Effects of human pregnancy on cardiac autonomic function above and below the ventilatory threshold

This study examined the effectsof human pregnancy on heart rate variability (HRV), spontaneousbaroreflex (SBR) sensitivity, and plasma catecholamines at rest andduring exercise. Subjects were 14 healthy, physically active pregnantwomen (PG; mean gestational age = 33.9 ± 1.0 wk). Resultswere compared with an age-matched nonpregnant control group (NPG;n = 14) with similar characteristics. Theelectrocardiographic R-wave-R-wave interval and systolic blood pressure (via finger plethysmograph) were measured on a beat-to-beat basis at rest and during upright cycling at 60 and 110% of the ventilatory threshold (T_vent). Parasympathetic nervoussystem (PNS) modulation (as reflected by HRV high-frequency/total power and SBR slope) was significantly reduced at rest in the PG vs. the NPG.During exercise, PNS modulation decreased significantly in both groups,but the magnitude of PNS withdrawal from rest to 110%T_vent was smaller in the PG vs. NPG. Sympathetic nervous system (SNS) modulation (reflected by the low-frequencypower-to-high-frequency power ratio) increased above resting values at60 and 110% T_vent in the NPG. SNS modulation at 110%T_vent was significantly lower in the PG compared with theNPG. Plasma norepinephrine and epinephrine levels were also lower at110% T_vent in the PG. It was concluded that healthypregnant women exhibit lower PNS modulation at rest and blunted SNSmodulation during exercise above T_vent in late gestation.

     

Plasma acid-base regulation above and below ventilatory threshold in late gestation.

Stewart's physicochemical approach was used to study the effects of pregnancy on acid-base regulation in arterialized blood. Responses of 15 healthy pregnant women (PG; gestational age, 37.1 +/- 0.2 wk) were compared with those of 15 nonpregnant controls (CG) at rest and during cycling at 70 and 110% of the ventilatory threshold (T(vent)). Hydrogen ion concentration ([H(+)]) was lower in the PG vs. CG at rest and during exercise (P < 0.05 at rest and 70% T(vent)). Exercise-induced changes in [H(+)] were similar between groups. Lower resting [H(+)] values in the PG vs. CG resulted from lower values for arterialized PCO(2) (Pa(CO(2))) and total weak acid ([A](tot)), which were partly offset by a lower strong-ion difference ([SID]). Reductions in [A](tot) and [SID] at rest were primarily the result of reductions in albumin [Alb] and sodium [Na(+)], respectively. In the transition from rest to 70% T(vent), small increases in Pa(CO(2)) and [A](tot) contributed to moderate increases in [H(+)] in both groups, however [SID] increased in the PG and decreased in the CG (P < 0.05 between groups). In the transition from rest to 110% T(vent), decreases in [SID] made a significantly greater contribution to changes in [H(+)] in the CG vs. PG. Exercise-induced increases in [H(+)] are similar in the pregnant vs. nonpregnant state, but there is a reduced contribution of [SID] both above and below T(vent) during pregnancy.     

Area summation in human vision at and above detection threshold

The initial image-processing stages of visual cortex are well suited to a local (patchwise) analysis of the viewed scene. But the world's structures extend over space as textures and surfaces, suggesting the need for spatial integration. Most models of contrast vision fall shy of this process because (i) the weak area summation at detection threshold is attributed to probability summation (PS) and (ii) there is little or no advantage of area well above threshold. Both of these views are challenged here. First, it is shown that results at threshold are consistent with linear summation of contrast following retinal inhomogeneity, spatial filtering, nonlinear contrast transduction and multiple sources of additive Gaussian noise. We suggest that the suprathreshold loss of the area advantage in previous studies is due to a concomitant increase in suppression from the pedestal. To overcome this confound, a novel stimulus class is designed where: (i) the observer operates on a constant retinal area, (ii) the target area is controlled within this summation field, and (iii) the pedestal is fixed in size. Using this arrangement, substantial summation is found along the entire masking function, including the region of facilitation. Our analysis shows that PS and uncertainty cannot account for the results, and that suprathreshold summation of contrast extends over at least seven target cycles of grating.     

