Maximum sustainable speed, energetics and swimming kinematics of a tropical carangid fish, the green jack Caranx caballus.

Maximum sustained swimming speeds, swimming energetics and swimming kinematics were measured in the green jack Caranx caballus (Teleostei: Carangidae) using a 41 l temperature-controlled, Brett-type swimming-tunnel respirometer. In individual C. caballus [mean ±s.d. of 22·1 ± 2·2 cm fork length (L(F) ), 190 ± 61 g, n = 11] at 27·2 ± 0·7° C, mean critical speed (U(crit)) was 102·5 ± 13·7 cm s⁻¹ or 4·6 ± 0·9 L(F) s⁻¹. The maximum speed that was maintained for a 30 min period while swimming steadily using the slow, oxidative locomotor muscle (U(max,c)) was 99·4 ± 14·4 cm s⁻¹ or 4·5 ± 0·9 L(F) s⁻¹. Oxygen consumption rate (M in mg O2 min⁻¹) increased with swimming speed and with fish mass, but mass-specific M (mg O2 kg⁻¹ h⁻¹) as a function of relative speed (L(F) s⁻¹) did not vary significantly with fish size. Mean standard metabolic rate (R(S) ) was 170 ± 38 mg O2 kg⁻¹ h⁻¹, and the mean ratio of M at U(max,c) to R(S) , an estimate of factorial aerobic scope, was 3·6 ± 1·0. The optimal speed (U(opt) ), at which the gross cost of transport was a minimum of 2·14 J kg⁻¹ m⁻¹, was 3·8 L(F) s⁻¹. In a subset of the fish studied (19·7-22·7 cm L(F) , 106-164 g, n = 5), the swimming kinematic variables of tailbeat frequency, yaw and stride length all increased significantly with swimming speed but not fish size, whereas tailbeat amplitude varied significantly with speed, fish mass and L(F) . The mean propulsive wavelength was 86·7 ± 5·6 %L(F) or 73·7 ± 5·2 %L(T) . Mean ±s.d. yaw and tailbeat amplitude values, calculated from lateral displacement of each intervertebral joint during a complete tailbeat cycle in three C. caballus (19·7, 21·6 and 22·7 cm L(F) ; 23·4, 25·3 and 26·4 cm L(T) ), were 4·6 ± 0·1 and 17·1 ± 2·2 %L(T) , respectively. Overall, the sustained swimming performance, energetics, kinematics, lateral displacement and intervertebral bending angles measured in C. caballus were similar to those of other active ectothermic fishes that have been studied, and C. caballus was more similar to the chub mackerel Scomber japonicus than to the kawakawa tuna Euthynnus affinis.

Metabolic indicators in the skeletal muscles of harbor seals (Phoca vitulina).

The goal of this study was to determine the distribution of citrate synthase (CS), beta-hydroxyacyl coenzyme A dehydrogenase (HOAD), and lactate dehydrogenase (LDH) activities and myoglobin (Mb) concentration in the locomotor muscles (epaxial muscles) and heart of harbor seals. The entire epaxial musculature, which produces most of the power for submerged swimming, was removed and weighed, and three transverse sections (cranial, middle, and caudal) were taken along the muscle bundle. Multiple samples were taken along points on a circular grid using a 6-mm biopsy. A single sample was taken from the left ventricle of the heart. Muscle groups of similar function were taken from three dogs as a control. Mean values were calculated for four roughly equal quadrants in each transverse section of the epaxial muscles. There were no significant differences among the quadrants within any of the transverse sections for the three enzymes or Mb. However, there were significant differences in the mean enzyme activities and Mb concentrations along the length of the muscle. The middle and caudal sections had significantly higher mean levels of CS, LDH, and Mb than the cranial section, which may be correlated with power production during swimming. The enzyme ratios CS/HOAD and LDH/CS exhibited no variation within transverse sections or along the length of the epaxial muscles. Relative to the dog, the epaxial muscles and heart of the harbor seal had higher HOAD levels and lower CS/HOAD, which, taken together, indicate an increased capacity for aerobic lipid metabolism during diving.

Swimming performance studies on the eastern Pacific bonito Sarda chiliensis, a close relative of the tunas (family Scombridae) I. Energetics.

