On the constancy of the evolutionary rate of cistrons

Summary The variations of evolutionary rates in hemoglobins and cytochrome c among various lines of vertebrates are analysed by estimating the variance. The observed variances appear to be larger than expected purely by chance.If the amino acid substitutions in evolution are the result of random fixation of selectively neutral or nearly neutral mutations, the evolutionary rate of cistrons can be represented by the integral of the product of mutation rate and fixation probability in terms of selective values around the neutral point. This integral is called the effective neutral mutation rate.The influence of effective population number and generation time on the effective neutral mutation rate is discussed. It is concluded that the uniformity of the rate of amino acid substitutions over diverse lines is compatible with random fixation of neutral or very slightly deleterious mutations which have some chance of being selected against during the course of substitution. On the other hand, definitely advantageous mutations will introduce significant variation in the substitution rate among lines. Approximately 10% of the amino acid substitutions of average cistrons might be adaptive and create slight but significant variations in evolutionary rate among vertebrate lines, although the uniformity of evolutionary rate is still valid as a first approximation.Contribution No. 813 from the National Institute of Genetics, Mishima, Shizuokaken 411 Japan. Aided in part by a grant-in-aid from the Ministry of Education, Japan.     

Changes in the metabolic rate of the foraging honeybee: effect of the carried weight or of the reward rate?

The metabolic rate of free flying honeybees (Apis mellifera ligustica) foraging on a multiple automatic feeder was measured in complete absence of perturbation. Each time the sucrose flow rate was doubled, the metabolic rate increased by 18.2 ± 2.0% (14.7 μl CO₂ min⁻¹) and final crop load by 25.1 ± 2.4% (7.04 μl). The possibility that the heavier load carried by the bees caused the increase in the metabolic rate was analyzed in detail. It was found that, for the same weight in the crop, the metabolic rate increased with the increasing reward rate. Therefore, a factor other than the carried weight might account for this increase: a motivational drive, whose intensity may depend on the reward rate at the food source. Although at higher reward rates metabolic rate increased during the visit, at lower reward rates it remained constant, suggesting that the effect of the carried weight on the metabolic rate might be controlled by this motivational drive. The hypothesis that honeybees maximize individual efficiency by reducing their crop load loses support, as foraging costs are not determined by the carried weight. The functional meaning of the reduction in crop load would be to increase the informational exchange at the hive. Accepted: 8 January 2000     

An experimental test of the thermoregulatory hypothesis for the evolution of endothermy

The thermoregulatory hypothesis proposes that endothermy in mammals and birds evolved as a thermoregulatory mechanism per se and that natural selection operated directly to increase body temperature and thermal stability through increments in resting metabolic rate. We experimentally tested this hypothesis by measuring the thermoregulatory consequences of increased metabolic rate in resting lizards (Varanus exanthematicus). A large metabolic increment was induced by feeding the animals and consequent changes in metabolic rate and body temperature were monitored. Although metabolic rate tripled at 32 degrees C and quadrupled at 35 degrees C, body temperature rose only about 0.5 degrees C. The rate of decline of body temperature in a colder environment did not decrease as metabolic rate increased. Thus, increasing the visceral metabolic rate of this ectothermic lizard established neither consequential endothermy nor homeothermy. These results are inconsistent with a thermoregulatory explanation for the evolution of endothermy.     

Rate of gluconeogenesis in the hepatocytes of the developing chick embryo

The rate of glucose formation from lactate was determined in the hepatocytes isolated from the liver of developing chick embryos and one day old chickens. The maximum rate of gluconeogenesis is observed in the hepatocytes of 16-17 day old chick embryos. A sharp fall in the rate of gluconeogenesis is noted in the hepatocytes of the embryos before hatching. It decreases still further in one day old chickens. The patterns of the gluconeogenesis rate are similar both for endogenous precursors and lactate.     

