Influence of hypoxia and of hypoxemia on the development of cardiac activity in zebrafish larvae

Cardiac activity and anaerobic metabolism were analyzed in zebrafish larvae raised under normoxia (PO(2) = 20 kPa) and under chronic hypoxia (PO(2) = 10 kPa) at three different temperatures (25, 28, and 31 degrees C). Heart rate increased with development and with temperature. Under normoxia, cardiac output increased significantly at high temperature (31 degrees C), but not at 28 or at 25 degrees C. Under chronic hypoxia, however, heart rate as well as cardiac output increased at all temperatures in larvae at about hatching time or shortly thereafter. Cardiac activity of larvae raised for 2 wk after fertilization with a reduced hemoglobin oxygen-carrying capacity in their blood (hypoxemia; due to the presence of CO or of phenylhydrazine in the incubation water) was not different from control animals. Whole body lactate content of these animals did not increase. Thus there was no indication of a stimulated anaerobic energy metabolism. The increase in cardiac activity observed during hypoxia suggests that at about hatching time receptors are present that sense hypoxic conditions, and this information can be used to induce a stimulation of convective oxygen transport to compensate for a reduction in bulk oxygen diffusion in the face of a reduced oxygen gradient between environmental water and tissues. Under normoxia, however, the PO(2) gradient between environmental water and tissues and diffusional oxygen transport assure sufficient oxygen supply even if hemoglobin oxygen transport in the blood is severely impaired. Thus, under normoxic conditions and with a normal metabolic rate of the tissues, convective oxygen transport is not required until approximately 2 wk after fertilization.     

Morphological development of the swim bladder in hatchery-reared striped trumpeter Latris lineata

This study examined swim bladder morphogenesis in three cohorts of striped trumpeter (Latris lineata), a euphysoclist species with physostomous larvae. The swim bladder was first discernible 1–2 days after hatching as an evagination on the dorsal surface of the incipient digestive tract. It comprised a cluster of mesenchymal cells surrounding an inner primordium of epithelial cells. At mouth opening in larvae of 5.3 mm standard length (SL), the swim bladder was noticeably enlarged. Histologically, the swim bladder lumen was dilated and liquid filled. The pneumatic duct was first seen during the dilation stage and the rete mirabile began forming among the connective tissue surrounding the swim bladder. Initial swim bladder inflation occurred on day 11 post‐hatching in Cohort 1, at 14°C, and day 9 post‐hatching, in Cohorts 2 and 3, at 16°C. Histologically, the lumens of inflated swim bladders were ellipsoid and the epithelium was squamous, except for cuboidal gas gland cells at the anterio‐ventral and anterio‐lateral regions of the swim bladder. During the initial inflation interval the pneumatic duct was dilated in larvae both with and without swim bladder inflation. The pneumatic duct began to regress in some larvae over 7.5 mm SL. The swim bladder of striped trumpeter was similar to larvae of other altricial perciform marine fish in respect to organ derivation, tissue differentiation, luminal dilation and initial gaseous inflation. However, variations, particularly the delay in initial swim bladder inflation until after the start of feeding, were observed that could be fundamental to problems encountered during larval rearing.     

Cold tolerance and the regulation of cardiac performance and hemolymph distribution in Maja squinado (Crustacea: decapoda)

Elevated Mg(2+) levels in the hemolymph ([Mg(2+)](HL)) of brachyuran crabs have recently been demonstrated to limit cold tolerance by reducing motor and circulatory activity. Therefore, the limiting function of elevated [Mg(2+)](HL) on circulatory performance and arterial hemolymph flow was investigated by the pulsed-Doppler technique in the spider crab Maja squinado during progressive cooling from 12 degrees to 0 degrees C. [Mg(2+)](HL) were reduced from control levels of 39.9 mmol L(-1) to levels of 6.1 mmol L(-1) by incubation in magnesium reduced seawater. At 12 degrees C cardiac output was 13.9+/-2.4 mL kg(-1) min(-1) and stroke volume 0.2+/-0.04 mL kg(-1) min(-1) in control animals. In [Mg(2+)](HL)-reduced animals cardiac output increased to 43.6+/-5.0 mL kg(-1) min(-1) and stroke volume rose to 0.6+/-0.1 mL kg(-1) min(-1). Temperature reduction in control animals revealed a break point at 8 degrees C linked to a major redirection of hemolymph flow from lateral to sternal and hepatic arteries. Cardiac output and heart rate dropped sharply during cooling until transiently constant values were reached. Further heart rate reduction occurred below 4.5 degrees C. Such a plateau was not detected in [Mg(2+)](HL)-reduced animals where the break point decreased to 6 degrees C, also indicated by a sharp drop in heart rate and cardiac output and the redirection of hemolymph flow. It is concluded that progressive cooling brings the animals from a temperature range of optimum cardiac performance into a deleterious range when aerobic scope for activity falls before critical temperatures are reached. Reduction of [Mg(2+)](HL) shifts this transition to lower temperatures. These findings support a limiting role for [Mg(2+)](HL) in thermal tolerance.     

