In the Dahl salt-sensitive hypertensive rat, a diet containing L-arginine, the natural substrate for nitric oxide synthase, abrogates the hypertension. We postulated that nitric oxide synthase inhibition might induce a salt-sensitive form of hypertension and that this salt sensitivity might be linked to a loss of the regulatory effect of sodium ingestion on angiotensin II (Ang II) and angiotensinogen. Male Wistar-Kyoto rats were randomised to a diet containing 0.008 %, 2.2 % or 4.4 % sodium chloride and to treatment with the NO synthase inhibitor L-NAME (10 mg kg-1 day-1) in the drinking water, or drinking water alone (Controls) for 4 weeks. Blood pressure was measured by tail cuff plethysmography twice weekly. After 4 weeks, the rats were anaesthetised and truncal blood collected for determination of angiotensinogen, renin, angiotensin I (Ang I), Ang II and aldosterone concentrations as well as angiotensin-converting enzyme (ACE) activity. Systolic blood pressure increased with increasing dietary sodium intake in the L-NAME-treated rats (P < 0.05). Plasma renin and aldosterone concentrations decreased with increasing dietary sodium intake in both Control and L-NAME-treated rats. Ang I and ACE activity were unchanged by increasing dietary sodium intake. In contrast, the plasma concentration of Ang II and angiotensinogen increased with increasing dietary sodium (P < 0.05 and P < 0.005, respectively). Treatment with the Ang II receptor blocker, losartan, reversed the blood pressure increase. We conclude that treatment with L-NAME induces an increase in blood pressure that is at least in part salt sensitive. Further, the salt-sensitive component appears to be Ang II-dependent, as it was associated with increasing plasma Ang II levels and could be reversed by treatment with an Ang II receptor antagonist. Experimental Physiology (2002) 87.1, 1-8.

The renal response to arginine vasopressin in the rat has been shown to depend on reproductive status. However there is no consensus as to when the kidney is most responsive. The varying results could depend on the protocol and the dose of hormone used. A study has been performed, with physiological doses of vasopressin, comparing the responses during infusion of hypotonic saline and glucose. After an equilibration period of 150 min, conscious rats were infused on each of the four days of the oestrous cycle with either isotonic saline (0.077 M) or 0.14 M glucose for a control period of 45 min. Vasopressin was then infused at 10-40 fmol min-1 for 1 h, followed by a recovery period of 90 min. Timed urine samples were collected for determination of volume, sodium concentration and osmolality. During the control period urine flow was greatest at oestrus and dioestrus day 2 and sodium excretion on dioestrus day 2 irrespective of the infusate. Vasopressin concentrations achieved lay within the physiological range and no difference was observed between the different days for a given dose. Infusion of vasopressin in both saline and glucose produced a dose-dependent antidiuresis, the greatest responses being seen of pro-oestrus and dioestrus day 2. It was only with the highest rate of infusion that a significant increase in sodium excretion was seen on each day of the cycle and the greatest responses were seen on pro-oestrus and dioestrus day 1 for both infusates. Thus the kidney shows the greatest response to physiological doses of vasopressin at pro-oestrus and dioestrus day 1 irrespective of the infusate employed. Experimental Physiology (2002) 87.1, 9-15.

Experiments were carried out to determine if endogenous pyrogen-induced fever impairs protective responses of newborn rats to hypoxia. Twenty-seven 5- to 6-day-old conscious rat pups received a subcutaneous injection of 0.20 µg of recombinant rat interleukin-1β (rrIL-1β) per kilogram of body weight to induce fever, or an equal volume of vehicle. They were then either exposed to a single period of hypoxia produced by breathing an anoxic gas mixture (97 % N2-3 % CO2) and their time to last gasp was determined, or they were exposed repeatedly to hypoxia and their ability to autoresuscitate from primary apnoea was determined. Core temperature increased significantly following administration of rrIL-1β but did not change following administration of vehicle (i.e. vehicle, 0.0 ± 0.1 °C; rrIL-1β, 0.7 ± 0.3 °C; P < 0.001) before exposure to hypoxia. IL-1β-induced fever did not alter the time to last gasp when the pups were exposed to a single period of hypoxia or the number of successful autoresuscitations upon repeated exposure to hypoxia. Thus, our data do not support the hypothesis that endogenous pyrogen-induced fever impairs the protective responses in newborns that may prevent death during hypoxia as may occur during single or repeated episodes of prolonged sleep apnoea. Experimental Physiology (2002) 87.1, 17-24.

