bluuebunny/crossref_metadata_2025 huggingface dataset:
Modulation of Delayed Rectifier Potassium Current, IK, by Isoprenaline in Rabbit Isolated Pacemaker Cells
Authors: Lei Ming , Brown Hilary F. , Terrar Derek A.
Published: May 2004

Permeabilized patch whole‐cell voltage clamp methods were used to investigate the effects of isoprenaline (ISO) on total delayed rectifier potassium current, iK, in rabbit sino‐atrial (SA) node pacemaker cells; total iK is composed of the rapidly activating iKr and the slowly activating iKs, but predominantly iKr in this species. ISO (20 nM) increased the amplitude of total iK and caused a negative shift of approximately 10 mV in the activation curve for iK, both in the absence and in the presence of 300 nM nisoldipine to block the L‐type Ca2+ current, iCa,L. The same concentration (20 nM) of ISO increased the spontaneous pacemaker rate of SA node pacemaker cells by 16%. In addition to increasing the amplitude of iK, ISO (20‐50 nM) also increased the rate of deactivation of this current. The stimulation of iK by ISO was reversed by 10 μM H‐89, a selective protein kinase A inhibitor, but not by 200 nM bisindolymaleimide I, a selective protein kinase C inhibitor. It therefore appears that the mechanisms by which β‐adrenoceptor agonists increase pacemaking rate in sinoatrial node pacemaker cells include an increase in the rate of deactivation of iK in addition to the well‐documented augmentation of iCa,L and the positive shift of the activation curve for the hyperpolarization‐activated inward current, if. The observations are also consistent with a role for protein kinase A in the stimulation of iK by ISO in SA node cells.
The Off‐Transient Pulmonary Oxygen Uptake (VO2) Kinetics Following Attainment of a Particular VO2 During Heavy‐Intensity Exercise in Humans
Authors: Cunningham D. A. , St Croix C. M. , Paterson D. H. , Özyener F. , Whipp B. J.
Published: May 2004

The oxygen uptake response to moderate‐intensity exercise (i.e. < anaerobic threshold (θan)) has been characterised with a gain (i.e. response amplitude per increment of work rate) and time constant that do not vary appreciably at different work rates or between the on‐ and off‐transients. Above θan, the response becomes more complex with an early component that typically projects to a value that has a gain similar to that of the < θan response, but which is supplemented by the addition of a delayed slow kinetic component. We therefore established a constant target VO2 (VO21) for each subject such that with different imposed work rates the contribution to VO21 from the slow phase varied over a wide range. Work rates were chosen so that VO21 was attained at 2‐24 min. Five subjects (aged 21‐58 years) cycled at four to five different work rates. VO2 was measured breath‐by‐breath, at VO21 the work rate was abruptly reduced and the subject recovered by cycling unloaded for 15 min. Unlike the on‐transient, for which the slow component shows a long delay, the off‐transient was best fitted as two simultaneous exponential components. The slower off‐transient component had a small amplitude and long time constant, but did not differ significantly among the various tests. The off‐transient kinetics for VO2 therefore was independent of the magnitude of the contribution to the slow phase from the on‐transient kinetics.
Effects of Lung Volume on Parasternal Pressure‐Generating Capacity in Dogs
Authors: DiMarco Anthony F. , Romaniuk Jaroslaw R. , Supinski Gerald , Kowalski Krzysztof E.
Published: May 2004

Previous studies have suggested that the optimum length for force generation of the parasternal intercostal (PS) muscles is well above functional residual capacity (FRC). We further explored this issue by examining the pressure‐generating capacity of the PS muscles as a function of lung volume in anaesthetized dogs. Upper thoracic spinal cord stimulation (SCS) was used to electrically activate the PS muscles. Changes in airway pressure and parasternal resting length (LR) during airway occlusion were monitored over a wide range of lung volumes during SCS. To assess the effects of parasternal contraction alone, SCS was performed following phrenicotomy and section of the external intercostal, levator costae and triangularis sterni muscles. With increasing lung volume, there were progressive decrements in the capacity of the PS muscles to produce changes in airway pressure. The relationship between PS pressure generation and lung volume was similar to a previous comparable assessment of the external intercostal muscles. The PS muscles shortened during passive inflation and also shortened further (by > 20% of LR) during SCS. Total shortening (passive plus active) increased progressively with increasing lung volume. Our results indicate that the capacity of the PS muscles to produce changes in airway pressure (a) falls progressively with increasing lung volume and (b) is similar to that of the external intercostal muscles. We speculate that the fall in PS pressure‐generating capacity is related, in part, to progressive reductions in end‐inspiratory length.
Effect of Reproductive Status on Plasma Oxytocin Concentrations and the Renal Response to Oxytocin in the Conscious Rat
Authors: Zhou Yan , Forsling Mary L.
Published: May 2004

The magnitude of diuresis and natriuretis produced by oxytocin in the female rat has been shown to be dependent on the stage of the oestrous cycle. A study has been performed to determine the role of ovarian steroids in modulating the renal response to oxytocin infused at a rate of 100 fmol min−1 in hypotonic saline. Observations were performed on ovariectomised rats with and without steroid treatment and rats with suppressed oestrous cycles following treatment with the long‐acting gonadotrophin‐releasing hormone analogue Zoladex, given as a 100 mg S.C. depot, or the antioestrogen tamoxifen, given as three daily injections of 1 mg. Steroid treatment comprised a single dose of 10 μg oestradiol benzoate or 2.0 mg progesterone, or a combination of the two hormones given 30 h apart. Ovariectomy had no significant effect on plasma oxytocin concentrations, although progesterone treatment produced an increase. The natriuresis in ovariectomised animals of 27% was smaller (P < 0.05) than that seen on the day of pro‐oestrus and closer to that seen in the intact rat at oestrus. The responses in animals with suppressed oestrous cycles following treatment with tamoxifen were not significantly different from those observed following ovariectomy, neither was the diuresis following Zoladex. The renal responses were greater following treatment with oestradiol than in the ovariectomised group. Thus ovarian steroids do influence the renal responsiveness to oxytocin with oestradiol augmenting the response.
The Secretory Response of the Rat Colon to the Flavonol Quercetin is Dependent on Ca2+‐Calmodulin
Authors: Cermak R. , Vujicic Z. , Kuhn G. , Wolffram S.
Published: May 2004

