To test the hypothesis that angiotensin (ANG) II modulates the arterial baroreflex control of heart rate (HR) in an age-dependent manner, various parameters governing the arterial baroreflex control of HR were assessed before and after removal of endogenously produced ANG II by administration of the angiotensin-converting enzyme (ACE) inhibitor, captopril, to conscious, chronically instrumented lambs aged ~1 week (8 ± 1 days; n = 8) or ~6 weeks (46 ± 5 days; n = 8). After administration of captopril, systolic, diastolic and mean arterial pressures decreased significantly from control levels and HR increased; however, the effects were greater in 1- than in 6-week-old lambs. In 1-week-old lambs, after administration of captopril, there was also a significant increase in the slope coefficient, a decrease in minimum HR and a decrease in the point of maximum gain. In 6-week-old lambs, there were no effects of captopril on any of the parameters governing the arterial baroreflex. Therefore, we accept our hypothesis and conclude that the role of ANG II in modulating cardiovascular homeostasis appears to be more predominant in the newborn than later in life. Experimental Physiology (2003) 88.6, 761-768.

Methods used to quantify the stress response in animals are vital tools in many areas of biology. Here we describe a new method of measuring the stress response, which provides rapid results and can be used in the field or laboratory. After a stressful event, we measure the capacity of circulating leukocytes to produce a respiratory burst in vitro in response to challenge by phorbol myristate acetate (PMA). During the respiratory burst leukocytes produce oxygen free radicals, and the level of production can be measured directly as chemiluminescence. When in vitro PMA-stimulated whole blood chemiluminescence is measured directly after a stressful event, we define the response as the leukocyte coping capacity (LCC). In an experiment badgers (Meles meles), which were caught as part of an on-going population study, were either transported to a central site prior to blood sampling or blood was collected at their site of capture. Transported animals had a significantly lower LCC and showed changes in leukocyte composition that were indicative of stress. We conclude that the stress of transport reduced LCC in badgers and that LCC serves as a quantitative measure of stress. Potential applications of this method are discussed. Experimental Physiology (2003) 88.4, 541-546.

Cardiac hypertrophy is a general term signifying an increase in cardiac mass in response to applied stress. In mild, early hypertrophy, cardiac myocyte contractile performance may be normal or enhanced, whereas in severe hypertrophy associated with cardiac failure, myocyte contraction is reduced in amplitude and increased in duration. In contrast to the varied contractile response, the duration of electrical excitation shows similar changes in both mild and severe hypertrophy. Action potential duration in mid-myocardial and sub-epicardial layers is increased, which is associated with ventricular arrhythmias (in a similar manner to the long QT syndromes from other causes), based on afterdepolarizations and enhanced automaticity. Single-cell studies following exercise training in animal models show that exercise-induced cardiac hypertrophy displays features similar to mild, compensated hypertrophy from other causes. Developed shortening of unloaded single cells is increased or unchanged, and developed force in single myocytes is enhanced. Action potential duration is increased, apart from in the sub-endocardial layer. As with mild hypertrophy from other causes, this will be pro-arrhythmic because of altered dispersion of repolarization and enhanced automaticity. Major abnormalities of the ECG in man include frequent and complex ventricular ectopy, ST segment changes and prolongation of repolarization. In this review a case is presented for regarding exercise-induced cardiac hypertrophy as being no different from mild cardiac hypertrophy resulting from other, pathological causes. The cellular electrophysiological changes are sufficient to account for many of the abnormalities of the ECG, including high-grade ventricular ectopy. Sudden death in trained athletes who have no evidence of specific heart disease may be a direct consequence of cardiac hypertrophy and altered repolarization. Experimental Physiology (2003) 88.5, 639-644.

