In healthy man, conditions that change muscle O2 delivery affect the achievable maximum rate of O2 uptake (,max) as well as the metabolic (e.g. lactate threshold, LT) and gas exchange (e.g. gas exchange threshold, The) responses to incremental exercise. Inclined (I) compared to level (L) running increases locomotory muscle EMG at a given speed in the horse, indicative of elevated metabolic demand. To our knowledge, the effect of treadmill incline on ,max, LT and The has not been addressed in the exercising quadruped. We used blood sampling and breath-by-breath expired gas analysis to test the hypothesis that I (10% gradient) would increase ,max and the rate of O2 uptake () at LT and Tge in six Thoroughbred horses during incremental running to volitional fatigue. ,max was significantly higher for I (I, 77.8 ± 4.1; L, 65.5 ± 5.3 l min−1; P<0.05), but peak plasma lactate concentration was not (I, 28.0 ± 3.7; L, 25.9 ± 3.0 mM). Arterial PCO2 increased to 62.1 ± 3.3 and 57.9 ± 2.7 Torr (I vs. L; P<0.05), yet despite this relative hypoventilation, a distinct The was present. This The occurred at a significantly different absolute (I, 49.6 ± 3.2; L, 42.4 ± 3.2 l min−1; P<0.05), but nearly identical relative (I, 63.6 ± 1.2; L, 63.9 ± 1.6% ,max) in I and L. Similarly, LT occurred at a significantly greater absolute (I, 37.3 ± 2.8; L, 26.9 ± 2.1 l min−1), but a relative that was not different (I, 47.9 ± 2.1; L, 43.9 ± 4.5% ,max). In addition, Tge occurred at a significantly higher (P ≤ 0.05) absolute and relative than LT for both I and L tests. In conclusion, ,max is higher during inclined than level running and both LT and Tge in the horse occur at a similar percentage of ,max irrespective of the absolute level of ,max. In contrast to humans, LT is a poor analogue of The in the horse.

We investigated the influence of ageing on the fatiguing characteristics of the mouse extensor digitorum longus (EDL) muscle as compared to those of the soleus muscle. Fatigue was produced by an intermittent stimulation protocol. We report for mature and aged animals the effects of fatigue on force produced during stimulation patterns that in non-fatigued muscle gave maximum force (Tmax, high frequency stimulation) and approximately half-maximum force (1/2Tmax, low frequency stimulation). In 15-month-old (mature) mice, fatiguing stimulation decreased Tmax in EDL and soleus muscle to 10.3 ± 1.0 % and 33.4 ± 3.0 % of control, respectively. In 30-month-old (aged) mice, the decrease in Tmax in EDL and soleus was statistically equal to that of the younger animals. Fatiguing stimulation decreased 1/2Tmax in EDL and soleus from 15-month-old animals to 22.5 ± 2.9 % and 45.7 ± 0.3 % of control, respectively. In 30-month-old animals, the 1/2Tmax in EDL and soleus muscle decreased to 18.2 ± 1.3 % and 35.0 ± 3.6 % of control, respectively. Under all conditions, the soleus fatigued significantly less. Contractile recovery from fatiguing stimulation was complete for the soleus in both age groups after 30 min, but incomplete for the EDL. The 1/2Tmax/Tmax ratio significantly increased in EDL and soleus muscle from 15-month-old animals after fatiguing stimulation. This increase was less significant in EDL, and absent in soleus muscle, from 30-month-old animals. These results indicate that fatiguing stimulation induces a leftward shift in the force-frequency relationship in the young animals; this shift is either significantly less (EDL) or absent (soleus) in the older animals. We speculate that the leftward shift of the force-frequency relationship may reflect a protective mechanism in younger animals against some of the damaging effects of fatiguing stimulation (i.e. oxidative stress). Experimental Physiology (2002) 87.1, 77-82.