The dietary flavonol quercetin induces chloride secretion in rat intestine. To clarify the underlying mechanisms, experiments were performed in Ussing chambers with tissue from rat proximal and distal colon. Quercetin induced an increase in short‐circuit current (Isc), which was largely independent of submucosal neurons, as it was not affected by the neurotoxin tetrodotoxin. The effect of quercetin was blocked by the calmodulin antagonists trifluoperazine and ophiobolin A and was diminished by a blocker of Ca2+ release from intracellular stores (TMB‐8), whereas the muscarinic receptor antagonist atropine was ineffective. The quercetin‐induced Isc was abolished in Ca2+‐free solution. The flavonol was able to further increase Isc after maximal stimulation of the cAMP pathway by forskolin. The Isc increase by the flavonol was differently affected by two analogous phosphodiesterase inhibitors. Whereas 3‐isobutyl‐1‐methylxanthine (IBMX) antagonized the effect of quercetin, 8‐methoxymethyl‐IBMX had no effect. Both phosphodiesterase inhibitors similarly influenced the Isc increase induced by forskolin. These results indicate that the chloride secretion induced by quercetin in rat colon depends on Ca2+ and calmodulin. The cAMP pathway and inhibition of phosphodiesterase appear not to be responsible for the secretory activity of the flavonol.
Effects of Shear Stress on [Ca2+]I and Membrane Potential of Vascular Endothelium of Intact Rat Blood Vessels
Authors: Marchenko Sergey M. , Sage Stewart O.
Published: May 2004

Shear stress is an important modulator of endothelial function which can affect vessel tone, endothelial morphology and gene transcription. However, the mechanism by which the endothelium detects changes in shear stress is the subject of conflicting reports. The possible contributions of changes in the cytosolic Ca2+ concentration ([Ca2+]i) and of membrane hyperpolarisation are disputed. Here we have investigated the effects of shear stress on these variables in native endothelium in situ in rat aorta and hepatic portal vein. Endothelial [Ca2+]i was recorded using the fluorescent indicator fura‐2. The [Ca2+]i in unstimulated rat aortic endothelium was 86 ± 12 nM (n= 9). Acetylcholine (ACh) evoked relaxation of the aorta and a biphasic increase in endothelial [Ca2+]i. Shear stress that evoked relaxation comparable to that evoked by ACh did not affect the endothelial [Ca2+]i. Endothelial membrane potential was recorded using the patch clamp technique. The resting membrane potentials of rat aortic and portal vein endothelium were ‐47 ± 3.8 mV (n= 14) and ‐68 ± 4.5 mV (n= 9), respectively. Shear stress did not affect the endothelial membrane potential of rat aorta, but evoked a small (‐1.6 ± 0.3 mV, n= 9) hyperpolarisation of the endothelium of rat portal vein in all preparations studied. These results suggest that neither changes in [Ca2+]i nor hyperpolarisation are involved in the reception of shear stress by native rat aortic endothelium in situ, but that shear stress induces small hyperpolarisations of endothelium of portal vein, a mechanism that may be present and of greater physiological significance in other parts of vascular system.
Differential Modification of Myosin Heavy Chain Expression by Tenotomy in Regenerating Fast and Slow Muscles of the Rat
Authors: Noirez Philippe , Agbulut Onnik , Ferry Arnaud
Published: May 2004

We have examined the effect of tenotomy on the expression of myosin heavy chains (MyHC) in regenerating fast and slow skeletal muscles. Degeneration/regeneration of the left soleus and plantaris of Wistar male rats was induced by an injection into the muscle belly of a myotoxin (snake venom: Notechis scutatus scutatus). MyHC isoform content of regenerating plantaris and soleus muscles were studied 21 days after muscle injury using an electrophoretic technique. Tenotomy of the regenerating plantaris (mechanical underload) did not alter its MyHC expression (P > 0.05). In contrast, tenotomy of the regenerating soleus increased its relative levels of MyHC‐2b (P < 0.05) and MyHC‐2x/d (P < 0.01), and decreased its relative level of MyHC‐1 (P < 0.01). Tenotomy of the synergistic gastrocnemius (overload) tended to decrease the relative level of MyHC‐2b in regenerating plantaris (P < 0.07). The effect of tenotomy of the synergistic gastronecmius on the regenerating soleus was different: a decrease in the relative levels of MyHC‐1 (P < 0.05) and an increase in the relative level of MyHC‐neonatal (P < 0.01). In conclusion, and in contrast to a regenerating slow muscle, a change of mechanical loading by tenotomy did not seem to markedly alter the expression of mature MyHC phenotype in a fast regenerating muscle.
Effect of Feeding Level and Thyroxine on Adipose Tissue Development and Growth in Postnatal Lambs
Authors: Gate J. J. , Clarke L. , Bird J. A. , Lomax M. A. , Symonds M. E.
Published: May 2004

