Bone growth and calcium balance during simulated weightlessness in the rat

Rats, age 28 days, experiencing tail suspension in modified metabolic cages for 1, 2, and 3 wk were compared with littermate controls. Food and water consumption, urinary and fecal Ca excretion, and serum Ca were measured; hearts, fore- and hindlimb bones, skulls, and mandibles were removed for determination of wet, dry, and ash weights and Ca concentration and for histological examination. Weight gain and Ca intake and excretion were the same for both groups; both displayed net Ca gain. Suspended rats had significantly lower wet, dry, and ash weights of femora and tibiae. Dry weights of the humeri and radii/ulnae were moderately higher, and the skull and mandible dry and ash weights were significantly higher in suspended than in control rats. Cortical thickness of the femur, but not humerus, was less in suspended rats. The data are consistent with the hypothesis that bone growth is influenced by the cardiovascular changes associated with tail suspension.     

Mechanism of the exercise hyperkalemia: an alternate hypothesis

Wasserman, Karlman, William W. Stringer, Richard Casaburi,and Yong-Yu Zhang. Mechanism of the exercisehyperkalemia: an alternate hypothesis. J. Appl.Physiol. 83(2): 631-643, 1997.A progressivehyperkalemia is observed as exercise intensity increases. The currentmost popular hypothesis for the hyperkalemia is that theNa⁺-K⁺pump cannot keep pace with the K⁺efflux from muscle during the depolarization-repolarization process ofthe sarcolemmal membrane during muscle contraction. In this report, wepresent data that suggest an alternate hypothesis to those previouslydescribed. Because phosphocreatine (PCr) is a highly dissociated acidand creatine is neutral at cell pH, the concentration of nondiffusibleanions decreases, and an alkaline reaction takes place when PCrhydrolyzes. This creates a state of cation(K⁺) excess andH⁺ depletion in the cell. Toexamine the balance of K⁺ andH⁺ for exercising muscle duringthe early period of exercise when PCr changes most rapidly, catheterswere inserted into the brachial artery and femoral vein (FV) in fivehealthy subjects who performed two 6-min cycle ergometer exercise testsat 40 and 85% of peak oxygen uptake. FV blood was sampled every 5 sduring the first 2 min, then every 30 s for the remaining 4 min ofexercise and the first 3 min of recovery, and then less frequently forthe next 12 min. Arterial sampling was every 30 s during exercise andsimultaneous with FV sampling during recovery. ArterialK⁺ concentration([K⁺]) increase laggedFV [K⁺]increase. The hyperkalemia observed during early exerciseresults from K⁺ release fromskeletal muscle. FV[K⁺] increased by 5 sof the start of exercise and followed the rate ofH⁺ loss from the FV blood for thefirst 30 s of exercise. FV lactate andNa⁺ kinetics differed fromK⁺ kinetics during exercise andrecovery. As predicted from the PCr hydrolysis reaction, the exercisinglimb took up H⁺ and releasedK⁺ at the start of exercise (first30 s) at both exercise intensities, resulting in a FV metabolicalkalosis. K⁺ release wasessentially complete by 3 min, the time at which oxygen uptake (and,presumably, PCr) reached its asymptote. These findings lead us tohypothesize that the early K⁺release by the cell takes place withH⁺ exchange and that the majormechanism for the exercise hyperkalemia is the reduction innondiffusible intracellular anions in the myocyte as PCr hydrolyzes.

