The specificity in location of angiogenesis to either glycolytic or oxidative fibre types, or muscle regions, was examined in the tibialis anterior (TA) and extensor digitorum longus (EDL) muscles of rat. Angiogenesis was induced by mechanical means either with (chronic muscle stimulation) or without (muscle stretch by overload) changes in blood flow, treatments which invoked only minor changes in fibre type and fibre size. Proliferation estimated by PCNA labelling of cells co-localised with capillaries was very rare in control muscles, where it occurred mainly in the glycolytic regions, but was increased in both models of angiogenesis. However, when labelled capillaries were scored according to the type of surrounding fibres, only muscle stimulation significantly accentuated proliferation of capillaries surrounded by glycolytic fibres. We conclude that while mechanical stimuli are important for proliferation in glycolytic regions in both models, capillary growth occurs specifically around glycolytic fibres in that region when the angiogenic stimulus includes increased blood flow and/or increased metabolic demand. Experimental Physiology (2003) 88.5, 565-568.

The Na+-K+-2Cl- cotransporter (NKCC1) is responsible for ion transport across the secretory and absorptive epithelia, the regulation of cell volume, and possibly the modulation of cell growth and development. It has been reported that a variety of cells, including osteoblasts, contain this cotransporter. In this study, the physiological role of NKCC1 in osteoclastogenesis was exploited in a co-culture system. Bumetanide, a specific inhibitor of NKCC1, reduced the number of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells. In order to investigate the mechanism by which bumetanide inhibits osteoclastogenesis, the mRNA expressions of the receptor activator of nuclear factor (NF)-κB ligand (RANKL) and osteoprotegerin (OPG) were analysed by RT-PCR. Exposure of osteoblastic cells to a medium containing 1 µM bumetanide reduced RANKL mRNA expression induced by 10 nM 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3, in a dose-dependent manner. In addition, RANKL expression was also analysed with enzyme-linked immunosorbant assay (ELISA) using anti-RANKL antibody. The expression of RANKL was decreased with the increase of bumetanide concentration. In contrast, the expression of OPG mRNA, a novel tumour necrosis factor (TNF) receptor family member was increased in the presence of bumetanide. These results imply that bumetanide inhibits osteoclast differentiation by reducing the RANKL/OPG ratio in osteoblastic cells. However, no significant difference in M-CSF mRNA expression was observed when bumetanide was added. Also, we found that the phosphorylation of c-Jun NH2-terminal kinase (JNK), which regulates the activity of various transcriptional factors, was reduced by bumetanide treatment. Conclusively, these findings suggest that NKCC1 in osteoblasts has a pivotal role in 1α,25(OH)2D3-induced osteoclastogenesis partly via the phosphorylation of JNK. Experimental Physiology (2003) 88.5, 569-574.

The effect of glucagon on renal haemodynamics in sheep is controversial. In this study we have examined the effects of perfused glucagon on renal blood flow (RBF) in six conscious sheep bilaterally implanted with transit-time ultrasonic flow metering probes around the renal arteries. Glucagon was perfused intravenously over 90 min at doses of 3.12, 6.25, 12.5, 25, 50 and 100 ng kg-1 min-1. Mean RBF was calculated over 10 min periods. Blood samples were taken to monitor the time course of the changes in glycaemia and glucagonaemia. The perfusions of glucagon induced rapid and progressive dose-dependent increases in RBF (9-19.2 %, P < 0.05) and glycaemia (29-155 %, P < 0.05) for doses of 25-100 ng kg-1 min-1. High positive correlations were found between the increases in RBF and glucagonaemia (R2 = 0.95) and between the increases in RBF and glycaemia (R2 = 0.96). At the lowest doses of glucagon (3.12-12.5 ng kg-1 min-1), the increase in RBF was highly significant; however, the rise in glucose level was not. At the highest doses (25-100 ng kg-1 min-1) the time course of the changes in RBF was parallel to that of glucagonaemia throughout the perfusion time. However, between minutes 45 and 90 of the glucagon perfusion, the increase in RBF was the inverse of the change in glycaemia, which decreased. One hour after the end of the 50 and 100 ng kg-1 min-1 perfusions, the glucose levels were still significantly higher than the baseline, while the RBF values were not. These results are consistent with the idea that the enhanced RBF cannot be attributed to a rise in blood glucose level. They also show that the haemodynamic response to glucagon perfusion was more sensitive than the metabolic response. It is concluded that the intravenous perfusion of physiological doses of glucagon induced a highly sensitive dose-dependent increase in RBF in sheep. Experimental Physiology (2003) 88.5, 575-580.

