RDX degradation in resting-cell incubations were analyzed and found to include nitrite, formaldehyde, and formate. No ammonium was excreted into the medium, and no dead-end metabolites were observed. The gene responsible for the degradation of RDX in strain 11Y is a constitutively expressed cytochrome P450-like gene, xpLA, which is found in a gene cluster with an adrenodoxin reductase homologue, xplB. The cytochrome P450 also has a flavodoxin domain at the N terminus. This study is the first to present a gene which has been identified as being responsible for RDX biodegradation. The mechanism of action of XplA on RDX is thought to involve initial denitration followed by spontaneous ring cleavage and mineralization

Calcium Homeostasis in a Local/Global Whole Cell Model of Permeabilized Ventricular Myocytes with a Langevin Description of Stochastic Calcium Release

Population density approaches to modeling local control of Ca2+ induced Ca2+ release in cardiac myocytes can be used to construct minimal whole cell models that accurately represent heterogeneous local Ca2+ signals. Unfortunately, the computational complexity of such local/global whole cell models scales with the number of Ca2+ release unit (CaRU) states, which is a rapidly increasing function of the number of ryanodine receptors (RyRs) per CaRU. Here we present an alternative approach based on a Langevin description of the collective gating of RyRs coupled by local Ca2+ concentration ([Ca2+]). The computational efficiency of this approach no longer depends on the number of RyRs per CaRU. When the RyR model is minimal, Langevin equations may be replaced by a single Fokker-Planck equation, yielding an extremely compact and efficient local/global whole cell model that reproduces and helps interpret recent experiments that investigate Ca2+ homeostasis in permeabilized ventricular myocytes. Our calculations show that elevated myoplasmic [Ca2+] promotes elevated network sarcoplasmic reticulum (SR) [Ca2+] via SR Ca2+ -ATPase-mediated Ca2+ uptake. However, elevated myoplasmic [Ca2+] may also activate RyRs and promote stochastic SR Ca2+ release, which can in turn decrease SR [Ca2+]. Increasing myoplasmic [Ca2+] results in an exponential increase in spark-mediated release and a linear increase in nonspark-mediated release, consistent with recent experiments. The model exhibits two steady-state release fluxes for the same network SR [Ca2+] depending on whether myoplasmic [Ca2+] is low or high. In the later case, spontaneous release decreases SR [Ca2+] in a manner that maintains robust Ca2+ sparks

Physicians’ Knowledge and Beliefs Regarding Athletic Trainers

Purpose: The relationship between athletic trainers (ATs) and physicians is a legal obligation and collaboration to improve patient outcomes. The objective of this study was to examine the knowledge of physicians regarding the educational preparation, legal obligations, and scope of practice for ATs and how it relates to previous experiences with ATs. Additionally physicians’ perceptions of Interprofessional Collaboration (IPC) were studied. Methods: 169 physicians medical doctors (MD)=133/169, 78.7%, doctor of osteopathy (DO)=36/169, 21%) completed a 36-question web-based survey, which included a validated IPC scale. Results: Respondents with experience working with an AT scored significantly higher (P \u3c 0.01) on the knowledge assessment, where physicians currently working with an AT scored higher (5.4/8) than those who previously worked with an AT (4.2/8) and those who had never worked with an AT (3.3/8). Additionally, physicians with previous exposure to an AT as an athlete had significantly higher knowledge scores than those without exposure (P \u3c 0.01). Two areas of weakness in IPC from the physician’s perspective included sharing of important information (2.48/4) and importance of work as compared to others on the team (2.38/4). Conclusions: Physicians who have a current working relationship with an AT and those that had access to an AT as an athlete demonstrated significantly higher knowledge about an AT’s academic preparation, legal obligations, and scope of practice. Moreover, physicians currently working with an AT report positive interprofessional collaborations

Adolescent and Young Adult Perceptions of Hookah and Little Cigars/Cigarillos: Implications for Risk Messages

Use of hookah and little cigars/cigarillos (LCCs) is high among adolescents and young adults. Although these products have health effects similar to cigarettes, adolescents and young adults believe them to be safer. This study examined adolescent and young adult perceptions of hookah and LCCs to develop risk messages aimed at discouraging use among users and at-risk nonusers. Ten focus groups with 77 adolescents and young adults were conducted to explore their perceptions about the perceived risks and benefits of hookah and LCC use. Participants were users of other (non-cigarette) tobacco products (n=47) and susceptible nonusers (n=30). Transcripts were coded for emergent themes on participants’ perceptions of hookah and LCCs. Participants did not perceive health effects associated with hookah and LCC use to be serious or likely to happen given their infrequency of use and perceptions that they are less harmful than cigarettes. Participants generally had positive associations with smoking hookah and LCCs for several reasons, including that they are used in social gatherings, come in various flavors, and can be used to perform smoke tricks. Because adolescents and young adults underestimate and discount the long-term risks associated with hookah and LCC use, effective messages may be those that focus on the acute/immediate health and cosmetic effects