The development of gene targeting technology, the exchange of an endogenous allele of a target gene for a mutated copy via homologous recombination, and the application of this technique to murine embryonic stem cells has made it possible to alter the germ-line of mice in a predetermined way. Gene targeting has enabled researchers to generate mouse strains with defined mutations in their genome allowing the analysis of gene function in vivo. This review presents the essential tools and methodologies used for gene targeting that have been developed over the past decade. Special emphasis has been laid on the available embryonic stem cell lines and the importance of the genetic background. Also, the state-of-the art of gene targeting approaches in species other than mice will be discussed.

The usual model of intermittent hypoxia (sleep apnoea) corresponds to repeated episodes of hypoxia from a few seconds to a few hours interspersed with episodes of normoxia. The aim of this study was to evaluate in rats the effect of two periods of intermittent exposure for 2 months to hypoxia (IHX1, 24 h in hypoxia (428 Torr), 24 h in normoxia; IHX2, 48 h in hypoxia (428 Torr), 24 h in normoxia) as a new model of hypoxia simulating intermittent exposure to high altitude experienced by Andean miners. We assessed the haematological parameters, time course of resting heart rate and systolic blood pressure. We also evaluated the expression of adrenergic and muscarinic receptors. IHX1 and IHX2 produced an increase in haematocrit, haemoglobin concentration and mean corpuscular volume as previously seen in most hypoxic models. IHX1 and IHX2 induced a similar sustained elevation of systolic blood pressure (132 ± 2 and 135 ± 3 mmHg, respectively, vs. the control level of 121 ± 16 mmHg) after 10 days of exposure without change in heart rate. Right ventricular (RV) hypertrophy (225 ± 13 and 268 ± 15 mg g−1, vs. 178 ± 7 mg g−1) and downregulation of α1-adrenoceptor (RV: 127 ± 21 and 94 ± 16 fmol mg−1vs. 157 ± 8 fmol mg−1; left ventricle (LV): 141 ± 5 and 126 ± 9 fmol mg−1vs. 152 ± 5 fmol mg−1) have been found in both groups, with right ventricular hypertrophy being greater and α1-adrenoceptor density being lower in IHX2 than in HX1 groups. These data indicate that both parameters are related to the time of exposure to hypoxia. IHX1 and IHX2 produced the same magnitude of upregulation of muscarinic receptors (LV, 60%; RV, 40%), and no change in β-adrenoceptors. In conclusion, exposure to intermittent hypoxia led to polycythaemia and RV hypertrophy as observed in other types of hypoxia. A specific cardiovascular response was seen, that is an increase in blood pressure without change in heart rate, which was different from the one observed in episodic and chronic hypoxia. Furthermore, this model involved specific modifications of α1-adrenergic and muscarinic expression.

To investigate the role of hypoxia as a stimulus to the early upregulation of vascular endothelial growth factor (VEGF) in fast skeletal muscles during chronic low frequency stimulation, blood flow, oxygen consumption, VEGF expression and capillary:fibre ratio were measured in chronically stimulated tibialis anterior and extensor digitorum longus (EDL) muscles in rabbits and rats. No differences were found in blood flow, oxygen consumption and extraction between rabbit muscles stimulated for 2 or 4 days (8 h on-16 h off) and controls. Muscle PO2 polarographically measured immediately at the end of stimulation on day 2 was also no different from control under resting conditions (10.7 ± 1.6 vs. 9.5 ± 1.2 Torr, n.s.). Unlike control muscles, however, PO2 in 2 day stimulated muscles did not increase immediately after a further acute bout of contractions. This difference was not apparent after similar acute contractions in 4 day stimulated muscles. The involvement of VEGF in early angiogenesis in stimulated muscles was studied in serial cryosections of rat EDL. The proportion of capillaries positively immunostained for VEGF increased from 25 ± 1 % to 40 ± 1 % (P < 0.05) in muscles removed on day 2 immediately at the end of chronic stimulation; it decreased slightly after 16 h rest, and increased again after 4 days of stimulation. Capillary:fibre ratio was unchanged throughout the experiment. Capillary cell proliferation increased only after the rest period on day 2 (20-fold increase) and day 4 (12-fold increase), indicating angiogenesis in progress. Thus the timing of transient hypoxia and increase in capillary-linked VEGF in stimulated muscles, albeit in different species, was similar, and increased VEGF staining and capillary cell proliferation occurred even after the hypoxia had resolved. This suggests (1) a connection between hypoxia and VEGF during the early stages of stimulation, although ensuing capillary proliferation may thereafter rapidly correct for local hypoxia, and (2) that the subsequent angiogenesis and VEGF expression are dependent on factors other than hypoxia. Experimental Physiology (2002) 87.3, 373-381.

