Contents Page

Introduction 1

Cardiac sympathetic imbalance and arrhythmia 2

Nitric oxide-cGMP pathway and the cholinergic modulation of cardiac excitability 3

Nitric oxide-cGMP pathway and the sympathetic modulation of cardiac excitability 7

Functional significance of nitric oxide in the autonomic regulation of cardiac excitability 9

Summary 9

References 10

Experimental Physiology (2001) 86.1, 1-12.

The dynamics and kinetics of thyroid hormone transport in the isolated rat heart were examined using the modified unidirectional paired tracer dilution method. The uptake of 125I-thyroxine (125I-T4) and 125I-triiodothyronine (125I-T3) from the extracellular space into heart cells was measured relative to the extracellular space marker 3H-mannitol. The thyroid hormone maximal uptake was 54.4 % for 125I-T4 and 52.15 % for 125I-T3. The thyroid hormone net uptake was 25.69 % for 125I-T4 and 25.49 % for 125I-T3. Backflux from the intracellular space was 53.17 % for 125I-T4 and 61.59 % for 125I-T3. In the presence of unlabelled thyroid hormones, 125I-T4 and 125I-T3 maximal uptakes were reduced from 10.1 to 59.74 % and from 34.6 to 65.3 %, respectively, depending on the concentration of the unlabelled hormone, suggesting a saturable mechanism of the thyroid hormone uptake by the heart cells, with Km(T4)= 105.46 µM and the maximal rate of 125I-thyroid hormone flux from the extracellular space to heart cells (Vmax(T4)) = 177.84 nM min-1 for 125I-T4 uptake, and Km(T3) = 80.0 µM and Vmax(T3) = 118.5 nM min-1 for 125I-T3 uptake. Experimental Physiology (2001) 86.1, 13-18.

Intense exercise or myocardial ischaemia can significantly increase both the concentration of extracellular potassium ([K+]o) and cardiac sympathetic nerve activity. Since changes in [K+]o modulate membrane currents involved in sino-atrial node pacemaking, in particular the voltage-sensitive hyperpolarization-activated current (If), we investigated whether raised [K+]o (from 4 mM to 8 or 12 mM) could directly affect the heart rate response to cardiac sympathetic nerve stimulation (SNS). In the isolated guinea-pig atrial-right stellate ganglion preparation, raised [K+]o significantly decreased the maximum diastolic potential, amplitude and maximum rate of rise of the upstroke of sino-atrial node pacemaker action potentials in 8 and 12 mM [K+]o (P < 0.05). At 12 mM [K+]o these effects were associated with significant decreases in baseline heart rate (4 mM [K+]o = 187 ± 5 beats min-1 (bpm); 12 mM = 144 ± 11 bpm; P < 0.05) and the heart rate response to SNS (1, 3 and 5 Hz; P < 0.05). A 10 % increase in the baseline heart rate with sympathetic activation (3 Hz) was associated with a significant enhancement of the slope of the pacemaker diastolic depolarization at 4 mM [K+]o (increased by 16 ± 6 %; n = 7; P < 0.05), but not with raised [K+]o. When the If current was blocked with 2 mM caesium (n = 8), 12 mM [K+]o had no effect on baseline heart rate and the heart rate response to 3 Hz SNS. The heart rate response to bath-applied noradrenaline (0.01-100 µM) was significantly attenuated by 12 mM [K+]o (at 4 mM [K+]o, EC50 = -6.31 ± 0.18; at 12 mM [K+]o, EC50 = -5.80 ± 0.10; n = 6, ANOVA, P < 0.05). In conclusion, extreme physiological levels of [K+]o attenuate the positive chronotropic response to cardiac sympathetic activation due to decreased activation of the If current. This is consistent with raised [K+]o protecting the myocardium from potentially adverse effects of excessive noradrenaline. Experimental Physiology (2001) 86.1, 19-25.

We have investigated the effect of acidosis on the ECG in isolated rat heart to determine whether acidosis has marked effects on the ECG, and have used pharmacological agents to investigate possible mechanisms whereby acidosis alters the ECG. Acidosis produced a marked decrease in heart rate and an increase in P-R interval with little apparent effect on the duration of the QRS complex. The effects of acidosis did not appear to be due to acidosis-induced changes in transmitter release from severed autonomic nerve terminals within the heart. Experimental Physiology (2001) 86.1, 27-31.