A large swim tunnel respirometer was used to quantify the swimming energetics of the eastern Pacific bonito Sarda chiliensis (tribe Sardini) (45-50 cm fork length, FL) at speeds between 50 and 120 cm s(-1) and at 18+/-2 degrees C. The bonito rate of oxygen uptake ((O(2)))-speed function is U-shaped with a minimum (O(2)) at 60 cm s(-1), an exponential increase in (O(2)) with increased speed, and an elevated increase in (O(2)) at 50 cm s(-1) where bonito swimming is unstable. The onset of unstable swimming occurs at speeds predicted by calculation of the minimum speed for bonito hydrostatic equilibrium (1.2 FL s(-1)). The optimum swimming speed (U(opt)) for the bonito at 18+/-2 degrees C is approximately 70 cm s(-1) (1.4 FL s(-1)) and the gross cost of transport at U(opt) is 0.27 J N(-1) m(-1). The mean standard metabolic rate (SMR), determined by extrapolating swimming (O(2)) to zero speed, is 107+/-22 mg O(2) kg(-1) h(-1). Plasma lactate determinations at different phases of the experiment showed that capture and handling increased anaerobic metabolism, but plasma lactate concentration returned to pre-experiment levels over the course of the swimming tests. When adjustments are made for differences in temperature, bonito net swimming costs are similar to those of similar-sized yellowfin tuna Thunnus albacares (tribe Thunnini), but the bonito has a significantly lower SMR. Because bonitos are the sister group to tunas, this finding suggests that the elevated SMR of the tunas is an autapomorphic trait of the Thunnini.

Review: Analysis of the evolutionary convergence for high performance swimming in lamnid sharks and tunas.

Elasmobranchs and bony fishes have evolved independently for more than 400 million years. However, two Recent groups, the lamnid sharks (Family Lamnidae) and tunas (Family Scombridae), display remarkable similarities in features related to swimming performance. Traits separating these two groups from other fishes include a higher degree of body streamlining, a shift in the position of the aerobic, red, locomotor muscle that powers sustained swimming to a more anterior location in the body and nearer to the vertebral column, the capacity to conserve metabolic heat (i.e. regional endothermy), an increased gill surface area with a decreased blood-water barrier thickness, a higher maximum blood oxygen carrying capacity, and greater muscle aerobic and anaerobic enzyme activities at in vivo temperatures. The suite of morphological, physiological, and biochemical specializations that define "high-performance fishes" have been extensively characterized in the tunas. This review examines the convergent features of lamnid sharks and tunas in order to gain insight into the extent that comparable environmental selection pressures have led to the independent origin of similar suites of functional characteristics in these two distinctly different taxa. We propose that, despite differences between teleost and elasmobranch fishes, lamnid sharks and tunas have evolved morphological and physiological specializations that enhance their swimming performance relative to other sharks and most other high performance pelagic fishes.

Enzyme activities support the use of liver lipid-derived ketone bodies as aerobic fuels in muscle tissues of active sharks.

Few data exist to test the hypothesis that elasmobranchs utilize ketone bodies rather than fatty acids for aerobic metabolism in muscle, especially in continuously swimming, pelagic sharks, which are expected to be more reliant on lipid fuel stores during periods between feeding bouts and due to their high aerobic metabolic rates. Therefore, to provide support for this hypothesis, biochemical indices of lipid metabolism were measured in the slow-twitch, oxidative (red) myotomal muscle, heart, and liver of several active shark species, including the endothermic shortfin mako, Isurus oxyrinchus. Tissues were assayed spectrophotometrically for indicator enzymes of fatty acid oxidation (3-hydroxy-o-acyl-CoA dehydrogenase), ketone-body catabolism (3-oxoacid-CoA transferase), and ketogenesis (hydroxy-methylglutaryl-CoA synthase). Red muscle and heart had high capacities for ketone utilization, low capacities for fatty acid oxidation, and undetectable levels of ketogenic enzymes. Liver demonstrated undetectable activities of ketone catabolic enzymes but high capacities for fatty acid oxidation and ketogenesis. Serum concentrations of the ketone beta-hydroxybutyrate varied interspecifically (means of 0.128-0.978 micromol mL(-1)) but were higher than levels previously reported for teleosts. These results are consistent with the hypothesis that aerobic metabolism in muscle tissue of active sharks utilizes ketone bodies, and not fatty acids, derived from liver lipid stores.