Adaptations to training at the individual anaerobic threshold

The individual anaerobic threshold (Th(an)) is the highest metabolic rate at which blood lactate concentrations can be maintained at a steady-state during prolonged exercise. The purpose of this study was to test the hypothesis that training at the Th(an) would cause a greater change in indicators of training adaptation than would training "around" the Th(an). Three groups of subjects were evaluated before, and again after 4 and 8 weeks of training: a control group, a group which trained continuously for 30 min at the Th(an) intensity (SS), and a group (NSS) which divided the 30 min of training into 7.5-min blocks at intensities which alternated between being below the Th(an) [Th(an) -30% of the difference between Th(an) and maximal oxygen consumption (VO2max)] and above the Th(an) (Th(an) +30% of the difference between Th(an) and VO2max). The VO2max increased significantly from 4.06 to 4.27 l.min-1 in SS and from 3.89 to 4.06 l.min-1 in NSS. The power output (W) at Th(an) increased from 70.5 to 79.8% VO2max in SS and from 71.1 to 80.7% VO2max in NSS. The magnitude of change in VO2max, W at Th(an), % VO2max at Th(an) and in exercise time to exhaustion at the pretraining Th(an) was similar in both trained groups. Vastus lateralis citrate synthase and 3-hydroxyacyl-CoA-dehydrogenase activities increased to the same extent in both trained groups. While all of these training-induced adaptations were statistically significant (P < 0.05), there were no significant changes in any of these variables for the control subjects.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of training at and above the lactate threshold on the lactate threshold and maximal oxygen uptake

Thirty-three college women (mean age = 21.8 years) participated in a 5 d X wk-1, 12 week training program. Subjects were randomly assigned to 3 groups, above lactate threshold (greater than LT) (N = 11; trained at 69 watts above the workload associated with LT), = LT (N = 12; trained at the work load associated with LT) and control (C) (N = 10). Subjects were assessed for VO2max, VO2LT, VO2LT/VO2max, before and after training, using a discontinuous 3 min incremental (starting at 0 watts increasing 34 watts each work load) protocol on a cycle ergometer (Monark). Respiratory gas exchange measures were determined using standard open circuit spirometry while LT was determined from blood samples taken immediately following each work load from an indwelling venous catheter located in the back of a heated hand. Body composition parameters were determined before and after training via hydrostatic weighing. Training work loads were equated so that each subject expended approximately 1465 kJ per training session (Monark cycle ergometer) regardless of training intensity. Pretraining, no significant differences existed between groups for any variable. Post training the greater than LT group had significantly higher VO2max (13%), VO2LT (47%) and VO2LT/VO2max (33%) values as compared to C (p less than .05). Within group comparisons revealed that none of the groups significantly changed VO2max as a result of training, only the greater than LT group showed a significant increase in VO2LT (48%) (p less than .05), while both the = LT and greater than LT group showed significant increases in VO2LT/VO2max (= LT 16%, greater than LT 42% (p less than .05)).(ABSTRACT TRUNCATED AT 250 WORDS)     

Blood glucose responses in humans mirror lactate responses for individual anaerobic threshold and for lactate minimum in track tests

The equilibrium point between blood lactate production and removal (La-(min)) and the individual anaerobic threshold (IAT) protocols have been used to evaluate exercise. During progressive exercise, blood lactate [La-]b, catecholamine and cortisol concentrations, show exponential increases at upper anaerobic threshold intensities. Since these hormones enhance blood glucose concentrations [Glc]b, this study investigated the [Glc] and [La-]b responses during incremental tests and the possibility of considering the individual glucose threshold (IGT) and glucose minimum (Glc(min)) in addition to IAT and La-(min) in evaluating exercise. A group of 15 male endurance runners ran in four tests on the track 3000 m run (v3km); IAT and IGT - 8 x 800 m runs at velocities between 84% and 102% of v3km; La-(min) and Glc(min) - after lactic acidosis induced by a 500-m sprint, the subjects ran 6 x 800 m at intensities between 87% and 97% of v3km; endurance test (ET) - 30 min at the velocity of IAT. Capillary blood (25 microl) was collected for [La-]b and [Glc]b measurements. The IAT and IGT were determined by [La-]b and [Glc]b kinetics during the second test. The La-(min) and Glc(min) were determined considering the lowest [La-] and [Glc]b during the third test. No differences were observed (P < 0.05) and high correlations were obtained between the velocities at IAT [283 (SD 19) and IGT 281 (SD 21) m. x min(-1); r = 0.096; P < 0.001] and between La-(min) [285 (SD 21)] and Glc(min) [287 (SD 20) m. x min(-1) r = 0.77; P < 0.05]. During ET, the [La-]b reached 5.0 (SD 1.1) and 5.3 (SD 1.0) mmol x l(-1) at 20 and 30 min, respectively (P > 0.05). We concluded that for these subjects it was possible to evaluate the aerobic capacity by IGT and Glc(min) as well as by IAT and La-(min).     

