The study was designed to develop an improved technique for perfusing the isolated caudal lobe of sheep liver. Twenty caudal lobes were perfused for 3-4 h, in a non-recirculating mode, with Krebs-Henseleit bicarbonate buffer. The perfusion system was designed to give a constant flow. The hepatic viability and functional normality of the perfused lobe were assessed by measuring the perfusion flow rate, pH, K+ efflux, O2 uptake, substrate uptake, gluconeogenesis from propionate and amino acids, and ureagenesis from ammonia and amino acids. Liver tissue was sampled for histological examination, as well as for the determination of liver glycogen and wet : dry weight ratio. The perfusion flow rate and pH were both stable throughout the perfusion. The potassium concentration in the effluent perfusate did not increase during the perfusion, suggesting that there was no loss of viability or hypoxia. The perfused lobe extracted more than 50 % of the O2 supply. The rate of oxygen consumption was comparable to the rate reported in vivo. The initial glycogen content was reduced by about 40 % after 4 h perfusion. The wet : dry weight ratio was 3.6, consistent with the absence of tissue oedema. Urea production was stimulated when NH4Cl (0.3 mM) was added to the medium but there was no significant increase in urea release when alanine (0.15 mM), glutamine (0.2 mM) or lysine (0.2 mM) was added. Urea production, however, increased by about 171 % when a physiological mixture of amino acids was added. Propionate (0.5 mM), alanine and glutamine stimulated glucose production but not lysine or the complete amino acid mixture. Glutamine release was lower than that reported in the rat liver. Changing the direction of flow also revealed an apparent difference between livers from sheep and rats in their metabolism of ammonia. The improved technique offers a simple practical and inexpensive approach to many problems in ruminant physiology and nutritional biochemistry. Experimental Physiology (2000) 85.5, 469-478.

Previous studies have shown that central administration of GABA (gamma -aminobutyric acid), an inhibitory neurotransmitter, preferentially reduces hindquarters and carotid vascular resistances but not renal and coeliac vascular resistances in conscious rats. This study tested the hypothesis that these preferential actions of central GABA receptors are related to differences between vessels in resting autonomic vascular tone in freely moving rats. Rats were chronically implanted with intracisternal cannulas and/or electromagnetic probes to measure regional blood flows. In response to GABA administration, the changes in vascular resistance (arterial blood pressure/regional blood flow) of the hindquarters (n = 23) and carotid (n = 12) vascular beds were significantly and negatively correlated with basal vascular resistance. No such relationship was found for the renal (n = 21), coeliac (n = 13) and superior mesenteric (n = 23) vascular beds. This finding indicates that the responsiveness to GABA of brainstem pathways controlling the hindquarters and carotid vascular beds co-varies with resting resistance in hindquarters and carotid vessels. A similar analysis was performed, correlating the ongoing vascular resistance of each vessel with its response to ganglionic blockade by chlorisondamine. In this case, a significant negative correlation was also found for the hindquarters (n = 26) and carotid (n = 15) vascular beds, but not for the coeliac (n = 17) or superior mesenteric (n = 19) vessels. Together, these findings suggest that central GABA receptors accessible from the cisterna magna preferentially affect two vascular beds which, in the freely moving rat, show resting autonomic vascular tone. Experimental Physiology (2000) 85.5, 479-485.