The paraventricular nucleus of the hypothalamus plays a pivotol role in the regulation of plasma volume. Part of the response to an increase in volume load is an inhibition of renal sympathetic nerve activity. The present experiments were designed to determine which subnuclei of the paraventricular nucleus are involved in this sympatho-inhibitory response. Experiments were performed on anaesthetised rats. Activated neurones were recognised by the expression of the early gene c-fos, identified by immunohistochemical labelling of its protein product Fos. Plasma volume loading with 4 % Ficoll 70, using an infusion-withdrawal procedure (2 ml over 1 min) repeated 15 times over 1 h revealed a total of 775 ± 101 (n = 6) Fos-positive neurones scattered throughout both the magnocellular and parvocellular subnuclei. In comparison, sustained hypertension resulted in 452 ± 56 (n = 3) Fos-positive neurones similarly distributed, whereas a normotensive control group (n = 3) displayed 115 ± 18 Fos-positive neurones. Because of this lack of a specific effect we used a more selective stimulation of right atrial receptors via a balloon placed at the junction of the superior vena cava and the right atrium so it did not impede venous return. Inflation of the balloon inhibited renal sympathetic nerve activity (36 ± 5 %, n = 7) and repetitive inflation over 1 h resulted in c-fos activation of a small number of neurones (54 ± 14) located only in the parvocellular subnuclei. Whether these are inhibitory interneurones acting within the paraventricular nucleus, or spinally projecting neurones which inhibit or excite renal sympathetic activity by an action in the spinal cord remains to be determined. Experimental Physiology (2002) 87.1, 25-32.

The sympathetic nervous system is believed to play a major role in regulating cardiovascular function during exercise. However, only a few direct measurements of sympathetic nervous activity during whole body dynamic exercise have been attempted. In the present study, we have established a method to allow routine measurement of renal sympathetic nerve activity (RSNA) and cardiovascular function during treadmill exercise in rats. We trained Wistar rats to run on the treadmill for a week before the surgery. At least 2 days before the experiment, electrodes for recording RSNA, electrocardiogram and electromyogram, and catheters for the measurements of systemic arterial and central venous pressures were implanted under aseptic conditions. Satisfactory signal to noise ratios were obtained in 80 %, 60 % and 40 % of the group at 1-3 days, 4-7 days and 8-10 days after the surgery, respectively. RSNA was successfully recorded without contamination by external noise during treadmill exercise. Treadmill exercise resulted in an abrupt increase in RSNA, by 82 % at 0.5 min, and then reached a stable level of ~40 % during the period of 5-30 min after the onset of treadmill exercise. This experimental model allows us to study the neural mechanisms involved in the regulation of cardiovascular function during dynamic exercise in rats. Experimental Physiology (2002) 87.1, 33-39.

It is well established that painful distension of hollow viscera such as the oesophagus can evoke a reflex tachycardia and pressor response; however, the nature of the oesophageal afferent pathway(s) remains controversial. This study investigated the afferent arc which mediates these reflex cardiovascular changes in the decerebrate rat. In addition, the effect of oesophageal distension on the respiratory activity of the costal diaphragm was studied. Focal distension of the oesophagus (volume of 0.3 ml applied for 10 s) just above the diaphragmatic hiatus evoked a reproducible presssor response and tachycardia in the decerebrate rat. Respiration was transiently inhibited at the beginning of oesophageal distension and prior to the rise in blood pressure. Neuromuscular blockade with the nicotinic acetylcholine receptor blocker α-bungarotoxin (140 µg bolus) had no effect on the magnitude of the cardiovascular response. Therefore the efferent supply to the striated muscle of the rat oesophagus was not essential in mediating this reflex. Signal averaging of the mean blood pressure response showed that neither selective ablation of oesophageal spinal afferents nor bilateral vagotomy altered the early trajectory of the pressure response. Bilateral vagotomy reduced the peak magnitude of the response to sustained oesophageal distension. In contrast, selective removal of spinal afferents had no effect on the response. Ablation of both neural pathways was essential to abolish the reflex cardiovascular and respiratory responses. It can be concluded that both vagal and spinal afferent pathways are utilised in the reflex cardiorespiratory response to painful oesophageal distension. Although ablation of one neural pathway had no effect on the response it was still implicated in the reflex, since ablation of both pathways was necessary to prevent the cardiorespiratory changes. This study emphasises the need for caution when inferences are made concerning single selective ablations of multiply innervated organs. Experimental Physiology (2002) 87.1, 41-48.