Phyto- and necromass above and below ground in a fen

Few investigations deal with the subterranean phytomass in fens (minerotrophic peatlands), their results differing widely. This study was carried out in a "rich fen" in central Sweden, and involved excavation of peat monoliths, depth-sectioning and hand-sorting as to species, type of organ, and living or dead condition. The below-ground living phytomass was large, over 4 kg m⁻² in each of two sample plots, being no <97–96% in the dominants, Carex limosa and C. lasiocarpa , 89% on average in Menyanthes trifoliata , and c. 93% for all species. Carex limosa had its rhizome maximum slightly higher than C. lasiocarpa , but a greater part of its roots penetrated deeper. For Menyanthes , also net annual production above and below ground was estimated. The subterranean phytomass decreased with depth, but living roots were found even at 42 cm. The recognizable necromass did not decrease with depth. These recent constituents would render a very unequal age to the peat. The imbalance between the small green aerial phytomass and the much greater subterranean phytomass was emphasized. It could be related to a relatively very great translocation of photosynthates from above to below ground, and/or to longevity of below-ground organs, the latter at least in Menyanthes.     

Peripheral changes above and below injury level lead to prolonged vascular responses following high spinal cord injury

Autonomic dysreflexia (AD) is a debilitating disorder producing episodes of extreme hypertension in patients with high-level spinal cord injury (SCI). Factors leading to AD include loss of vasomotor baroreflex control to regions below injury level, changes in spinal circuitry, and peripheral changes. The present study tested for peripheral changes below and above injury level 6 wk after a transection at the fourth thoracic spinal level. Changes in vascular conductance were recorded in the femoral, renal, brachial, and carotid arteries in response to intravenous injections of two alpha-adrenergic agonists, phenylephrine (PE; 0.03-100 microg/kg) and methoxamine (Meth; 1-300 microg/kg). Unlike PE, Meth is not subject to neuronal reuptake. Ganglionic blockade (0.6 mg/kg chlorisondamine) was used to eliminate the central component of the cardiovascular response. After ganglionic blockade, SCI animals exhibited prolonged vasoconstriction in response to PE in all blood vessels measured compared with those in intact animals (all, P < 0.035). However, the PE dose-response curves obtained after ganglionic blockade revealed no significant difference in the potency between the two groups (all, P > 0.06), indicating that the prolonged vasoconstriction was not due to supersensitivity to PE. In contrast to PE, vascular responses to Meth did not vary between intact and SCI groups (all P > 0.108). These results show the development of a widespread peripheral change producing prolonged vasoconstriction in response to PE, but not Meth, possibly due to reduced neuronal reuptake of PE after SCI. This is the first study to report such a change in blood vessels not only below but also above injury level. Interventions to correct this reduced reuptake may help limit the development of AD.     

Muscle metabolic responses to exercise above and below the "critical power" assessed using 31P-MRS

We tested the hypothesis that the asymptote of the hyperbolic relationship between work rate and time to exhaustion during muscular exercise, the "critical power" (CP), represents the highest constant work rate that can be sustained without a progressive loss of homeostasis [as assessed using (31)P magnetic resonance spectroscopy (MRS) measurements of muscle metabolites]. Six healthy male subjects initially completed single-leg knee-extension exercise at three to four different constant work rates to the limit of tolerance (range 3-18 min) for estimation of the CP (mean +/- SD, 20 +/- 2 W). Subsequently, the subjects exercised at work rates 10% below CP (CP) for as long as possible, while the metabolic responses in the contracting quadriceps muscle, i.e., phosphorylcreatine concentration ([PCr]), P(i) concentration ([P(i)]), and pH, were estimated using (31)P-MRS. All subjects completed 20 min of CP exercise was 14.7 +/- 7.1 min. During CP exercise, however, [PCr] continued to fall to the point of exhaustion and [P(i)] and pH changed precipitously to values that are typically observed at the termination of high-intensity exhaustive exercise (end-exercise values = 26 +/- 16% of baseline [PCr], 564 +/- 167% of baseline [P(i)], and pH 6.87 +/- 0.10, all P < 0.05 vs. 相似文献   

Hemodynamic responses during exercise at and above VO2max in swine

Mean arterial pressure (Pa), heart rate, cardiac output (Q), and Q distribution (with radiolabeled microspheres) were measured in miniature swine as they ran at high levels on a motor-driven treadmill. Each animal ran on two occasions: once during exercise at maximal O2 uptake (VO2max) and once at an intensity estimated to require approximately 115% VO2max. The purpose was to assess these cardiovascular variables to determine whether the calculated resistance to blood flow during supramaximal exercise was different from that during maximal exercise. A total of 114 tissues/organs were dissected for blood flow analysis. Pa and Q were unaltered between the two exercise conditions. Blood flow to all but one of the 62 skeletal muscles sampled was unchanged between conditions as were the blood flows to the visceral organs and brain. The results demonstrate that vascular resistance was constant in all these tissues between maximal and supramaximal exercise intensities. Elevated blood flows were measured in 7 of the 11 coronary sites sampled. Calculated resistance to blood flow indicated that a decrease in resistance occurred in most of the samples having elevated blood flow. Because heart rate was elevated during the supramaximal exercise, the increase in blood flow was probably in response to the greater myocardial work and concomitant elevation in O2 demand. In summary, it was shown that Pa, Q, and Q distribution in most tissues remained unchanged during exercise at intensities above VO2max. Thus a precise matching occurs between the increasingly powerful vasoconstrictor drive initiated by the sympathetic nervous system and the elevated local vasodilatory drive responding to the greater O2 demand during the supramaximal exercise.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of foundation species above and below tree line