The response of pulmonary arteries to hypoxia varies as a function of vessel diameter. Small intrapulmonary resistance arteries are thought to be the main site of hypoxic pulmonary vasoconstriction (HPV), with hypoxia causing minimal contraction or even dilatation in large, conduit vessels. This has been proposed to reflect a differential distribution of morphologically and electrophysiologically distinct pulmonary artery smooth muscle (PASM) cells. We investigated longitudinal heterogeneity in smooth muscle cells isolated from five regions of the rabbit pulmonary vasculature and could find no evidence of morphological heterogeneity at the level of the light microscope. PASM cells from main (8 mm outer diameter) and branch (5 mm) arteries and large ( 400 m) intrapulmonary arteries (IPA) were similar in shape and size, as indicated by cell capacitance (25 pF). PASM cells from medium (200-400 m) and small ( 200 m) IPA were significantly smaller (15 pF), but had the same classical spindle shape. Cells from all five regions also had similar resting membrane potentials and displayed voltage-activated K+ currents of similar amplitude when recorded in standard physiological solution. Longitudinal heterogeneity in K+ current became apparent when tetraethylammonium ions (TEA; 10 mM) and glibenclamide (10 M) were added. The remaining delayed rectifier current (IK(V)) doubled in amplitude upon moving down the pulmonary arterial tree from the main artery (9 pA pF-1 at 40 mV) to the large IPA (17 pA pF-1), but remained constant throughout the intrapulmonary vasculature. The O2-sensitive, non-inactivating K+ current (IK(N)) showed a similar trend, but was significantly reduced in the smallest IPA, where its amplitude was comparable with the main artery. Thus the IK(N)/IK(V) ratio was relatively constant, at around 0.14, from the main pulmonary artery to medium IPA, but fell by 50 % in the smallest vessels. The amplitude of the TEA-sensitive K+ current was similar (16 pA pF-1 at 40 mV) at all levels of the pulmonary arterial tree, except in the medium sized vessels where it was 50 % smaller. These variations in K+ current expression correlate with reported variations in sensitivity to hypoxia and may contribute to the regional heterogeneity of HPV in the rabbit lung. Experimental Physiology (2000) 85.5, 487-496.

The purpose of the present study was to characterize the Ca2+ channels in smooth muscle cells from human stomach and to examine the effects of osmotic swelling on the channel activity. Ca2+ channel current with either Ca2+ or Ba2+ as charge carrier was recorded from freshly isolated smooth muscle cells using the conventional whole-cell patch clamp technique. The degree of cell swelling as a result of hypotonic challenge was monitored using a video image analysis system. The changes in intracellular Ca2+ concentration ([Ca2+]i) were measured by microfluorimetry. The pharmacological and voltage activation profile suggests a typical dihydropyridine-sensitive L-type Ca2+ current. Cell swelling, induced by hypotonic challenge, enhanced the amplitude of currents through L-type Ca2+ channels without significant effects on steady-state voltage dependency. After treatment with the L-type Ca2+ channel agonist Bay K 8644 (0.1-2 µM), no further significant increase in calcium channel current or corresponding [Ca2+]i transients were provoked by the swelling. The above results demonstrated that the presence of L-type Ca2+ current in smooth muscle cells of the human stomach and the augmentation of the current are closely associated with the volume increase resulting from hypotonic swelling. Experimental Physiology (2000) 85.5, 497-504.

Species differences in response to hypoxic damage have been observed in studies using whole hearts. The aims of this study were to determine whether (i) species differences in response to simulated hypoxia could be detected at the level of the single myocyte, and (ii) there were any interspecies differences in the Ca2+ handling properties of the cells. Ventricular myocytes were isolated from hearts of adult rats and guinea-pigs and electrically stimulated on the stage of a fluorescence microscope. Cell length was measured using an edge-tracking device, and total intracellular [Ca2+] ([Ca2+]i) determined using indo-1. Cells were exposed to metabolic inhibition (MI) (2.5 mM NaCN and no glucose) to simulate hypoxia followed by washout of CN and re-addition of glucose ('reperfusion'). Following exposure to MI, rat cells underwent rigor contracture in 18.8 ± 0.8 min (n = 80 cells), whereas the time was longer for guinea-pig cells (32.9 ± 1.2 min, n = 83) (P < 0.001). If cells were reperfused after 1-5 min in rigor, then rat cells showed improved morphological recovery compared with guinea-pig cells (P < 0.05); thereafter recovery decreased with increasing time spent in rigor, and was similar in both groups. In indo-1 loaded cells, [Ca2+]i was significantly increased in cells from both species at the end of MI; however, the actual increase was much higher in guinea-pig cells. Upon reperfusion, [Ca2+]i recovered fully in rat cells, but in guinea-pig cells there was no significant decrease. The restoration of [Ca2+]i to normal levels in rat cells following MI was associated with improved contractile recovery compared with guinea-pig cells. We conclude that rat cells are more resistant to effects of MI than are guinea-pig cells; this may be related to species differences in Ca2+ handling during and following exposure to MI. Experimental Physiology (2000) 85.5, 505-510.