Cross-correlation of the discharges of muscle spindle afferents in ankle extensor and flexor muscles has been used to reveal the activity of static gamma (γS) motoneurones innervating chain intrafusal muscle fibres during locomotion. In the anaesthetised cat, the cross-correlation of spindle afferents, jointly innervated by a γS-efferent with chain fibre contacts, showed short duration synchrony (2-8 ms) when the efferent was stimulated repetitively. In pre-mammillary decerebrated cats, the cross-correlograms of discharges of some pairs of spindle afferents showed similar short duration peaks of synchronisation and these were interpreted as being due to a common γS drive to chain intrafusal muscle fibres. The incidence of synchrony was low, and was similar at rest (5 % of pairs) and during treadmill locomotion (7 % of pairs). Phase dependence of synchrony was evident during locomotion in the flexor muscle. The synchrony of muscle spindle afferent discharge is discussed in relation to estimates of the numbers of spindles contacted by individual γS-efferents. Experimental Physiology (2003) 88.2, 285-296.

Regularly performed exercise in the form of endurance training produces a well-established adaptation in skeletal muscle termed mitochondrial biogenesis. The physiological benefit of this is an enhanced performance of muscle when subject to endurance exercise. This is not only of great advantage for athletic endeavours, but it also clearly improves the quality of life of previously sedentary individuals and those involved in injury rehabilitation. Here we review the molecular basis for mitochondrial biogenesis in muscle, from the initial signals arising in contracting muscle, to the transcription factors involved in mitochondrial and nuclear DNA transcription, as well as the post-translational import mechanisms required for the synthesis of the organelle. We discuss specific protein components associated with reactive oxygen species production, and suggest some questions which remain unanswered with respect to the role of exercise-induced mitochondrial biogenesis in ageing, apoptosis and disease. Experimental Physiology (2003) 88.1, 99-107.

This study was designed to investigate the effect of both hypertension and ageing on the efficiency of glucose metabolism. A 12-sample, 120 min intravenous glucose tolerance test (IVGTT) was applied to 36 rats: two groups of nine young (12 weeks) spontaneously hypertensive and Wistar Kyoto rats (Y-SHR and Y-WKY group, respectively) and two groups of nine old (40 weeks) SHR and WKY rats (O-SHR and O-WKY group, respectively). Insulinaemia and glycaemia data were interpreted in terms of estimates of glucose effectiveness, SG, and insulin sensitivity, SI, provided by the minimal model of glucose kinetics. The possible link between insulin resistance and hypertension was investigated by comparing Y-SHR vs. Y-WKY and O-SHR vs. O-WKY groups. Comparison of O-SHR vs. Y-SHR and O-WKY vs. Y-WKY groups enabled us to investigate the role of age in the development of abnormalities in glucose metabolism. No significant differences (P > 0.05) were observed in the mean SG and SI estimates between SHR and age-matched WKY groups. This finding indicates that exposure of SHR to high blood pressure levels does not necessarily lead to the development of insulin resistance and impaired glucose effectiveness. Similarly, no significant differences (P > 0.05) were observed in SG and SI estimates between old and young SHR and WKY groups. This finding indicates that, in this animal model of hypertension, insulin sensitivity and glucose effectiveness do not even deteriorate with ageing. Experimental Physiology (2003) 88.3, 399-404.

The purpose of this study was to determine the effect of baroreceptor unloading on the sensitivity of the cardiovagal and sympathetic arms of the baroreflex during upright posture. Beat-by-beat R-R interval, arterial blood pressure and cardiac output (Doppler ultrasound), as well as muscle sympathetic nerve activity (MSNA) were recorded during periods in supine (Supine) and 60 deg head-up tilt (HUT) positions (n = 8 volunteers). Cardiovagal baroreflex sensitivity (BRS) was measured by the spontaneous sequence analysis method using systolic blood pressure and R-R interval, while sympathetic BRS was determined using the slope of the linear relationship between decreasing segments of diastolic blood pressure (DBP) and corresponding increases in MSNA. On changing to HUT, mean R-R interval and cardiac output decreased, while mean measures of MSNA, DBP and total peripheral resistance increased (P < 0.05). Cardiovagal BRS decreased from Supine to 60 deg HUT (19 ± 2 ms mmHg-1 versus 7.6 ± 1.2 ms mmHg-1; P < 0.01). In contrast, sympathetic BRS increased from -6.1 ± 1.4 a.u. mmHg-1 in Supine to -14 ± 2 a.u. mmHg-1 in HUT (P < 0.01). Thus, HUT produced differential effects on cardiac versus sympathetic BRS. The data suggest that dynamic baroreflex-mediated cardiovascular control is dominated by sympathetic control during baroreceptor unloading. Experimental Physiology (2003) 88.6, 769-774.