We investigated the influence of exogenous thyroxine (T4) administration in conjunction with level of feeding on adipose tissue and liver growth in postnatal lambs. Pairs of lambs were fed either 100 g (i.e. low fed) or 200 g (i.e. high fed) of milk powder per litre of reconstituted milk replacer over the first month of life. Half of the pairs of lambs were fed a bolus dose of T4 (15 mg (kg body weight)‐1) daily until 8 days of age. Perirenal adipose tissue and hepatic tissue were sampled at either 8 or 35 days of age. High fed lambs grew faster, possessed more adipose tissue and had larger livers than low fed lambs at 8 and 35 days of age. T4 administration resulted in a lower thermogenic activity (i.e. GDP binding) in adipose tissue at 8 days of age in low, but not high fed lambs. There was no difference between groups in colonic temperature or oxygen consumption. Between 17 and 35 days of age high fed lambs previously treated with T4 had lower daily milk consumption than untreated siblings, but still attained the same growth rate. Plasma insulin‐like growth factor‐I concentrations were greater in high than low fed lambs, a relationship that was not influenced by T4 treatment. In adipose tissue, iodothyronine 5′ deiodinase activity was not influenced by T4 administration and was greater in high than low fed lambs. Hepatic iodothyronine 5′ deiodinase activity was not influenced by T4 administration in low fed lambs, but was reduced by T4 in high fed lambs. In conclusion, T4 administration over the first 8 days of life can accelerate the rate of decline in thermogenic activity of uncoupling protein‐1. This effect is not observed when the level of feeding is increased. Following withdrawal of T4 treatment, high fed T4‐treated lambs were able to maintain the same growth rate as untreated lambs despite having a lower food intake.
Effects of Capsaicin‐Induced Sensory Denervation on Osteoclastic Resorption in Adult Rats
Authors: Adam C. , Llorens A. , Baroukh B. , Cherruau M. , Saffar J. L.
Published: May 2004

Many recent findings suggest that the nervous system has efferent effects on bone. A putative role of the sensory innervation has been assessed by using a synchronised rat model of bone resorption after treating adult animals with the neurotoxin capsaicin. Fourteen days after capsaicin treatment (50 mg kg−1) the right maxillary molars were extracted to activate a wave of resorption along the mandibular cortex. The rats were killed 4 days later (i.e. at the peak of resorption in this model), and their right mandibles were processed for histometric evaluation of resorption along the cortex and of calcitonin gene‐related peptide (CGRP)‐ and substance P (SP)‐immunoreactive (IR) fibres in the dental pulp. CGRP‐IR and SP‐IR fibres were significantly reduced in numbers by the capsaicin treatment (by 58 and 49%, respectively), confirming the success of sensory denervation. The resorption surface was significantly reduced (P < 0.005) versus the sham‐treated animals. Although the size of the osteoclast population recruited in the site was not modified, the number of actively resorbing osteoclasts was significantly reduced (P < 0.03). However, the activity of the resorbing cells was not modified. Non‐specific esterase‐positive osteoclast precursors were also significantly few after capsaicin treatment. These data show that the sensory nervous system is involved in the control of bone resorption at two different levels: (1) in the recruitment of osteoclast precursors, and (2) in regulating the access of recruited cells to the bone surface.
Body Position and Cardiac Dynamic and Chronotropic Responses to Steady‐State Isocapnic Hypoxaemia in Humans
Authors: Lucy S. Deborah , Hughson Richard L. , Kowalchuk John M. , Paterson Donald H. , Cunningham David A.
Published: May 2004

Neural mediation of the human cardiac response to isocapnic (IC) steady‐state hypoxaemia was investigated using coarse‐graining spectral analysis of heart rate variability (HRV). Six young adults were exposed in random order to a hypoxia or control protocol, in supine and sitting postures, while end‐tidal PCO2 (PET,CO2) was clamped at resting eucapnic levels. An initial 11 min period of euoxia (PET,O2 100 mmHg; 13.3 kPa) was followed by a 22 min exposure to hypoxia (PET,O2 55 mmHg; 7.3 kPa), or continued euoxia (control). Harmonic and fractal powers of HRV were determined for the terminal 400 heart beats in each time period. Ventilation was stimulated (P < 0.05) and cardiac dynamics altered only by exposure to hypoxia. The cardiac interpulse interval was shortened (P < 0.001) similarly during hypoxia in both body positions. Vagally mediated high‐frequency harmonic power (Ph) of HRV was decreased by hypoxia only in the supine position, while the fractal dimension, also linked to cardiac vagal control, was decreased in the sitting position (P < 0.05). However, low‐frequency harmonic power (Pl) and the HRV indicator of sympathetic activity (Pl/Ph) were not altered by hypoxia in either position. These results suggest that, in humans, tachycardia induced by moderate IC hypoxaemia (arterial O2 saturation Sa,O2≈ 85%) was mediated by vagal withdrawal, irrespective of body position and resting autonomic balance, while associated changes in HRV were positionally dependent.
Modelling the Influence of Age, Body Size and Sex on Maximum Oxygen Uptake in Older Humans
Authors: Johnson Patrick J. , Winter Edward M. , Paterson Don H. , Koval John J. , Nevill Alan M. , Cunningham David A.
Published: May 2004