Secretion of fluid and proteins by salivary cells is under the control of parasympathetic and sympathetic autonomic nerves. In a recent study we have shown that, in the rat submandibular gland, autonomic nerves can also increase the secretion of IgA, a product of plasma cells secreted into saliva as SIgA (IgA bound to Secretory Component, the cleaved poly-immunoglobulin receptor). The present study aimed to determine if parotid secretion of SIgA is increased by autonomic nerves and to compare SIgA secretion with other parotid proteins stored and secreted by acinar and ductal cells. Assay of IgA in saliva evoked by parasympathetic nerve stimulation immediately following an extended rest period under anaesthesia indicated that it had been secreted into intraductal saliva in the absence of stimulation during the rest period. The mean rate of unstimulated IgA secretion (2.77 ± 0.28 µg min-1 g-1) and the 2.5-fold increase in IgA secretion evoked by parasympathetic stimulation were similar to results found previously in the rat submandibular gland. Sympathetic nerve stimulation increased SIgA secretion 2.7-fold, much less than in the submandibular gland. SDS-PAGE and Western blot analysis with anti-IgA and anti-Secretory Component antibodies confirmed that SIgA was the predominant form of IgA in saliva. Acinar-derived amylase and ductal-derived tissue kallikrein were more profoundly increased by parasympathetic and particularly sympathetic stimulation than SIgA. Overall, the results of the present study indicate that SIgA forms a prominent component of unstimulated parotid salivary protein secretion and that its secretion is similarly increased by stimulation of either autonomic nerve supply. The secretion of other parotid salivary proteins that are synthesized and stored by acinar or ductal cells is upregulated to a much greater extent by parasympathetic and particularly sympathetic stimulation. Experimental Physiology (2000) 85.5, 511-518.

The deterioration of the immune system with ageing, which leads to an increased morbidity and mortality from infections, appears to be related to decreases in specific lymphocyte functions. However, the alteration of non-specific immunity is a more controversial subject. Our purpose was to investigate the age-related changes of different functions of the non-specific immune response in peritoneal macrophages (adherence to tissues, mobility directed to a chemical gradient from an infectious focus or chemotaxis, phagocytosis of foreign agents and destruction of these agents by superoxide anion production) and in lymphocytes (adherence and chemotaxis) from peritoneum, axillary lymph nodes, spleen and thymus. We used young (12 weeks), adult (22 weeks), mature (48 weeks) and old (72 weeks) female BALB/c mice. The adherence capacity of macrophages and lymphocytes was greater in adult and old mice than in young animals. The chemotaxis of macrophages showed higher values in cells from young mice than in those from adult mice, increasing again in macrophages from mature and old animals. A similar behaviour was shown by phagocytosis, which reached its highest values in old animals. Anion superoxide production increased with age and again the highest values were obtained in the oldest mice. Conversely, chemotaxis of lymphocytes was higher in the adult and mature animals than in the young and old animals. We conclude that, although there is a decrease in lymphocyte chemotaxis in old animals, the non-specific immune response of macrophages instead of decreasing, may increase in aged mice with respect to the values seen in adult mice. Experimental Physiology (2000) 85.5, 519-525.

In mammals that use the ventilatory system as the principal means of increasing heat loss, raising body temperature causes the adoption of a specialised breathing pattern known as panting and this is mediated by the thermoregulatory system in the preoptic area of the hypothalamus. In these species an additional respiratory drive is also present at raised body temperature, since breathing can reappear at low Pa,CO2 levels, when stimulation of chemoreceptors is minimal. It is not known whether the preoptic area is also the source of this additional drive. Rats do not pant but do possess this additional respiratory drive at raised body temperatures. We have therefore tested whether the preoptic area of the hypothalamus is the source of this additional respiratory drive in rats. Urethane anaesthesia and hyperoxia were used in eleven rats to minimise behavioural and chemical drives to breathe. The presence of the additional respiratory drive was indicated if rhythmic diaphragmatic EMG activity reappeared during hypocapnia (a mean Pa,CO2 level of 21 ± 2 mmHg, n = 11), induced by mechanical ventilation. The additional respiratory drive was absent at normal body temperature (37°C). When the temperature of the whole body was raised using an external source of radiant heat, the additional respiratory drive appeared at 40.6 ± 0.5°C (n = 3). In two further rats this drive was induced at normal body temperature by localised warming in the preoptic area of the intact hypothalamus. The additional respiratory drive appeared at similar temperatures to those in control rats in three rats following isolation of the hypothalamus from more rostral areas of the brain. In contrast, the additional respiratory drive failed to appear at these temperatures in three rats after isolating the hypothalamus from the caudal brainstem, by sectioning pathways medial to the medial forebrain bundle. Since the preoptic area is known to contain thermoreceptors and to receive afferents from peripheral thermoreceptors, the results show that this area is also the source of the additional respiratory drive at raised body temperature in anaesthetised rats. Experimental Physiology (2000) 85.5, 527-537.