Measurement of the velocity of the pulse wave

Pulse wave transmission rate has been measured "in vivo" in 6 male anaesthetized Wistar rats, by means of cannulae, inserted both in carotid and femoral arteries. The measure has been done by calculating the time delay of several corresponding levels in the two pressure pulses: at the foot, during the ascending phase, at the peak. Three different intervention have been carried out: Ach, NA, blood transfusion. The transmission rate outcame higher at the pulse foot than at the peak, in control animals and in Ach and blood transfusion interventions. The NA intervention inverted this behaviour, where the increase in transmission rate did not correspond to the rate of pressure growing. The possible influences of a vegetative component on arterial viscoelastic behaviour are discussed.     

Modulation of the rate of peptidyl transfer on the ribosome by the nature of substrates

The ribosome catalyzes peptide bond formation between peptidyl-tRNA in the P site and aminoacyl-tRNA in the A site. Here, we show that the nature of the C-terminal amino acid residue in the P-site peptidyl-tRNA strongly affects the rate of peptidyl transfer. Depending on the C-terminal amino acid of the peptidyl-tRNA, the rate of reaction with the small A-site substrate puromycin varied between 100 and 0.14 s(-1), regardless of the tRNA identity. The reactivity decreased in the order Lys = Arg > Ala > Ser > Phe = Val > Asp > Pro, with Pro being by far the slowest. However, when Phe-tRNA(Phe) was used as A-site substrate, the rate of peptide bond formation with any peptidyl-tRNA was approximately 7 s(-1), which corresponds to the rate of binding of Phe-tRNA(Phe) to the A site (accommodation). Because accommodation is rate-limiting for peptide bond formation, the reaction rate is uniform for all peptidyl-tRNAs, regardless of the variations of the intrinsic chemical reactivities. On the other hand, the 50-fold increase in the reaction rate for peptidyl-tRNA ending with Pro suggests that full-length aminoacyl-tRNA in the A site greatly accelerates peptide bond formation.     

Mechanism of the alcohol cyclic pattern: role of the hypothalamic-pituitary-thyroid axis

The cause of the cycle of urinary alcohol levels (UALs) in rats fed ethanol continually at a fixed rate is unknown. Rats were fed ethanol intragastrically at a constant dose for 2 mo, and daily body temperatures and UALs were recorded. Body temperature cycled inversely to UAL, suggesting that the rate of metabolism could be mechanistically involved in the rate of ethanol elimination during the cycle. To document this, whole body O(2) consumption rate was monitored daily during the cycle. The rate of O(2) consumption correlated positively with the change in body temperature and negatively with the change in UAL. Since the metabolic rate responds to changes in body temperature, thyroid hormone levels were measured during the UAL cycle. T(4) levels correlated positively with the O(2) consumption rate and negatively with the UALs. In a second experiment using propylthiouracil treatment, UALs did not cycle and a fall in body temperature failed to stimulate an increase in the rate of ethanol elimination. Consequently, rats died of overdose. Likewise, in a third experiment using rats with severed pituitary stalks, UALs failed to cycle and rats died of overdose. From these observations it was concluded that the UAL cycle depends on an intact hypothalamic-pituitary-thyroid axis response to the ethanol-induced drop in body temperature by increasing the rate of ethanol elimination.     

A statistical analysis of the effect of substrate utilization and shear stress on the kinetics of biofilm detachment

One of the least understood processes affecting biofilm accumulation is detachment. Detachment is the removal of cells and cell products from an established biofilm and subsequent entrainment in the bulk liquid. The goal of this research was to determine the effects of shear stress and substrate loading rate on the rate of biofilm detachment.Monopopulation Pseudomonas aeruginosa and undefined mixed population biofilms were grown on glucose in a RotoTorque biofilm reactor. Three levels of shear stress and substrate loading rate were used to determine their effects on the rate of detachment. Suspended cell concentrations were monitored to determine detachment rates, while other variables were measured to determine their influence on the detachment rate. Results indicate that detachment rate is directly related to biofilm growth rate and that factors which limit growth rate will also limit detachment rate. No significant influence of shear on detachment rate was observed.A new kinetic expression that incorporates substrate utilization rate, yield, and biofilm thickness was compared to published detachment expressions and gives a better correlation of data obtained both in this research and from previous research projects, for both mono- and mixed-population biofilms. (c) John Wiley & Sons, Inc.     