Cardiac function and critical swimming speed of the winter flounder (Pleuronectes americanus) at two temperatures

Using Transonic flow probes and a uniquely designed swimming flume, we directly measured cardiac parameters (Q, cardiac output; SV, stroke volume; and fH, heart rate) in winter flounder (Pleuronectes americanus) before and during critical swim speed (Ucrit) tests at 4 and 10 degrees C. Resting Q, SV and fH averaged 9.8 ml min(-1) kg(-1), 0.5 ml kg(-1) (1.0 ml g ventricle(-1)) and 21 beats min(-1) at 4 degrees C and 15.5 ml min(-1) kg(-1), 0.5 ml kg(-1) (0.95 ml g ventricle(-1)) and 34 beats min(-1) at 10 degrees C (Q10 values of 2.13, 0.91 and 2.35, for Q, SV and fH, respectively). Cardiac output, SV and fH increased by approx. 170%, 70% and 60% at both temperatures during the Ucrit test. However, cardiac parameters generally reached near maximal levels almost immediately upon swimming and remained at these levels until Ucrit (0.65 +/- 0.06 bl s(-1) at 4 degrees C and 0.73 +/ -0.07 bl s(-1) at 10 degrees C). This rapid rise in cardiac function to near maximal levels did not appear to be the result of stress alone, as Q only fell slightly when flounder were swum for 75 min at < 0.4 bl s(-1), speeds at which they appeared to swim comfortably. Our results suggest that both Q and Ucrit have been significantly overestimated in flatfishes, and that "lift-off"/slow swimming is energetically expensive. Furthermore, they show that maximum and resting stroke volume (per g of ventricle) are extremely high in the flounder as compared with other teleosts.     

Postnatal development of intestinal bile salt transport. Relationship to membrane physico-chemical changes

The postnatal development of intestinal bile salt transport in the rat was examined using the villus technique. Jejunal uptake of taurocholate was linear with respect to incubation concentration at all study ages. Ileal uptake was linear with taurocholate concentration during the first 2 postnatal weeks; a curvilinear relationship indicating the presence of saturable transport appeared during the third week. With the appearance of ileal active transport at age 3 weeks, the Km (app) was constant at 0.49 mM, 0.59 mM, and 0.50 mM in 3-week, 4-week, and adult animals, respectively. The V(app) was 14.65 nmol X mg-1 (dry wt) X min-1 at 3 weeks and declined with age to 11.40 and 10.51 nmol X mg-1 (dry wt) X min-1 in 4-week and adult animals, respectively. The role of physico-chemical changes in the microvillus membrane in the development of ileal active transport was examined. With increasing postnatal age, microvillus membrane cholesterol content rose while the phospholipid content remained unchanged in both ileum and jejunum. Corresponding rises in the cholesterol/phospholipid ratio were observed in both sites. Simultaneously, the microvillus membrane fatty acid composition was changing from predominantly saturated to unsaturated species in both ileum and jejunum. The microvillus membrane fluorescence anisotropy (r) increased with postnatal age in jejunum when measured at 25 degrees C and 37 degrees C and ileum when measured at 25 degrees C; however, no change was noted in ileum when measured at 37 degrees C. Ileal active bile salt transport develops during the third postnatal week, and is associated with concurrent changes in membrane lipid composition and fluidity when measured at 25 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of progressive hypoxia on respiration in the dogfish (scyliorhinus canicula) at different seasonal temperatures.

