This study compared the ventilatory response to 20 min of acute isocapnic hypoxia (end-tidal PO2, 50 mmHg) using the technique of dynamic end-tidal forcing in young (Y) and old (O) men. Two groups of non-smoking male subjects (mean ±plusmn; s.d. age: Y, 29.8 ±plusmn; 6.9 years; O, 73.4 ± 2.8 years) with similar body size, normal age-predicted spirometry, and normal moderate levels of physical activity were studied. Compared with baseline ventilation in euoxia (10.79 ± 1.99 and 11.88 ± 0.91 l min-1) both groups responded to the abrupt onset of isocapnic hypoxia with peak ventilatory responses of 22.58 ± 2.60 and 24.56 ± 2.54 l min-1 for Y and O, respectively (not significant, n.s.). Both groups demonstrated a significant increment in neuromuscular drive (i.e. tidal volume (VT)/inspiratory time (TI); 0.46 ± 0.06 to 0.91 ± 0.15 and 0.48 ± 0.06 to 0.91 ± 0.12 l s-1 for Y and O, respectively) with a small (but also significant) change in central timing (TI/total ventilation time (Ttot); 0.38 ± 0.02 to 0.41 ± 0.02 and 0.42 ± 0.02 to 0.45 ± 0.02 for Y and O, respectively). Oxygen sensitivity was assessed using Weil's equation, and gave a hyperbolic factor (A) of 282 ± 75 and 317 ± 72, and using the linear equation: change in expiratory minute volume (ΔVE)/change in arterial O2 saturation (ΔSa,O2) which gave -1.17 ± 0.57 and -1.17 ± 0.42 l min-1 %-1 (n.s.) for Y and O, respectively. After 20 min of sustained isocapnic hypoxia, ventilation declined to 14.29 ± 1.92 and 16.85 ±plusmn; 2.34 l min-1 for Y and O, respectively (n.s.). The acute response to hypoxia was characterised by similar time constants (16.0 ±plusmn; 5.4 and 18.5 ±plusmn; 6.7 s) and time delays (4.8 ±plusmn; 2.1 and 4.6 ±plusmn; 1.9 s) for Y and O, respectively. Thus, the dynamic ventilatory response to acute isocapnic hypoxia is maintained into the eighth decade in a group of habitually active elderly men. Experimental Physiology (2001) 86.1, 117-126.

The effects of carbonic anhydrase inhibition on the responsiveness to CO2 of pressure-sensitive laryngeal receptors were examined in anaesthetised, paralysed cats. Laryngeal CO2-sensitive receptors from the superior laryngeal nerve were selected by their responsiveness to intralaryngeal pressure and to perfusion of solution equilibrated with 9 % CO2. The carbonic anhydrase inhibitor, methazolamide, when given intralaryngeally at 10-4 M, diminished or abolished the responses to the CO2-equilibrated solution in four of six pressure-sensitive receptors. Histochemical staining for carbonic anhydrase activity showed that the larynges perfused with methazolamide had diminished carbonic anhydrase activity, especially on the superficial layers of surface epithelium. Compared to untreated controls, when given intravenously (50 mg kg-1) methazolamide diminished or abolished the responses to CO2 of five of the six fibres studied. Histochemical staining of these larynges showed no carbonic anhydrase reactivity at the sites of laryngeal receptors. These results suggest that the responses to CO2 of laryngeal pressure-sensitive receptors are dependent on the presence of carbonic anhydrase. Inhibition of laryngeal receptor carbonic anhydrase activity by methazolamide is more reliably achieved by systemic rather than by luminal administration. Experimental Physiology (2001) 86.5, 641-649.