The purpose of this study was to describe the influence of body size and sex on the decline in maximum oxygen uptake (V̇O2,max) in older men and women. A stratified random sample of 152 men and 146 women, aged 55‐86 years, was drawn from the study population. Influence of age on V̇O2,max, independent of differences in body mass (BM) or fat‐free mass (FFM), was investigated using the following allometric model: V̇O2,max= BMb (or FFMb) exp(a + (c × age) + (d × sex)) [epsilon]. The model was linearised and parameters identified using standard multiple regression. The BM model explained 68.8% of the variance in V̇O2,max. The parameters (± s.e.e., standard error of the estimate) for lnBM (0.563 ± 0.070), age (‐0.0154 ± 0.0012), sex (0.242 ± 0.024) and the intercept (‐1.09 ± 0.32) were all significant (P < 0.001). The FFM model explained 69.3% of the variance in V̇O2,max, and the parameters (± s.e.e) lnFFM (0.772 ± 0.090), age (‐0.0159 ± 0.0012) and the intercept (‐1.57 ± 0.36) were significant (P < 0.001), while sex (0.077 +/− 0.038) was significant at P = 0.0497. Regardless of the model used, the age‐associated decline was similar, with a relative decline of 15% per decade (0.984 exp(age)) in V̇O2,max in older humans being estimated. The study has demonstrated that, for a randomly drawn sample, the age‐related loss in V̇O2,max is determined, in part, by the loss of fat‐free body mass. When this factor is accounted for, the loss of V̇O2,max across age is similar in older men and women.
Mechanisms Associated with the Negative Inotropic Effect of Deuterium Oxide in Single Rat Ventricular Myocytes
Authors: Hongo K. , Brette F. , Haroon M. M. , White E.
Published: May 2004

Deuterium oxide (D2O) is known to cause a negative inotropic effect in muscle although the mechanisms associated with this response in cardiac muscle are not well understood. We studied the effects of D2O in single rat ventricular myocytes in order to characterise the mechanisms associated with its negative inotropic effect and to assess its possible use as an acute modulator of microtubules. D2O rapidly reduced the magnitude of contraction in rat ventricular myocytes, and there was some recovery of contraction in the presence of D2O. Colchicine, an agent known to depolymerise microtubules, did not modify the effect of D2O. D2O decreased the L‐type Ca2+ current (ICa), measured under whole cell and perforated patch clamp conditions. Slowing of the time to peak and a delay in inactivation of ICa were observed. Intracellular calcium ([Ca2+]i) and sodium ([Na+]i) were measured using the fluorescent indicators fura‐2 and SBFI, respectively. The fall in contraction upon exposure to D2O was not associated with a fall in the [Ca2+]i transient; this response is indicative of a reduction in myofilament Ca2+ sensitivity. Both the [Ca2+]i transient and [Na+]i increased during the partial recovery of contraction in the presence of D2O. We conclude that a decrease in the myofilament sensitivity for Ca2+ and a reduction in Ca2+ influx via ICa are principally responsible for the negative inotropic effect of D2O in cardiac muscle. We found no evidence to explain the negative inotropic effect of D2O in terms of microtubule proliferation. In addition we suggest that acute application of D2O is not a useful procedure for the investigation of the role of microtubules in excitation‐contraction coupling in cardiac muscle.
Exercise Training Enhances Relaxation of the Isolated Guinea‐Pig Saphenous Artery in Response to Acetylcholine
Authors: Choate J. K. , Kato K. , Mohan R. M.
Published: May 2004

The effects of exercise training were investigated on the vascular responses in the isolated guinea‐pig saphenous artery. Exercising animals swam 5 days week‐1 for 6 weeks (60 min day‐1 for weeks 1 and 2; 75 min day‐1 for weeks 3 and 4; 90 min day‐1 for weeks 5 and 6), while control animals were placed into shallow water for the same duration. Trained animals had significantly higher ventricular:body weight ratios, increased citrate synthase activity in the latissimus dorsi, and enhanced Na+ pump concentrations in the latissimus dorsi and gastrocnemius muscles (P < 0.05). In vitro isometric techniques were used to measure constriction and relaxation responses of saphenous artery rings from trained and control animals. There were no significant differences in the constriction responses to KCl (50 mm) and phenylephrine (0.3‐100 μM) in arterial rings from control versus trained animals. Relaxation responses to acetylcholine (10 μM; ACh‐relaxation), following preconstriction with phenylephrine (10 μM), were significantly enhanced in rings from trained animals (P < 0.05). Acetylcholine relaxed the vessels to 47 ± 6% (control) and 18 ± 3% (trained) of the preconstriction responses to phenylephrine. The nitric oxide synthase inhibitor N G‐nitro‐L‐arginine (L‐NA; 50 μM) significantly attenuated the ACh‐relaxation in control and trained animals (P < 0.05). The effect of L‐NA on the ACh‐relaxation was significantly larger in trained (change in ACh‐relaxation with L‐NA = 29 ± 9%) than control (14 ± 3%) animals (P < 0.05). In conclusion, exercise training enhanced the ACh‐relaxation of the isolated guinea‐pig saphenous artery. Inhibition of nitric oxide synthase attenuated the ACh‐relaxation of rings from control and trained animals, but this effect was significantly larger in the vessels from trained animals. These results are consistent with the idea that nitric oxide could contribute to the enhanced ACh‐relaxation of the saphenous artery with exercise training.
A Respiratory Drive in Addition to the Increase in Co2 Production at Raised Body Temperature in Rats
Authors: Boden A. G. , Harris M. C. , Parkes M. J.
Published: May 2004