Uptakes of oxygen, glucose and lactate by the gravid uterus, fetus and uteroplacental tissues were measured in chronically catheterized pregnant ponies and their fetuses at mid- and late gestation (term 335 days). Rates of O2 uptake by the gravid uterus, fetus and uteroplacental tissues were significant at both gestational ages and were 2- to 3-fold higher in late gestation than the mid-gestation values of 3338 ± 794, 1352 ± 258 and 2035 ± 602 µmol min-1, respectively (n = 4). Similarly, there were significant uptakes of glucose by the gravid uterus, fetus and uteroplacental tissues at both mid- and late gestation. However, unlike O2 uptake, glucose uptake by the uterus and uteroplacental tissues did not increase between mid- and late gestation. No significant uptakes or outputs of lactate were observed by the uterus or uteroplacental tissues at either gestational age, although there was a significant umbilical uptake of lactate in late but not mid-gestation. There was no change in the distribution of uterine O2 uptake between the fetus and uteroplacental tissues with increasing gestational age. The uteroplacental tissues accounted for about 50 % of the uterine O2 uptake at both gestational ages. In contrast, the proportion of the uterine glucose uptake used by the uteroplacental tissues decreased from 73.2 ± 2.1 % (n = 5) at mid-gestation to 61.1 ± 1.9 % (n = 4, P < 0.02) in late gestation. The gestational changes in uteroplacental carbohydrate metabolism in the mare differ from those seen in the ewe and may have important consequences for the duration and outcome of pregnancy in the mare. Experimental Physiology (2000) 85.5, 539-545.

We investigated anaerobic threshold (thetaL) gas exchange kinetics and maximal oxygen uptake (VO2,max) among older men with reduced left ventricular end-diastolic filling (LVDF). Ten men (mean age, 73 years) with LVDF impairment and low fitness, but without other cardiovascular dysfunction were studied. Treatments compared to control included: 5 days, high intensity exercise training protocol; 5 days, calcium channel blockade (240 mg verapamil); 21 days, detraining/washout; and 5 days, combined treatments. Results indicated no changes in resting left ventricular systolic function with any treatment. Significant resting diastolic function changes included increased early:late flow velocity (control, 0.87; training, 1.28; verapamil, 1.32), and a decreased isovolumic relaxation time (control, 0.10 s; training, 0.08 s; verapamil, 0.08 s). The combined treatments were not additive. Sub-threshold oxygen uptake kinetics (tauVO2, s) were significantly faster following either training or verapamil (tauVO2,control, 62 ± 12; tauVO2,training, 44 ± 9; tauVO2,verapamil, 48 ± 10) and combined treatments (tauVO2, 41 ± 8). V O2,max (ml kg-1 min-1) was significantly increased (control, 21.8 ± 2.2; training, 27.3 ± 2.2; verapamil, 25.2 ± 3.4; combined treatments, 26.9 ± 2.3). Increasing ventricular preload with either exercise training or calcium channel blockade was coincident with faster tauVO2 and increased VO2,max. Experimental Physiology (2000) 85.5, 547-555.