The effect of exposure to hypobaric hypoxia for 4 weeks (oxygen pressure = 106 hPa), equivalent to 5500 m in altitude) on myocardial total lactate dehydrogenase (tLDH) activity and isoform (H and M) composition was comparatively studied in growing (4.5 weeks old) and in adult (4.5 months old) male rats. The consequences of the hypoxia-induced anorexia were checked in growing rats using a pair-fed group. Exposure to hypoxia induced a significant decrease in the H/tLDH ratio in the left (LV) and right ventricle (RV) of growing and adult rats. In adult rats this alteration was mainly a consequence of the significant increase in the specific activity of the M isomer, which resulted in an increase in the overall LDH activity. In contrast, in the LV of young rats exposed to hypoxia, the specific activity of the M isomer was similar to that of normoxic animals while the H isomer activity was significantly lower than in normoxic rats, and the overall LDH activity remained unchanged. These effects were specifically due to hypoxia per se since no significant alterations were observed in pair-fed animals. In the hypertrophied RV, the alteration of H and M isomers following hypoxia was similar to that observed in adults (i.e. no change in H and an increase in M isoform). We conclude that the well-known hypoxia-induced decrease in the H/tLDH ratio is governed by different age-dependent mechanisms. In adult rats, hypoxia may induce in both ventricles a stimulating effect on M isomer expression. In the LV of growing rats this stress could inhibit the H isomer maturation without any effect on the M isomer. In the RV of growing rats this effect could have been counteracted by the growth effect of the hypertrophying process. Experimental Physiology (2003) 88.3, 405-413.

Resistance arteries isolated from patients with critical limb ischaemia (CLI), a hypotensive/hypoperfusion state of the lower leg, have been shown to undergo morphological changes opposite to those observed in hypertension, that is, decreased wall thickness, reduced cross-sectional area and a decreased wall : lumen ratio. The aim of this present study was to use laser scanning confocal microscopy (LSCM) to study intact resistance arteries isolated from patients with CLI, specifically to identify the cellular aspects of the morphological changes identified in ischaemic subcutaneous and skeletal muscle resistance vessels. Using LSCM, a significant reduction in adventitial and medial thickness, cross-sectional area and wall : lumen ratio was confirmed in resistance arteries from both distal ischaemic subcutaneous and skeletal muscle vascular beds when compared with corresponding arteries from the proximal non-ischaemic sites. The cellular composition of the adventitial, medial and intimal layers of these distal ischaemic arteries was significantly different compared with proximal non-ischaemic arteries. These differences in the distal arteries were characterised by hypoplasia in the adventitial and medial layers of the arterial wall and hypertrophy in the intimal layer. The differences observed in both distal ischaemic vascular beds (subcutaneous and skeletal muscle) were similar. Experimental Physiology (2003) 88.4, 547-554.

This review explores the current concepts relating the structural and functional modifications of skeletal muscle mitochondria to the molecular mechanisms activated when organisms are exposed to a hypoxic environment. In contrast to earlier assumptions it is now established that permanent or long-term exposure to severe environmental hypoxia decreases the mitochondrial content of muscle fibres. Oxidative muscle metabolism is shifted towards a higher reliance on carbohydrates as a fuel, and intramyocellular lipid substrate stores are reduced. Moreover, in muscle cells of mountaineers returning from the Himalayas, we find accumulations of lipofuscin, believed to be a mitochondrial degradation product. Low mitochondrial contents are also observed in high-altitude natives such as Sherpas. In these subjects high-altitude performance seems to be improved by better coupling between ATP demand and supply pathways as well as better metabolite homeostasis. The hypoxia-inducible factor 1 (HIF-1) has been identified as a master regulator for the expression of genes involved in the hypoxia response, such as genes coding for glucose transporters, glycolytic enzymes and vascular endothelial growth factor (VEGF). HIF-1 achieves this by binding to hypoxia response elements in the promoter regions of these genes, whereby the increase of HIF-1 in hypoxia is the consequence of a reduced degradation of its dominant subunit HIF-1α. A further mechanism that seems implicated in the hypoxia response of muscle mitochondria is related to the formation of reactive oxygen species (ROS) in mitochondria during oxidative phosphorylation. How exactly ROS interfere with HIF-1α as well as MAP kinase and other signalling pathways is debated. The current evidence suggests that mitochondria themselves could be important players in oxygen sensing. Experimental Physiology (2003) 88.1, 109-119.