The effects of neuronal inhibition of nitric oxide (NO) production on the bradycardia resulting from stimulation of preganglionic and postganglionic parasympathetic fibres were investigated in an anaesthetised dog preparation following transection of the cervical vagi and in the presence of a β-adrenoreceptor antagonist. Injection of 1-(2-trifluoromethylphenyl) imidazole (TRIM), an inhibitor of neuronally released NO, into the sinus node artery reduced the bradycardia evoked by right cervical vagal stimulation. In contrast, when the response to preganglionic stimulation had been abolished by hexamethonium (10 mg kg-1), the bradycardia following stimulation of postganglionic parasympathetic fibres on the atrial epicardium was unaffected by TRIM. First, these results confirm the facilitatory actions of neuronally released NO on vagal heart responses in the dog. Second, they indicate that this modulatory and facilitatory role of NO is likely to be exerted at vagal preganglionic-postganglionic synaptic mechanisms in the cardiac parasympathetic ganglia and not at the postganglionic-sinoatrial node synapse. Experimental Physiology (2002) 87.1, 49-52.

Under the conditions of both an increased red cell affinity for O2 at a constant rate of O2 delivery (arterial O2 content × flow) and a decrease in the rate of O2 delivery induced by hypoxic hypoxia at constant blood flow, we have obtained a linear relationship between the partial pressure of O2 in the muscle venous effluent (Pv,O2) and O2 uptake (V˙O2). The relationship is described by the equation V˙O2 = Da × Pv,O2 + V˙O2,conv where Da is the apparent O2 diffusion capacity and V˙O2,conv is O2 delivery-limited V˙O2, and Da × Pv,O2 represents the O2 diffusion-limited V˙O2 (V˙O2,diff). From these observations, we propose the hypothesis that V˙O2 consists of two additive values, V˙O2,conv and V˙O2,diff. The mechanism underlying the reduction in V˙O2 that is induced by reducing O2 delivery to markedly below the V˙O2,conv value has only been investigated using a model based on the single compartment of diffusion-limited V˙O2, and has not been investigated in terms of this additive V˙O2 model. The single compartment analysis appears to overestimate the role of O2 diffusion in limiting the reduction of V˙O2 that occurs in response to a decrease in O2 diffusion capacity, as reflected by the V˙O2/Pv,O2 ratio. To gain better insight into the mechanism involved, we altered the rate of O2 delivery by changing arterial PO2 from normoxia (with inhalation of air) to hypoxia (by inhalation of 10-11 % O2) and blood flow (with high and low flow rates (n = 7 for both groups), and very low and ischaemic flow rates (n = 4 for both groups)) in pump-perfused dog gastrocnemius preparations during tetanic isometric contractions at 1 Hz. As rates of O2 delivery were reduced from 23.2 to 10.9 ml min-1 (100 g)-1, significant decreases in Pv,O2 and V˙O2 were observed (P < 0.05). From the data of Pv,O2 and V˙O2 values within this range of O2 delivery rates, we obtained the regression equation V˙O2 = 0.22 × Pv,O2 + 8.14 (r = 0.58). From the equation, the intercept of the V˙O2-axis was significantly different from zero (P < 0.05), in accordance with the observation that the V˙O2/Pv,O2 ratio (ml min-1 (100 g)-1 Torr-1) increased from 0.54 to 1.35 (P < 0.05). However, at extremely low rates of O2 delivery (5.6 and 7.3 ml min-1 (100 g)-1 the V˙O2/Pv,O2 ratio was 1.51 and 2.80 (P < 0.05), respectively. This indicates a break in the linear V˙O2-Pv,O2 relationship as the rate of O2 delivery was reduced to below the V˙O2,conv value of the V˙O2-axis intercept. These results suggest that the reduction in V˙O2 caused by extreme reductions in the rate of O2 delivery is not attributable to a reduction in O2 diffusion capacity, as expected from the V˙O2/Pv,O2 ratio, but to a reduction in the O2 delivery-limited V˙O2 component, as evaluated by the V˙O2-axis intercept of the linear V˙O2-Pv,O2 relationship. Experimental Physiology (2002) 87.1, 53-61.