The interstitial fluid pressure (Pisf) has been measured in the exposed superfused mesenteries of anaesthetised rats using the micropipette servo-null technique. When mesenteries were superfused with Ringer-Locke solutions, Pisf was close to atmospheric pressure with mean ± S.E.M. values of -0.46 ± 0.14 cmH2O (n = 22). Superfusing with paraffin oil did not alter Pisf significantly, but Pisf could be lowered considerably by removing fluid from the upper surface of the mesentery. Measurements of Pisf were also made in the tissues immediately outside mesenteric venules as the pressure inside these vessels and the filtration of fluid through their walls was varied. No significant changes in perivascular Pisf could be detected even though the intravascular pressure varied from 20 to 70 cmH2O. Addition of histamine or the mast cell degranulating agent compound 48/80 to the superfusate had no significant effect on Pisf. The findings are relevant to experiments on the permeability of single perfused mesenteric microvessels. They strengthen the assumption, which is made in these studies, that Pisf is close to atmospheric pressure and does not change significantly with changes in the filtration and reabsorption of fluid through the vessel walls. Experimental Physiology (2001) 86.1, 33-38.

The intracellular concentration of cholesterol is regulated by the balance between endogenous synthesis and exogenous uptake. Oestrogens have been reported to be involved in the physiological regulation of cellular cholesterol content. Relevant reports have focused on long-term responses and there is a lack of information about the relationship between the timing of the oestrogen effect and the regulation of cholesterol homeostasis. The aim of this work has been to set up a systematic picture of the short-term effects induced by oestrogen on hepatic lipid metabolism in vivo and the involvement of some relevant signal transduction pathways. At intervals after oestrogen administration (30 min to 6 h), oestrogen receptor expression and changes in liver cAMP, IP3 and protein kinase C-α (PKC-α) were followed. Changes in the expression of the low density lipoprotein receptor at mRNA and protein levels, and of hydroxy-methyl-glutaryl-CoA reductase activity have been verified. At the same time, the content of hepatic cholesterol, ubiquinone and dolichol and of plasma cholesterol have been determined. Changes of rab 5 and rab 8, small GTP-binding prenylated proteins involved in the transfer of neosynthesised proteins through the cell, have been also checked. In vivo treatment with oestradiol produced no change in cyclic AMP but a rapid increase in IP3, increased PKC-α localisation on the membranes and enhanced expression of the low density lipoprotein receptor in the liver occurred. PKC inhibition completely prevented any increase in low density lipoprotein receptor mRNA in isolated and perfused rat liver. Early changes of ubiquinone and dolichol content and a later reduction in hepatic hydroxy-methyl-glutaryl-CoA reductase activity and plasma cholesterol content were also detectable. A functional role of the IP3 -protein kinase C-α pathway in the induction of the low density lipoprotein receptor is suggested. Experimental Physiology (2001) 86.1, 39-45.

An investigation was undertaken into the potential role of nitric oxide (NO) and its interaction with renal sympathetic nerves in mediating renal responses to acute saline volume expansion (VE). Groups of anaesthetized Wistar rats with innervated and denervated kidneys were subjected to VE, 0·25 % body wt min-1 for 40 min, in the presence and absence of nitric oxide synthase (NOS) inhibitors, NG-nitro-{fontsize L-arginine-methyl-ester ({fontsize L-NAME, non-selective), aminoguanidine (AG, relatively selective for inducible NOS (iNOS)), and 7-nitroindazole (7-NI, relatively selective for neuronal NOS (nNOS)). Pretreatment with {fontsize L-NAME or AG enhanced the cumulative sodium excretion (CuUNaV) after 40 min VE in the innervated kidneys by 27 and 23 % (both P < 0·001), respectively, compared to the untreated control group, whereas they were without effect in the denervated kidneys. Cumulative urine flow (CuUV) after VE in {fontsize L-NAME- and AG-treated groups was enhanced in both kidneys, by some 17-21 % in the denervated (P < 0·01) and 37-39 % in the innervated kidneys (P < 0·001) by comparison with the corresponding untreated controls. 7-NI had no effect on CuUV, but reduced CuUNaV in the denervated kidneys by 25 % (P < 0·01) when compared to the control group. The results suggested that NO, possibly generated by endothelial NOS (eNOS) and iNOS, was a contributory factor in mediating the renal response to VE. There appeared to be a tonic inhibitory action of NO on water excretion which was renal nerve independent, whereas its impact on sodium handling appeared to be dependent upon a background level of renal nerve activity. Experimental Physiology (2001) 86.1, 47-54.