Mammals that use the ventilatory system as the principal means of increasing heat loss, i.e. that pant, show two fundamental changes in the control of breathing at raised temperatures. First, alveolar ventilation increases by more than, rather than in proportion to, the increase in CO2 production. Second, hypocapnia no longer causes apnoea. Rats do not use the ventilatory system as the principal means of increasing heat loss, so we have investigated whether rats also show these two changes at raised temperatures. Breathing was detected from diaphragmatic electromyogram (EMG) activity. Anaesthesia and hyperoxia were used to minimise behavioural and hypoxic drives to ventilation and arterial PCO2 (Pa,CO2) was controlled using mechanical ventilation. At 36.6 ± 0.1 °C, breathing was absent as long as Pa,CO2 was held below a threshold level of 32.9 ± 0.7 mm Hg (n= 14) under steady‐state conditions. When body temperature in rats was raised above 37 °C, both fundamental changes in the control of breathing became apparent. First, at 39 °C the mean Pa,CO2 level during spontaneous breathing (39.6 ± 5.4 mm Hg, n= 4) fell by 3.9 ± 1.4 mm Hg (P < 0.05, Student's paired t test). Second, at 39.9 ± 0.1 °C breathing was present when mean Pa,CO2 levels were only 18.2 ± 1.5 mm Hg (n= 14), the lowest mean Pa,CO2 level we could achieve with mechanical ventilation. We calculate, however, that at 39.9 °C, the drive to breathe from the increased CO2 production alone would not sustain breathing below a Pa,CO2 level of 27.8 ± 1.4 mm Hg (n= 13). In rats at raised body temperatures therefore a respiratory drive exists that is in addition to that related to the increase in CO2 production.
Effects of Placental Insufficiency on the Ovine Fetal Renin‐Angiotensin System
Authors: Zhang David Y. , Lumbers Eugenie R. , Simonetta Giuseppe , Wu June J. , Owens Julie A. , Robinson Jeffrey S. , McMillen I. Caroline
Published: May 2004

We postulated that chronic placental insufficiency would be associated with reduced expression of renal renin and angiotensinogen genes in the fetal sheep. Placental development was restricted in ewes by removing the majority of caruncles prior to mating (placentally restricted (PR) group). The weights of PR fetuses were significantly reduced (P < 0.05, 2.98 ± 0.33 kg) compared to control fetuses (4.20 ± 0.30 kg). Kidney weights were also significantly reduced in the PR fetuses (P < 0.05, 8.4 ± 0.9 g) compared with control fetuses (12.2 ± 1.3 g). The ratios of renal renin β‐actin mRNA levels were significantly reduced in PR fetuses (P < 0.001, 0.35 ± 0.02) when compared to control animals (0.98 ± 0.13). The renal angiotensinogen mRNA/18S rRNA ratio was significantly lower (P < 0.05, 0.28 ± 0.13) in PR fetuses compared with control fetuses (0.72 ± 0.10), while hepatic angiotensinogen was unaffected. There was a positive correlation between renal renin mRNA and renal angiotensinogen mRNA levels (r= 0.65, P < 0.05, n= 12). It is unlikely that these changes in renal angiotensinogen and renin mRNA were due to the small increment in plasma cortisol levels (< 5 nmol l‐1). There was, however, a positive correlation between arterial PO2 and renal renin mRNA (r2= 0.77, P < 0.01). Plasma renin levels were not different between the two groups. Thus, restriction of nutrient and oxygen supply throughout fetal life was associated with suppression of renal renin and renal angiotensinogen gene expression, with no effect on hepatic angiotensinogen mRNA levels. This specific suppression of fetal renal renin and angiotensinogen expression could alter the activity of the intrarenal RAS and so affect growth and development of the kidney.
Influence of Muscle Fibre Type and Fitness on the Oxygen Uptake/Power Output Slope During Incremental Exercise in Humans
Authors: Barstow Thomas J. , Jones Andrew M. , Nguyen Paul H. , Casaburi Richard
Published: May 2004

We recently reported that a higher percentage of type I fibres in vastus lateralis and a greater peak oxygen uptake (V̇O2) were associated with a greater initial rise in V̇O2 (ΔV̇O2/ΔW, where W is work rate) following the onset of heavy constant power output exercise (above the lactate threshold, LT). It was unclear if these results were true only for heavy exercise, or if the association between fibre type and/or fitness and ΔV̇O2/ΔW would also be seen for moderate (< LT) exercise. The purpose of the present study was to compare the relationships between fibre type or peak V̇O2 and ΔV̇O2/ΔW determined for moderate (< LT) and heavy (> LT) exercise intensities during incremental exercise. Nine healthy subjects performed an incremental ramp test on a cycle ergometer. The V̇O2/Wslope was calculated for the domain of power outputs up to the LT (S1), from the LT towards peak V̇O2 (S2), and over the entire linear portion of the ΔV̇O2/ΔW response (ST), and compared to fibre type distribution determined from biopsy of the vastus lateralis, and to peak V̇O2 (as ml kg‐1 min‐1). Significant correlations between ΔV̇O2/ΔW and the proportion of type I fibres were found for each exercise domain (r is 0.69, 0.71 and 0.84 for S1, S2 and ST, respectively, P < 0.05). S1 ranged between about 9 ml min‐1 W‐1 for a low proportion of type I fibres and 11 ml min‐1 W‐1 for a high proportion of type I fibres. Similar correlations were also found between S2 (r = 0.70) and ST (r = 0.76) and peak V̇O2. These results are consistent with our previous findings during > LT constant power output exercise, and suggest that the proportion of type I fibres, and possibly fitness as indicated by peak V̇O2, is associated with greater ΔV̇O2/ΔW during the initial adjustment to < LT as well as > LT exercise. These results do not appear to be explained by classical descriptions of the kinetics of adjustment of V̇O2 following the onset of ramp or constant power output exercise. They might reflect enhanced motor unit recruitment in subjects with a greater percentage of type I fibres, and/or who are more aerobically fit. However, the underlying mechanism for these findings must await further study.
Hyperosmotic but not Hyposmotic Stress Evokes a Rise in Cytosolic Ca2+ Concentration in Endothelium of Intact Rat Aorta
Authors: Marchenko Sergey M. , Sage Stewart O.
Published: May 2004