In this review the evidence for structural adaptations of the coronary circulation in the healthy adult heart in response to exercise and training is examined. Previously, it was thought that expansion of the coronary arteries and resistance vasculature occurred without angiogenesis. Detailed studies of the time course of coronary vascular remodelling now reveal that capillary proliferation is an integral response to exercise training, but is disguised by concurrent transformation of capillaries into arterioles by 'arteriolarization'. Increases in numbers of small arterioles < 30 µm in diameter are accompanied by expansion in calibre of resistance and large coronary arteries. Stimuli related to increases in blood flow - shear stress and wall tension - and to mechanical deformation of the myocardium - stretch and compression - provide the main impetus for coronary vascular remodelling. Despite the technical difficulties of measuring such parameters in the heart in vivo, intervention studies using specific exercise components such as vasodilator, inotropic and chronotropic manipulations have allowed some insight into the differential regional effects of haemodynamic factors throughout the vascular tree. It remains to establish more clearly the involvement of mediators such as nitric oxide and growth factors in the temporal relationship to endothelial and smooth muscle growth and proliferation so that the endogenous attributes of exercise can be exploited for therapeutic purposes. Experimental Physiology (2003) 88.5, 645-658.

Maximal exercise reduces the cerebral metabolic ratio (O2/(glucose + 1/2lactate)) to < 4 from a resting value close to 6, and only part of this decrease is explained by the 'intent' to exercise. This study evaluated whether sensory stimulation of brain by muscle ischaemia would reduce the cerebral metabolic ratio. In 10 healthy human subjects the cerebral arterial-venous differences (a-v differences) for O2, glucose and lactate were assessed before, during and after three bouts of 10 min cycling with equal workload: (1) control exercise at light intensity, (2) exercise that elicited a high rating of perceived exertion due to a 100 mmHg thigh cuff, and (3) exercise followed by 5 min of post-exercise muscle ischaemia that increased blood pressure by ~ 20 %. Control exercise did not significantly affect the a-v differences. However, during the recovery from exercise with thigh cuffs the cerebral metabolic ratio decreased from a resting value of 5.4 ± 0.2 to 4.0 ± 0.4 (mean ± S.E.M.; P < 0.05) as a discrete lactate efflux from the brain at rest shifted to a slight uptake. Also, following post-exercise muscle ischaemia, the cerebral metabolic ratio decreased to 4.5 ± 0.3 (P < 0.05). The results support the hypothesis that during exercise, cerebral metabolism is influenced both by the mental effort to exercise and by sensory input from skeletal muscles. Experimental Physiology (2003) 88.2, 297-302.

In 48 conscious resting subjects we examined the temporal coupling of heart beat timing and the onset of inspiration (cardioventilatory coupling), and the relationship between coupling and spectral indices of autonomic function. Using the proportional Shannon entropy (SHα) of the RI-1 interval (interval between inspiration and the preceding ECG R wave) as a measure of coupling we detected statistically significant coupling in 32 of the 48 subjects. This was confirmed by visual inspection of time series plots of RI intervals, in which coupling was evident as horizontal banding. Coupling resulted in a significant preference for whole number heart rate/respiratory frequency ratios. The strongest coupling was associated with low ventilatory frequency and high heart rate variability in the high (0.15-0.40 Hz) and low (0.04-0.15 Hz) frequency ranges, but was not related to blood pressure variability, or to a spectral measure of baroreflex sensitivity (α-index, low frequency range). There was no difference in coupling strength between males and females. We have previously described cardioventilatory coupling in spontaneously breathing anaesthetised subjects. The current study extends those observations by demonstrating that the qualitative features of coupling seen during anaesthesia are also observed in the conscious state. We conclude that the role of coupling in normal physiological respiratory control needs to be more widely explored. Experimental Physiology (2003) 88.6, 775-782.

