Maximal steady state versus state of conditioning.

Criteria for the identification of maximal steady state as related to state of conditioning were evaluated. 13 volunteers walker and/or ran during a series of 15 min tests on a treadmill. The speeds ranged from mild to exhaustive. Heart rate was monitored continuously; VO2 was determined from 6 min to 9 min; and venous blood was obtained at 10 min and 15 min for lactate analyses. Max VO2 was established for each subject. Subjects were classified on level of conditioning according to the quantity and quality of their activity record for the previous 6 months. The 10 min heart rate associated with a blood lactate level of 2.2 mM/L (MSSHR) was the best predictor of conditioning. The relative VO2 (% of max VO2) found with a 10 min blood lactate concentration of 2.2 mM/L (RMSSVO2) was almost as accurate as MSSHR in predicting state of conditioning. Changes in blood lactate levels between 10 min and 15 min were not significantly related to conditioning.     

Transient versus steady state NOE in paramagnetic molecules Cu2Co2SOD as an example

Truncated, steady state and transient NOE experiments have been performed on bovine Cu2Co2 superoxide dismutase. The effectiveness of the different NOE experiments in the general case of paramagnetic macromolecules is discussed. It is concluded that steady state NOEs give superior results. The validity of the two spins approximation is discussed, and NOE values for a fully coupled set of nuclei have been calculated. Transient NOE experiments, when properly performed, confirm the previous assignment of the hyperfine shifted signals in Cu2Co2SOD based on steady state NOE measurements [(1989) Inorg. Chem. 28, 4650] and eliminate any further reason for controversy on an important issue as the assignment of the 1H NMR signals of protons of metal-coordinated imidazoles.     

An algorithm for detecting non-steady state moieties in steady state subnetworks

An algorithm is presented for the automatic detection of dynamic moiety pools within steady state metabolic subnetworks which may be embedded within larger dynamic networks. This is an aid in the quantitative development of the hierarchical structure of metabolic networks. The algorithm can also be used to test for the physical readability of a stoichiometric matrix of a closed metabolic network.     

Analysis of steady state photosynthesis in alfalfa leaves

A method for carrying out kinetic tracer studies of steady state photosynthesis in whole leaves has been developed. An apparatus that exposes whole leaves to ¹⁴CO₂ under steady state conditions, while allowing individual leaf samples to be removed as a function of time, has been constructed. Labeling data on the incorporation of ¹⁴C into Medicago sativa L. metabolite pools are reported. A carbon dioxide uptake rate of 79 micromoles ¹⁴CO₂ per milligram chlorophyll per hour was observed at a CO₂ level slightly below that of air. Several actively turning over pools of early and intermediate metabolites, including 3-phosphoglyceric acid, glycerate, citrate, and uridine diphosphoglucose, showed label saturation after approximately 10 to 20 minutes of photosynthesis with ¹⁴CO₂ under steady state conditions. Alanine labeling increased more rapidly at first, and then at a lower rate as saturation was approached. Sucrose was a major product of photosynthesis and label saturation of the sucrose pool was not observed. Labeled carbon appeared rapidly in secondary metabolites. The steady state apparatus used has numerous advantages, including leaf temperature control, protection against leaf dehydration, high illumination, known ¹⁴CO₂ specific radioactivity, and provision for control and adjustment of ¹⁴CO₂ concentration. The apparatus allows for experiments of long duration and for sufficient sample points to define clearly the metabolic steady state.     

Structural fluctuations in the steady state of muscular contraction.

Recent studies of the intensity fluctuation spectra of coherent light scattered from striated muscle have demonstrated the existence of large scale fluctuations in position and polarizability at the level of the myofibrillar sarcomere and its major structural subunits during the steady state of contraction. The existence of these fluctuations implies a fluctuating driving force. Various possible fluctuating motions of the thick and thin filaments, A and I bands, and entire sarcomeres are described. The magnitude of the fluctuating forces associated with the making and breaking of cross bridges is estimated. A mechanical model is proposed for coupling structural elements of a single sarcomere to one another and for coupling myofibrillar sarcomeres to one another. It is shown that the fluctuating force generated by the spontaneous making and breaking of cross bridges in conjunction with the model accounts for some of the features of the observed intensity fluctuation spectra.     

Hysteresis in systems with a single steady state

It is usually stated that only systems with multiple steady states can exhibit hysteresis. For protein conformations, this would violate the idea that, in fixed environments, primary structure uniquely determines tertiary structure. It is shown that hysteresis-like phenomena can be exhibited by systems possessing only a single steady-state configuration. This property is placed in a more general theoretical setting of recognition and classification systems, and some implications for processes such as memory, learning and pattern generation (morphogenesis) are discussed.     

Mutation rates of bacteria in steady state populations

The breeder and the chemostat have been used to measure mutation rates for two mutations under a variety of steady state growth conditions. These rates have been found to be higher in complex medium than in minimal (F) medium. The effects of changes in nutritional conditions on these high rates have been described. In addition, the mutation rates at short generation times, in complex medium, have been shown to decrease with increasing generation time.     