     

Redox-optimized ROS balance: A unifying hypothesis

While it is generally accepted that mitochondrial reactive oxygen species (ROS) balance depends on the both rate of single electron reduction of O₂ to superoxide (O₂^?) by the electron transport chain and the rate of scavenging by intracellular antioxidant pathways, considerable controversy exists regarding the conditions leading to oxidative stress in intact cells versus isolated mitochondria. Here, we postulate that mitochondria have been evolutionarily optimized to maximize energy output while keeping ROS overflow to a minimum by operating in an intermediate redox state. We show that at the extremes of reduction or oxidation of the redox couples involved in electron transport (NADH/NAD⁺) or ROS scavenging (NADPH/NADP⁺, GSH/GSSG), respectively, ROS balance is lost. This results in a net overflow of ROS that increases as one moves farther away from the optimal redox potential. At more reduced mitochondrial redox potentials, ROS production exceeds scavenging, while under more oxidizing conditions (e.g., at higher workloads) antioxidant defenses can be compromised and eventually overwhelmed. Experimental support for this hypothesis is provided in both cardiomyocytes and in isolated mitochondria from guinea pig hearts. The model reconciles, within a single framework, observations that isolated mitochondria tend to display increased oxidative stress at high reduction potentials (and high mitochondrial membrane potential, ?Ψ_m), whereas intact cardiac cells can display oxidative stress either when mitochondria become more uncoupled (i.e., low ?Ψ_m) or when mitochondria are maximally reduced (as in ischemia or hypoxia). The continuum described by the model has the potential to account for many disparate experimental observations and also provides a rationale for graded physiological ROS signaling at redox potentials near the minimum.     

Leaf water balance during oscillation of stomatal aperture

Continuous transpiration and β-guage measurements were made on cotton plants, the stomatal apertures of which had been induced to oscillate, thus allowing a water balance to be made, and leaf potential to be measured as functions of time.     

Implications of the gene balance hypothesis for dosage compensation

Dosage compensation refers to the equal expression between the sexes despite the fact that the dosage of the X chromosome is different in males and females. In Drosophila there is a twofold upregulation of the single male X. In triple X metafemales, there is also dosage compensation, which occurs by a two-thirds downregulation. There is a concomitant reduction in expression of many autosomal genes in metafemales. The male specific lethal (MSL) complex is present on the male X chromosome. Evidence is discussed showing that the MSL complex sequesters a histone acetyltransferase to the X chromosome to mute an otherwise increased expression by diminishing the histone acetylation on the autosomes. Several lines of evidence indicate that a constraining activity occurs from the MSL complex to prevent overcompensation on the X that might otherwise occur from the high level of acetylation present. Together, the evidence suggests that dosage compensation is a modification of a regulatory inverse dosage effect that is a reflection of intrinsic gene regulatory mechanisms and that the MSL complex has evolved in reaction in order to equalize the expression on both the X and autosomes of males and females.     

Validation of the doubly labeled water method in rats during isolation and simulated weightlessness

Total energy expenditure (TEE) of rats during simulated microgravity is unknown. The doubly labeled water method (DLW) reliably measures TEE, but the results depend on the methods of calculation. These methods were validated and appraised by indirect calorimetry in eight rats during isolation (7 days) and simulated microgravity (10 days). There were no effects on CO(2) production in the method used to derive constant flux rates as in the regression models. r(CO(2)) estimates were dependent on the assumed fractionation processes, the derivation of constant flux rate methods, and the selected pool models. Use of respiratory or food quotients did not influence TEE estimations, which were similar during isolation and simulation. During either isolation with growth or simulation with a stabilized mass, the one-pool model of Speakman (Speakman JR. Doubly Labelled Water. Theory and Practice. London: Chapman and Hall, 1997) resulted in the more reliable validation (0.8 +/- 2.2 and 2.2 +/- 3.4% vs. calorimetry, respectively). However, during simulation, agreement was also observed with the single pool model of Lifson (Lifson N, Gordon GB, and McClintock R. J Appl Physiol 7: 704-710, 1955) (-2.5 +/- 2.5%), and two two-pool models [Schoeller (Schoeller DA. J Nutr 118: 1278-1289, 1988) (0.5 +/- 3.1%) and Speakman (Speakman, JR. Doubly Labelled Water. Theory and Practice. London: Chapman and Hall, 1997) (-1.9 +/- 2.7%)]. This latter finding seems linked to the stable body mass and to fractionation consideration close to the single-pool model of Speakman. During isolation or simulated microgravity, the other equations underestimated TEE by 10-20%.     