The effects of osmotic stress on the cytosolic Ca2+ concentration ([Ca2+]i) in the endothelium of excised intact rat aorta were investigated using the indicator fura‐2 and the patch clamp technique. Hyperosmotic stress evoked a reversible rise in endothelial [Ca2+]i in the presence but not absence of extracellular Ca2+, indicating that it evoked Ca2+ entry without release from intracellular stores. Hyposmotic stress was without significant effect. Cytochalasins B and D reduced the effect of hyperosmotic stress but not acetylcholine on endothelial [Ca2+]i. In endothelium isolated from underlying smooth muscle, hyperosmotic stress produced a rise in the [Ca2+]i and depolarisation of the endothelial membrane potential whereas hyposmotic stress was without effect. Mechanosensitive cationic channels recorded in cell‐attached patches were activated by hyperosmotic solutions applied to the endothelium and inhibited by hyposmotic solutions. These data suggest that shrinkage of endothelial cells evokes an increase in [Ca2+]i by opening a pathway for Ca2+ entry from the extracellular space. The mechanosensitive ionic channels which we have previously described may be responsible for this response.
Forskolin has a Bimodal Camp‐Independent Effect on Superoxide Anion Generation in Isolated Osteoclasts
Authors: Berger Christine E. M. , Datta Harish K.
Published: May 2004

The mode of action of forskolin is of clinical and scientific interest since forskolin has been shown to have potentially therapeutic bone anti‐resorptive and anti‐hypertensive properties. Forskolin is thought to inhibit the bone resorptive activity of osteoclasts by elevating cytosolic cAMP and to mimic as well as augment the anti‐resorptive effect of calcitonin (CT). Other studies have found that forskolin has a dose‐dependent dual effect in mouse calavaria, stimulating bone resorption at low doses and having an inhibitory effect at high doses. However, the acute effect of forskolin on osteoclast functional modality has never been studied. The present investigation examined the effect of low (1 mM) and high doses (10 mM) of forskolin on superoxide anion (O2·−) generation in isolated bone‐resorbing rat osteoclasts. Forskolin was found to have a bimodal cAMP‐independent effect on O2·− generation, being stimulatory at a low dose and having an inhibitory effect at a higher dose. These findings also suggest that CT‐induced inhibition of O2·− generation in the osteoclasts is likely to be mediated by cAMP‐independent pathways, perhaps involving [Ca2+]i modulation.
Reflex Responses from the Main Pulmonary Artery and Bifurcation in Anaesthetised Dogs
Authors: McMahon N. C. , Drinkhill M. J. , Myers D. S. , Hainsworth R.
Published: May 2004

This study was undertaken to determine the reflex cardiovascular and respiratory responses to discrete stimulation of pulmonary arterial baroreceptors using a preparation in which secondary modulation of responses from other reflexes was prevented. Dogs were anaesthetised with α‐chloralose, artificially ventilated, the chests widely opened and a cardiopulmonary bypass established. The main pulmonary arterial trunk, bifurcation and extrapulmonary arteries as far as the first lobar arteries on each side were vascularly isolated and perfused through the left pulmonary artery and drained via the right artery through a Starling resistance which controlled pulmonary arterial pressure. Pressures distending systemic baroreceptors and reflexogenic regions in the heart were controlled. Reflex vascular responses were assessed from changes in perfusion pressures to a vascularly isolated hind limb and to the remainder of the subdiaphragmatic systemic circulation, both of which were perfused at constant flows. Respiratory responses were assessed from recordings of efferent phrenic nerve activity. Increases in pulmonary arterial pressure consistently evoked increases in both perfusion pressures and in phrenic nerve activity. Both vascular and respiratory responses were obtained when pulmonary arterial pressure was increased to above about 30 mmHg. Responses increased at higher levels of pulmonary arterial pressures. In 13 dogs increases in pulmonary arterial pressure to 45 mmHg increased systemic perfusion pressure by 24 ± 7 mmHg (mean ± S.E.M.) from 162 ± 11 mmHg. Setting carotid sinus pressure at different levels did not influence the vascular response to changes in pulmonary arterial pressure. The presence of a negative intrathoracic pressure of ‐20 mmHg resulted in larger vascular responses being obtained at lower levels of pulmonary arterial pressure. This indicates that the reflex may be more effective in the intact closed‐chest animal. These results demonstrate that stimulation of pulmonary arterial baroreceptors evokes a pressor reflex and augments respiratory drive. This reflex is likely to be elicited in circumstances where pulmonary arterial pressure increases and the negative excursions of intrathoracic pressure become greater. They are likely, therefore, to be involved in the cardio‐respiratory response to exercise as well as in pathological states such as pulmonary hypertension or restrictive or obstructive lung disease.
Absence of Reflex Vascular Responses from the Intrapulmonary Circulation in Anaesthetised Dogs
Authors: McMahon N. C. , Drinkhill M. J. , Myers D. S. , Hainsworth R.
Published: May 2004