On the Distribution of the Menstruating Interval

This article is concerned with the development of some stochastic models better suited to describe the observed distributions of the menstruating interval relating to last closed-birth interval of two types of females. In this context, two models suited for couples using or not using contraception have been proposed. In the first case, the model assumes a constant conception rate (analogous to the constant hazard rate in the life-testing problem) over time whereas the second case assumes a time-dependent form of the conception rate. The models have been applied to the data collected in a survey conducted in Lucknow, and estimates of conception rate for the two types of females have also been obtained.     

A re-interpretation of the effect of temperature on the metabolism of certain marine invertebrates

Measurements by nieans of an oxygen electrode of the rate of oxygen consumption of a wide variety of common intertidal invertebrates show that at least two rates of uptake can be distinguished in the intact animal. First, there is a rapid rate of oxygen uptake corresponding with activity and second there is a slower rate which corresponds with periods of quiescence and which alternates with the fast rate. Intermediate rates correspond with intermediate rates of overt activity. The maximal and minimal rates of oxygen uptake are affected differently by temperature change. The fast rate corresponding to activity ("active metabolism") increases with temperature in approximate agreement with Arrhenius's law. The slow rate corresponding to quiescence ("maintenance metabolism") does not vary with temperature over much of the range 7 to 22.5C. It is concluded that, contrary to common belief, the intertidal invertebrates studied have a "basal metabolic rate" with a Q₁₀ of approximately one over much of the normal environmental temperature range and in this respect are well suited to life in a habitat where rapid fluctuations in temperature occur.     

Dynamics of the rythm of the isolated and intact frog heart

This study was designed to examine the changes of the isolated frog's heart rate as a function of the time in the two time intervals: 20-95 min and 95-170 min after hearts were prepared. The heart rate decreased in these intervals quite linear but average slope between 20-75 min was significantly steeper than in the time interval 95-170 min. It was shown that the difference in the heart rate response induced by the increasing temperature in intact animals and isolated hearts partly might be explored by this decrease in the isolated frog's heart rate with time.     

Effect of fentolamin on the resistance of the coronary vessels and on the frequency of the heart rate

The effects of phentolamine on coronary resistance and heart rate, as well as the influence of beta 1-adrenoceptor blockade on phentolamine effects were studied on the isolated cat hearts. Phentolamine reduced coronary resistance in 100% of cases (-25.9 +/- 4.4%; p less than 0.001) and increased heart rate only in 31.6% of cases (+9.1 +/- 1.4%); on average the alterations of heart rate were nonsignificant (p less than 0.1). After beta 1-blockade with Cordanum the rate of coronary dilation reduced (p less than 0.001), the changes in coronary resistance being nonsignificant (p greater than 0.1). Basal coronary resistance and tone during phentolamine infusion without or after beta 1-blockade were identical (p greater than 0.1). Thus, the reduction of coronary resistance during phentolamine infusion was due to beta 1-adrenomimetic activity of the drug.     

Estimation of the contribution of the bioluminescent reaction rate and quantum yield to the enhancement of firefly bioluminescence in the presence of cationic liposomes.

Cationic liposomes containing phosphatidylcholine, cholesterol and distearyldimethylammonium chloride (DSDAC) enhanced maximum light emission (BL intensity) and total light emission from the firefly bioluminescence (BL) reaction. The increase in BL intensity was interpreted on the basis of the increase in both BL reaction rate and BL quantum yield (PhiBL) of the BL reaction. The increase in BL reaction rate was due to the increase in the localized concentration of BL reactants on the surface of cationic liposomes by electrostatic interaction. On the other hand, the increase in PhiBL was due to the change of light-emitting species in the presence of cationic liposomes. Each contribution of BL reaction rate and PhiBL to the enhancement of the BL intensity was estimated by measuring the BL reaction rate and PhiBL in the presence of cationic liposomes containing various amounts of DSDAC. The contribution of the BL reaction rate to the increase in the BL intensity was found to be two-fold greater than that of PhiBL.     