1. Dogfish were acclimated to 7, 12 or 17 degrees C and exposed to progressive hypoxia at the temperature to which they had been acclimated. During normoxia, the Q10 values for oxygen uptake, heart rate, cardiac output and respiratory frequency over the full 10 degrees C range were: 2.1, 2.1, 2.1 and 2.5 respectively. Increased acclimation temperature had no effect on cardiac stroke volume or systemic vascular resistance, although there was a decrease in branchial vascular resistance, pHa and pHv. 2. Progressive hypoxia had no effect on heart rate or oxygen uptake at 7 degrees C, whereas at 12 degrees C and 17 degrees C there was bradycardia, and a reduction in O2 uptake, with the critical oxygen tension for both variables being higher at the higher temperature. Cardiac stroke volume increased during hypoxia at each temperature, such that cardiac output did not change significantly at 12 and 17 degrees C. Neither pHa nor pHv changed significantly during hypoxia at any of the three temperatures. 3. The influence of acclimation temperatures on experimental results from poikilotherms is pointed out. Previously-published results show quantitative differences. 4. The significance of the present results with respect to the functioning and location of oxygen receptors is discussed. It is argued that as the metabolic demand and critical oxygen tension of the whole animal are increased at high acclimation temperatures the same must be the case with the oxygen receptor. This would raise the stimulation threshold and could account for the bradycardia seen during hypoxia becoming manifest at higher values of PI,O2, Pa,O2 and Pv,O2 as the acclimation temperature is raised.     

Cardiovascular correlates of exercise in snapping turtles, Chelydra serpentina

1. Heart rate increased with a rise in body temperature (10-30 degrees C) and with induced physical exercise in snapping turtles. 2. Maximum heart rate increment occurred at 30 degrees C. 3. Standard oxygen pulse did not change with a rise in temperature. 4. Oxygen pulse during exercise and oxygen pulse increment were maximal at 10 degrees C and minimal at 20 degrees C. 5. The increase in heart rate with exercise accounted for only 9-22% of the increase in oxygen transport during activity; the remainder was provided by a rise in cardiac stroke volume and/or A-V difference.     

Man's physiologic response to long-term work during thermal and pollutant stress.

Metabolic, temperature, and cardiorespiratory responses of 19 healthy males, age range 18-30 yr for one group and 40-55 yr for another, were studied during 210 minutes submaximal work at 35% Vo2 max. The subjects were exposed to four different pollutant gas mixtures at two different temperatures, 25 degrees C and 35 degrees C (relative humidity 30%). The four gas mixtures were filtered air (FA), 50 ppm carbon monoxide in filtered air (CO), 0.24 ppm peroxyacetyl nitrate in filtered air (PAN), and a combination of all three mixtures (PANCO). In the CO exposure, the heart rate was significantly greater than that observed during FA conditions (P less than 0.05). Metabolic and thermoregulatory responses to long-term work were not different in the various pollutant environments. Significant decreases in stroke volume and increases in heart rate were observed during the course of the 25 degrees C exposures with no alteration in cardiac output. Heart rates were higher during 35 degrees C exposures while cardiac output remained at the same level with a consequent further reduction in stroke output.     

Differential metabolic capacity of mice selected for magnitude of swim stress-induced analgesia.

Maximum oxygen consumption (Vo(2)) elicited by swimming in 20 degrees C water or by exposure to -2.5 degrees C in helium-oxygen (Helox) atmosphere is higher in mice selected for low (LA) than for high (HA) stress-induced analgesia (SIA) produced by swimming. However, this line difference is greater with respect to swim- than to cold-elicited Vo(2). To study the relationship between the analgesic and thermogenic mechanisms, we acclimated HA and LA mice to 5 degrees C or to daily swimming at 20 or 32 degrees C. Next, the acclimated mice were exposed to a Helox test at -2.5 degrees C and to a swim test at 20 degrees C to compare Vo(2) and hypothermia (DeltaT). Cold acclimation raised Vo(2) and decreased DeltaT. These effects were similar in both lines in the Helox test but were smaller in the HA than in the LA line in the swim test. HA and LA mice acclimated to 20 or 32 degrees C swims increased Vo(2) and decreased DeltaT elicited by swimming, but only HA mice acclimated to 20 degrees C swims increased Vo(2) and decreased DeltaT in the Helox test. We conclude that the between-line difference in swim Vo(2) results from a stronger modulation of thermogenic capacities of HA mice by a swim stress-related mechanism, resulting in SIA. We suggest that the predisposition to SIA observed in laboratory as well as wild animals may significantly affect both the results of laboratory measurements of Vo(2) and the interpretation of its intra- and interspecific variation.     