This study seeks to confirm the progressive changes in cardiac output and heart rate previously reported with 8 h exposures to constant hypoxia, and to examine the role of sympathetic mechanisms in generating these changes. Responses of ten subjects to four 8 h protocols were compared: (1) air breathing with placebo; (2) isocapnic hypoxia (end-tidal PO2 = 50 mmHg) with placebo; (3) isocapnic hypoxia with beta-blockade; and (4) air breathing with beta -blockade. Regular measurements of heart rate and cardiac output (using ultrasonography and N2O rebreathing techniques) were made with subjects seated in the upright position. The sensitivity of heart rate to rapid variations in hypoxia (GHR) and heart rate in the absence of hypoxia were measured at times 0, 4 and 8 h. No significant progressive effect of hypoxia on cardiac output was detected. There was a gradual rise in heart rate with hypoxia of 11 ± 2 beats min-1 in the placebo protocol and of 10 ± 2 beats min-1 in the beta-blockade protocol over 8 h, compared to the air breathing protocols. The rise in heart rate was progressive (P < 0.001) and accompanied by progressive increases in both GHR (P < 0.001) and heart rate measured in the absence of hypoxia (P < 0.05). No significant effect of beta-blockade was detected on any of these progressive changes. We conclude that sympathetic mechanisms that act via beta -receptors play little role in the progressive changes in heart rate observed over 8 h of moderate hypoxia. Experimental Physiology (2000) 85.5, 557-565.

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.

The response to strength training varies widely between individuals and is considerably influenced by genetic variables, which until now, have remained unidentified. The deletion (D), rather than the insertion (I), variant of the human angiotensin-converting enzyme (ACE) genotype is an important factor in the hypertrophic response of cardiac muscle to exercise and could also be involved in skeletal muscle hypertrophy - an important factor in the response to functional overload. Subjects were 33 healthy male volunteers with no experience of strength training. We examined the effect of ACE genotype upon changes in strength of quadriceps muscles in response to 9 weeks of specific strength training (isometric or dynamic). There was a significant interaction between ACE genotype and isometric training with greater strength gains shown by subjects with the D allele (mean ± S.E.M.: II, 9.0 ± 1.7 %; ID, 17.6 ± 2.2 %; DD, 14.9 ± 1.3 %, ANOVA, P 0.05). A consistent genotype and training interaction (ID DD II) was observed across all of the strength measures, and both types of training. ACE genotype is the first genetic factor to be identified in the response of skeletal muscle to strength training. The association of the ACE I/D polymorphism with the responses of cardiac and skeletal muscle to functional overload indicates that they may share a common mechanism. These findings suggest a novel mechanism, involving the renin-angiotensin system, in the response of skeletal muscle to functional overload and may have implications for the management of conditions such as muscle wasting disorders, prolonged bed rest, ageing and rehabilitation, where muscle weakness may limit function. Experimental Physiology (2000) 85.5, 575-579.

This study examined the effect of pre-exercise carbohydrate (CHO) ingestion on pyruvate dehydrogenase complex (PDC) activation, acetyl group availability and substrate level phosphorylation (glycogenolysis and phosphocreatine (PCr) hydrolysis) in human skeletal muscle during the transition from rest to steady-state exercise. Seven male subjects performed two 10 min treadmill runs at 70 % maximum oxygen uptake (VO2,max), 1 week apart. Each subject ingested 8 ml (kg body mass (BM))-1 of either a placebo solution (CON trial) or a 5.5 % CHO solution (CHO trial) 10 min before each run. Muscle biopsy samples were obtained from the vastus lateralis at rest and immediately after each trial. Muscle PDC activity was higher at the end of exercise in the CHO trial compared with the CON trial (1.78 ± 0.18 and 1.27 ± 0.16 mmol min-1 (kg wet matter (WM))-1, respectively; P 0.05) and this was accompanied by lower acetylcarnitine (7.1 ± 1.2 and 9.1 ± 1.1 mmol kg-1 (dry matter (DM))-1 in CHO and CON, respectively; P 0.05) and citrate concentrations (0.73 ± 0.05 and 0.91 ± 0.10 mmol (kg DM)-1 in CHO and CON, respectively; P 0.05). No difference was observed between trials in the rates of muscle glycogen and PCr breakdown and lactate accumulation. This is the first study to demonstrate that CHO ingestion prior to exercise augments the exercise-induced activation of muscle PDC and reduces acetylcarnitine accumulation during the transition from rest to steady-state exercise. However, those changes did not affect the contribution of substrate level phosphorylation to ATP resynthesis. Experimental Physiology (2000) 85.5, 581-586.