Cardiac vagal preganglionic neurones are found in two locations in the medulla oblongata. The neurones of the ventral group near the nucleus ambiguus (NA) have a discharge pattern which reflects strong respiratory and baroreceptor inputs. In contrast the neuronal discharge of the dorsal group near the dorsal vagal motor nucleus (DVMN) is not modulated by either of these inputs. Electrophysiological analysis of these groups has shown that the DVMN group possesses C-fibre axons (conduction velocity, < 2 m s-1) whilst the NA group has B-fibre axons (conduction velocity, 10-30 m s-1). Therefore, selective stimulation of preganglionic inputs to cardiac ganglia is possible using anodal block techniques. The results of these kinds of experiments have shown that both populations have similar functions (related to cardiac chronotropy, dromotropy and inotropy), although the magnitude and time course of the effects differ substantially. Anterograde tracing experiments from brainstem to heart have confirmed these striking differences, as the potent NA group of neurones has a greater divergence of innervation. Both populations project to similar clusters of ganglion cells on the atrial epicardium suggesting functional overlap. On-going levels of sinus arrhythmia produced by vagus activity may depend on the recruitment and relative combination of these two populations of neurones. Much work remains to be done to investigate preganglionic convergence from neurones of the NA and the DVMN to their postganglionic targets on the heart.

Experimental Physiology (2001) 86.6, 797-801.

The contractility of the human uterus is under the fine control of a variety of interacting bioactive agents. During labour, the excitability of the uterus is drastically transformed in comparison with the non-labour state and is manifest at the membrane level via the acivity of uterine ion channels. This article reviews the contribution of potassium (K+) channels to human uterine excitability. Experimental Physiology (2001) 86.2, 255-264.

Neurological testing tools for measuring and monitoring somatosensory function lack resolution and are often dependent on the clinician testing. In this study we have measured perceptual threshold (PT) to electrical stimulation of the skin and compared it with two-point discriminative ability (TPDA) in 12 control subjects. Tests were made on both sides of the body at American Spinal Injury Association (ASIA) key points on seven spinal dermatomes (C3 (neck), C4 (shoulder), C5 (upper arm), C6 (thumb), T8 (abdomen), L3 (knee), L5 (foot)) and in the mandibular (chin) and maxillary (cheek) fields of the trigeminal (V) nerve. Electrical stimulation (0·5 ms pulse width; 3 Hz) was applied via a self-adhesive cathode and an anode strapped to the wrist or ankle. The stimulus intensity was adjusted and PT was recorded as the lowest current at which the subject reported sensation. Sites were tested in random order. Indices for both TPDA and PT differed according to the dermatome tested but there was no correlation between TPDA and PT for any dermatome. There was good correlation between results from equivalent dermatomes on left and right sides for both PT and TPDA. Women frequently had lower mean (± s.e.) PTs and better TPDA than men; differences were significant (P < 0·05) for PT on the knee (women, 1·31 ± 0·15 mA; men, 2·05 ± 0·26 mA) and the foot (women, 2·90 ± 0·19 mA; men, 4·13 ± 0·28 mA) and for TPDA on the thumb (women, 3·8 ± 0·2 mm; men, 7·8 ± 1·3 mm) and the knee (women, 17·8 ± 1·6 mm; men, 27·1 ± 4·0 mm). Four subjects repeated the experiment on another day and the results correlated well with the first test for PT (r2, 0·62) and TPDA (r2, 0·48). PT differs between dermatomes in a predictable way but does not relate to TPDA. PT is easy to measure and may be a useful assessment tool with which to monitor recovery or deterioration in neuropathies, neurotrauma or after surgery. Experimental Physiology (2001) 86.1, 127-130.