Ontogenetic changes in characteristics required for endothermy in juvenile black skipjack tuna (Euthynnus lineatus).

To characterize better the development of endothermy in tunas, we assessed how the abilities to generate heat and to conserve heat within the aerobic, slow-twitch (red) myotomal muscle using counter-current heat exchangers (retia) change with size in juvenile black skipjack tuna (Euthynnus lineatus) above and below the hypothesized minimum size for endothermy of 207 mm fork length (FL). Early juvenile scombrids (10-77 mm FL) collected off the Pacific coast of Panama were raised to larger sizes at the Inter-American Tropical Tuna Commission Laboratory at Achotines Bay, Panama. Evidence of central and lateral rete blood vessels was found in E. lineatus as small as 95.9 mm FL and 125 mm FL, respectively. In larger E. lineatus juveniles (up to 244 mm FL), the capacity for heat exchange increased with fork length as a result of increases in rete length, rete width and the number of vessel rows. The amount (g) of red muscle increased exponentially with fork length in both E. lineatus (105-255 mm FL) and a closely related ectothermic species, the sierra Spanish mackerel Scomberomorus sierra (151-212 mm FL), but was greater in E. lineatus at a given fork length. The specific activity (international units g(-)(1)) of the enzyme citrate synthase in red muscle, an index of tissue heat production potential, increased slightly with fork length in juvenile E. lineatus (84. 1-180 mm FL) and S. sierra (122-215 mm FL). Thus, total red muscle heat production capacity (red muscle citrate synthase activity per gram times red muscle mass in grams) increased with fork length, primarily because of the increase in red muscle mass. Below 95.9 mm FL, E. lineatus cannot maintain red muscle temperature (T(m)) above the ambient water temperature (T(a)) because juveniles of this size lack retia. Above 95.9 mm FL, the relationship between T(x) (T(m)-T(a)) and FL for E. lineatus diverges from that for the ectothermic S. sierra because of increases in the capacities for both heat production and heat retention that result in the development of endothermy.

Maximum sustainable speeds and cost of swimming in juvenile kawakawa tuna (Euthynnus affinis) and chub mackerel (Scomber japonicus).

Tunas (Scombridae) have been assumed to be among the fastest and most efficient swimmers because they elevate the temperature of the slow-twitch, aerobic locomotor muscle above the ambient water temperature (endothermy) and because of their streamlined body shape and use of the thunniform locomotor mode. The purpose of this study was to test the hypothesis that juvenile tunas swim both faster and more efficiently than their ectothermic relatives. The maximum sustainable swimming speed (U(max), the maximum speed attained while using a steady, continuous gait powered by the aerobic myotomal muscle) and the net cost of transport (COT(net)) were compared at 24 degrees C in similar-sized (116-255 mm fork length) juvenile scombrids, an endothermic tuna, the kawakawa (Euthynnus affinis) and the ectothermic chub mackerel (Scomber japonicus). U(max) and COT(net) were measured by forcing individual fish to swim in a temperature-controlled, variable-speed swimming tunnel respirometer. There were no significant interspecific differences in the relationship between U(max) and body mass or fork length or in the relationship between COT(net) and body mass or fork length. Muscle temperatures were elevated by 1.0-2.3 degrees C and 0.1-0.6 degrees C above water temperature in the kawakawa and chub mackerel, respectively. The juvenile kawakawa had significantly higher standard metabolic rates than the chub mackerel, because the total rate of oxygen consumption at a given swimming speed was higher in the kawakawa when the effects of fish size were accounted for. Thus, juvenile kawakawa are not capable of higher sustainable swimming speeds and are not more efficient swimmers than juvenile chub mackerel.

Swimming kinematics of juvenile kawakawa tuna (Euthynnus affinis) and chub mackerel (Scomber japonicus).