To investigate the role of hypoxia as a stimulus to the early upregulation of vascular endothelial growth factor (VEGF) in fast skeletal muscles during chronic low frequency stimulation, blood flow, oxygen consumption, VEGF expression and capillary:fibre ratio were measured in chronically stimulated tibialis anterior and extensor digitorum longus (EDL) muscles in rabbits and rats. No differences were found in blood flow, oxygen consumption and extraction between rabbit muscles stimulated for 2 or 4 days (8 h on-16 h off) and controls. Muscle PO2 polarographically measured immediately at the end of stimulation on day 2 was also no different from control under resting conditions (10.7 ± 1.6 vs. 9.5 ± 1.2 Torr, n.s.). Unlike control muscles, however, PO2 in 2 day stimulated muscles did not increase immediately after a further acute bout of contractions. This difference was not apparent after similar acute contractions in 4 day stimulated muscles. The involvement of VEGF in early angiogenesis in stimulated muscles was studied in serial cryosections of rat EDL. The proportion of capillaries positively immunostained for VEGF increased from 25 ± 1 % to 40 ± 1 % (P < 0.05) in muscles removed on day 2 immediately at the end of chronic stimulation; it decreased slightly after 16 h rest, and increased again after 4 days of stimulation. Capillary:fibre ratio was unchanged throughout the experiment. Capillary cell proliferation increased only after the rest period on day 2 (20-fold increase) and day 4 (12-fold increase), indicating angiogenesis in progress. Thus the timing of transient hypoxia and increase in capillary-linked VEGF in stimulated muscles, albeit in different species, was similar, and increased VEGF staining and capillary cell proliferation occurred even after the hypoxia had resolved. This suggests (1) a connection between hypoxia and VEGF during the early stages of stimulation, although ensuing capillary proliferation may thereafter rapidly correct for local hypoxia, and (2) that the subsequent angiogenesis and VEGF expression are dependent on factors other than hypoxia. Experimental Physiology (2002) 87.3, 373-381.

Previous studies have suggested that the uptake of prolactin from the blood into the milk may be restricted when the alveolus is distended with milk. Therefore the aim of this study was to determine the relationship between prolactin in milk and milk production by measuring the concentration of prolactin in samples of fore- and hind-milk as well as the volume of milk removed for each breast, at each breastfeed over a 24 h period. The mean (± S.D.) concentration of prolactin in milk for all women (n = 15) over the 24 h period was 18.5 ± 11.6 µg l-1 (fore-milk) and 16.8 ± 12.8 µg l-1 (hind-milk). The variation between women masked small changes within women in the concentration of prolactin in milk over the 24 h period, therefore a prolactin ratio (individual fore- or hind-milk value divided by the mean for all fore- or hind-milk samples collected over a 24 h period) was determined. The concentration of prolactin was highest in milk between 02.01 and 06.00 h (prolactin ratio for fore- to hind-milk, 1.5), and lowest between 10.01 and 18.00 h (prolactin ratio for fore- to hind-milk, 0.8). Furthermore, we observed that the difference in prolactin concentration between the fore- and hind-milk (fore-hind gradient) was greatest between 06.01 and 10.00 h (4 µg l-1). To ensure that this effect was not due to permeability in the paracellular pathway, the concentrations of serum albumin and sodium in milk were measured. No significant (P > 0.05) changes over the 24 h period, or with increasing time since last feed were observed. We therefore concluded that when the breast is most drained of milk (in the late evening), and the rate of milk synthesis is greatest, the fore-hind prolactin gradient in the milk of the following feed (in the early morning) is highest. This is consistent with the observation that prolactin uptake from the blood by the lactocyte is dependent on the fullness of the breast, such that prolactin uptake may be inhibited in full alveoli. Experimental Physiology (2002) 87.2, 207-214.

Dendritic spines are morphologically and functionally heterogeneous. Here, we use two-photon imaging of layer V pyramidal neurons in slices from mouse visual cortex to characterize differences in spine calcium dynamics between individual spines. By measuring action potential-evoked [Ca2+]i transients in spines, we find different calcium dynamics in spines from proximal apical and distal apical dendrites. Using a mathematical multi-compartmental model, we demonstrate that these differences are even more pronounced in the absence of exogenous calcium buffers. We also find that these different calcium dynamics cause different susceptibility to synaptic depression in proximal and distal apical synapses, and that modifying calcium decay kinetics in spines changes the expression of long-term depression. We conclude that the location of the spine determines its time window of calcium compartmentalization and degree of calcium-dependent synaptic plasticity. Our results highlight the precision of the design of neocortical neurons.

