Effects Of Chronic Changes Research Articles

A framework approach to address the trend and causes of flood stage change in a river reach downstream of a dam influenced by tributaries

The evaluation of the trend of flood stage changes in alluvial rivers downstream of dams is important for flood management. However, the flood stage associated with a given discharge generally is nonstationary in river reaches with multiple tributaries. This is not only because of the dam-induced shifting in the cross-sectional area and/or channel roughness but also because of the backwater induced by high flows from the tributaries. To determine the total trend of the flood stage and quantify the separate contributions of hydrological and geomorphic effects, the current study proposed a framework approach consisting of hydrological analysis and multiscenario numerical modeling. By this means, the trend in the flood stage could be distinguished from the stage oscillation driven by varying factors, including extreme hydrologic events. The effects of chronic changes, including channel incision and flow resistance increase, also were quantitatively separated. This framework was applied to the Chenglingji–Datong (CD) reach downstream of the Three Gorges Dam (TGD) in the Yangtze River, China. The results indicated that the effect of the roughness increase counterbalanced the effect of channel incision when the flow discharge was beyond the bankfull level. The backwater effect induced by tributary inflow was the major cause of the flood stage rise in recent years. The method presented in the current study provides a useful tool for managers and engineers to obtain better insight into the driving mechanisms of flood stage changes in river reaches that are downstream of dams. These findings indicate that the flood stage may not decline or may even occasionally increase, although the cross-sectional area was enlarged by channel incision. Special attention should be given to the flood risk situation in the study reach after the TGD began operation.

Adaptations in the passive mechanical properties of skeletal muscle to altered patterns of use.

The aim of this mini-review is to describe the present state of knowledge regarding the effects of chronic changes in the patterns of muscle use (defined as changes lasting >1 wk), including muscle stretching, strengthening, and others, on the passive mechanical properties of healthy human skeletal muscles. Various forms of muscle stretch training and some forms of strength training (especially eccentric training) are known to strongly impact the maximum elongation capacity of muscles in vivo (i.e., maximum joint range of motion), largely by increasing our ability to tolerate higher stretch loads. However, only small effects are observed in the passive stiffness of the muscle-tendon unit (MTU) or the muscle itself, although a reduction in muscle stiffness has been observed in the plantar flexors after both stretching and eccentric exercise interventions. No changes have yet been observed in viscoelastic properties such as the MTU stress-relaxation response, although a minimum of evidence indicates that hysteresis during passive stretch-relaxation cycles may be reduced by muscle stretching training. Importantly, data exist for relatively few muscle groups, and little is known about the effects of age and sex on the adaptive process of passive mechanical properties. Despite the significant research effort afforded to understanding the effects of altered physical activity patterns on the maximum range of motion at some joints, further information is needed before it will be possible to develop targeted physical activity interventions with the aim of evoking specific changes in passive mechanical properties in individuals or in specific muscles and muscle groups.

Stable Control of Firing Rate Mean and Variance by Dual Homeostatic Mechanisms

Homeostatic processes that provide negative feedback to regulate neuronal firing rates are essential for normal brain function. Indeed, multiple parameters of individual neurons, including the scale of afferent synapse strengths and the densities of specific ion channels, have been observed to change on homeostatic time scales to oppose the effects of chronic changes in synaptic input. This raises the question of whether these processes are controlled by a single slow feedback variable or multiple slow variables. A single homeostatic process providing negative feedback to a neuron’s firing rate naturally maintains a stable homeostatic equilibrium with a characteristic mean firing rate; but the conditions under which multiple slow feedbacks produce a stable homeostatic equilibrium have not yet been explored. Here we study a highly general model of homeostatic firing rate control in which two slow variables provide negative feedback to drive a firing rate toward two different target rates. Using dynamical systems techniques, we show that such a control system can be used to stably maintain a neuron’s characteristic firing rate mean and variance in the face of perturbations, and we derive conditions under which this happens. We also derive expressions that clarify the relationship between the homeostatic firing rate targets and the resulting stable firing rate mean and variance. We provide specific examples of neuronal systems that can be effectively regulated by dual homeostasis. One of these examples is a recurrent excitatory network, which a dual feedback system can robustly tune to serve as an integrator.