Using microdialysis, we measured nitric oxide (NO) levels in healthy human skin, in vivo, before and during the local inflammatory response to histamine. Basal dialysate NO concentration, assayed using an amperometric technique, was 0·49 ± 0·06 µM (mean ± S.E.M., 21 probes, 14 subjects). Histamine injection produced transient increases in NO concentration within both the weal and flare which was blocked by the NO synthase inhibitor, NG-nitro-L-arginine-methyl ester (L-NAME). Dialysate NO concentration also increased following transdermal delivery of the nitrosovasodilator, glyceryl trinitrate. Thus, using microdialysis, it is possible to quantify NO production in human skin in vivo and study its modulation during the acute inflammatory response.

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.

The mechanism by which pulmonary blood flow increases and pulmonary vascular resistance decreases after birth is not fully understood. The aim of this study was to simulate the decrease in lung volume caused by the onset of air-breathing at birth and determine whether it can duplicate the changes in pulmonary blood flow and vascular resistance that occur at this time. In chronically catheterized fetal sheep near term (145 days of gestation), fetal pulmonary arterial blood flow was measured, using coloured microspheres, before and after fetal lung liquid volumes were reduced from 52·2 ± 2·7 to 21·2 ± 1·6 ml kg-1. During the 30 min period following the reduction in lung liquid volume, the pulmonary-to-systemic arterial pressure difference decreased from 6·8 ± 1·2 mmHg (pulmonary > systemic) to 1·6 ± 0·5 mmHg. Reducing the volume of fetal lung liquid increased pulmonary blood flow from 59·1 ± 10·5 to 204·2 ± 40·4 ml min-1 (100 g tissue)-1 and reduced pulmonary vascular resistance from 0·53 ± 0·20 to 0·14 ± 0·04 mmHg min ml-1 (100 g tissue)-1. We conclude that a reduction in fetal lung liquid volume, which simulates the reduction in lung volume that occurs at birth, causes a 3- to 4-fold increase in pulmonary blood flow and a reduction in pulmonary vascular resistance of a similar magnitude. Thus, the reduction in lung volume associated with the lung changing from a liquid- to an air-filled organ, may partly account for the increase in pulmonary blood flow and decrease in pulmonary vascular resistance at birth.

Previous studies have suggested that the uptake of prolactin from the blood into the milk may be restricted when the alveolus is distended with milk. Therefore the aim of this study was to determine the relationship between prolactin in milk and milk production by measuring the concentration of prolactin in samples of fore- and hind-milk as well as the volume of milk removed for each breast, at each breastfeed over a 24 h period. The mean (± S.D.) concentration of prolactin in milk for all women (n = 15) over the 24 h period was 18.5 ± 11.6 µg l-1 (fore-milk) and 16.8 ± 12.8 µg l-1 (hind-milk). The variation between women masked small changes within women in the concentration of prolactin in milk over the 24 h period, therefore a prolactin ratio (individual fore- or hind-milk value divided by the mean for all fore- or hind-milk samples collected over a 24 h period) was determined. The concentration of prolactin was highest in milk between 02.01 and 06.00 h (prolactin ratio for fore- to hind-milk, 1.5), and lowest between 10.01 and 18.00 h (prolactin ratio for fore- to hind-milk, 0.8). Furthermore, we observed that the difference in prolactin concentration between the fore- and hind-milk (fore-hind gradient) was greatest between 06.01 and 10.00 h (4 µg l-1). To ensure that this effect was not due to permeability in the paracellular pathway, the concentrations of serum albumin and sodium in milk were measured. No significant (P > 0.05) changes over the 24 h period, or with increasing time since last feed were observed. We therefore concluded that when the breast is most drained of milk (in the late evening), and the rate of milk synthesis is greatest, the fore-hind prolactin gradient in the milk of the following feed (in the early morning) is highest. This is consistent with the observation that prolactin uptake from the blood by the lactocyte is dependent on the fullness of the breast, such that prolactin uptake may be inhibited in full alveoli. Experimental Physiology (2002) 87.2, 207-214.