The effect of added respiratory work on leg muscle oxygenation during constant-load cycle ergometry was examined in six healthy adults. Exercise was initiated from a baseline of 20 W and increased to a power output corresponding to 90 % of the estimated lactate threshold (moderate exercise) and to a power output yielding a tolerance limit of 11.8 min (± 1.4, S.D.) (heavy exercise). Ventilation and pulmonary gas exchange were measured breath-by-breath. Profiles of leg muscle oxygenation were determined throughout the protocol using near-infrared (NIR) spectroscopy (Hamamatsu NIRO 500) with optodes aligned midway along the vastus lateralis of the dominant leg. Four conditions were tested: (i) control (Con) where the subjects breathed spontaneously throughout, (ii) controlled breathing (Con Br) where breathing frequency and tidal volume were matched to the Con profile, (iii) increased work of breathing (Resist Br) in which a resistance of 7 cmH2O l-1 s-1 was inserted into the mouthpiece assembly, and (iv) partial leg blood flow occlusion (Leg Occl), where muscle perfusion was reduced by inflating a pressure cuff (~90 mmHg) around the upper right thigh. During Resist Br and Leg Occl, subjects controlled their breathing pattern to reproduce the ventilatory profile of Con. An ~3 min period with respiratory resistance or pressure cuff was introduced ~4 min after exercise onset. NIR spectroscopy data for reduced haemoglobin-myoglobin (Δ[Hb]) were extracted from the continuous display at specific times prior to, during and after removal of the resistance or pressure cuff. While the Δ[Hb] increased during moderate- and heavy-intensity exercise, there was no additional increase in Δ[Hb] with Resist Br. In contrast, Δ[Hb] increased further with Leg Occl, reflecting increased muscle O2 extraction during the period of reduced muscle blood flow. In conclusion, increasing the work of breathing did not increase leg muscle deoxygenation during heavy exercise. Assuming that leg muscle O2 consumption did not decrease, this implies that leg blood flow was not reduced consequent to a redistribution of flow away from the working leg muscle. Experimental Physiology (2002) 87.5, 601-611.

An increase in the concentration of serotonin in the brain has been shown to cause fatigue during exercise in humans and experimental animals. This type of fatigue is referred to as central fatigue and is likely to be mediated by the concentration of serotonin as well as serotonin receptor sensitivity. Serotonin (5-HT) receptor antagonism in humans and experimental animals has been shown to improve endurance performance. A previous report has shown decreased receptor sensitivity in athletes compared to sedentary controls. It is unclear whether this is due to a training adaptation or if individuals are predisposed to enhanced athletic performance due to their inherent decreased receptor sensitivity. The present study investigated changes in 5-HT receptor sensitivity in response to aerobic exercise. Subjects completed 3 × 30 min of stationary cycling at 70 % of their peak aerobic power (V˙O2,peak) for 9 weeks. Serotonin receptor sensitivity was assessed indirectly by measuring the neuroendocrine response following administration of a serotonin agonist (buspirone hydrochloride). The neuroendocrine response following administration of a placebo was also investigated in a blind crossover design. A group of sedentary control subjects was also recruited to control for seasonal variations in central receptor sensitivity. The training caused a significant increase in V˙O2,peak (3.1 ± 0.16 to 3.6 ± 0.15 l min-1, P < 0.05) and endurance capacity (93 ± 8 to 168 ± 11 min, P < 0.05), but there was no change (P > 0.05) in the neuroendocrine response in the presence of a serotonin agonist. However, one-quarter of the subjects in the training group demonstrated decreases in receptor sensitivity. These results suggest that despite increases in V˙O2,peak and endurance performance, there was no measurable change in 5-HT receptor sensitivity in the presence of a serotonin agonist. In addition, it is possible that changes in receptor sensitivity may take longer to occur, that the training stimulus used in the present investigation was inadequate and/or that changes occurred in receptor subtypes that were not probed by the agonist used in the present investigation. Experimental Physiology (2002) 87.1, 83-89.