Distinct synaptic and neurochemical changes to the granule cell-CA3 projection in Bassoon mutant mice.

Proper synaptic function depends on a finely-tuned balance between events such as protein synthesis and structural organization. In particular, the functional loss of just one synaptic-related protein can have a profound impact on overall neuronal network function. To this end, we used a mutant mouse model harboring a mutated form of the presynaptic scaffolding protein Bassoon (Bsn), which is phenotypically characterized by: (i) spontaneous generalized epileptic seizure activity, representing a chronically-imbalanced neuronal network; and (ii) a dramatic increase in hippocampal brain-derived neurotrophic factor (BDNF) protein concentration, a key player in synaptic plasticity. Detailed morphological and neurochemical analyses revealed that the increased BDNF levels are associated with: (i) modified neuropeptide distribution; (ii) perturbed expression of selected markers of synaptic activation or plasticity; (iii) subtle changes to microglial structure; and (iv) morphological alterations to the mossy fiber (MF) synapse. These findings emphasize the important contribution of Bassoon protein to normal hippocampal function, and further characterize the Bsn-mutant as a useful model for studying the effects of chronic changes to network activity.

Effect of permissive hypercapnia on background cerebral electrical activity in premature babies.

Permissive hypercapnia is routinely practiced in neonatal intensive care units. The effect of permissive hypercapnia on the preterm brain and brain electrical activity is unknown. In this study, we aimed to determine the effect of chronic changes in partial pressure of blood carbon dioxide (PcO2) on brain electrical activity in preterm newborn babies born at or before 32 wk gestation. Eighty-three 1-h long digital electroencephalography (EEG) recordings were performed once a week for 4 wk on 25 babies with median gestational age of 29 wk (range: 23-32) after 48 h of age. Capillary blood gas measurements were performed midway through EEG recordings. There are associations between EEG parameters and blood pH, PcO2, and blood glucose concentration. However, there are also strong and complex associations with gestational age and substantial individual patient effects that make it difficult to demonstrate predictive associations. PcO2 and bicarbonate are significantly correlated with relative power of θ EEG band and Δ EEG band respectively after adjustment for age and intrababy correlations, but after allowing for multiple testing these relationships are of borderline statistical significance. Compensated respiratory acidosis may affect EEG by increased delta wave activity in preterm babies born before 32 wk gestation.

Bioindicators of changes in water quality on coral reefs: review and recommendations for monitoring programmes

Effective environmental management requires monitoring programmes that provide specific links between changes in environmental conditions and ecosystem health. This article reviews the suitability of a range of bioindicators for use in monitoring programmes that link changes in water quality to changes in the condition of coral-reef ecosystems. From the literature, 21 candidate bioindicators were identified, whose responses to changes in water quality varied spatially and temporally; responses ranged from rapid (hours) changes within individual corals to long-term (years) changes in community composition. From this list, the most suitable bioindicators were identified by determining whether responses were (i) specific, (ii) monotonic, (iii) variable, (iv) practical and (v) ecologically relevant to management goals. For long-term monitoring programmes that aim to quantify the effects of chronic changes in water quality, 11 bioindicators were selected: symbiont photophysiology, colony brightness, tissue thickness and surface rugosity of massive corals, skeletal elemental and isotopic composition, abundance of macro-bioeroders, micro- and meiobenthic organisms such as foraminifera, coral recruitment, macroalgal cover, taxonomic richness of corals and the maximal depth of coral-reef development. For short-term monitoring programmes, or environmental impact assessments that aim to quantify the effects of acute changes in water quality, a subset of seven of these bioindicators were selected, including partial mortality. Their choice will depend on the specific objectives and the timeframe available for each monitoring programme. An assessment framework is presented to assist in the selection of bioindicators to quantify the effects of changing water quality on coral-reef ecosystems.