Hemodynamic and hormonal effects of prolonged anti-G suit inflation in humans.

This study examined the hemodynamic consequences of prolonged lower body positive-pressure application and their relationship to changes in the plasma concentration of the major vasoactive hormones. Six men [36 +/- 2 (SE) yr] underwent 30 min of sitting and then 3 h of 70 degrees head-up tilt. An antigravity suit was applied (60 Torr legs, 30 Torr abdomen) during the last 2 h of tilt. In a similar noninflation experiment, the endocrine responses were measured in the suited subjects tilted for 3 h. Two-dimensional echocardiography was used to calculate ventricular volume and cardiac output. Measurements were made 30 min before and 30 and 90 min after inflation. Immediately after inflation, mean arterial pressure increased by 7 +/- 2 Torr and heart rate decreased by 16 +/- 4 beats/min. Left ventricular end-diastolic volume and systolic volume increased significantly (P less than 0.05) at 30 and 90 min of inflation. Cardiac output increased after 30 min of inflation and returned to the preinflation level at 90 min. Plasma norepinephrine and plasma renin activity were maximally suppressed after 15 and 90 min of inflation, respectively (P less than 0.05). No such hormonal changes occurred during control. Plasma sodium, potassium, and osmolality remained unchanged during both experiments. Thus, prolonged application of lower body positive pressure induces 1) a transient increase in cardiac output and 2) a marked and sustained decrease in plasma norepinephrine and plasma renin activity, which reflect an inflation-induced decrease in sympathetic activity.     

Hemodynamic effects of anti-G suit inflation in a 1-G environment

This study evaluated effects of various anti-G inflation pressures on cardiac volumes and the relationship of these volume changes to mean arterial pressure changes. Ventricular volumes were calculated using two-dimensional echocardiography. An anti-G suit was inflated to 2, 4, and 6 psi in the standing and supine positions for 10 male subjects. In the supine position, mean arterial pressure increased from base line for all three inflation pressures (P = 0.05). The end-diastolic volume increased after 2-psi inflation (P = 0.03). Cardiac output or stroke volume did not change. After standing, mean arterial pressure (P = 0.002), end-diastolic volume (P = 0.002), and stroke volume (P = 0.05) fell after suit deflation. Peripheral vascular resistance fell in the 2- and 4-psi inflation profiles. In the standing protocol, mean arterial pressure, end-diastolic volume, stroke volume, and cardiac output rose with all three inflation pressures (P less than 0.05). After reclining, heart rate increased (P = 0.02) and mean arterial pressure fell (P less than 0.05) in the 4- and 6-psi inflation profiles after suit deflation. Increases in mean arterial pressure are caused by increases in cardiac preload and cardiac output after inflation of the anti-G suit while subjects were standing. Increased cardiac preload was not consistently seen after inflation while subjects were supine. Changes in end-diastolic volume and mean arterial pressure were dependent on the pressure used to inflate the anti-G suit.     

Cardiac output during labour.

Serial measurements of cardiac output and mean arterial pressure were performed in 15 women during the first stage of labour and at one and 24 hours after delivery. Cardiac output was measured by Doppler and cross sectional echocardiography at the pulmonary valve. Basal cardiac output (between uterine contractions) increased from a prelabour mean of 6.99 l/min to 7.88 l/min at greater than or equal to 8 cm of cervical dilatation as a result of an increase in stroke volume. Over the same period basal mean arterial pressure also increased. During uterine contractions there was a further increase in cardiac output as a result of increases in both stroke volume and heart rate. The increment in cardiac output during contractions became progressively greater as labour advanced. At greater than or equal to 8 cm of dilatation cardiac output increased from a basal mean of 7.88 l/min to 10.57 l/min during contractions. There were also further increases in mean blood pressure during contractions. One hour after delivery heart rate and cardiac output had returned to prelabour values, though mean arterial pressure and stroke volume remained raised. By 24 hours after delivery all haemodynamic variables had returned to prelabour values. Haemodynamic changes of the magnitude found in this series are of considerable clinical relevance in managing mothers with complicated cardiovascular function.     