A re-interpretation of the effect of temperature on the metabolism of certain marine invertebrates

Measurements by nieans of an oxygen electrode of the rate of oxygen consumption of a wide variety of common intertidal invertebrates show that at least two rates of uptake can be distinguished in the intact animal. First, there is a rapid rate of oxygen uptake corresponding with activity and second there is a slower rate which corresponds with periods of quiescence and which alternates with the fast rate. Intermediate rates correspond with intermediate rates of overt activity. The maximal and minimal rates of oxygen uptake are affected differently by temperature change. The fast rate corresponding to activity ("active metabolism") increases with temperature in approximate agreement with Arrhenius's law. The slow rate corresponding to quiescence ("maintenance metabolism") does not vary with temperature over much of the range 7 to 22.5°C. It is concluded that, contrary to common belief, the intertidal invertebrates studied have a "basal metabolic rate" with a Q₁₀ of approximately one over much of the normal environmental temperature range and in this respect are well suited to life in a habitat where rapid fluctuations in temperature occur.     

The role of phosphate in the regulation of the independent calcium-efflux pathway of liver mitochondria

The rate of spontaneous efflux of Ca from liver mitochondria incubated in the absence of ATP and Mg increases with time and is associated with a synchronous collapse of membrane potential and with Pi efflux. In the presence of Mg and ATP the ruthenium-red-induced Ca efflux does not change with time. The activity of the Ca efflux pathway in Pi-depleted mitochondria is 15-fold greater than in mitochondria equilibrated with 3.3 mM Pi. 50% inhibition is caused by 0.3 mM Pi. The membrane potential is not affected by changes in Pi concentration, although the steady-state extra-mitochondrial free Ca concentration reflects the alterations in efflux rate. In the presence of Pi, the ruthenium-red-induced efflux rate is independent of the total matrix Ca content; however in Pi-depleted mitochondria, with acetate substituting as permeant anion, the efflux rate increases with total matrix Ca content. The lowered efflux rate in the presence of Pi is not due to a limitation in the rate of dissociation of the matrix Ca-phosphate complex. The efflux pathway is activated by a lowered membrane potential, but the relative effect of Pi is retained. Under the present conditions Na slightly inhibits the efflux rate. The lack of an effect of total matrix Ca content on the efflux rate in the presence of Pi is used as the basis of a highly accurate determination of the activity of the Ca uniporter as a function of external free Ca concentration.     

Influence of motor unit firing statistics on the median frequency of the EMG power spectrum

Changes in the EMG power spectrum during static fatiguing contractions are often attributed to changes in muscle fibre action potential conduction velocity. Mathematical models of the EMG power spectrum, which have been empirically confirmed, predict that under certain conditions a distinct maximum occurs in the low-frequency part of the spectrum, indicating the dominant firing rate of the motor units. The present study investigated the influence of this firing rate peak on the spectral changes during a static fatiguing contraction at 50% of maximum EMG amplitude in the frontalis and corrugator supercilii muscles. An exponential decrease of the median frequency (MF) of the EMG power spectrum was observed when the firing rate peak was absent. When the firing rate peak was present, an exaggerated decrease of MF in the beginning of the contraction was found, which was associated with an increase in firing rate peak magnitude. In later stages of the contraction, a partial recovery of MF occurred, concomitant with a decrease in firing rate peak magnitude. The influence of the firing rate peak on MF was also investigated during nonfatiguing contractions of the frontalis muscle at 20, 40, 60, and 80% of maximum EMG amplitude. A curvilinear relationship between MF and contraction strength was found, whether firing rate peaks were present or absent. The presence of firing rate peaks, however, was associated with a decrease in MF which was inversely related to contraction strength, due to the inverse relationship between firing rate peak magnitude and contraction strength.     