Several lines of evidence suggest that central serotoninergic neurons may mediate fatigue signals during prolonged exercise. In this study we examined the effects of diet and ambient temperature on peripheral modulators and indices of serotoninergic function and their relationship to exercise performance. Six well-trained cyclists participated, in randomised order, in two diet and exercise regimens each lasting 8 days and comprising four cycle tests to exhaustion at 70 % of maximum oxygen uptake. On days 1 and 5, subjects exercised to exhaustion to deplete muscle glycogen. For 3 days after the first depletion trial a diet providing 10 % of energy in the form of carbohydrate (CHO) was consumed (low CHO), and for 3 days after the second depletion trial a diet providing 80 % (high CHO) of energy as CHO was consumed, and each diet was followed by a performance trial at the same ambient temperature, either 10 °C or 30 °C (days 4 and 8). This schedule was repeated after 1 week, but performance trials were carried out at the other ambient temperature. In the cold, cycling time increased (P < 0.01) from 89.2 (78.0-129.5) min (median (range)) in the low CHO trial to 158.2 (116.9-165.6) min in the high CHO trial. In the heat, cycling time increased from 44.0 (31.8-51.4) min in the low CHO trial to 53.2 (50.2-82.2) min on the high CHO trial (P = 0.02). The serum prolactin (Prl) concentration was higher at exhaustion during the two trials in the heat than in the two trials in the cold. Serum Prl levels were unrelated to the purported peripheral modulators of serotoninergic function (plasma concentrations of total tryptophan (Trp), free Trp, branched-chain amino acids (BCAAs), free Trp/BCAA ratio and total Trp/BCAA ratio) but were significantly related to the rectal temperatures measured during the two trials in the heat. This finding provides indirect evidence that the serotoninergic system may be involved in fatigue during exercise under conditions of heat stress. Experimental Physiology (2002) 87.2, 215-226.

The strength of synaptic connections in the brain is not fixed, but can be modulated by numerous mechanisms. Traditionally, electrophysiology has been used to characterize connections between neurons. Electrophysiology typically reports the activity of populations of synapses, while most mechanisms of plasticity are thought to operate at the level of single synapses. Recently, two-photon laser scanning microscopy has enabled us to perform optical quantal analysis of individual synapses in intact brain tissue. Here we introduce the basic principle of the two-photon microscope and discuss its main differences compared to the confocal microscope. Using calcium imaging in dendritic spines as an example, we explain the advantages of simultaneous dual-dye imaging for quantitative calcium measurements and address two common problems, dye saturation and background fluorescence subtraction.

The rapid increase in body mass that often occurs following creatine (Cr) supplementation is believed to be due to intracellular water retention. The purpose of this study was to determine whether Cr consumption alters the magnetic resonance (MR) transverse relaxation (T2) distribution of skeletal muscle. Transverse relaxation can be used to model water compartments within a cell or tissue. In this double-blind study, subjects were asked to supplement their normal diet with creatine monohydrate (20 g day-1 for 5 days) mixed with a grape drink (Creatine group, n = 7), or the grape drink alone (Placebo group, n = 8). Phosphorous MR spectroscopy was used to determine the effectiveness of the supplementation protocol. Subjects that responded to the Cr supplementation (i.e. showed a > 5 % increase in the ratio of the levels of phosphocreatine (PCr) and ATP) were placed in the Creatine group. Both proton MR imaging and spectroscopy were used to acquire T2 data, at 1.89 T, from the flexor digitorum profundus muscle of each subject before and after supplementation. Following the supplementation period, the Creatine group showed a gain in body mass (1.2 ± 0.8 kg, P < 0.05, mean ± S.D.), and an increase in PCr/ATP ratio (23.8 ± 16.4 %, P < 0.001). Neither group showed any changes in intracellular pH or T2 calculated from MR images. However, the spectroscopy data revealed at least three components (> 5 ms) at approximately 20, 40 and 125 ms in both groups. Only in the Creatine group was there an increase in the apparent proton concentration of the two shorter components combined (+5.0 ± 4.7 %, P < 0.05). According to the cellular water compartment model, the changes observed in the shorter T2 components are consistent with an increase in intracellular water. Experimental Physiology (2002) 87.3, 383-389.