Although vagal afferents are active during intrauterine life, vagal denervation does not lead to gross changes in the occurrence of fetal breathing movements. However, lack of volume feedback has profound and deleterious effects on pulmonary compliance and impairs gas exchange during the early neonatal period. Intrathoracic vagal denervation, which spares vocal cord paralysis and upper airway obstruction in spontaneously breathing unanaesthetised neonatal lambs, leads to reduced pulmonary compliance and persistent hypoxaemia. The underlying mechanisms of these adverse effects include slow breathing frequency, prolonged expiratory times, decreased augmented breaths and the inability to maintain dynamic functional residual capacity leading to progressive atelectasis and regional ventilation- perfusion mismatch. Our data suggest that intrathoracic vagal denervation does not lead to pulmonary oedema in newborn lambs which most probably occurred because of upper airway obstruction in previous studies. We provide evidence that the vagal afferent fibre-receptor system provides critical feedback for the maintenance of normal breathing patterns and pulmonary gas exchange during the early neonatal period.

Experimental Physiology (2001) 86.6, 803-810.

Abnormal loading and distension of the right ventricle may induce arrhythmia through the process of mechanoelectrical feedback. Nonetheless, the electrophysiological effects of right ventricular distension are ill-defined and the mechanisms which underpin mechanoelectrical feedback in the right ventricle are unknown. We examined the effects of changes in right ventricular load (complete occlusion of both caval veins or the main pulmonary artery) in 14 anaesthetised lambs, instrumented with right ventricular surface electrodes and strain gauges for recording monophasic action potential and segment length, and an integrated conductance and micromanometer-tipped catheter for measurement of right ventricular pressure and volume. Caval occlusion did not alter right ventricular segment length and monophasic action potential duration. By contrast, pulmonary arterial occlusion increased the segment length and decreased the monophasic action potential duration at 25, 50 and 70 % repolarisation by 29 ± 6, 22 ± 4 and 17 ± 3 ms, respectively (all P < 0.01). Of the 42 pulmonary arterial occlusions, 38 were associated with early afterdepolarisations (EADs) which increased progressively in magnitude as the occlusion was maintained until, in 32, overt arrhythmia was observed. By contrast, none of the four occlusions in which EADs were not observed resulted in arrhythmia. As a result, the proportion of occlusions which resulted in arrhythmia were greater in those associated with EADs than in those which were not (P = 0.002). Right ventricular distension alters the pattern of repolarisation, precipitates early afterdepolarisations and results in a variety of ventricular arrhythmia, including ventricular tachycardia. Experimental Physiology (2001) 86.5, 651-657.

It is accepted that whilst hormones such as oxytocin, vasopressin and prostaglandin F2α induce myometrial contractions, essentially via an elevation of intracellular calcium, other ligands, such as β-adrenoceptor agonists, calcitonin gene-related peptide, and prostaglandin E2, promote uterine quiescence via their ability to increase intracellular cyclic AMP levels. At present, the exact factors initiating human parturition remain unknown, and labour may occur due to a loss of uterine quiescence, an increase in uterine contractility, or a combination of both. Whilst many studies have aimed to understand the mechanisms underlying uterine contractility there is a relative paucity of data regarding myometrial relaxation. We have verified the presence of mRNA encoding adenylyl cyclase (AC) isoforms I, II, III, V, VI, VII, VIII and IX in both non-pregnant and pregnant human myometrium, and in isolated myometrial cells maintained in cell culture. Furthermore, by means of immunoblotting and immunocytochemistry, we have demonstrated the expression of these isoforms as membrane-associated AC proteins, and identified changes in individual AC isoform expression during gestation. These findings illustrate the diversity of potential cAMP generating pathways in human myometrium, and the complexity of the signal transduction systems underlying uterine quiescence. Experimental Physiology (2001) 86.2, 265-272.

