Voltage and Calcium Dynamics Both Underlie Cellular Alternans in Cardiac Myocytes

Cardiac alternans, a putative trigger event for cardiac reentry, is a beat-to-beat alternation in membrane potential and calcium transient. Alternans was originally attributed to instabilities in transmembrane ion channel dynamics (i.e., the voltage mechanism). As of this writing, the predominant view is that instabilities in subcellular calcium handling are the main underlying mechanism. That being said, because the voltage and calcium systems are bidirectionally coupled, theoretical studies have suggested that both mechanisms can contribute. To date, to our knowledge, no experimental evidence of such a dual role within the same cell has been reported. Here, a combined electrophysiological and calcium imaging approach was developed and used to illuminate the contributions of voltage and calcium dynamics to alternans. An experimentally feasible protocol, quantification of subcellular calcium alternans and restitution slope during cycle-length ramping alternans control, was designed and validated. This approach allows simultaneous illumination of the contributions of voltage and calcium-driven instability to total cellular instability as a function of cycle-length. Application of this protocol in in vitro guinea-pig left-ventricular myocytes demonstrated that both voltage- and calcium-driven instabilities underlie alternans, and that the relative contributions of the two systems change as a function of pacing rate.     

Attenuation of cGAS‐STING signaling is mediated by a p62/SQSTM1‐dependent autophagy pathway activated by TBK1

Negative regulation of immune pathways is essential to achieve resolution of immune responses and to avoid excess inflammation. DNA stimulates type I IFN expression through the DNA sensor cGAS, the second messenger cGAMP, and the adaptor molecule STING. Here, we report that STING degradation following activation of the pathway occurs through autophagy and is mediated by p62/SQSTM1, which is phosphorylated by TBK1 to direct ubiquitinated STING to autophagosomes. Degradation of STING was impaired in p62‐deficient cells, which responded with elevated IFN production to foreign DNA and DNA pathogens. In the absence of p62, STING failed to traffic to autophagy‐associated vesicles. Thus, DNA sensing induces the cGAS‐STING pathway to activate TBK1, which phosphorylates IRF3 to induce IFN expression, but also phosphorylates p62 to stimulate STING degradation and attenuation of the response.     

Intracellular diffusion restrictions in isolated cardiomyocytes from rainbow trout

Background  

Restriction of intracellular diffusion of adenine nucleotides has been studied intensively on adult rat cardiomyocytes. However, their cause and role in vivo is still uncertain. Intracellular membrane structures have been suggested to play a role. We therefore chose to study cardiomyocytes from rainbow trout (Oncorhynchus mykiss), which are thinner and have fewer intracellular membrane structures than adult rat cardiomyocytes. Previous studies suggest that trout permeabilized cardiac fibers also have diffusion restrictions. However, results from fibers may be affected by incomplete separation of the cells. This is avoided when studying permeabilized, isolated cardiomyocytes. The aim of this study was to verify the existence of diffusion restrictions in trout cardiomyocytes by comparing ADP-kinetics of mitochondrial respiration in permeabilized fibers, permeabilized cardiomyocytes and isolated mitochondria from rainbow trout heart. Experiments were performed at 10, 15 and 20°C in the absence and presence of creatine.     

Temperature acclimation has no effect on ryanodine receptor expression or subcellular localization in rainbow trout heart

In cardiomyocytes, ryanodine receptors (RYRs) mediate Ca²⁺-induced Ca²⁺-release (CICR) from the sarcoplasmic reticulum (SR) during excitation–contraction (e–c) coupling. In rainbow trout heart, the relative importance of CICR increases with cold-acclimation. Thus, the aim of this study was to investigate the effect of temperature acclimation (4, 11 and 18°C) on RYR intracellular localization and expression density. We used immunocytochemistry to assess intracellular localization in ventricular myocytes and Western blotting to assess RYR expression in both atrial and ventricular tissue. In ventricular myocytes, RYRs were localized peripherally in transverse bands aligning with sarcomeric m-lines and centrally around mitochondria and the nucleus. Localization did not change with temperature acclimation. RYR expression was also unaffected by temperature acclimation. The localization of RYRs at the m-line is similar to neonatal mammalian cardiomyocytes. We suggest this positioning is indicative of myocytes which rely predominantly on transsarcolemmal Ca²⁺-influx, rather than CICR, during e–c coupling.     

Assessing re-introductions of the African Wild dog (Lycaon pictus) in the Limpopo Valley Conservancy, South Africa, using the stochastic simulation program VORTEX

The African wild dog (Lycaon pictus) is one of Africa's most endangered species and therefore classified as endangered by IUCN. Earlier distributions included most of Africa but currently the African wild dog only has populations larger than 300 individuals in three countries (Botswana, Tanzania and South Africa). In 1998, a plan was launched in South Africa to manage sub-populations of the African wild dog in several small, geographically isolated, conservation areas. This management program involved the reintroduction of wild dogs into suitable conservation areas and periodic translocations among them. We used the stochastic population simulation model VORTEX to evaluate the Limpopo Valley Conservancy in the north of South Africa, as a possible reintroduction site for African wild dogs. The simulations showed that the size of the initial population released only had a small effect on the population dynamics. However, when individuals were supplemented and harvested over a longer period the probability of persistence increased. Number of females breeding, male mortality, and carrying capacity were key factors in the population dynamics, but according to VORTEX the severity of natural catastrophes had the greatest influence on the extinction risk and inbreeding. We suggest that the reintroduction program may be successful, if areas are properly secured, the dogs are held in a boma before release, wild animals or at least a mix of wild and captive animals are used for the release and the animals are vaccinated against rabies. It is, however, essential to continue monitoring followed by modelling efforts to re-evaluate the success of the reintroduction program.     

Adult body growth and reproductive investment vary markedly within and across Atlantic and Pacific herring: a meta-analysis and review of 26 stocks

Reviews in Fish Biology and Fisheries - Life-history traits of Pacific (Clupea pallasii) and Atlantic (Clupea harengus) herring, comprising both local and oceanic stocks subdivided into...     

Towards the cloning of genes underlying murine QTLs

Many quantitative trait loci (QTLs), including those for ethanol-related traits, have been mapped in the mouse. In light of rapidly developing tools and resources, we briefly review the strategy for identifying the genes underlying these QTLs. We note that positional cloning will soon be a matter of testing candidate genes rather than discovering genes; therefore, we describe a ``congenic test' to support that a candidate gene is indeed a QTL. Considering the rapid development of congenics and mutants, we also identify four areas of investigation—phenotypes, ethanol specificity, environment, and gene interactions—that might be exploited during the course of positional cloning to gain insights into QTL pathways. In particular, we note that multiple mutants of nearly every major neurotransmitter pathway have now been made. These mutants are not only useful for phenotypic tests, but also could be used to conduct ``gene dependence' tests of QTLs. We also consider potential applications for the very recently developed ability to clone mice. Received: 15 September 1998 / Accepted: 8 October 1998