Isometric exercise training has been shown to reduce resting blood pressure, but the effect that this might have on orthostatic tolerance is poorly understood. Changes in orthostatic tolerance may also be dependent on whether the upper or lower limbs of the body are trained using isometric exercise. Twenty-seven subjects were allocated to either a training or control group. A training group first undertook 5 weeks of isometric exercise training of the legs, and after an 8 week intervening period, a second training group containing six subjects from the initial training group, undertook 5 weeks of isometric arm-training. The control group were asked to continue their normal daily activities throughout the 18 weeks of the study. In all subjects orthostatic tolerance, assessed using lower body negative pressure (LBNP), and resting blood pressure were measured before and after each of the 5 week training or control periods. Estimated lean leg volume was determined before and after leg-training. During all LBNP tests, heart rate and blood pressure were recorded each minute, and the time taken to reach the highest heart rate was derived (time to peak HR). Resting systolic blood pressure (mean ± S.D.), when measured during the last week of training, was significantly reduced after both leg (−10 ± 8.7 mmHg) and arm (−12.4 ± 9.3 mmHg; P<0.05) isometric exercise training, compared to controls. This reduction disappeared when blood pressure was measured immediately before the LBNP tests, which followed training. Orthostatic tolerance only increased after leg-training (20.8 ± 16.4 LTI; P<0.05) and was accompanied by an increased time to peak HR (119.8 ± 106.3 beats min−1; P<0.05) in this group. Blood pressure responses to LBNP did not change after arm-training, leg-training or in controls (P>0.05). There was a small but significant increase in estimated lean leg volume after leg-training (0.1 ± 0.1 l; P<0.05). These results suggest that lower resting blood pressure is probably not responsible for the increased orthostatic tolerance after isometric exercise training of the legs. Rather, it is possible that the training altered some other aspect of cardiovascular control during orthostatic stress that was apparent in the changes in heart rate. Legtraining was accompanied by increases in estimated lean leg volume. The effects of isometric training on orthostatic tolerance appear to be specific to limbs that are directly involved in LBNP testing.

Recent evidence indicates that endurance-trained athletes are able to increase their stroke volume (SV) throughout incremental upright exercise, probably due to a progressively greater effect of the Frank-Starling mechanism. This is contrary to the widely held belief that SV reaches a plateau at a submaximal heart rate (irrespective of fitness level), owing to a limitation in the time for diastolic filling. The purpose of this investigation was to evaluate whether endurance-trained athletes rely on a progressively greater effect of the Frank-Starling mechanism throughout incremental exercise. A secondary purpose was to evaluate the effects of postural position on the cardiovascular responses to incremental exercise. Ten male cyclists participated in this investigation. Left ventricular function was assessed throughout incremental exercise in the supine and upright positions (counterbalanced) using radionuclide ventriculography. Stroke volume increased in a linear fashion during incremental exercise in both the upright and supine positions. The increases in cardiac output (Q) throughout incremental to maximal exercise (in both the supine and upright positions) were significantly related to changes in heart rate, myocardial contractility and the Frank-Starling mechanism. Percentage changes in end-diastolic volume and SV were significantly greater in the upright position versus the supine position, reflecting an increased reliance on the Frank-Starling effect to increase Q. We conclude from this investigation that highly trained endurance athletes are able to make progressively increasing usage of the Frank-Starling effect throughout incremental exercise. Postural position has a significant effect on the relative contribution of heart rate, myocardial contractility and the Frank-Starling mechanism to the increase in Q during exercise conditions. Experimental Physiology (2002) 87.5, 613-622.

Leg muscles were occluded (33 kPa) prior to exercise to determine whether the induced metabolic changes, and reactive hyperaemia upon occlusion release just prior to the exercise, would accelerate the subsequent oxygen consumption (V˙O2) response. Eight subjects performed double bouts (6 min duration, 6 min rest in-between) of square wave leg cycle ergometry both below and above their lactate threshold (LT). Prior to exercise, large blood pressure cuffs were put around the upper thighs. Occlusion durations were 0 min (control), 5 min and 10 min. Ischaemia was terminated within 5 s prior to exercise onset. Heart rate, V˙O2, ventilatory rate (V˙E), electromyogram (EMG) and haemoglobin/myoglobin (Hb/Mb) saturation were recorded continuously. Single exponential modelling demonstrated that, compared to control (time constant = 53.9 ± 13.9 s), ischaemia quickened the V˙O2 response (P < 0.05) for the first bout of exercise above LT (time constant = 48.3 ± 14.5 s) but not to any other exercise bout below or above LT. The 3-6 min integrated EMG (iEMG) slope was correlated to the 3-6 min V˙O2 slope (r = 0.73). Hb/Mb saturation verified the ischaemia but did not show a consistent relation to the V˙O2 time course. Reactive hyperaemia induced a faster V˙O2 response for work rates above LT. The effect, while significant, was not large considering the expected favourable metabolic and circulatory changes induced by ischaemia. Experimental Physiology (2002) 87.2, 227-235.