Increased fetal lung expansion, induced by tracheal obstruction (TO), is a potent stimulus for fetal lung growth, but rapidly reduces surfactant protein (SP) mRNA levels. Our aim was to determine the time course for the re-expression of the surfactant proteins in fetal lung tissue following the release of a TO and to relate these to the changes in lung liquid volume. Fetal sheep were exposed to either: (1) no treatment (controls); (2) 4 days of TO; (3) 4 days of TO, followed by release of the obstruction for 24 h; (4) 4 days of TO followed by release of the obstruction for 3 days. Four days of TO increased lung liquid volumes from 26.8 ± 1.9 to 72.0 ± 5.6 ml kg-1 and reduced SP-A, SP-B and SP-C mRNA levels to 38.5 ± 10.7, 56.8 ± 10.3 and 18.3 ± 5.3 % of control values, respectively. One day after TO release, lung liquid volumes were reduced to 17.4 ± 5.3 ml kg-1 (control 128 days, 31.0 ± 3.8 ml kg-1) and SP-A and SP-B mRNA levels were not different from control levels. In contrast, SP-C mRNA levels only increased to 45.4 ± 17.3 % of control. Three days after TO release, lung liquid volumes increased to 48.0 ± 8.5 ml kg-1 and SP-A and SP-B mRNA levels were reduced to 48.8 ± 10.2 % and 71.5 ± 19.8 % of control, respectively; SP-C mRNA levels remained at 35.3 ± 12.3 % of control. Following the release of a TO, SP-A, SP-B and SP-C mRNA levels were closely and inversely related to the volume of lung liquid. Based on these relationships, the lung liquid volumes that equate to 100 % expression were considerably less than control lung volumes (< 10 vs. 30-40 ml kg-1) in fetuses of this age. Thus, the changes in fetal lung SP-A, SP-B and SP-C mRNA levels following the release of a TO are variable, differ between the proteins and are closely related to the changes in lung liquid volumes. We conclude that the re-expression of surfactant proteins following TO is variable and that the change in lung liquid volume is potentially a good indicator for surfactant protein re-expression. Experimental Physiology (2001) 86.1, 55-63.

We examined the effect of time after birth and β3-adrenergic agonist (Zeneca D7114) administration on uncoupling protein-1 (UCP1) abundance and thermoregulation in the lamb. Forty twin lambs, all born normally at term, were maintained at a cold ambient temperature of between 3 and 8 °C. At 0.5, 1.75, 5.25, 11.25 and 23.25 h after birth eight sets of twins were fed 20 ml of formula milk ± 10 mg kg-1 of β3-adrenergic agonist, and 45 min after feeding brown adipose tissue (BAT) was sampled. Colonic temperature was measured and BAT analysed for UCP1 abundance, GDP-binding to mitochondrial protein (i.e. thermogenic activity) and catecholamine content. Colonic temperature declined between 1.25 and 6 h from 40.2 °C to 39.2 °C and then increased to 39.8 °C at 12 h, but increased after feeding at all ages. UCP1 abundance increased from 1.25 h after birth, to peak at 2 h after birth in controls, compared with 6 h after birth in β3-adrenergic agonist-treated lambs. The level of GDP-binding to mitochondrial protein did not change significantly with age but was increased by β3-adrenergic agonist treatment. The noradrenaline (norepinephrine) content of BAT increased between 1.25 and 12 h after birth, irrespective of β3-adrenergic agonist administration. The total weight of perirenal BAT plus its lipid, protein and mitochondrial protein content declined over the first 6 h of life. UCP1 development continues over the first 24 h of neonatal life, and can be manipulated by β3-adrenergic agonist administration. This may represent one method of improving thermoregulation in newborn lambs. Experimental Physiology (2001) 86.1, 65-70.