Aside from its role as a hypothalamic stress hormone, corticotrophin releasing hormone (CRH) is also a placental hormone, at least in primates. Although the function of placentally derived CRH remains to be fully elucidated, elevated CRH levels have been associated with premature labour, suggesting that the hormone may be involved in regulating the duration of pregnancy. Indeed, pregnant human myometrium expresses functional CRH receptors (CRH R1 and CRH R2 subtypes) thought to signal predominantly via the second messenger cAMP. Thus, like other cAMP-producing hormones in the myometrium such as β2 agonists, CRH may play a part in maintaining uterine quiescence. However, several of the CRH receptor isoforms identified to date have a reduced ability to activate adenylate cyclase, raising the question as to whether they are linked to other signal transduction pathways. Here, we discuss critically the evidence for the peptide's role in regulating contractility, both directly at the myometrium and indirectly via the fetal membranes and decidua. The possibility of a role in myometrial growth modulation is also described. Experimental Physiology (2001) 86.2, 273-281.

Transcranial magnetic stimulation (TMS) of the human motor cortex was used to study facilitation of motor-evoked potentials (MEPs) in the rectus abdominis (RA) muscle, a trunk flexor, during voluntary activation. MEPs could be produced in the relaxed RA muscles of all six normal subjects studied. The MEPs had short latencies (18-22 ms) which are consistent with other studies suggesting a fast corticospinal input to the trunk muscles. Marked facilitation was observed in the MEPs when subjects were asked to produce graded levels of voluntary contractions. The two tasks used to produce voluntary contractions were a forced expiration during a breath-holding task (FEBH) and bilateral trunk flexion (BTF). Maximal voluntary EMG activity during the BTF task produced around 4.2 times more integrated EMG than during the FEBH task. Similarly the MEP amplitude at MVC was 2.3 times greater during BTF than FEBH. The pattern of MEP facilitation with increasing voluntary EMG was not linear and a maximal MEP amplitude was observed at a level of voluntary contraction around 30 % MVC in both tasks. There were some subtle differences in the pattern of facilitation in the two tasks. When TMS was applied to the right cortex only, MEPs were seen in both left and right RA muscles suggesting some ipsilateral corticospinal innervation. The latency of the right (ipsilateral) response was approximately 2 ms longer than the left. Comparison with studies in hand and leg muscles suggests that the facilitation pattern in RA may reflect a substantial degree of corticospinal innervation. Experimental Physiology (2001) 86.1, 131-136.

We hypothesised that the observed acceleration in the kinetics of exercise on-transient oxygen uptake (V˙O2) of five older humans (77 ± 7 years (mean ± S.D.) following 9 weeks of single-leg endurance exercise training was due to adaptations at the level of the muscle cell. Prior to, and following training, subjects performed constant-load single-limb knee extension exercise. Following training V˙O2 kinetics (phase 2, τ) were accelerated in the trained leg (week 0, 92 ± 44 s; week 9, 48 ± 22 s) and unchanged in the untrained leg (week 0, 104 ± 43 s; week 9, 126 ± 35 s). The kinetics of mean blood velocity in the femoral artery were faster than the kinetics of V˙O2, but were unchanged in both the trained (week 0, 19 ± 10 s; week 9, 26 ± 11 s) and untrained leg (week 0, 20 ± 18 s; week 9, 18 ± 10 s). Maximal citrate synthase activity, measured from biopsies of the vastus lateralis muscle, increased (P < 0.05) in the trained leg (week 0, 6.7 ± 2.0 µmol (g wet wt)-1 min-1; week 9, 11.4 ± 3.6 µmol (g wet wt)-1 min-1) but was unchanged in the untrained leg (week 0, 5.9 ± 0.5 µmol (g wet wt)-1 min-1; week 9, 7.9 ± 1.9 µmol (g wet wt)-1 min-1). These data suggest that the acceleration of V˙O2 kinetics was due to an improved rate of O2 utilisation by the muscle, but was not a result of increased O2 delivery. Experimental Physiology (2001) 86.5, 659-665.