The development of the reaction to thermal nociceptive stimulation was studied in neonatal rats of two strains (albino Wistar and pigmented Long-Evans). Using the tail-flick method in neonates of both strains the effects of daily repeated thermal stimulation (sensitisation) were compared to those of acute stimulation applied on 1 day only. During the whole postnatal development period studied (3-28 days) the pigmented rat pups were more sensitive to a repeated thermal nociceptive stimulation than the albinos. The latency of the response did not change linearly during the developmental time studied, but had a pronounced peak (decreased sensitivity) in the period of eye-lid opening. In the pigmented animals the peak appeared 1 day earlier than in the albinos. At the time of the weaning period the latencies of the response were again shorter. Experimental Physiology (2002) 87.1, 63-67.

Spermine is a low molecular weight polyamine involved in the postnatal maturation of the gut. When it is administered orally to suckling rats, it induces maturation of the intestinal tract (liver, pancreas and small intestine). Here we show that this polyamine is able to induce precocious intestinal and splenic development in suckling mice. In fact, in 15-day-old mice which had received spermine orally twice daily for 3 days we observed an increase in the ratio of white pulp surface to total spleen surface in comparison with untreated mice. The two macrophage subsets of the marginal zone and the B-cell population were more developed and reached the development level of 5- or 10-week-old mice. The proliferation rate of B-cells was increased by spermine administration to pups. These observations suggest that spermine might play a role in immune system development; further investigation of its effects are intended, namely the evaluation of its capacity to enhance defence during the neonatal period. Experimental Physiology (2002) 87.1, 69-75.

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

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

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

The effects of 7 days of 'dry' water immersion were investigated in six subjects. Changes in the contraction properties were studied in the triceps surae muscle. After immersion, the maximal voluntary contraction (MVC) was reduced by 18.9 % (P < 0.01), and the electrically evoked (150 impulses s-1) maximal tension during tetanic contraction (Po) was reduced by 8.2 % (P > 0.05). The difference between Po and MVC expressed as a percentage of Po and referred to as force deficiency was also calculated. The force deficiency increased by 44.1 % (P < 0.001) after immersion. The decrease in Po was associated with increased maximal rates of tension development (7.2 %) and relaxation. The twitch time-to-peak was not significantly changed, and half-relaxation and total contraction time were decreased by 5.3 % and 2.8 %, respectively, but the twitch tension (Pt) was not significantly changed and the Pt/Po ratio was decreased by 8.7 %. The 60 s intermittent contractions (50 impulses s-1) decreased tetanic force to 57 % (P < 0.05) of initial values, but force reduction was not significantly different in the two fatigue-inducing tests: fatigue index (the mean loss of force of the last five contractions, expressed as a percentage of the mean value of the first five contractions) was 36.2 ± 5.4 % vs. 38.6 ± 2.8 %, respectively (P > 0.05). While identical force reduction was present in the two fatigue-inducing tests, it would appear that concomitant electrical failure was considerably different. Comparison of the electrical and mechanical alterations recorded during voluntary contractions, and in contractions evoked by electrical stimulation of the motor nerve, suggests that immersion not only modifies the peripheral processes associated with contraction, but also changes central and/or neural command of the contraction. At peripheral sites, it is proposed that the intracellular processes of contraction play a role in the contractile impairment recorded during immersion. Experimental Physiology (2002) 87.1, 101-111.