The aim of this investigation was to determine whether reflex cardiovascular responses were obtained to localised distension of the intrapulmonary arterial and venous circulations in a preparation in which the stimuli to other major reflexogenic areas were controlled and the lung was shown to possess reflex activity. Dogs were anaesthetised with α‐chloralose, artificially ventilated, the chests widely opened and a cardiopulmonary bypass established. The intrapulmonary region of the left lung was isolated and perfused through the left pulmonary artery and drained through cannulae in the left pulmonary veins via a Starling resistance. Intrapulmonary arterial and venous pressures were controlled by the rate of inflow of blood and the pressure applied to the Starling resistance. Pressures to the carotid, aortic and coronary baroreceptors and heart chambers were controlled. Responses of vascular resistance were assessed from changes in perfusion pressures to a vascularly isolated hind limb and to the remainder of the subdiaphragmatic circulation (flows constant). The reactivity of the preparation was demonstrated by observing decreases in vascular resistance to large step changes in carotid sinus pressure (systemic vascular resistance decreased by ‐40 ± 5%), chemical stimulation of lung receptors by injection into the pulmonary circulation of veratridine or capsaicin (resistance decreased by ‐32 ± 4%) and, in the four dogs tested, increasing pulmonary stroke volume to 450 ml (resistance decreased by ‐24 ± 6%). However, despite this evidence that the lung was innervated, increases in intrapulmonary arterial pressure from 14 ± 1 to 43 ± 3 mmHg or in intrapulmonary venous pressure from 5 ± 2 to 34 ± 2 mmHg or both did not result in any consistent changes in systemic or limb vascular resistances. In two animals tested, however, there were marked decreases in efferent phrenic nerve activity. These results indicate that increases in pressure confined to the intrapulmonary arterial and venous circulations do not cause consistent reflex vascular responses, even though the preparation was shown to be reflexly active and the lung was shown to be innervated.
Carbachol‐Stimulated Chloride Secretion in Mouse Colon: Evidence of a Role for Autocrine Prostaglandin E2 Release
Authors: Carew Mark A. , Thorn Peter
Published: May 2004

We used the short‐circuit current technique to investigate the possible facilitatory role of epithelium‐derived prostaglandin E2 (PGE2) release on Cl− secretion in the mouse colon. Carbachol‐ (CCh)‐stimulated Cl− secretion was reduced by pretreatment with either indomethacin (10 μM), or TTX (1 μM), and when added together, these inhibitors revealed net CCh‐stimulated K+ secretion. CCh‐stimulated Cl− secretion was partially restored to TTX/indomethacin‐treated colons by addition of a subsecretory concentration of PGE2 (1 nM). In acutely isolated, unstimulated crypt cells, we measured PGE2 release at a similar level. We conclude that autocrine release of PGs from epithelial cells is sufficient to support the CCh‐induced Cl− secretory response and is a likely co‐factor in this response.
The Effect of Carbachol and α‐Bungarotoxin on the Frequency of Miniature Endplate Potentials at the Frog Neuromuscular Junction
Authors: Bukharaeva Ela , Ipatova Tania , Nikolsky E. E. , Vyskocil F.
Published: May 2004

The effects of an acetylcholine analogue, carbachol (CCh), and a purified irreversible nicotinic antagonist, α‐bungarotoxin (BTX), on the frequency of the miniature endplate potentials (mEPPs) at the neuromuscular junction of the frog were tested at 20 and 10°C. CCh (5 ± 10‐6 m) reduced the frequency of mEPPs to about 60%; this reduction was not affected by 1 ± 10‐7 g ml‐1 BTX. BTX also reversibly decreased the mEPP frequency by 40%, but not in the presence of CCh or in Ringer solution with 0 or 8 mM Ca2+. The present data show that BTX, which inhibits a class of nicotinic ACh receptors, does not block the decrease of mEPP frequency evoked by CCh and can itself suppress the frequency of spontaneous quantal release.
Growth Induced by Incremental Static Stretch in Adult Rabbit Latissimus Dorsi Muscle
Authors: Cox Valerie M. , Williams Pamela E. , Wright Helena , James Robert S. , Gillott Kay L. , Young Iain S. , Goldspink David F.
Published: May 2004

Incrementally applied static stretch over 3 weeks resulted in a 72% increase in the weight of the in situ latissimus dorsi muscle in rabbits. True growth rather than tissue oedema was confirmed by increases in the protein content (130%), the cross‐sectional area of the type I fibres (30%) and the muscle length (i.e. number of sarcomeres in series increased 25%). Despite an increase in the proportion of fibres staining positive for the enzyme succinate dehydrogenase (SDH), the myosin ATPase stain showed no appreciable fibre type transformation. While total power output in the stretched muscle was unchanged, its maximum mass specific power output, as determined by oscillatory work loops, was decreased by 50%. The cross‐sectional area that was occupied by connective tissue increased from 15 to 19% in the stretched muscles, with a concomitant increase in passive energy dissipation. Some incrementally stretched muscles were then allowed an additional 3 weeks of maintained stretch to determine whether the adaptive changes would be preserved or reversed. Previous gains in muscle weight, length and area of type I fibres all remained. In contrast, the connective tissue content and the passive properties returned to control values during this period.
Permeability Characteristics of Monovalent Cations in Atrial Myocytes of the Rat Heart
Authors: Youm Jae Boum , Ho Won‐Kyung , Earm Yung E.
Published: May 2004

We investigated the permeability of Cs+ and Na+ through various ion channels in rat atrial myocytes using the whole‐cell voltage‐clamp technique. With isotonic CsCl (140 mM) on both sides of the membrane and nominally [Ca2+]o‐free conditions, depolarising clamp pulses induced an increase of outward currents which showed a biphasic time course. Repolarisation to the holding potential induced inward tail currents. With isotonic NaCl, depolarisation also induced outward currents which showed a monotonic decay, but inward tail currents were not observed. Both in NaCl and CsCl, currents were hardly affected by TEA (10 mM), 4‐AP (5 mM) and DIDS (100 μM). Nicardipine (1 M) almost completely blocked time‐dependent outward currents in isotonic NaCl solution, leaving only time‐independent currents which showed linear I‐V relationship. In isotonic CsCl conditions, nicardipine blocked outward current considerably, but there still remained time‐dependent outward currents and inward tail currents. Addition of E‐4031 (2‐20 M) which is known as a specific blocker of the rapidly activating delayed rectifier K+ current (IKr) completely blocked these time‐dependent outward and inward currents, leaving only a time‐independent current. Time‐independent currents recorded in the presence of nicardipine and E‐4031 were inhibited by GdCl3, which is known to block non‐selective cation (NSC) currents. From these results, it was suggested that NSC current in atrial myocytes can be investigated in isotonic Cs+ or Na+ solution in the presence of Ca2+ channel and IKr blockers.
Paraventricular Neurones Elicit a Volume Expansion‐Like Change of Activity in Sympathetic Nerves to the Heart and Kidney in the Rabbit
Authors: Deering J. , Coote J. H.
Published: May 2004