The facilitative effects of “foundation species” on community diversity and composition in alpine systems can be strong, but the strength of these effects often varies along gradients that occur above tree line. Here, we explore the effects of foundation species above and below tree line, a globally consistent and often abrupt climatic and ecological transition. The experiment was developed in Northern Rocky Mountains of Wyoming and Montana, USA, in vegetation with similar herbaceous physiognomies without trees, but containing dominant foundation species with “cushion” morphologies. In these systems, we compared the diversity and structure of plant communities occurring within these foundation species to that in the open at three sites below tree line and three sites above tree line. Above tree line, 69–94% of all species were significantly spatially associated with foundation species, whereas below tree line, 29–62% of the species were associated with foundation species. Similar patterns were observed in species accumulation curves with foundation species affecting the accumulation of species by +17 to +47% above tree line vs. –27 to +24% below tree line. Our results were consistent with the idea that facilitative interactions are more common with greater abiotic stress. However, our results also suggest the possibility that the climatic transition representing tree line might also represent a threshold, rather than a simple linear response, in how foundation species affect community diversity. A much larger sample size is warranted to thoroughly test such thresholds, but the fundamental ecological and physiological transition that plants experience at the climate of the tree line may also be reflected in small-scale species interactions that organize communities.     

Interaction of proflavine and 9-aminoacridine with DNA at temperatures below and above the melting temperature

The influence of temperature on the binding of 9-aminoacridine and of proflavine to E. coli DNA in 10^?3M NaCl solution has been determined by a spectrophotometric technique. The inadequacy of the expression normally used for the determination of the extent of binding is discussed with reference to measurements at temperatures above which dissociation of the double helix occurs. A method of determining the relative extents of binding to native and denatured DNA at elevated temperatures is described.     

Conceptualization of above and below relationships by an insect

Relational rules such as 'same' or 'different' are mastered by humans and non-human primates and are considered as abstract conceptual thinking as they require relational learning beyond perceptual generalization. Here, we investigated whether an insect, the honeybee (Apis mellifera), can form a conceptual representation of an above/below spatial relationship. In experiment 1, bees were trained with differential conditioning to choose a variable target located above or below a black bar that acted as constant referent throughout the experiment. In experiment 2, two visual stimuli were aligned vertically, one being the referent, which was kept constant throughout the experiment, and the other the target, which was variable. In both experiments, the distance between the target and the referent, and their location within the visual field was systematically varied. In both cases, bees succeeded in transferring the learned concept to novel stimuli, preserving the trained spatial relation, thus showing an ability to manipulate this relational concept independently of the physical nature of the stimuli. Absolute location of the referent into the visual field was not a low-level cue used by the bees to solve the task. The honeybee is thus capable of conceptual learning despite having a miniature brain, showing that such elaborated learning form is not a prerogative of vertebrates.     

Modulation of network pacemaker neurons by oxygen at the anaerobic threshold

Previous in vitro and in vivo studies showed that the frequency of rhythmic pyloric network activity in the lobster is modulated directly by oxygen partial pressure (PO(2)). We have extended these results by (1) increasing the period of exposure to low PO(2) and by (2) testing the sensitivity of the pyloric network to changes in PO(2) that are within the narrow range normally experienced by the lobster (1 to 6?kPa). We found that the pyloric network rhythm was indeed altered by changes in PO(2) within the range typically observed in vivo. Furthermore, a previous study showed that the lateral pyloric constrictor motor neuron (LP) contributes to the O(2) sensitivity of the pyloric network. Here, we expanded on this idea by testing the hypothesis that pyloric pacemaker neurons also contribute to pyloric O(2) sensitivity. A 2-h exposure to 1?kPa PO(2), which was twice the period used previously, decreased the frequency of an isolated group of pacemaker neurons, suggesting that changes in the rhythmogenic properties of these cells contribute to pyloric O(2) sensitivity during long-term near-anaerobic (anaerobic threshold, 0.7-1.2?kPa) conditions.