The aim of the present study was to examine the pattern of cardiovascular recovery from exercise in 15 women (age, 20.3 ± 1.4 years; body mass, 61.5 ± 4.3 kg) across two phases of oral contraceptive (OC) use: 21 days of consumption and 7 days of withdrawal. Cardiovascular recovery was measured in the supine position for 60 min following 30 min of exercise at 60 % maximal rate of oxygen consumption (V˙O2,max). Central and peripheral haemodynamics were assessed during consumption and withdrawal of the OC pill using occlusion plethysmography, Doppler flowmetry and echocardiography. Significant hypotension occurred following exercise (P < 0.05), returning to baseline values after 60 min. The peak hypotension occurred 5 min into recovery. Cardiac output and heart rate were elevated for 60 min following exercise (P < 0.05), whilst stroke volume remained at baseline values. Heart rate was greater throughout recovery during consumption compared to withdrawal (P < 0.05); however, although there was a trend for greater responses during consumption, phase of OC use did not affect the other central cardiovascular variables (P > 0.05). Post-exercise blood flow parameters were not significantly affected by exercise or OC phase; however, calf blood flow was greater, and resistance to flow lower during consumption (P > 0.05). The pattern of post-exercise fluctuations in cardiovascular parameters may differ from those seen in men, whilst oestrogen variation may influence research findings. Experimental Physiology (2002) 87.5, 623-632.

We have investigated the febrile responses of New Zealand White rabbits to a Gram-negative pyrogen (bacterial lipopolysaccharide (LPS) from Salmonella typhosa), commonly associated with systemic infection, and a Gram-positive pyrogen (Staphylococcus aureus), more frequently associated with superficial soft tissue infection, each administered via one of four different routes (intravenous, intramuscular, subcutaneous or intraperitoneal) at each of three different doses (LPS: 0.1, 1 and 10 µg kg-1; S. aureus: 1.5 × 107, 1.5 × 108 and 1.5 × 109 cell walls kg-1). Intravenous administration of LPS evoked rapid, dose-dependent biphasic fever. Injection of LPS by the other routes also evoked dose-dependent fever. However, these fevers were monophasic, had increased latency of onset, and were of lower amplitude. It is important to note that a dose of approximately 10 and 100 times that of the standard intravenous dose was required to produce a similar peak rise in temperature when administered subcutaneously and intraperitoneally, respectively. Intravenous injection of the highest dose of S. aureus evoked dose-dependent biphasic fever, with short latency of onset, which was very similar to that induced by intravenous LPS. At lower doses, intravenous S. aureus induced monophasic fever. No fever occurred when the same doses of S. aureus were administered by any other route. We conclude that any of the four routes may be used for the study of LPS-induced fever, provided that the doses are adjusted. However, studies of S. aureus-induced fever, and detection of contamination with either pyrogen, requires intravenous injection. Experimental Physiology (2002) 87.3, 391-399.

This study examined the relationship between muscle recruitment patterns using surface electromyography (EMG) and the excess O2 uptake (ExV˙O2) that accompanies slow (SR, 8 W min-1) but not fast (FR, 64 W min-1) ramp increases in work rate (WR) during exercise on a cycle ergometer. Nine subjects (2 females) participated in this study (25 ± 2 years, ± S.E.M.). EMG was obtained from the vastus lateralis and medialis and analysed in the time (root mean square, RMS) and frequency (median power frequency, MDPF) domain. Results for each muscle were averaged to provide an overall response and expressed relative to a maximal voluntary contraction (%MVC). ΔV˙O2/ΔWR was calculated for exercise below (S1) and above (S2) the lactate threshold (LT) using linear regression. The increase in RMS relative to the increase in WR for exercise below the LT (ΔRMS/ΔWR-S1) was determined using linear regression. Due to non-linearities in RMS above the LT, ΔRMS/ΔWR-S2 is reported as the difference in RMS (ΔRMS) and the difference in WR (ΔWR) at end-exercise and the LT. SR was associated with a higher (P < 0.05) ΔV˙O2/ΔWR (S1, 9.3 ± 0.3 ml min-1 W-1; S2, 12.5 ± 0.6 ml min-1 W-1) than FR (S1, 8.5 ± 0.4 ml min-1 W-1; S2, 7.9 ± 0.4 ml min-1 W-1) but a similar ΔRMS/ΔWR-S1 (SR, 0.11 ± 0.01% W-1; FR, 0.10 ± 0.01 % W-1). ExV˙O2 was greater (P < 0.05) in SR (3.6 ± 0.7 l) than FR (-0.7 ± 0.4 l) but was not associated with a difference in either ΔRMS/ΔWR-S2 (SR, 0.14 ± 0.01% W-1; FR, 15 ± 0.02 % W-1) or MDPF (SR, 2.6 ± 5.9 %; FR, -15.4 ± 4.5 %). The close matching between power output and RMS during SR and FR suggests that the ExV˙O2 of heavy exercise is not associated with the recruitment of additional motor units since ExV˙O2 was observed during SR only. Compared to the progressive decrease in MDPF observed during FR, the MDPF remained relatively constant during SR suggesting that either (i) there was no appreciable recruitment of the less efficient type II muscle fibres, at least in addition to those recruited initially at the onset of exercise, or (ii) the decrease in MDPF associated with fatigue was offset by the addition of a higher frequency of type II fibres recruited to replace the fatigued motor units. Experimental Physiology (2002) 87.1, 91-100.