Effect of Chronic Changes in Heart Rate on Congestive Heart Failure

Heart rate can affect cardiac function, but the importance of rates lower than 100 paced beats per minute is unknown. We therefore sought to evaluate the impact of different heart rates on ejection fraction, 6-minute walk, and peak oxygen consumption (VO2) in heart failure patients. We studied 13 pacemaker-dependent New York Heart Association Class III patients with ejection fraction <40%, age 66 +/- 13. Eligible patients included those pacing at least 75% of the time at a lower set rate of 60 ppm. This was a 3-period randomized blinded crossover study. Patients were assigned to pace at 60, 75, or 90 ppm (with rate responsivity to 20 ppm above the lower rate) for 2 months at each setting. At the end of each period, ejection fraction (by nuclear ventriculography) and exercise tolerance (by peak VO2 and 6-minute walk) were assessed. Ejection fraction, peak VO2, and 6-minute walk distance were significantly different among the 3 heart rates. All 3 were depressed at 90 ppm. A heart rate of 90 also led to more clinical deterioration and premature discontinuation from that period. Pacing at a heart rate of 90 led to lower ejection fraction, VO2, 6-minute walk distance and clinical evidence of worsening heart failure as compared with slower heart rates.

Central Peptidergic Neurons Are Hyperactive during Collateral Sprouting and Inhibition of Activity Suppresses Sprouting

Little is known regarding the effect of chronic changes in neuronal activity on the extent of collateral sprouting by identified CNS neurons. We have investigated the relationship between activity and sprouting in oxytocin (OT) and vasopressin (VP) neurons of the hypothalamic magnocellular neurosecretory system (MNS). Uninjured MNS neurons undergo a robust collateral-sprouting response that restores the axon population of the neural lobe (NL) after a lesion of the contralateral MNS (). Simultaneously, lesioned rats develop chronic urinary hyperosmolality indicative of heightened neurosecretory activity. We therefore tested the hypothesis that sprouting MNS neurons are hyperactive by measuring changes in cell and nuclear diameters, OT and VP mRNA pools, and axonal cytochrome oxidase activity (COX). Each of these measures was significantly elevated during the period of most rapid axonal growth between 1 and 4 weeks after the lesion, confirming that both OT and VP neurons are hyperactive while undergoing collateral sprouting. In a second study the hypothesis that chronic inhibition of neuronal activity would interfere with the sprouting response was tested. Chronic hyponatremia (CH) was induced 3 d before the hypothalamic lesion and sustained for 4 weeks to suppress neurosecretory activity. CH abolished the lesion-induced increases in OT and VP mRNA pools and virtually eliminated measurable COX activity in MNS terminals. Counts of the total number of axon profiles in the NL revealed that CH also prevented axonal sprouting from occurring. These results are consistent with the hypothesis that increased neuronal activity is required for denervation-induced collateral sprouting to occur in the MNS.

Psychological effects of snacks and altered meal frequency.

Over the past two decades, substantial research has been conducted to investigate the idea that alterations in short-term nutritional intake play a role in influencing cognitive behaviour and mood. A portion of this research has examined specifically the effect of meal intake on the performance of mental tasks and subjective feelings of mood. Results of this research indicate that a number of variables including the timing and nutritional composition of the meal, nutritional status, habitual patterns of feeding behaviour, beliefs about food, and the nature of the mental tasks, can influence the effects of meals on cognitive behaviour. For example, studies have demonstrated that breakfast intake generally is associated with an improvement in cognitive performance later in the morning, while lunch intake is associated with an impairment in mid-afternoon performance on mental tasks and more negative reports of mood. Intake of nutrients late in the afternoon appears to have a positive effect on subsequent performance on tasks involving sustained attention or memory. Although research has provided insights into the role of meal intake on cognitive behaviour and mood, there are a number of factors which remain to be studied. These include the interaction of age, gender, activity level, meal composition, personality factors, stress with the effects of meals on cognitive behaviour. Additionally, more work is needed on the time-course of short-term nutrient effects, and the effects of chronic changes in meal intake on behaviour.

Regulation of angiotensin II receptor AT1 subtypes in renal afferent arterioles during chronic changes in sodium diet.