Thyroid hormone (T3) has a profound effect on mitochondrial biogenesis. T3-regulated gene expression is mediated by thyroid hormone receptor (TR) binding to thyroid hormone response elements (TREs). In concert with the action of various coactivators and corepressors this interaction leads to a modulation of the chromatin structure and subsequently to a modulation of gene expression of adjacent target genes. However, as numerous genes are endogenous regulated by T3 whereas a TRE appears to be absent in their regulatory elements, a TR-independent pathway of T3-mediated gene regulation is likely. In this review, we discuss the direct mechanisms of TR-dependent regulation of gene expression on the nuclear and mitochondrial genome by T3. We also summarise recent observations on an indirect mechanism of T3 action via intermediate factor(s). We discuss the regulation of nuclear respiratory factor 1 (NRF-1) and peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α) by T3, suggesting NRF-1 and PGC-1α as attractive candidate factors for an intermediate factor of T3 action in vivo. Experimental Physiology (2003) 88.1, 121-128.

Thyroid hormone (T3) has a profound effect on mitochondrial biogenesis. T3-regulated gene expression is mediated by thyroid hormone receptor (TR) binding to thyroid hormone response elements (TREs). In concert with the action of various coactivators and corepressors this interaction leads to a modulation of the chromatin structure and subsequently to a modulation of gene expression of adjacent target genes. However, as numerous genes are endogenously regulated by T3, and a TRE appears to be absent in their regulatory elements, a TR-independent pathway of T3-mediated gene regulation is likely. In this review, we discuss the direct mechanisms of TR-dependent regulation of gene expression on the nuclear and mitochondrial genome by T3. We also summarise recent observations on an indirect mechanism of T3 action via intermediate factor(s). We discuss the regulation of nuclear respiratory factor 1 (NRF-1) and peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α) by T3, suggesting NRF-1 and PGC-1α as attractive candidates for an intermediate factor of T3 action in vivo.

We tested the hypotheses that: (1) Ca2+ handling and force production would be irreversibly altered in skeletal muscle during steady-state contractions when subjected to severe, prolonged hypoxia and subsequent reoxygenation; and (2) application of the cardio-protective drug trimetazidine would attenuate these alterations. Single, living skeletal muscle fibres from Xenopus laevis were injected with the Ca2+ indicator fura 2, and incubated for 1 h prior to stimulation in 100 µM TMZ-Ringer solution (TMZ; n = 6) or standard Ringer solution (CON; n = 6). Force and relative free cytosolic Ca2+ concentration ([Ca2+]c) were measured during continuous tetanic contractions produced every 5 s as fibres were sequentially perfused in the following manner: 3 min high extracellular PO2 (159 mmHg), 15 min hypoxic perfusion (3-5 mmHg) then 3 min high PO2. Hypoxia caused a decrease in force and peak [Ca2+]c in both the TMZ and CON fibres, with no significant (P < 0.05) difference between groups. However, basal [Ca2+]c was significantly lower during hypoxia in the TMZ group vs. the CON group. While reoxygenation generated only modest recovery of relative force and peak [Ca2+]c in both groups, basal [Ca2+]c remained significantly less in the TMZ group. These results demonstrated that in contracting, single skeletal muscle fibres, TMZ prevented increases in basal [Ca2+]c generated during a severe hypoxic insult and subsequent reoxygenation, yet failed to protect the cell from the deleterious effects of prolonged hypoxia followed by reoxygenation. Experimental Physiology (2003) 88.3, 415-421.