In both medical research and diagnostics characterization of biological tissue on the cellular level relies on high-resolution optical microscopy. In most cases, however, tissue is excised for microscopic investigation, in part because conventional microscopes are bulky instruments. Imaging of cells in the intact living organism has been difficult. Over the last decade several groups have developed miniature confocal microscopes that use fibre optics to deliver light to the specimen and to measure either reflected or excited fluorescence light. In addition, two-photon excitation recently has been employed in a small portable ‘fibrescope’. A potential clinical application of these microscope probes is their endoscopic use for optical biopsy of inner organs or guidance of conventional biopsy. As a mobile research tool they may permit imaging of neuronal activity in the brain of awake, behaving animals. Here, we review technological approaches to build miniaturized fluorescence microscopes and discuss their potential applications.

During acclimatization to the hypoxia of altitude, the cerebral circulation is exposed to arterial hypoxia and hypocapnia, two stimuli with opposing influences on cerebral blood flow (CBF). In order to understand the resultant changes in CBF, this study examined the responses of CBF during a period of constant mild hypoxia both with and without concomitant regulation of arterial PCO2. Nine subjects were each exposed to two protocols in a purpose-built chamber: (1) 48 h of isocapnic hypoxia (Protocol I), where end-tidal PO2 (PET,O2) was held at 60 Torr and end-tidal PCO2 (PET,CO2) at the subject's resting value prior to experimentation; and (2) 48 h of poikilocapnic hypoxia (Protocol P), where PET,O2 was held at 60 Torr and PET,CO2 was uncontrolled. Transcranial Doppler ultrasound was used to assess CBF. At 24 h intervals during and after the hypoxic exposure CBF was measured and the sensitivity of CBF to acute variations in PO2 and PCO2 was determined. During Protocol P, PET,CO2 decreased by 13 % (P < 0.001) and CBF decreased by 6 % (P < 0.05), whereas during Protocol I, PET,CO2 and CBF remained unchanged. The sensitivity of CBF to acute variations in PO2 and PCO2 increased by 103 % (P < 0.001) and 28 % (P < 0.01), respectively, over the 48 h period of hypoxia. These changes did not differ between protocols. In conclusion, CBF decreases during mild poikilocapnic hypoxia, indicating that there is a predominant effect on CBF of the associated arterial hypocapnia. This fall occurs despite increases in the sensitivity of CBF to acute variations in PO2/PCO2 arising directly from the hypoxic exposure. Experimental Physiology (2002) 87.5, 633-642.

Understanding the relationship of the structure of organs to their function is a key component of integrative physiological research. The structure of the organs of the body is not constant but changes, both during growth and development and under conditions of sustained stress (e.g. high altitude exposure and disease). Recently, powerful new techniques have become available in molecular biology, which promise to provide novel insights into the mechanisms and consequences of these altered structure–function relationships. Conventionally structure–function relationships are studied by microscopic examination of tissue sections. However, drawing conclusions about the three-dimensional structure of an organ based on this two-dimensional information frequently leads to serious errors. The techniques of stereology allow precise and accurate quantification of structural features within three-dimensional organs that relate in a meaningful way to integrated function. For example, knowledge of changes in the total surface area of the capillary endothelium in an organ can be related directly to changes in fluid filtration and permeability, or knowledge of total vessel length and mean radius allows deductions about vascular resistance. Confocal microscopy adds enormously to the power of stereological approaches. It reduces the difficulties and labour involved in obtaining suitable images. Moreover, when used in conjunction with new analytical software, it allows convenient application of stereology to small samples and those in which it is essential to maintain a specific orientation for interpretation. The information obtained will allow us to examine in a quantitative manner the altered structure–function relationships produced by manipulation of single genes and regulatory pathways in whole organisms.