Studies determined the effects of chronic changes in sodium diet on the expression, regulation, and function of different angiotensin II (ANG II) receptor subtypes in renal resistance vessels. Rats were fed low- or high-sodium diets for 3 wk before study. Receptor function was assessed in vivo by measuring transient renal blood flow responses to bolus injections of ANG II (2 ng) into the renal artery. ANG II produced less pronounced renal vasoconstriction in rats fed a low- compared with high-sodium diet (16% vs. 56% decrease in renal blood flow, P < 0.001). After acute blockade of ANG II formation by iv enalaprilat injection in sodium-restricted animals, ANG II produced a 40% decrease in renal blood flow, a level between untreated dietary groups and less than high salt diet. Intrarenal administration of angiotensin II receptor type 1 (AT1) receptor antagonists losartan or EXP-3174 simultaneously with ANG II caused dose-dependent inhibition of ANG II responses. Based on maximum vasoconstriction normalized to 100% ANG II effect in each group, AT1 receptor antagonists produced the same degree of blockade in all groups, with an apparent maximum of 80-90%. In contrast, similar doses of the angiotensin II receptor type 2 (AT2) receptor ligand CGP-42112 had only a weak inhibitory effect. In vitro equilibrium-saturation 125I-ANG II binding studies on freshly isolated afferent arterioles indicated that ANG II receptor density was lower in the low- vs. high-sodium animals (157 vs. 298 fmol/mg, P < 0.04); affinity was similar (0.65 nM). Losartan and EXP-3174 displaced up to 80-90% of the ANG II binding; fractional displacement was similar in both diet groups. In contrast, the AT2 receptor analogues PD-123319 and CGP-42112 at concentrations < 10(-6) M had no effect on ANG II binding. RT-PCR assays revealed the expression of both angiotensin II receptor type 1A (AT(1A)) and angiotensin II receptor type 1B (AT(1B)) subtypes in freshly isolated afferent arterioles, while there was very little AT2 receptor expression. Total AT1 receptor mRNA expression was suppressed by low sodium intake to 66% of control levels, whereas it was increased to 132% of control by high-sodium diet, as indicated by ribonuclease protection assay. Receptor regulation was associated with parallel changes in AT(1A) and AT(1B) expression; the AT(1A)/AT(1B) ratio was stable at 3.7. We conclude that AT1 receptors are the predominant ANG II receptor type in renal resistance vessels of 7-wk-old rats. Chronic changes in sodium intake caused parallel regulation of expression and amount of receptor protein of the two AT1 receptor genes that modulate receptor function and altered reactivity of renal vessels to ANG II.

Molecular biology of parathyroid hormone.

The entire biosynthetic pathway of PTH has been elucidated from the determination of the chromosomal location to the eventual secretion of the hormone from the cell. The human gene is present on the short arm of chromosome 11, and restriction site polymorphisms near the gene have been detected. The PTH genes and cDNAs have been isolated and characterized in the bovine, human, and rat species. The gene contains two introns, which are in the same position in each species, and dissect the gene into 3 exons that code, respectively, for the 5' untranslated region, the signal peptide, and PTH plus the 3' untranslated region. The mRNAs are about twice as long as necessary to code for preProPTH and contain a 7-methylquanosine cap at the 5' terminus and polyadenylic acid at the 3' terminus. The 5' termini of the bovine and human mRNAs are heterogeneous at the 5' terminus, the basis of which is two TATA sequences in the 5' flanking regions of the gene. In contrast, the rat gene contains a single TATA sequence and the mRNA has a single 5' terminus. The initial translational product of the mRNA is preProPTH, and the pre-peptide of 25 amino acids is equivalent to signal peptides of other secreted and membrane proteins. The genes of the three species are very homologous in the region that codes for preProPTH. Substantial homology is also retained in the gene flanking regions, introns, and mRNA untranslated regions. Silent sites are also conserved more than would be expected, particularly between the human and bovine sequences. The bovine and human sequences are more closely related than the rat is to either the human or bovine. These studies of the basic molecular biology of PTH will provide the framework for future analysis of significant biological and medical questions. In vitro mutagenesis techniques should soon provide information about the elements of the gene involved in regulating transcription and about functional elements of the signal peptide. Eventually, signals involved in directing the ProPTH molecule to secretory granules as well as the biologically active regions of PTH, itself, will be examined by these methods. The molecular biological studies, combined with the development of dispersed cell cultures, provide the opportunity to study the effects of chronic changes in calcium on gene transcription and mRNA metabolism. The restriction site polymorphisms associated with the human PTH gene will allow a search for correlations between PTH gene structure and parathyroid disease.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of chronic changes in dietary electrolytes and acute stress on plasma levels of corticosterone and aldosterone in the duck ( Anas platyrhynchos)