The swimming kinematics of two active pelagic fishes from the family Scombridae were compared to test the hypothesis that the kawakawa tuna (Euthynnus affinis) uses the thunniform mode of locomotion, in which the body is held more rigid and undergoes less lateral movement in comparison with the chub mackerel (Scomber japonicus), which uses the carangiform swimming mode. This study, the first quantitative kinematic comparison of size-matched scombrids, confirmed significantly different swimming kinematics in the two species. Ten kawakawa (15.1-25.5 cm fork length, FL) and eight chub mackerel (14.0-23.4 cm FL), all juveniles, were videotaped at 120 Hz while swimming at several speeds up to their maximum sustained speed at 24 degrees C. Computerized motion analysis was used to digitize specific points on the body in sequential video frames, and kinematic variables were quantified from the progression of the points over time. At a given speed, kawakawa displayed a significantly greater tailbeat frequency, but lower stride length, tailbeat amplitude and propulsive wavelength, than chub mackerel when size effects were accounted for. Midline curvatures subdivided on the basis of X-rays into individual vertebral elements were used to quantify axial bending in a subset of the fish studied. Maximum intervertebral lateral displacement and intervertebral flexion angles were significantly lower along most of the body in kawakawa than in chub mackerel, indicating that the kawakawa undergoes less axial flexion than does the chub mackerel, resulting in lower tailbeat amplitudes. However, lateral movement at the tip of the snout, or yaw, did not differ significantly interspecifically. Despite these differences, the net cost of transport was the same in the two species, and the total cost was higher in the kawakawa, indicating that the tuna juveniles are not more efficient swimmers.

Effects of endurance training in the leopard shark, Triakis semifasciata.

This study is the first to examine the effects of endurance training in an elasmobranch fish. Twenty-four leopard sharks (Triakis semifasciata) were divided randomly into three groups. Eight sharks were killed immediately, eight were forced to swim continuously for 6 wk against a current of 35 cm s-1 (60%-65% of maximal sustainable swimming speed), and eight were held for 6 wk in a tank without induced current. There were no changes due to training in maximal sustainable speed, oxygen consumption rates, percentage of the myotome composed of red and white muscle fibers, blood oxygen-carrying capacity, liver mass, liver lipid, glycogen, and protein concentrations, white muscle protein content, heart ventricle mass, or the specific activities of the enzymes citrate synthase, pyruvate kinase, and lactate dehydrogenase in the heart ventricle. In red myotomal muscle, citrate synthase activity increased 17% as a result of training, but there was no change in muscle fiber diameter. The greatest effects occurred in white myotomal muscle, in which a 34% increase in fiber diameter and a 36% increase in the activities of citrate synthase and lactate dehydrogenase occurred as a result of training. The conditioned fish also had significantly higher growth rates. The observed effects within the myotomal muscle may reflect the higher growth rates of the trained leopard sharks, or they may be a specific response to the increased energetic demands of the training activity, indicating characteristics that limit swimming performance in leopard sharks.

Fetal and maternal fluid balance in sheep during hyperthermia with and without water deprivation.

Our aim was to determine the effect of maternal hyperthermia, both with and without maternal water deprivation, on fetal fluid balance. Seven pregnant ewes (131.8 +/- 1.0 days gestation) were studied during a control period and periods of maternal heating (MH, 42-44 degrees C for 8 h, water freely available), maternal water deprivation (MWD, 30 h) and maternal heating combined with water deprivation (MH + MWD, 30 h deprivation with heating during last 8 h). Relative to control values, MH increased maternal water intake and urine output, and [K+] in fetal plasma and fetal urine. Relative to control values, MH decreased maternal plasma osmolality, [Na+] and [K+]; fetal plasma osmolality and [Na+]; fetal lung liquid [Na+] and [Cl-]; and fetal production rates of lung liquid and urine. In response to MH + MWD, the osmolality, [Na+] and [Cl-] of maternal and fetal plasma, fetal lung liquid and fetal urine (excluding urinary [Cl-]) increased compared with control values. In the fetus, MH + MWD increased plasma and urinary [K+], and decreased production rates of lung liquid and urine compared with control values. During MH + MWD, compared with MH alone, greater alterations were seen in maternal rectal temperature, water input and urine output; osmolality, [Na+] and [Cl-] of maternal and fetal plasma, fetal lung liquid and fetal urine (excluding urinary [Cl-]); and fetal urinary [K+]. During MH + MWD, compared with MWD alone, greater alterations were seen in maternal plasma [Cl-] and [K+]; fetal urinary osmolality and [K+]; and fetal plasma [K+]. Our results show that, when water is available, maternal hyperthermia stimulates ewes to drink substantially more than under normal conditions, thereby decreasing their plasma osmolality; water transfer to the fetus may increase, thereby decreasing fetal plasma osmolality. When drinking water is unavailable, maternal hyperthermia and associated dehydration may decrease water transfer to the fetus. Thus, the fetus becomes not only hyperthermic, but also hyperosmotic and possibly hypovolaemic. Maternal hyperthermia, irrespective of the availability of drinking water, decreases production rates of lung liquid and urine in the fetus.