Previous studies in fetal sheep have concluded that (a) the vascular AT1 angiotensin II (Ang II) receptor subtype is present in the external umbilical artery, but not in other systemic blood vessels, and (b) carotid arterial rings contract in vitro in response to Ang II. These contractions are blocked by the AT1 specific receptor antagonist losartan. The aim of the present study was to resolve the apparent contradiction of these earlier conclusions, by examining the distribution of Ang II receptor subtypes in different regions of the ovine fetal cardiovascular system, and to find out at what stage in development AT1 receptors first appear. We measured AT1 and AT2 receptors in hearts, carotid arteries, aortae and umbilical vessels from fetal sheep aged 65-144 days (term ~150 days), and in hearts and aortae from lambs, and adult pregnant and non-pregnant ewes. Both AT1 and AT2 receptors were present in aortae of fetuses > 118 days gestation, and carotid arteries of fetuses > 121 days gestation, while in younger fetuses only AT2 receptors were found. The proportion of carotid artery and aortic AT1 receptors increased with age, while the proportion of AT2 receptors decreased. The internal umbilical artery contained both subtypes, but there was no relationship between receptor density and gestational age. The external umbilical artery had only AT1 receptors. The highest density of Ang II receptors was found in the fetal heart where the AT2 subtype predominated. The density of fetal cardiac Ang II receptors declined with age (r = -0.44, P < 0.02) due to the decrease in the AT2 subtype. The density in late gestation fetal hearts was greater than in lamb or adult hearts (P < 0.001). Our study shows that fetal systemic blood vessels contain AT1 receptors, and we have documented for the first time that the appearance of AT1 receptors is both different in different regions of the fetal cardiovascular system and is developmentally regulated. Together with the in vitro contractile studies, this suggests that Ang II can play an important role in fetal blood pressure regulation via AT1 receptors in the fetal systemic vasculature, as well in the umbilicoplacental vessels. Experimental Physiology (2001) 86.1, 71-82.

To determine to what extent exposure to high ambient temperature and feeding level affect tissue energy metabolism in piglets, regional blood flow and oxidative capacity of tissues were evaluated in sixteen 21.8 ± 2.8 kg pigs. At 5 weeks of age, littermates were divided into three groups and acclimated to the treatment for 25 days. One group was reared at 33°C and fed ad libitum (33AL, n = 6) while the other two groups were maintained at 23°C and either pair-fed on the basis of the food consumption of their 33AL littermates (23PF, n = 5), or fed ad libitum (23AL, n = 5). Regional blood flow was determined in conscious pigs by injection of coloured microspheres, which were recovered in different tissues after slaughter. Activities of cytochrome oxidase and cytochrome aa3 content were measured in tissue homogenates of heart, longissimus dorsi and rhomboideus muscles, liver and small intestine. There was decreased blood flow to internal adipose tissue (42 %) and increased blood flow to peripheral tissues (skin, 44 %) and tissues implicated in respiratory evaporative heat loss (diaphragm, 45 %, lungs, 59 %) at 33°C compared to 23°C, which can be viewed as an effective mechanism for increasing heat loss at high temperature. In addition, the concomitant decrease in blood flow (49 %) and slight reduction of oxidative capacities in both muscles at 33°C might contribute to the reduction in thermogenesis, but these effects were also observed when the feeding level was reduced at thermal neutrality (23PF group). In the viscera (intestine, liver), blood flow was decreased in the two groups on a restricted food intake (about 50 % of 23AL), independently of environmental temperature. The results suggest that most of the mechanisms associated with the reduction in energy expenditure during warm acclimation are related to the adaptive reduction in food intake. Experimental Physiology (2001) 86.1, 83-91.