Plasma levels of corticosterone and aldosterone were determined by radioimmunoassay in ducks consuming diets containing different concentrations of sodium and potassium. Compared with control diet birds, maintenance on a high-Na + diet for 5 days caused a 2-fold increase in the basal plasma corticosterone concentration, while adaptation for 8 days to a low-Na + diet resulted in a 2.6-fold increase in the basal plasma concentration of aldosterone. Both corticosterone and aldosterone basal plasma levels were greatly elevated in birds denied access to drinking water for 4 days. Adaptation to a high-Na + diet or deprivation of water resulted in hyperosmolality and hypernatremia, while the high-K +, low-Na + low-K + , and low-Na + diets did not significantly alter the plasma sodium or potassium levels from the control levels. In addition, birds were stressed by semi-immobilization to determine the effects of acute stress-induced ACTH secretion on the adrenocortical response following changes in dietary sodium and potassium intake. In ducks adapted to low-Na + low-K + , high-Na +, and low-Na + diets, stress-induced adrenocorticotrophic hormone (ACTH) increased the aldosterone, but not the corticosterone, response to a level significantly greater than in the controls. These results demonstrate that in the duck secretion of corticosterone and aldosterone can be independently regulated. Furthermore, the endocrine changes that are induced by altered sodium and potassium intake are reflected in the adrenocortical responses to acute stress.

Effect of chronic changes in rate of enterohepatic cycling on bile acid kinetics and biliary lipid composition in the Rhesus monkey

Although several studies have demonstrated an inverse relationship between size of the bile salt pool and frequency of enterohepatic cycling, most such data have been obtained in conditions where control of cycling frequency was not possible. In this study, we diverted bile from the bile duct of Rhesus monkeys into a reservoir and returned it to the duodenum at varying rates, a model that mimicked differences in frequency of gallbladder emptying. As cycling frequency increased from 1 to 8 cycles/day, size of the bile salt pool decreased from 1756 μM to 369 μM. Changes in fractional turnover rate parallelled changes in cycling frequency. The relative composition of the biliary lipids remained the same throughout the range of cycling frequencies. These data show that direct manipulation of cycling frequency had the predicted effects on other parameters of the enterohepatic circulation but did not give rise to lithogenic bile.

Effects of chronic changes in hemoglobin-O2 affinity in rats

We have assessed the characteristics of oxygen transport following chronic intraperitoneal administration of sodium cyanate (NaCNO, 90 mg/kg; P50 decreased), o-iodosodium benzoate (OISB, 300 mg/kg; P50 increased), or sodium chloride (NaCl; P50 unchanged) to rats at a rate of 5 times/wk for 16 wk. At the end of this period, the animals were exposed to a low inspired O2 concentration and were subjected to exercise stress. Hill's n50 at pH 6.90-7.60, hematocrit, and the saturation dependency of the Bohr effect (PCO2 constant) were altered by NaCNO only. Cyanate-treated rats gained less weight, probably due to a toxic side effect of the drug. Hypoxemia-induced lactatemia was greatest with a high P50 (OISB). Physical effort (swimming) was tolerated poorly at normal arterial PO2 when P50 was low (NaCNO). Although a left-shifted curve appears beneficial when FIO2 is reduced, reasons for the physiological advantage may be the result of other characteristics of the O2 dissociation curve, not P50 alone.

Relationship of Serum Glucose Concentration to Changes in Refraction

The effect of chronic changes in serum glucose concentration on refraction was studied by increasing the dose of insulin or chlorpropamide in 10 diabetic patients who initially had relatively high glucose concentrations. In every case when serum glucose concentration was reduced the vision became less myopic or more hyperopic. To assess acute changes. 10 diabetics (including four with aphakic eyes) were given an intravenous injection of glucose. In patients with intact lenses the vision became more myopic or less hyperopic following the administration of glucose, but in the aphakic eyes hyperopia increased. It is concluded from both the acute and chronic studies that higher levels of serum glucose concentration produce myopia and lower levels produce hyperopia. Furthermore, these changes are related to changes in the optical properties of the crystallin lens.