The aim of our study was to develop and validate a simple surgical model in the sheep which allows control of the gas composition of the blood supplying the carotid and central chemosensitive area independently of the rest of the body. This approach was made possible due to the specific features of the cranial circulation in the sheep. An extracorporeal circuit, consisting of a pump and a gas exchanger, was placed at the level of the two common carotid arteries to create a pressure gradient between the carotid and the systemic systems and to reverse blood flow in the vertebral vessels via the occipital arteries. When a pressure gradient of about 40 Torr was created between the systemic and carotid circulation, we found that no blood could reach the carotid bodies and the medulla without passing though the extracorporeal circulation. This was established (1) by measuring vertebral blood flow; and (2) by injecting either a coloured suspension or particles labelled with 99m*Tc into the systemic or the carotid circulation. The slope of the relationship between minute ventilation (V˙E) and systemic arterial PCO2 (Pa,CO2) during high CO2 inhalation in seven hyperoxic vagotomised and anaesthetised sheep was dramatically reduced, but remained above zero, when Pa,CO2 was maintained constant in the cephalic circuit (0.11 ± 0.15 vs. 0.70 ± 0.35 l min-1 Torr-1 for the control tests). This residual V˙E response to CO2 inhalation remains to be explained since it could not be accounted for by any of the chemical or circulatory changes occurring in the cephalic circulation. Nevertheless, this preparation provides an easy method of maintaining chemical and circulatory homeostasis at the chemoreceptor level. Experimental Physiology (2003) 88.5, 581-594.

Although square-wave sustained and R wave-triggered pulsatile stimuli have been used to assess carotid baroreflex (CBR) function in humans, it remains unclear whether these different stimulus protocols elicit comparable responses and whether CBR responses adapt during prolonged stimulation. Thus, we measured muscle sympathetic nerve activity (MSNA), heart rate (HR) and mean arterial pressure (MAP) in response to +30 Torr neck pressure (NP) and -30 Torr neck suction (NS) delivered for 20 s either as a sustained or pulsatile stimulus. CBR-mediated changes in MSNA, HR and MAP were similar with sustained and pulsatile stimuli. The time course of MSNA and HR responses identified that significant changes occurred during the initial 5 s and were better maintained over 20 s with NP than with NS. Changes in MAP exhibited a slower onset with the peak increase during NP occurring at 10 s (sustained, 7 ± 1 mmHg; pulsatile, 7 ± 1 mmHg; P > 0.05) and the nadir during NS occurring at 20 s (sustained, -7 ± 1 mmHg; pulsatile, -9 ± 2 mmHg; P > 0.05). These data demonstrate that sustained and pulsatile NP and NS produce comparable CBR-mediated responses. Furthermore, despite MSNA and HR returning towards baseline during NS, CBR-mediated changes in MAP are well maintained over 20 s of NS and NP. Experimental Physiology (2003) 88.5, 595-601.

The aims of this study were: (1) to determine the within-subject, between-day variability in end-tidal PCO2 (PET,CO2); (2) to determine whether the within-subject, between-day variability in PET,CO2 was caused by variations in metabolic acid-base status; and (3) to determine whether between-subject variations in blood gas variables arose predominantly through variations in respiratory or renal pH control mechanisms. Fourteen healthy males were studied, of whom 12 provided adequate data for further analysis. Each subject was studied on at least six different occasions, with each visit separated by at least 1 week. On each occasion, PET,CO2 was determined over a 4-10 min period using a fine nasal catheter taped just inside the nose, and an arterialised capillary blood sample was obtained from each ear and analysed for blood gases. The following results were obtained. (1) PET,CO2 showed a standard deviation (S.D.) for the within-subject, between-day variation of 1.06 mmHg. (2) Less than 5 % (P = NS) of the variability in PET,CO2 could be explained by underlying variations in metabolic acid-base status. (3) The variation in blood gas values between individuals did not fit a pattern associated with either a predominantly respiratory or a predominantly renal origin. Furthermore, the pattern of variation in the blood gas data suggested that variations in the renal controller for pH and the respiratory controller for pH were not independent. The precise origins of variability in blood gas regulation remain obscure. Experimental Physiology (2003) 88.5, 603-610.