There is much evidence showing that the rapid rise in heart rate at the onset of exercise is due to a withdrawal of cardiac vagal tone. This short review discusses the main afferent mechanisms involved in this effect. In addition to signals from central motor command it is shown that muscle mechanoreceptors of group III afferent fibres also play a significant role. Recent studies in man demonstrating this by stretching muscles such as the triceps surae or by direct compression of muscles are briefly reviewed. The evidence also supports the idea that these small fibre mechanoreceptors inhibit the baroreceptor-heart rate reflex. Several studies suggest that skeletal muscle metaboreceptors (mainly group IV) are more important for increasing cardiac sympathetic and vasoconstrictor nerve activity. At the conclusion of exercise the cessation of mechanoreceptor stimulation is an important factor in determining the rapid return of heart rate to resting level.

Experimental Physiology (2001) 86.6, 811-815.

Until about 40 years ago, it was thought that the parasympathetic innervation of the mammalian heart was confined to supraventricular structures. Hence, neither the vagus nor its primary neurotransmitter, acetylcholine (ACh), were believed to have significant effects on the inotropic state of the ventricles or on their excitability. However, it is now well-established that vagal/muscarinic stimulation prolongs ventricular refractoriness in humans and has a small but distinct negative inotropic effect on the left ventricle, which is accentuated in the presence of elevated sympathetic activity (Löffelholz & Pappano, 1985).

Experimental Physiology (2001) 86.6, 817-823.

A comprehensive understanding of the mechanisms by which agonists control uterine contraction is essential for the successful clinical management of parturition and for the timely treatment of situations involving inappropriate uterine performance. In this review we discuss some of the key stimulatory mechanisms linking receptor occupation at the myometrial plasma membrane with alteration of myofilament activation. We focus on evidence that receptor-induced membranous recruitment of the small G-protein rhoA, and its downstream effector rho-associated kinase (ROK) is crucial to agonist-induced Ca2+-sensitisation of uterine contraction and that co-ordination of this signal transduction pathway may be mediated by the actions of caveolins, proteins integral to specialised membranous regions termed caveolae. Experimental Physiology (2001) 86.2, 283-288.

We compared estimates for the phase 2 time constant (τ) of oxygen uptake (V˙O2) during moderate- and heavy-intensity exercise, and the slow component of V˙O2 during heavy-intensity exercise using previously published exponential models. Estimates for τ and the slow component were different (P < 0.05) among models. For moderate-intensity exercise, a two-component exponential model, or a mono-exponential model fitted from 20 s to 3 min were best. For heavy-intensity exercise, a three-component model fitted throughout the entire 6 min bout of exercise, or a two-component model fitted from 20 s were best. When the time delays for the two- and three-component models were equal the best statistical fit was obtained; however, this model produced an inappropriately low ΔV˙O2/ΔWR (WR, work rate) for the projected phase 2 steady state, and the estimate of phase 2 τ was shortened compared with other models. The slow component was quantified as the difference between V˙O2 at end-exercise (6 min) and at 3 min (ΔV˙O2 (6-3 min); 259 ml min-1), and also using the phase 3 amplitude terms (truncated to end-exercise) from exponential fits (409-833 ml min-1). Onset of the slow component was identified by the phase 3 time delay parameter as being of delayed onset ~2 min (vs. arbitrary 3 min). Using this delay ΔV˙O2 (6-2 min) was ~400 ml min-1. Use of valid consistent methods to estimate τ and the slow component in exercise are needed to advance physiological understanding. Experimental Physiology (2001) 86.5, 667-676.