Our recent data have confirmed that maximum oxygen delivery in racing Thoroughbreds is positively correlated to left ventricular mass measured by echocardiography. A similar, but weaker relationship also exists between left ventricular mass and Timeform performance rating in commercial racehorses. The relationship of the Thoroughbred heart to racing success and the special problems that selective breeding for aerobic capacity have had in this species are reviewed in this article. Experimental Physiology (2003) 88.5, 659-663.

The aim of this study was to determine the effect of acute plasma volume expansion on arterial blood-gas status during 6.5 min strenuous cycling exercise comparing six athletes with and six athletes without exercise-induced arterial hypoxaemia (EIAH). We hypothesized that plasma volume expansion could improve arterial oxygen pressure in a homogeneous sample of athletes - those with EIAH. In this paper we have extended the analysis and results of our recently published surprising findings that lengthening cardiopulmonary transit time did not improve arterial blood-gas status in a heterogeneous sample of endurance cyclists. One 500 ml bag of 10 % Pentastarch (infusion condition) or 60 ml 0.9 % saline (placebo) was infused prior to exercise in a randomized, double-blind fashion on two different days. Power output, cardiac output, oxygen consumption and arterial blood gases were measured during strenuous exercise. Cardiac output and oxygen consumption were not affected by acute hypervolaemia. There were group × condition interaction effects for arterial oxygen pressure and alveolar-arterial oxygen pressure difference, suggesting that those with hypoxaemia experienced improved arterial oxygen pressure (+4 mmHg) and lower alveolar-arterial oxygen pressure difference (-2 mmHg) with infusion. In conclusion, acute hypervolaemia improves blood-gas status in athletes with EIAH. The impairment of gas exchange occurs within the first minute of exercise, and is not impaired further throughout the remaining duration of exercise. This suggests that arterial oxygen pressure is only minimally mediated by cardiac output. Experimental Physiology (2003) 88.4, 555-564.

Regulation of energy metabolism is one of the major functions of steroid hormones. In this process, mitochondria, by way of oxidative phosphorylation, play a central role. Depending on the energy needs of the cell, on the tissue, on the developmental stage and on the intensity of the hormonal stimulus, the response can be an activation of pre-existing respiratory chain components, an increased transcription of nuclear-encoded and/or mitochondrial-encoded respiratory chain enzyme (OXPHOS) genes and of biosynthesis of the respective enzyme subunits or, in extreme cases of high energy needs, an increase in the number of mitochondria and mitochondrial DNA content per cell. Some of the hormonally regulated systems involving effects on nuclear and mitochondrial OXPHOS genes are reviewed in this paper. The possible molecular mechanisms of steroid hormone action on nuclear and mitochondrial gene transcription and possible ways of coordination of transcription in these two separate cell compartments involving direct interaction of steroid receptors with hormone response elements in nuclear OXPHOS genes and in mitochondria and induction/activation of nuclear-encoded regulatory factors affecting mitochondrial gene transcription are presented. Experimental Physiology (2003) 88.1, 129-140.

Metabolic heat production (calculated from oxygen consumption), dry heat loss (measured in a calorimeter) and body temperature (measured by telemetry) were recorded simultaneously at 6 min intervals over five consecutive days in rats maintained in constant darkness. Robust circadian rhythmicity (confirmed by chi square periodogram analysis) was observed in all three variables. The rhythm of heat production was phase-advanced by about half an hour in relation to the body temperature rhythm, whereas the rhythm of heat loss was phase-delayed by about half an hour. The balance of heat production and heat loss exhibited a daily oscillation 180 deg out of phase with the oscillation in body temperature. Computations indicated that the amount of heat associated with the generation of the body temperature rhythm (1.6 kJ) corresponds to less than 1 % of the total daily energy budget (172 kJ) in this species. Because of the small magnitude of the fraction of heat balance associated with the body temperature rhythm, it is likely that the daily oscillation in heat balance has a very slow effect on body temperature, thus accounting for the 180 deg phase difference between the rhythms of heat balance and body temperature. Experimental Physiology (2003) 88.3, 423-429.