Effects of cold exposure on heart clearing factor lipase and triglyceride utilization in the rat

The clearing factor lipase activity of the rat heart was measured in animals kept at 4 degrees C for several hours and was compared with that in control animals kept at 25 degrees C. The total activity of the enzyme in the heart increased markedly on exposure to the low temperature, whether the animals were in a fed or a fasted state. The activities of both the heparin-releasable and the heparin-nonreleasable enzyme fractions were usually raised. However, only increases in the former could be correlated satisfactorily with corresponding increases in the capacity of the heart to utilize chylomicron triglyceride fatty acids perfused through it. Cold exposure also raised the plasma clearing factor lipase activity and reduced the plasma triglyceride concentration. These changes may have been due, at least in part, to the alterations in the activity of the tissue enzyme.     

Calmodulin-dependent protein phosphatase: a developmental study

Calmodulin-dependent protein phosphatase, one of the major calmodulin-binding proteins in bovine brain, dephosphorylates casein with a specific activity of 15 nmol mg-1 min-1 at 30 degrees C. The stimulation of phosphatase activity by calmodulin is reversed by ethylene glycol bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid or trifluoperazine, a calmodulin antagonist. Antibodies raised in rabbit against the phosphatase inhibit the enzyme activity. The levels of the protein in brain extracts from various animals, determined by a radioimmunoassay, range from 20 micrograms/g of tissue in chick and fish brains to 143 micrograms in rat cerebrum. The ontogeny of the phosphatase was studied in nervous tissues from rat and chick, animals in which synaptogenesis takes place at different times during their development. The levels of the protein increased significantly in rat cerebrum and cerebellum and in chick brain and retina during the periods corresponding to major synapse formation. In rat cerebrum, the enzyme appeared to be equally distributed between the cytosol and the particulate fraction; the level in both compartments increased during the major period of synapse formation. Thus, the development of calmodulin-dependent protein phosphatase closely parallels synaptogenesis, implicating a role in some synaptic function.     

秸秆预处理对土壤微生物量及呼吸活性的影响

冬小麦秸秆经8．0g·L^-1H2O2(pH11．0)溶液、12．5g·L^-1 NaOH溶液或H2SO4溶液浸泡8h并80℃烘干后,与无机N一起加入土壤,进行室内25℃恒温培养试验,在不同时间测定土壤微生物量C、N和CO2释放速率。结果表明,培养前期,秸秆预处理使土壤微生物量C数量增加了1．0～1．4倍,但降低了土壤微生物的呼吸活性;培养后期,NaOH和H2SO4处理使土壤微生物量C分别下降了28％和42％,但增加了土壤微生物的呼吸活性;H2O2处理则使土壤微生物量N增加90％;土壤微生物区系中的真菌比例在不同时刻有所增加,表明将秸秆预处理后施入土壤,将对土壤中微生物数量和呼吸活性产生一定影响。     

Effect of sympathectomy on the heart pump function in rats during postnatal ontogenesis

The influence of guanetidine sympathectomy (30 mg/kg) on the heart pump function in rats during 3 weeks in postnatal ontogenesis has been investigated. Sympathectomy restrains age-dependent establishment of stroke volume, cardiac output and heart rate. The adaptation effects of regular physical training do not develop in the animals with sympathectomy, i.e. heart rate does not decrease and stroke volume does not increase. The initial stage of adaptation of the sympathectomized animals to physical training is accompanied by decrease in stroke volume and remarkable increase in heart rate which indicates the reduction of contractile activity in the myocardium.     

Changes in ornithine decarboxylase activity in response to temperature stress and stimulation of juvenile hormone in Anastrepha fraterculus (Diptera, Tephritidae)

Ornithine decarboxylase (ODC) activity was analyzed in Anastrepha fraterculus (Diptera) females (4 days old) submitted to temperature stress (6 degrees C and 20/6 degrees C) and the topical application of juvenile hormone (JH). ODC activity and ejaculatory apodeme measurements (length and width) were made in males (15 days old) after 6 degrees C stress. JH dose of 500 ng and incubation of 3, 7, and 18 h increased ODC activity. Females reared at 6 degrees C and 20/6 degrees C had higher ODC activity than those reared at 25 degrees C. The treatment of 6 degrees C and JH incubation for 1 h increased ODC activity when compared to 6 degrees C treatments only. However, the treatment of 20/6 degrees C only after 3 or 18 h of JH incubation resulted in higher ODC activity than controls (20/6 degrees C) or 20/6 degrees C plus 1 h of JH incubation. Males did not undergo differences in ODC activity when reared at 6 degrees C or 25 degrees C but the ejaculatory apodeme measurements was higher in those reared at 25 degrees C than in those reared at 6 degrees C. The results can be considered an adaptive process to environmental changes.     