Role of fetal sac fluids during maternal water deprivation in sheep.

Our aim was to determine the importance of amniotic and allantoic fluids for the maintenance of fetal plasma composition during maternal dehydration when water transfer from mother to fetus is likely to be reduced. Eight pregnant ewes were studied before, during and after water deprivation (36 h), firstly with the fetal fluid sacs intact and then with them drained of fluid for 5 days. When water deprivation was combined with drainage, the increases in the osmolalities, [Na+] and [Cl-] in maternal plasma, in fetal plasma and in lung liquid; the increases in fetal urinary osmolality and [Na+]; and the increases in maternal plasma and fetal plasma arginine vasopressin concentrations were greater than those resulting from water deprivation alone. Our results show that during maternal water deprivation, an absence of fluid in the fetal sacs increases both the osmotic stimulus to the fetus and the fetal responses resulting in conservation of water and salt. We conclude that, when the mother is deprived of water, fluid in the fetal sacs is used to limit the degree of maternal and fetal dehydration.

Compliances of the liquid-filled lungs and chest wall during development in fetal sheep.

Our aim was to measure the compliance of the liquid-filled lungs (CL), and the compliance of the chest wall (CW) in fetal sheep in utero. CL and CW were measured in 6 fetuses. The compliance of the lungs and chest wall combined (respiratory system, Crs) was measured in 9 fetuses. Pressure differences across the lungs (PL), chest wall (PW) and respiratory system (Prs) were measured while the lungs were deflated and inflated with liquid from their resting lung liquid volume (V1). V1 was measured using an indicator dilution technique. Specific compliance values were obtained by normalizing the values of CL, CW and Crs with respect to values of V1. From values obtained during stepwise inflation from V1, specific compliances (ml/cm H2O/ml of lung liquid) were: lungs, 0.22 +/- 0.02; chest wall, 0.41 +/- 0.07; respiratory system, 0.13 +/- 0.01. Specific compliances of the lungs, chest wall and respiratory system did not change significantly with advancing gestational age from 120 to 143 days. Our baseline data will be valuable in assessing the in utero progress of the structural development of the lungs following manipulations known to cause altered lung growth.

Fetal breathing and pressures in the trachea and amniotic sac during oligohydramnios in sheep.

Oligohydramnios commonly leads to fetal lung hypoplasia, but the mechanisms are not fully understood. Our aim was to determine, in fetal sheep, the effects of prolonged oligohydramnios on the incidence and amplitude of tracheal pressure fluctuations associated with fetal breathing movements (FBM), on tracheal flow rate during periods of FBM (VtrFBM) and periods of apnea (Vtrapnea), on tracheal pressure relative to amniotic sac pressure, and on amniotic sac pressure relative to atmospheric pressure. In five sheep, oligohydramnios was induced by draining amniotic and allantoic fluids from 107 to 135 days of gestation (411.8 +/- 24.4 ml/day), resulting in fetal lung hypoplasia. In five control sheep, amniotic fluid volume was 732.3 +/- 94.4 ml. Oligohydramnios increased the incidence of FBM by 14% at 120 and 125 days and the amplitude of FBM by 30-34% at 120-130 days compared with controls. From 120 days onward, VtrFBM was 35-55% lower in experimental fetuses than in controls. Influx of lung liquid during FBM was 87% lower in experimental fetuses than in controls. Vtrapnea, tracheal pressure, and amniotic sac pressure were not significantly altered by oligohydramnios. Our tracheal flow rate data suggest that transient changes in lung liquid volume during periods of FBM and periods of apnea were diminished by oligohydramnios. We conclude that the primary factor in the etiology of oligohydramnios-induced lung hypoplasia is not an inhibition of FBM (as measured by tracheal pressure fluctuations) or a reduction in amniotic fluid pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

A mechanism leading to reduced lung expansion and lung hypoplasia in fetal sheep during oligohydramnios.