The role of the paraventricular nucleus (PVN) of the hypothalamus in the cardiovascular response induced by blood volume expansion was examined in anaesthetised rabbits, in which simultaneous recordings were made from a renal sympathetic nerve and one of other sympathetic nerves, the inferior cardiac nerve, a splanchnic nerve and an adrenal nerve. Activation of PVN neurones, by discrete injections (25‐100 nl) of d,l‐homocysteic acid (DLH, 0.2 M) produced one main pattern of sympathetic nerve activity accompanying a pressor response (57‐86% of PVN sites). This was a decrease in renal sympathetic activity (27 +/− 12%) and an increase in splanchnic (60 +/− 12%), adrenal (31 +/− 9%) and cardiac (42 +/− 8%) sympathetic activity. Sites in the PVN from which these combinations of nerve activity were obtained were not confined to a specific subnucleus. An increase in renal sympathetic activity which was reversed to a decrease by reducing the volume of DLH injected was obtained at 10 sites in the PVN. These sites were mainly located in the dorsal parvocellular subnucleus. Varying combinations of sympathetic activation were obtained at a minority of sites. It is concluded that the PVN can evoke a differential pattern of sympathetic discharge which may be functionally significant in the control of blood volume regulation, as it mimics that seen on acute volume expansion.
Absence of Selective Brain Cooling in Free‐Ranging Zebras in Their Natural Habitat
Authors: Fuller Andrea , Maloney Shane K. , Kamerman Peter R. , Mitchell Graham , Mitchell Duncan
Published: May 2004

We used implanted miniature data loggers to measure brain and arterial blood temperatures in three free‐ranging zebras (Equus burchelli) in their natural habitat, every 5 min for 9 days. The animals experienced globe temperatures exceeding 40°C, and radiant heat load of about 1000 W m‐2. Arterial blood exhibited a moderate amplitude (1.7°C) nychthemeral rhythm, with an acrophase at 19.00 h and a nadir late in the morning, at 10.00 h. Brain temperature consistently exceeded blood temperature, on average by 0.2‐0.4°C, and changes in brain temperature closely tracked changes in blood temperature. There was no evidence of selective brain cooling, even during the hyperthermia which followed surgery or that associated with intense, short‐duration exercise. The relationship between brain and arterial blood temperatures in free‐ranging zebras was unlike that reported for horses in the laboratory. Our results do not support the view that mammals lacking a carotid rete can achieve selective brain cooling.
The Role of Nitric Oxide in the Coronary Vasoconstriction Caused by Growth Hormone in Anaesthetized Pigs
Authors: Molinari C. , Battaglia A. , Bona G. , Grossini E. , Mary D. A. S. G. , Vacca G.
Published: May 2004

Intravenous injection of growth hormone in anaesthetized pigs has been shown to cause coronary vasoconstriction by antagonizing the vasodilatory effects of β2‐adrenergic receptors. Because nitric oxide is believed to modulate or mediate β2‐adrenergic effects, the present study was undertaken in the same experimental model to determine the role of nitric oxide in the above response to growth hormone. In fourteen pigs anaesthetized with sodium pentobarbitone, changes in left circumflex or anterior descending coronary blood flow caused by intravenous injection of 0.05 i.u. kg‐1 of growth hormone at constant heart rate and arterial blood pressure were assessed using electromagnetic flowmeters. In a first control group of six pigs, growth hormone caused a decrease in coronary blood flow which averaged 13.1% of the baseline values. In a second group of eight pigs, intravenous administration of Nω‐nitro‐L‐arginine methyl ester (L‐NAME) was used to block the endothelial release of nitric oxide. In these pigs, the subsequent injection of growth hormone did not cause any significant changes in coronary blood flow, 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. These results indicated that the coronary vasoconstricting effect of growth hormone, known to involve antagonism of β2‐adrenergic vasodilatory effect, was mediated by inhibition of nitric oxide release.
Epidermal Growth Factor Regulates Amino Acid Transport in Chick Embryo Hepatocytes via Protein Kinase C
Authors: Marino Maria , Mele Raffaella , Spagnuolo Silvana , Pulcinelli Fabio Maria , Mangiantini Maria Teresa , Leoni Silvia
Published: May 2004

System A‐mediated amino acid transport, activation of different steps of signal transduction and involvement of different isoforms of protein kinase C (PKC) have been investigated in chick embryo hepatocytes after epidermal growth factor (EGF) stimulation. EGF rapidly (10 min) increased the rate of aminoisobutyric acid (AIB) uptake in chick embryo hepatocytes freshly isolated on the 19th day of embryonic life, while no change was detectable at other embryonal stages. The growth factor stimulation was abolished by PKC and tyrosine kinase inhibitors and was mimicked by 4‐phorbol‐12‐myristate‐13‐acetate, dimethyl‐2 (PMA). EGF treatment did not modify the phosphorylation of the γ isoform of phospholipase C (PLC‐γ), and inositol trisphosphate (IP3) and intracellular calcium levels, but it induced an increase in PKC activity. Our data show that EGF regulates amino acid uptake, via PKC and without PLC‐γ activation, only in the last period of chick embryo hepatocyte development. The effects of growth factor on PKC activity suggest the involvement of PKC‐α and ‐ε isoforms in EGF modulation of amino acid transport.