Short chain fatty acids (SCFAs) and CO2 have been shown to stimulate net Mg2+ efflux from the isolated reticulorumen in vivo. To investigate the underlying mechanisms of Mg2+ transport we performed Ussing chamber and microelectrode experiments and measured 28Mg2+ fluxes across sheep rumen epithelium in vitro. In the presence of SCFAs mucosal-to-serosal Mg2+ flux (JMgm-s) amounted to 82·3 ± 7·8 nmol cm-2 h-1 and serosal-to-mucosal Mg2+ flux (JMgs-m) to 3·2 ± 0·7 nmol cm-2 h-1. Replacing SCFAs with gluconate caused a 50 % reduction of JMgm-s, whereas JMgs-m was not affected. Among the SCFAs, n-butyrate was more effective in stimulating JMgm-s than acetate, propionate or iso-butyrate. Eliminating HCO3--CO2 from SCFA-containing solutions did not affect Mg2+ fluxes, whereas the same replacement in SCFA-free solutions led to a further reduction in JMgm-s. JMgm-s decreased after the addition of ethoxyzolamide to SCFA-free, bicarbonate buffered solutions. Decreasing mucosal pH from 6·4 to 5·4 increased JMgm-s in SCFA-free, bicarbonate buffered solutions. SCFAs had no effect on the apical membrane potential of rumen epithelial cells. The experiments show that both SCFAs and CO2 stimulate Mg2+ transport through an increase in JMgm-s, most probably via stimulation of a Mg2+-2H+ exchange mechanism. SCFAs may have additional metabolic effects on Mg2+ transport.

Salivary secretion of proteins from rat submandibular glands was studied using graded stimulation of the parasympathetic nerve in isolation, and then at a fixed rate in combination with graded sympathetic nerve stimulation. Increasing the frequency of parasympathetic nerve stimulation per se caused a gradual increase in the secretion of peroxidase (from acini) but only small increases in proteinase (from ductal cells) and IgA outputs. Dual stimulations, with an increasing frequency of sympathetic nerve stimulation on a background of low frequency parasympathetic nerve stimulation, showed that maximal acinar secretion of peroxidase required only a low frequency of additional sympathetic stimulation, whereas ductal secretion of kallikrein was greatest with the highest frequency of additional sympathetic stimulation (20 Hz in bursts). IgA secretion also required high frequency additional sympathetic stimulation in bursts for greatest output. Although a synergism occurred with parasympathetic plus sympathetic nerve stimulation for the secretion of both peroxidase and kallikrein it was not evident for the secretion of IgA. This presumably reflects a difference for exocytosis of proteins stored in granules (e.g. peroxidase and kallikrein) compared to those proteins continuously transported across the plasma membrane in vesicles by transcytosis. This work confirms that vesicular movement of secretory IgA can be increased by both parasympathetic and sympathetic nerve stimulation, but the frequency parameters differ for each nerve. Experimental Physiology (2000) 85.3, 281-286.