Development of the ear, hearing capabilities and laterophysic connection in the spotfin butterflyfish (Chaetodon ocellatus)

The ontogeny of the ear, swim bladder and laterophysic connection was investigated in the spotfin butterflyfish, Chaetodon ocellatus in order to determine how the development of the laterophysic connection (a Chaetodon synapomorphy) is correlated with ontogenetic changes in the hearing capabilities in these abundant and ecologically important coral reef fishes. Histological and cleared and stained material revealed that the medial opening in the lateral line canal in the supracleithrum (which defines the laterophysic connection), an inflated physoclistous swim bladder, and the three otolithic organs are already present in the smallest individuals examined (7?C15?mm SL). The medial opening in the supracleithrum increases in size and the cylindrical swim bladder horns form after the loss of the head plates characteristic of the tholichthys stage, in individuals ??29?mm SL. The three sensory maculae of the ear increase in size, and the shape of the sacculus changes most dramatically with fish growth; hair cell density is highest in the utriculus. Physiological analysis of the reponse to sound pressure showed that larval and juvenile C. ocellatus had a hearing sensitivity peak at 100?C200?Hz, which was ~30?C40?dB more sensitive than that measured in larval coral reef fishes (e.g., damselfishes) that lack swim bladder horns. C. ocellatus did not show any ontogenetic changes in sensitivity to sound pressure, which may be explained by the fact that the growth of the swim bladder horns maintains the small distance between the swim bladder and ear that was established earlier during the larval stage. The timing of the development of the swim bladder horns suggests that if the laterophysic connection has a sensory acoustic function, its presence in individuals >29?mm SL suggests that its role is limited to post-settlement, reef-based behaviors.     

Effect of temperature on lipids,proteins, and carbohydrates levels during development from egg extrusion to juvenile stage of Cherax quadricarinatus (Decapoda: Parastacidae)

The influence of temperature on biochemical composition, survival and duration of development of Cherax quadricarinatus from egg extrusion to juvenile was analyzed. Berried females were individually subjected to each of 22, 25, 28 and 31 degrees C (n=5 per temperature). Egg samples were obtained every 3 days from egg extrusion to juvenile stage for biochemical analysis. Duration of development and survival decreased with increasing temperature. At 22 and 25 degrees C half of the initial lipid content was consumed during development. At 28 and 31 degrees C, 80% of the initial amount of lipids was consumed. For proteins, depletion rate was significantly lower at 25 degrees C (36% of the initial amount) than at 22, 28 and 31 degrees C (61-65% of the initial amount). For carbohydrates, a significant consumption was observed only at 22 degrees C. Total energy consumption was lower at 22 and 25 degrees C than at 28 and 31 degrees C. We conclude that 22-25 degrees C is the optimal temperature range for C. quadricarinatus egg incubation, although 25 degrees C might be better in terms of development duration in terms of survival, energy cost and protein consumption.     

Effects of high-frequency oscillatory ventilation on vagal and phrenic nerve activities

This study was undertaken to define the mechanism for the respiratory inhibition observed during high-frequency oscillatory ventilation (HFOV). The effects of HFOV on the activities of single units in the vagus (Vna) and phrenic nerves (Pna) were examined in pentobarbital-anesthetized dogs. The animals were either ventilated by intermittent positive-pressure ventilation (IPPV) with and without positive end-expiratory pressure (PEEP), or by HFOV at a frequency of 25 Hz and pump displacement volume of 3 ml/kg. In 13 vagal units the Vna was much higher during HFOV than during IPPV or airway occlusion at a matched airway pressure. Ten units in the phrenic nerves were examined, and Pna (expressed as bursts/min) was attenuated by HFOV in all of them. In four of them, the effect of cooling the vagi to 8-10 degrees C on Pna was examined, and it was found that HFOV failed to alter the Pna. We conclude that 1) HFOV stimulates the pulmonary vagal afferent fibers continuously and to a degree greater than that due to static lung inflation and increased airway pressure and 2) the increased vagal activity during HFOV probably causes phrenic nerve activity inhibition.