Our aim was to determine the mechanism whereby oligohydroamnios causes reduced fetal lung expansion and eventual lung hypoplasia. We studied 20 fetal sheep during 2 to 9 days of oligohydramnios produced by drainage of amniotic and allantoic fluids during the last third of gestation. Oligohydramnios led to a reversible reduction in lung liquid volume of 19.5% within 48 hours. During oligohydramnios tracheal pressure, relative to amniotic pressure, rose by 1.7 mm Hg (p less than 0.001); pressures also tended to rise in the fetal pleural space and abdomen, relative to amniotic pressure, and to fall in the amniotic sac. Pressure increments, relative to amniotic pressure, which normally occur in the fetal trachea, pleural cavity, and abdomen during nonlabor uterine contractions, were significantly increased by 1.9 to 2.5 mm Hg during oligohydramnios. Oligohydramnios increased flexion of the fetal thoracolumbar spine, quantified as a reduction in the ratio of spinal radius of curvature to spine length (0.76 in controls vs 0.40 after oligohydramnios, p less than 0.001). In three sets of twins, only the fetus exposed to oligohydramnios was affected. A similar degree of spinal flexion imposed on normal fetal sheep cadavers increased abdominal (1.6 mm Hg), pleural (1.5 mm Hg), and tracheal (2.0 mm Hg) pressure, and caused a significant reduction in fetal lung expansion. We conclude that oligohydramnios in fetal sheep increases spinal flexion, leading to compression of abdominal contents, upward displacement of the diaphragm, and lung compression, favoring loss of fetal lung liquid. These changes, which are accentuated during nonlabor uterine contractions and are reversible, may lead to pulmonary hypoplasia if prolonged.

Endocrine and fluid-balance responses to amniotic and allantoic fluid loss in sheep.

Our aim was to determine fetal and maternal endocrine and fluid-balance responses to prolonged loss of amniotic and allantoic fluids in sheep. In seven sheep, amniotic and allantoic fluids were drained [379.1 +/- 20.1 (SE) ml/day] from 107 to 135.3 +/- 0.6 days of gestation (term: 145 days). The results from these sheep were compared with those from seven control sheep. Maternal water intake, urine production, and urine osmolality were not altered by fluid drainage, nor were fetal and maternal arterial blood gases, pH, or plasma osmolalities. Fluid drainage increased amniotic, but not allantoic, fluid osmolality. Maternal plasma cortisol concentration increased with fluid drainage, but maternal plasma concentrations of prolactin and arginine vasopressin were unchanged. Fluid drainage increased prolactin concentrations in fetal plasma and amniotic fluid, but fetal plasma concentrations of cortisol (hydrocortisone), arginine vasopressin, norepinephrine, and epinephrine were unchanged. Our results show that the fetus is capable of maintaining its plasma osmolality despite prolonged loss of fluid from its amniotic and allantoic sacs and that this is associated with alterations in the production rate and the composition of amniotic fluid.

Decline in lung liquid volume and secretion rate during oligohydramnios in fetal sheep.

Reduced amniotic fluid volume often results in fetal lung hypoplasia. Our aim was to examine the effects of prolonged drainage of amniotic and allantoic fluids on lung liquid volume (Vl), secretion rate (Vs), and tracheal flow rate (Vtr) in fetal sheep. In five experimental animals, amniotic and allantoic fluids were drained from 107 to 135 days of gestation. The volume of fluid drained from the experimental animals was 411.8 +/- 24.4 ml/day (n = 140). In six control animals, amniotic fluid volume was 747.7 +/- 89.7 ml (n = 15). Wet and dry lung weights were 20-25% lower in experimental fetuses than in control fetuses. Fetal hemoglobin, O2 saturation, arterial PO2, pH, and hematocrit were unchanged by drainage. During the drainage period, Vl was up to 65% lower, Vs was up to 35% lower, and Vtr was up to 40% lower in experimental fetuses than in control fetuses. We conclude that prolonged drainage of amniotic and allantoic fluids decreases Vl, Vs, and Vtr in fetal sheep. These findings indicate that fetal lung hypoplasia associated with oligohydramnios may be the result of a prolonged reduction in Vl.