The passive and active electrical properties of left ventricular myocardium were measured, using conducted action potentials and current clamp of isolated myocytes. The objective was to quantify changes of intracellular resistivity, Ri, during hypertrophic growth and the simultaneous imposition of cellular hypoxia. Ri was estimated from the time course of the rising phase of a conducted action potential using a solution of the two-dimensional cable equation. The thoracic aorta of guinea-pigs was constricted to induce left ventricular hypertrophy (LVH) and myocardium used 50 and 150 days post-operation. Conduction velocity increased in the earlier stage of LVH and declined in the later stage, compared with age-matched controls. Hypoxia reduced conduction velocity in all experimental groups. Ri increased only in the later stage of hypertrophy (253 ± 39 [Omega] cm to 544 ± 130 [Omega] cm) and was additionally increased by hypoxia in all groups (e.g. control myocardium 252 ± 39 [Omega] cm to 506 ± 170 [Omega] cm). The magnitude of the increase of Ri in hypertrophied, hypoxic myocardium can create conditions required to generate re-entrant arrhythmias.

The purpose of this study was to quantify the possible reduction in ventilation that could be attributed to changes in cerebral blood flow (CBF) with hypoxia to determine whether it could be of sufficient magnitude to underlie hypoxic ventilatory decline (HVD). Six subjects underwent 20 min of isocapnic hypoxia (end-tidal PO2, 50 mmHg). An index of CBF was obtained using transcranial Doppler ultrasound of the middle cerebral artery. The CBF sensitivities to hypoxia and hypercapnia were obtained from the percentage changes in CBF between the last 3 min of the hypoxic or hypercapnic exposure and the 3 min period prior to the exposure. The magnitude of HVD during hypoxia was estimated by fitting a simple model of the ventilatory response to the hypoxic stimulus. The predicted fall in expiratory ventilation ([Vdot]E) due to a reduction in brain PCO2 generated by the increase in CBF with hypoxia in all subjects was less than the measured magnitude of HVD (33 %). Thus, the results from this study suggest that, in awake humans, changes in CBF during acute isocapnic hypoxia are quantitatively insufficient to underlie HVD in humans.

Glycolytic fibres in rat extensor digitorum longus (EDL) and tibialis anterior (TA) were selectively activated, as demonstrated by glycogen depletion, by indirect electrical stimulation via electrodes implanted in the vicinity of the peroneal nerve using high frequency (40 Hz) trains (250 ms at 1 Hz) and low voltage (threshold of palpable contractions). This regime was applied 10 times per day, each bout being of 15 min duration with 60 min recovery, for 2 weeks. Cryostat sections of muscles were stained for alkaline phosphatase to depict capillaries, succinate dehydrogenase (SDH) to demonstrate oxidative fibres, and periodic acid-Schiff reagent (PAS) to verify glycogen depletion. Specific activity of hexokinase (HK), 6-phosphofructokinase, pyruvate kinase, glycogen phosphorylase and cytochrome c oxidase (COX) were estimated separately in homogenates of the EDL and the predominantly glycolytic cortex and oxidative core of the TA. Stimulation increased the activity of HK but not that of oxidative enzymes in fast muscles. Comparison of changes in oxidative capacity and capillary supply showed a dissociation in the predominantly glycolytic TA cortex. Here, COX was 3.9 ± 0.68 µM min-1 (g wet wt)-1 in stimulated muscles compared with 3.7 ± 0.52 µM min-1 (g wet wt)-1 in contralateral muscles (difference not significant), while the percentage of oxidative fibres (those positively stained for SDH) was also similar in stimulated (14.0 ± 2.8 %) and contralateral (12.2 ± 1.9 %) muscles. In contrast, the capillary to fibre ratio was significantly increased (2.01 ± 0.12 vs. 1.55 ± 0.04, P < 0.01). We conclude that capillary supply can be increased independently of oxidative capacity, possibly due to haemodynamic factors, and serves metabolite removal to a greater extent than substrate delivery. Experimental Physiology (2000) 85.5, 567-574.