Echocardiography and Doppler measurements were performed to investigate changes in heart dimensions and function during pregnancy, lactation and the non-pregnant, non-lactating (dry) period in dairy goats. Eight Swedish domestic goats (Capra hircus) were studied with two-dimensional (2DE), M-mode and continuous wave Doppler measurements. Cardiac dimensions did not differ between periods. The mean cardiac outputs were 35 and 28 % greater during pregnancy and lactation, respectively, vs. the dry period. Arterial blood pressure (measured by telemetry) did not differ between reproductive periods before echocardiographic measurements, but became elevated during the measurements, with the smallest rise during lactation. Heart rate was elevated during pregnancy and increased further during echocardiographic measurements, but was not elevated during the other periods. The cranial location of the heart influenced the selection of examination window and caused some difficulties in positioning the ultrasound transducer. This may have led to underestimation of the velocity trace integral and therefore to underestimation of cardiac output. However, our values were within the range of these obtained with dye- and thermodilution methods. The goats tolerated the investigations well during lactation and the dry period, but showed increased blood pressure and heart rate during pregnancy. Therefore, the non-invasive and safe echocardiographic-Doppler method appears to be a good alternative for studies of heart dimensions and function during lactation and the dry period in the dairy goat. Experimental Physiology (2001) 86.1, 93-99.

The present study was designed to investigate the effect of progesterone on the coronary circulation and to determine the mechanisms involved. In pigs anaesthetized with sodium pentobarbitone, changes in left circumflex or anterior descending coronary blood flow caused by intravenous infusion of progesterone at constant heart rate and arterial blood pressure were assessed using an electromagnetic flowmeter. In 14 pigs, infusion of 1 mg h-1 of progesterone caused an increase in coronary blood flow without affecting left ventricular dP/dtmax (rate of change of left ventricular systolic pressure) and filling pressures of the heart. In a further four pigs, this vasodilatory coronary effect was enhanced by graded increases in the dose of the hormone of between 1, 2 and 3 mg h-1. The mechanisms of the above response were studied in the 14 pigs by repeating the experiment after haemodynamic variables had returned to the control values observed before infusion. In six pigs, blockade of muscarinic cholinoceptors and adrenoceptors with atropine, propranolol and phentolamine did not affect the coronary vasodilatation caused by progesterone. In the remaining eight pigs, this response was abolished by intracoronary injection of Nω-nitro-L-arginine methyl ester (L-NAME) even when performed after reversing the increase in arterial blood pressure and coronary vascular resistance caused by L-NAME with continuous intravenous infusion of papaverine. The present study showed that intravenous infusion of progesterone primarily caused coronary vasodilatation. The mechanism of this response was shown to involve the endothelial release of nitric oxide. Experimental Physiology (2001) 86.1, 101-108.

The contribution of automatic drives to breathing at rest, relative to behavioural drives such as 'wakefulness', has been a subject of debate. We measured the combined central and peripheral chemoreflex contribution to resting ventilation using a modified rebreathing method that included a prior hyperventilation and addition of oxygen to maintain isoxia at a PET,O2 (end-tidal partial pressure of oxygen) of 100 mmHg. During rebreathing, ventilation was unrelated to PET,CO2 (end-tidal partial pressure of carbon dioxide) in the hypocapnic range, but after a threshold PET,CO2 was exceeded, ventilation increased linearly with PET,CO2. We considered the sub-threshold ventilation to be an estimate of the behavioural drives to breathe (mean ± S.E.M. = 3.1 ± 0.5 l min-1), and compared it to ventilation at rest (mean ± S.E.M. = 9.1 ± 0.7 l min-1). The difference was significant (Student's paired t test, P < 0.001). We also considered the threshold PCO2 observed during rebreathing to be an estimate of the chemoreflex threshold at rest (mean ± S.E.M. = 42.0 ± 0.5 mmHg). However, PET,CO2 during rebreathing estimates mixed venous or tissue PCO2, whereas the resting PET,CO2 during resting breathing estimates Pa,CO2 (arterial partial pressure of carbon dioxide). The chemoreflex threshold measured during rebreathing was therefore reduced by the difference in PET,CO2 at rest and at the start of rebreathing (the plateau estimates the mixed venous PCO2 at rest) in order to make comparisons. The corrected chemoreflex thresholds (mean ± S.E.M. = 26.0 ± 0.9 mmHg) were significantly less (paired Student's t test, P < 0.001) than the resting PET,CO2 values (mean ± S.E.M. = 34.3 ± 0.5 mmHg). We conclude that both the behavioural and chemoreflex drives contribute to resting ventilation. Experimental Physiology (2001) 86.1, 109-116.

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.

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.

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 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.