Prospective timing in pigeons: Isolating temporal perception in the time-left procedure

In the time-left procedure, a task used to study prospective timing, animals choose between two stimuli that signal different delays to reinforcement. Trials begin with one stimulus signaling C seconds to reinforcement and, at different moments since its onset, another stimulus, signaling S seconds to reinforcement, with C > S, is introduced. Optimal performance consists in choosing the stimulus signaling the shorter time to reinforcement. Animals have been found to perform in this optimal way. However, this procedure is complex and variables other than time may be responsible for the results. In two experiments with pigeons we sought to improve the time-left procedure to better isolate the effect of time in the animals’ behavior. We attempted to control for two confounding variables, the asymmetry in the time markers from training to testing and the cost of switching between the two response alternatives. We conclude that in the time-left task pigeons seem indeed to regulate their behavior based on time because, with our improved procedure, they still chose the stimulus associated with the shorter time to food. However, our version of the procedure created new interpretative difficulties, strengthening the idea that the time-left procedure may be too complex to study timing.     

Microbial viability and steady state chemostat culture

Summary A model of a two-stage chemostat incorporating cell viability as a function of the rate limiting substrate concentration was developed and compared with a model considering viability as a function of the dilution rate. Both models were applied to the data obtained from a chemostatic culture of Klebsiella pneumonia. Although both models successfully predicted the values of the biomass and substrate concentrations in the first stage, only the model relating viability to substrate concentration gave a satisfactory fit for the second stage.     

Reliability of the calculated maximal lactate steady state in amateur cyclists

Complex performance diagnostics in sports medicine should contain maximal aerobic and maximal anaerobic performance. The requirements on appropriate stress protocols are high. To validate a test protocol quality criteria like objectivity and reliability are necessary. Therefore, the present study was performed in intention to analyze the reliability of maximal lactate production rate (V.L_amax) by using a sprint test, maximum oxygen consumption (V.O_2max) by using a ramp test and, based on these data, resulting power in calculated maximum lactate-steady-state (PMLSS) especially for amateur cyclists. All subjects (n = 23, age 26 ± 4 years) were leisure cyclists. At three different days they completed first a sprint test to approximate V.La_max. After 60 min of recreation time a ramp test to assess V.O_2max was performed. The results of V.La_max-test and V.O_2max-test and the body weight were used to calculate PMLSS for all subjects. The intra class correlation (ICC) for V.La_max and V.O_2max was 0.904 and 0.987, respectively, coefficient of variation (CV) was 6.3% and 2.1%, respectively. Between the measurements the reliable change index of 0.11 mmol·l ^-1s ^-1 for V.La_max and 3.3 mlkg ^-1min ^-1 for V.O_2max achieved significance. The mean of the calculated PMLSS was 237 ± 72 W with an RCI of 9 W and reached with ICC = 0.985 a very high reliability. Both metabolic performance tests and the calculated PMLSS are reliable for leisure cyclists.     

Relaxation times in bacteriological culture and the approach to steady state

The differential equations governing the evolution of a population of bacteria, both in static and continuous culture, are studied. Asymptotic solutions, valid in the limit of large time t, are derived, and hence the characteristic times with which such systems approach their steady-state configurations are obtained. It is suggested from these results that the anomalous experimental results obtained by Herbert, Elsworth &; Telling (1956) near the critical dilution rate D, can be explained in terms of the enormous times which the system will take to reach steady-state in these circumstances.     

Regulation of the microtubule steady state in vitro by ATP.

ATP increases microtubule steady state assembly and disassembly rates in vitro in a concentration-dependent manner. Bovine brain microtubules, composed of 75% tubulin and 25% high molecular weight microtubule-associated proteins (MAPs), were purified by three cycles of assembly and disassembly in the absence of ATP. When assembled to steady state, these microtubules add dimers at one end and lose them at the other in a unidirectional assembly-disassembly process. In the presence of 1.0 mM ATP the unidirectional flow of tubulin from one end of the microtubules to the other increases as much as 20 fold, as revealed by loss of 3H-GTP from uniformly labeled microtubules under GTP chase conditions and by the rate of disassembly following addition of 50 microM podophyllotoxin. UTP, CTP and 5' adenylylimidodiphosphate (AMP-PNP) cannot substitute for ATP in producing this effect. Furthermore, the increase in steady state flow rate persists afer ATP is removed. Thus microtubules assembled in ATP and centrifuged through sucrose cushions to separate them from nucleotides continue to exhibit increased rates in the next assembly cycle in the absence of ATP. It is possible that an ATP-dependent microtubule protein kinase is responsible for the observed increase in tubulin flow rate. A kinase activity associated with brain MAPs has been reported to be cAMP-dependent (Sloboda et al., 1975). We have found an adenylate cyclase activity associated with these microtubules. Whether the adenylate cyclase is a contaminant or due to a specific microtubules-associated protein, and whether its activity is functionally linked to the increased rate of assembly and disassembly in the presence of ATP, remain to be determined.