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 influence of skin surface cooling and heating on heart rate variability (HRV), blood pressure variability (BPV), and baroreflex sensitivity (BRS) were studied in 11 healthy supine volunteers in air temperatures of 18°C (cool), 24°C (mild), 48°C (warm), and 60°C (hot) in a styrene foam chamber. The high-frequency component of HRV (HFRR) decreased and the ratio of low- to high-frequency components, LFRR/HFRR, increased with skin surface heating. Therefore, a suppression of cardiac vagal nervous activity and an enhancement of cardiac sympathetic nervous activity can be caused by skin surface heating. The low-frequency component of BPV (LFSBP), i.e. the fluctuations of the Mayer waves, increased with skin surface heating. The responses between the very low-frequency components of HRV (VLFRR) and systolic BPV (VLFSBP) to thermal skin stimulation were different, although these variables both were considered to be indicators of the thermoregulatory vasomotor control or renin-angiotensin system. Baroreflex sensitivity (BRS) did not increase with skin surface heating, at least in humans.

The effect of different pedalling rates (40, 60, 80, 100 and 120 rev min-1) on power generating capability, oxygen uptake ([Vdot]O2) and blood lactate concentration [La]b during incremental tests was studied in seven subjects. No significant differences in [Vdot]O2,max were found (mean ± S.D., 5.31 ± 0.13 l min-1). The final external power output delivered to the ergometer during incremental tests (PI,max) was not significantly different when cycling at 60, 80 or 100 rev min-1 (366 ± 5 W). A significant decrease in PI,max of ∼60 W was observed at 40 and 120 rev min-1 compared with 60 and 100 rev min-1, respectively (P < 0.01). At 120 rev min-1 there was also a pronounced upward shift of the [Vdot]O2-power output ([Vdot]O2-P) relationship. At 50 W δ[Vdot]O2 between 80 and 100 rev min-1 amounted to +0.43 l min-1 but to +0.87 l min-1 between 100 and 120 rev min-1. The power output corresponding to 2 and 4 mmol l-1 blood lactate concentration (P[La]2 and P[La]4 ) was also significantly lower (> 50 W) at 120 rev min-1 (P < 0.01) while pedalling at 40, 60, 80 and 100 rev min-1 showed no significant difference. The maximal peak power output (PM,max) during 10 s sprints increased with pedalling rate up to 100 rev min-1. Our study indicates that with increasing pedalling rate the reserves in power generating capability increase, as illustrated by the PI,max/PM,max ratio (54.8, 44.8, 38.1, 34.6, 29.2 %), the P[La]4/PM,max ratio (50.4, 38.9, 31.0, 27.7, 22.9 %) and the P[La]2/PM,max ratio (42.8, 33.5, 25.6, 23.1, 15.6 %) increases. Taking into consideration the [Vdot]O2,max, the PI,max and the reserve in power generating capability we concluded that choosing a high pedalling rate when performing high intensity cycling exercise may be beneficial since it provides greater reserve in power generating capability and this may be advantageous to the muscle in terms of resisting fatigue. However, beyond 100 rev min-1 there is a decrease in external power that can be delivered for an given [Vdot]O2 with an associated earlier onset of metabolic acidosis and clearly this will be disadvantageous for sustained high intensity exercise.

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

The endogenous pain control system is composed of multiple functionally distinct brain regions, which are thought to integrate nociceptive information with various brain functions. The clear involvement of some pain control centres in cardiovascular modulation has been claimed as a strong indication of their role in nociceptive-cardiovascular integration. Particular emphasis has been given to their putative function in triggering cardiovascular reactions to pain. However, the possibility of their participation in the less-studied influence of cardiovascular conditions in pain perception has been largely ignored. We have recently addressed this issue by investigating the involvement of the caudal ventrolateral medullary reticular formation (cVLM) in hypertension-induced hypoalgesia. Circuits capable of conveying cVLM-elicited antinociception include a direct reciprocal cVLM-spinal loop, and two disynaptic spinal pathways relaying in rostroventromedial medullary (RVM) neurones and A5 noradrenergic neurones. In the three pathways, the cVLM neurones involved are circumscribed to a small area of reticular formation located laterally to the lateral reticular nucleus, the VLMlat. The VLMlat has a vasodepressor effect similar to that obtained from the cVLM. In the spinal cord dorsal horn, c-fos expression evoked by noxious stimuli is decreased in hypertensive animals, as compared to normotensive animals. In hypertensive animals following lesion of the VLMlat, spinal c-fos expression is identical to that observed in normotensive animals. The collected data point to a role for the VLMlat in the depression of spinal nociceptive processing in response to rises in blood pressure. Since hypertension-induced hypoalgesia is mediated by spinal α2-adrenoreceptors, this effect could be conveyed by the cVLM-A5-spinal pathway. Experimental Physiology (2002) 87.2, 267-274.