We examined the effect of caffeine co-ingested with either carbohydrate or fat on metabolism and performance in eight endurance-trained subjects who performed a random order of four experimental trials consisting of 120 min of steady-state ergometer cycling at 70 % of maximal O2 uptake (SS) followed by a time trial in which subjects completed a set amount of work (7 kJ kg-1) as quickly as possible. One hour before SS subjects ingested either 2.6 g kg-1 carbohydrate (CHO); 2.6 g kg-1 CHO + 6 mg kg-1 caffeine (CHO + CAF); 1.2 g kg-1 fat with 2000 U I.V. heparin (FAT); or 1.2 g kg-1 fat with 2000 U I.V. heparin + 6 mg kg-1 caffeine (FAT + CAF). The rate of carbohydrate oxidation was higher (µmol kg-1 min-1: CHO, 243 ± 39 and CHO + CAF, 239 ± 30 vs. FAT, 196 ± 48 and FAT + CAF, 191 ± 55; P < 0.05, values are means ± S.D.) and the rate of fat oxidation lower (µmol kg-1 min-1: CHO, 19 ± 8 and CHO + CAF, 22 ± 7 vs. FAT, 35 ± 19 and FAT + CAF, 37 ± 17; P < 0.05) with carbohydrate than fat ingestion. Yet despite lower carbohydrate use with fat feeding, the time taken to complete the time trial was less after carbohydrate than after fat ingestion (min: CHO, 30.37 ± 7.42 and CHO + CAF, 29.12 ± 5.62 vs. FAT, 33.02 ± 8.50 and FAT + CAF, 32.78 ± 7.70; P < 0.05). We conclude that (1) caffeine co-ingested with either carbohydrate or fat meals has no additive effect on substrate utilization or exercise performance and (2) carbohydrate ingestion before exercise improves subsequent time trial performance compared with fat ingestion. Experimental Physiology (2001) 86.1, 137-144.

Orthostatic stress, including standing, head-up tilting and lower body suction, results in increases in peripheral vascular resistance but little or no change in mean arterial pressure. This study was undertaken to determine whether the sensitivity of the carotid baroreceptor reflex was enhanced during conditions of decreased venous return. We studied eight healthy subjects and determined responses of pulse interval (ECG) and forearm vascular resistance (mean finger blood pressure divided by Doppler estimate of brachial artery blood velocity) to graded increases and decreases in carotid transmural pressure, effected by a neck suction/pressure device. Responses were determined with and without the application of lower body negative pressure (LBNP) at -40 mmHg. Stimulus-response curves were determined as the responses to graded neck pressure changes and the differential of this provided estimates of reflex sensitivity. Changes in carotid transmural pressure caused graded changes in R-R interval and vascular resistance. The cardiac responses were unaffected by LBNP. Vascular resistance responses, however, were significantly enhanced during LBNP and the peak gain of the reflex was increased from 1.2 ± 0.3 (mean ± S.E.M.) to 2.2 ± 0.3 units (P < 0.05). The increased baroreflex gain may contribute to maintenance of blood pressure during orthostatic stress and limit the pressure decreases during prolonged periods of such stress. Experimental Physiology (2001) 86.5, 677-681.

Experimental Physiology (2001) 86.6, 825-825.

The oxytocin receptor (OTR) is part of an ancient hormone system expressed in diverse phyla in relation to acute reproductive smooth muscle responses, such as egg-laying, birth, or milk letdown. The regulation of the OTR gene, while correlating with steroid levels in vivo, remains elusive. There appear to be both inhibitory and stimulatory influences acting upon a constitutive pattern of basal expression. We have found no evidence, however, for an effect of the sex steroids either directly on gene transcription, or on the receptor itself at the protein level. In the prostatic carcinoma cell line Du145, we have shown that up-regulation of the OTR gene transcription can be effected by cAMP. In an attempt to characterize the expression of the OTR protein in vivo, we have shown, using ligand-blotting, that the OTR can be expressed at different sizes in transfected cells and in myometrium. Also, in the myometrium at term, immunohistochemistry suggests that there is both an increase in OTR protein per cell, as well as in the number of smooth muscle cells expressing OTR, emphasizing that perinatal changes are the results of both individual gene activation events and gross cellular differentiation. The OTR is a valuable model system reflecting molecular changes in the perinatal period. When we understand how this important molecule is regulated, we will also be a long way towards understanding the mechanisms controlling myometrial contractility at birth. Experimental Physiology (2001) 86.2, 289-296.

Experimental Physiology (2001) 86.6, 826-826.