Water balance during vitellogenesis by the American cockroach: translocation of water during the cycle

The female American cockroach undergoes harmonious cycles of ingestion and elimination during the 6-day vitellogenic cycle. More water is lost through transpiration than through faeces, and under dessicating conditions the female cockroach is able to reduce water loss by regulation of transpiration. The use of radioactive water permits a comparison of ‘gross’ and ‘net’ water loss during transpiration.     

Plant chemical defense: monoterpenes and the growth-differentiation balance hypothesis

Recent studies of allocation to defensive chemicals in plants have provided insights into the ecological controls over plant defensive chemicals. Both developmental and ecological studies now suggest that we can understand the factors influencing allocation to defense by examining the relative availability of resources, external needs for chemical defense, and the internal demands for growth that plants face. These studies have also shed light on one of the more popular theories in plant evolutionary ecology, the growth-differentiation balance hypothesis of plant resource allocation.     

Neurotrophic influence on proliferation-differentiation decisions during amphibian forelimb regeneration: an hypothesis

Amphibian forelimb regeneration is under neural influence. Although the precise role of nerves is unknown, clear influence on macromolecular synthesis and on mitosis have been demonstrated. The hypothesis presented here proposes that neural input is directed primarily at influencing the decision of blastemal cells either to proliferate or to prepare to express differentiated phenotypes. This is considered to be accomplished by an interplay between the catecholamine neurotransmitters and neurotrophic peptides using cyclic nucleotides (cAMP) and Ca2+, respectively, as intracellular mediators. This coordination of proliferation-differentiation decisions in regenerating limbs is proposed to be primarily, but not exclusively, the function of nerves.     

Hexapoda phylogeny: implications for an aquatic origin hypothesis

Emergence from an aquatic environment to the land is one of the major evolutionary transitions within the arthropods. It is often considered that the first hexapods, and especially the first Collembola, went from the sea through intermediate freshwater environments to colonize fully terrestrial ecosystems. To understand the ancestral ecology of hexapods, a phylogenetic framework is used. By mapping ecological attributes onto the phylogeny, it is shown that hexapods colonized terrestrial environments directly from marine environment without a transition through freshwater. An edaphic life-style is the basal state for Collembola and more generally for hexapods as a whole. Aquatic ecology is inferred to be a secondary change that occurred several times independently, particularly in some group of Collembola and Pterygota. The answer is ambiguous for Pterygota, whether the first Pterygota had aquatic larvae and reversed toward fully terrestrial in Neoptera, or aquatic larvae appeared independently in Odonata and Ephemeroptera. Subsequently, aquatic larvae were secondarily acquired in various groups independently (e.g. Plecoptera, Trichoptera, Coleoptera).     

植物群落物种共存机制:负密度制约假说

负密度制约假说主要描述由于资源竞争、有害生物侵害(比如病原微生物、食草动物捕食)等, 同种个体之间发生的相互损害行为; 它主要强调同种个体之间的相互作用, 解释自然群落物种共存的机理; 负密度制约机制主要在小尺度上降低群落内同种个体生长率, 同时提高个体死亡率, 从而为其他物种的生存提供空间和资源, 促进物种共存。目前负密度制约假说的检验研究主要侧重密度制约、距离制约、群落补偿效应等三个方面。最近, 研究者又探讨了近缘物种之间由于对相似资源的竞争所产生的负效应, 扩展了负密度制约假说, 进而提出异群保护假说和谱系多样性制约假说。负密度制约假说引起生态学家长久的探讨和关注, 世界范围内大尺度森林动态样地的建立, 又为探索不同尺度上密度制约效应的研究提供了条件。然而目前的研究仍然存在不足, 比如负密度制约假说的检验受到其他因素的干扰、区域研究不平衡等。因此, 生态学家们仍然怀疑负密度制约效应调节群落物种共存的重要性, 但是目前的研究还没有发现否定负密度制约假说的充分证据。