Viscoelastic properties determine the dynamic behaviour of the arterial wall under pulsatile pressure and flow, suggesting time- or frequency-dependent responses to changes in wall stress and strain. The objectives of the present study were: (i) to develop a simplified model to derive simultaneously the elastic, viscous and inertial wall moduli; (ii) to assess Young's modulus as a function of frequency, in conscious, chronically instrumented dogs. Parametric discrete time models were used to characterise the dynamics of the arterial system based on thoracic aortic pressure (microtransducer) and diameter (sonomicrometry) measurements in control steady state and during activation of smooth muscle with the α-adrenoceptor agonist phenylephrine (5 µg kg-1 min-1, I.V.), in eight conscious dogs. The linear autoregressive model and a physically motivated non-linear model were fitted to the input-output (stress-strain) relationship. The aortic buffering function (complex Young's modulus) was obtained in vivo from the identified linear model. Elastic, viscous and inertial moduli were significantly increased from control state ((44.5 ± 7.7) × 104 Pa; (12.3 ± 4.7) × 104 Pa s; (0.048 ± 0.028) × 104 Pa s2 ) to active state ((85.3 ± 29.5) × 104 Pa, P < 0.001; (22.4 ± 8.3) × 104 Pa s, P < 0.05; (0.148 ± 0.060) × 104 Pa s2, P < 0.05). These moduli, obtained using the linear model, did not present significant differences compared with those derived using the non-linear model. In control conditions, the magnitude of the normalised complex Young's modulus was found to be similar to that reported in previous animal studies ranging from 1 to 10 Hz. During vascular smooth muscle activation, this modulus was found to be increased with regard to control conditions (P < 0.01) in the frequency range used in this study. The frequency-dependent Young's modulus of the aortic wall was obtained for the first time in conscious, unsedated dogs. The parametric modelling approach allows us to verify that vascular smooth muscle activation increases the elastic, viscous and inertial moduli with the advantage of being able to track their time evolution. Furthermore, under activation, the aortic wall remains stiff in the physiological frequency range, suggesting the impairment of the arterial buffering function. Experimental Physiology (2001) 86.4, 519-528.

Long-term administration of NG-nitro-L-arginine methyl ester (L-NAME) induces development of hypertension and hypertrophy of the left ventricle in rats. The aim of the present study was to demonstrate the effect of chronic L-NAME treatment on DNA and RNA concentration, and protein synthesis in the rat heart, aorta, brain and kidney and to determine the effect of angiotensin converting enzyme (ACE) inhibitor captopril on these potential alterations. Four groups of rats were investigated: control, L-NAME (40 mg kg-1 day-1), captopril (100 mg kg-1 day-1), and L-NAME (40 mg kg-1 day-1) + captopril (100 mg kg-1 day-1). NO synthase activity in the heart, aorta, brain and kidney was found to be decreased in the L-NAME group. In the group of rats treated with L-NAME + captopril, captopril did not affect NO synthase inhibition. Captopril, however, completely prevented development of hypertension and left ventricular hypertrophy in this group. In the L-NAME group, DNA and RNA concentrations, as well as [14C]leucine incorporation, were significantly increased in all the tissues investigated. In the L-NAME + captopril group, captopril completely prevented the enhancement of DNA and RNA concentrations and [14C]leucine incorporation in all tissues compared to the L-NAME group. Moreover, a significant decrease in RNA concentration and [14C]leucine incorporation below control values was found in the captopril group as well as the L-NAME + captopril group in all the tissues investigated. We conclude that captopril prevented the development of hypertension and increase in nucleic acid concentration and protein synthesis in the heart, aorta, brain and kidney in rats treated with L-NAME + captopril. However, this protective effect of captopril was not associated with increased NO synthase activity in this model of hypertension.

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.