The activation of cardiovascular reflexes for postural adaptation questions whether, in healthy humans, the central blood volume is optimised to support the upright position. A functional definition of an 'optimal circulating volume' that provides the heart with enough central blood volume to establish a maximal cardiac output (CO) and mixed venous oxygen saturation (Sv,O2) at rest was evaluated in nine healthy subjects. Preload to the heart was varied by passively changing the body position from 70 deg head-up to 20 deg head-down tilt. The Sv,O2 was compared with simultaneously measured estimates of CO by computer-controlled thermodilution. The CO was in the range 8.7-3.8 l min-1 and Sv,O2 was in the range 79-58 %. Neither CO (median 6.0 (range 5.3-8.7) l min-1) nor Sv,O2 (mean ± S.D. 73.6 ± 2.6 %) changed from the supine to the 20 deg head-down position. During sustained 70 deg head-up tilt, Sv,O2 decreased to 64 ± 4 % together with a decline in CO to 4.7 (3.9-5.6) l min-1 (P < 0.05). Under conditions of varying tilt angles, a change in CO is paralleled by concordant changes in Sv,O2. Maximal values for CO and Sv,O2 during supine rest suggest that the horizontal position provides for an 'optimal' central blood volume. Experimental Physiology (2003) 88.5, 611-616.

The human finger pad is highly vascularized so it might be expected that the on-going cardiac pulsations in the vicinity of a cutaneous mechanoreceptor would be reflected in its spontaneous or evoked discharge. The purpose of this study was to examine the prevalence of this potential cardiac rhythmicity in a random sample of tactile afferents from the finger pad. Unitary recordings were made from 21 rapidly ('fast') adapting (18 FA I; 3 FA II) and 44 slowly adapting (17 SA I; 27 SA II) afferents via tungsten microelectrodes in the median nerve. Skin blood flow was measured over the pulp of a contralateral finger; ECG activity, blood pressure and respiration were also recorded. Cardiac modulation, present either as a simple pulse rhythm or as modulation of an on-going discharge, was expressed by 44 % of the afferents. Only two out of 18 FA I units, and two out of three FA II units, exhibited cardiac rhythmicity, but their temporal coupling to the pulse was very tight. Modulation was more common for the slowly adapting afferents (57 %), and more prevalent among the SA II (65 %) than the SA I (47 %) classes. Nine spontaneously active SA II afferents exhibited respiratory rhythmicity, their background discharge falling during inspiration. It is concluded that cardiac modulation is common for both classes of slowly adapting tactile afferents (but less common for the rapidly adapting afferents), which may have implications for the sensory signalling of tactile information. Experimental Physiology (2003) 88.5, 617-625.

Pituitary hormones have an important role during exercise yet relatively little is known about the stimulus for their release. Body temperature progressively increases during prolonged steady-state exercise in the heat and we have investigated the role that this may play in the release of prolactin, growth hormone and cortisol (as an indicator of adrenocorticotropic hormone) into the circulation. Fit young male subjects exercised at 73 %O2,max until volitional fatigue at 20 °C and at 35 °C (30 % relative humidity at both temperatures). Rectal temperature and mean skin temperature were monitored and blood samples analysed for lactate, glucose, cortisol, growth hormone and prolactin concentrations. During the first 20 min, core temperature rose continuously and to a similar extent at both temperatures, while mean skin temperature was approximately 4 °C lower during exercise in the cool. Blood glucose concentration was essentially constant throughout the period of exercise while lactate concentration increased in the first 10 min and then remained constant with very similar changes in the two exercise conditions. Prolactin and growth hormone concentrations both increased during the exercise period while the concentration of cortisol declined slightly before rising slightly over the 40 min period. Prolactin release was significantly greater when exercise was carried out in the heat while there was no difference in the release of growth hormone or cortisol in the two conditions. When plotted as a function of rectal temperature, growth hormone concentration showed a linear relationship which was the same at ambient temperatures of 35 °C and 20 °C. Prolactin concentration had a curvilinear relationship with rectal temperature and this differed markedly at the two ambient temperatures. Cortisol concentration showed no dependence on any measure of body temperature. Our results are consistent with some aspect of body temperature being a stimulus for growth hormone and prolactin secretion; however, the precise mechanism clearly differs between the two hormones and we suggest that skin temperature modulates prolactin release, but does not affect the release of growth hormone. Experimental Physiology (2003) 88.5, 627-635.

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.

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.

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.

Regular physical training is associated with several physiological and biochemical adaptations which enable an increase in cardiac output and widening of the systemic arterio-venous oxygen difference. An increase in cardiac chamber size is fundamental to the generation of a sustained increase in cardiac output for prolonged periods. Echocardiographic studies have shown that the vast majority of athletes have modest cardiac enlargement although a small proportion exhibit substantial increases in heart size. Recognised determinants of cardiac size include age, sex, ethnicity and type of sport. Cardiac dimensions vary considerably amongst athletes, even when allowances are made for these variables, suggesting that genetic, endocrine and biochemical factors also influence heart size. This review discusses the effects of age, sex, ethnicity and sporting discipline on cardiac dimensions in athletic individuals. Experimental Physiology (2003) 88.5, 665-669.