Effect of shear on the inactivation kinetics of the enzyme dextransucrase

An inactivation model previously developed to characterize the rate of enzyme activity loss in unstirred solutions was extended to take into account orthokinetic interactions resulting from convective mixing. A synergistic relationship between shear rate and temperature was observed; the rate of inactivation of the enzyme dextransucrase was unaffected by the action of shear below 25 degrees C, but was increased by the shear rate at 30 degrees C. Shear rate does not appear to influence the equilibrium between native and denatured dextransucrase either directly in solution or indirectly by augmenting the turnover of the gas-liquid interface. However, a second-order plot of the inverse of relative activity (A(O)/A) versus Gt (shear rate x time) of dextransucrase at a constant temperature was linear because of the influence of shear on the coagulation of the denatured enzyme. The addition of 0.01 g L(-1) of polyethylene glycol (MW 20,000) blocked this coagulation reaction, thereby completely inhibiting the shear-induced inactivation of dextransucrase at 30 degrees C. (c) 1993 John Wiley & Sons, Inc.     

Effect of some hormones on the rate of the triacylglycerol/fatty-acid substrate cycle in adipose tissue of the mouse in vivo

A method is described for the measurement of the rate of the triacylglycerol/fatty-acid cycle in adipose tissue of the mouse in vivo, which depends upon the incor-poration of tritium from [³H]H₂O into the glycerol and fatty-acid moieties of triacyiglycerol. The rate of the cycling is increased two-fold by feeding, an effect that is completely abolished by the β-adrenergic blocker propranolol. The β-adrenergic agonist fenoterol increased the rate of cycling five-fold in white adipose tissue and three-fold in brown adipose tissue. Cold exposure had no effect on the rate of cycling in white adipose tissue but increased the rate almost two-fold in brown adipose tissue. The increased rate of cycling during feeding, which may be due to increased sympathetic nervous activity, is consistent with the view that the role of cycling is to increase sensitivity of metabolic control systems when required.     

Mechanism of the cardiovascular response to systemic intravenous administration of leucine-enkephalin in the conscious dog

Leucine-enkephalin (Leu⁵-ENK) (35 μg/kg) increased heart rate and mean systemic arterial blood pressure following intravenous injection into chronically-instrumented, conscious dogs. Repeated injections at five-minute intervals were not associated with a diminished response. Naloxone (1 mg/kg) pre-treatment inhibited both heart rate and blood pressure increases. Prazosin (1 mg/kg) attenuated the increase in blood pressure but did not influence the heart rate response. Propranolol (1 mg/kg) attenuated the heart rate response but not the pressor response. Clonidine (30 μg/kg) attenuated the positive chronotropic effect of Leu⁵-ENK. Atropine (1 mg/kg) plus propranolol (1 mg/kg) blocked the heart rate response but the pressor effect was still present. The attenuation of the heart rate response by propranolol and the pressor response by prazosin suggests an adrenergic component to the enkephalin response; the reduction in the heart rate response by clonidine and atropine-propranolol indicates a role for cholinergic mechanisms in the chronotropic response. Hexamethonium (10 mg/kg) blocked the heart rate response and markedly inhibited the pressor response. Vagal interruption attenuated both heart rate and blood pressure responses. It is concluded that intravenous Leu⁵-ENK stimulates afferent pathways located in fibers which are contained in the vagosympathetic trunk to reflexly increase heart rate and blood pressure.     

On the constancy of cell division rate in the root meristem

This review examines under what circumstances the rate of cell division among cells of the root meristem is known to vary. First, methods are compared that have been used to quantify cell division rate. These can be grouped as being either cytological, in which the rate of accumulation of cells in a particular phase of the cell cycle is determined based on some form of cytological labeling, or kinematic, in which the rate of cell accumulation is determined from the net movement of cells. Then, evidence is reviewed as to whether cell division rates vary between different tissues or cell types, between different positions in the root, or finally between different environments. The evidence is consistent with cells dividing at a constant rate, and well documented examples where cell division rate changes substantially are rare. The constancy of cell division rate contrasts with the number of dividing cells, which varies extensively, and implies that a major point for cell cycle control is governing the exit from the proliferative state at the basal boundary of the meristem.