Lung liquid secretion, flow and volume in response to moderate asphyxia in fetal sheep.

The effects of moderate fetal asphyxia, induced by constriction of the maternal common internal iliac artery, on lung liquid secretion, tracheal fluid efflux and lung liquid volume have been investigated in unanaesthetized fetal sheep (111-142 days) in utero. During periods of fetal asphyxia the percent oxygen saturation, PO2, pH, and PCO2 of fetal carotid arterial blood changed from 57.2 +/- 1.3% (mean +/- SEM), 22.9 +/- 0.6 mmHg, 7.35 +/- 0.01 and 45.6 +/- 1.0 mmHg to 26.3 +/- 0.5% (P less than 0.001), 14.7 +/- 0.2 mmHg (P less than 0.001), 7.28 +/- 0.02, (P less than 0.001) and 47.8 +/- 0.4 mmHg (P less than 0.02), respectively. Fetal asphyxia, over 6 h, decreased the efflux of tracheal fluid from 7.07 +/- 0.47 ml/h to 3.97 +/- 0.36 ml/h (P less than 0.01) and, over 4 h, decreased the rate of lung liquid secretion from 9.42 +/- 1.76 ml/h to 4.91 +/- 1.54 ml/h (P less than 0.005), whereas it had no significant effect on lung liquid volume. The incidence of fetal breathing movements decreased from 52.9 +/- 2.5% to 22.6 +/- 3.5% during 6-h periods of fetal asphyxia. Thus, although fetal asphyxia decreased the net production of lung liquid, lung liquid volume was maintained probably, because the net efflux of fluid from the lungs via the trachea decreased to a similar extent.

Restoration of lung liquid volume following its acute alteration in fetal sheep.

1. The experiments were aimed at determining the means by which lung liquid volume is controlled in fetal sheep. Six unanaesthetized chronically catheterized fetuses between 121 and 144 days of gestation were used in experiments in which lung liquid volume was acutely reduced or increased. 2. The effects on lung liquid volume, secretion and flow in the trachea were continuously monitored until the original volume was restored. Tracheal pressure, breathing activity, electrocortical activity and laryngeal adductor muscle activity were also monitored. 3. Following a mean reduction in lung liquid volume of 54.1% (40.0 +/- 4.3 ml) there was a reduction in tracheal pressure of 1.91 +/- 0.26 mmHg. There was no change in the rate of lung liquid secretion and there was a near cessation in its rate of efflux from the trachea. Influx of liquid from the upper airway was rarely seen. 4. 5 h after its reduction, lung liquid volume had returned to control values. Tracheal pressure and the rate of liquid flow in the trachea returned to control values over 7-8 h. 5. In the hour following the reduction in lung liquid volume the incidence and amplitude of fetal inspiratory muscle electromyogram (e.m.g.) activity were reduced by 19% and 28% respectively. There was no change in laryngeal adductor muscle activity or in the duration of fetal sleep states. 6. When lung liquid volume was increased by 25.4 +/- 3.1 ml tracheal pressure increased by 2.01 +/- 0.17 mmHg. Although there was no change in the rate of liquid secretion there was a rapid efflux of liquid from the trachea principally during epochs of fetal breathing activity thus returning lung liquid volume to its control value. 7. Following the increase in lung liquid volume there were no changes in the incidence or amplitude of fetal inspiratory muscle activity, the activity of laryngeal adductor muscles or in the duration of sleep states. 8. It is concluded that following its alteration in fetal sheep lung liquid volume is restored by passive means. There was no evidence of volume-receptive neural reflexes being activated in defence of lung liquid volume.

State-related changes in lung liquid secretion and tracheal flow rate in fetal lambs.

The volume of liquid in the fetal lungs depends on the rate of liquid secretion (Vs) across the pulmonary epithelium and the rate of flow out of the trachea (Vtr). We measured Vs, by an isotope-dilution technique, and Vtr, with a bubble flowmeter, during low-voltage (LV) and high-voltage (HV) electrocortical activity. In nine chronically instrumented fetal lambs, Vtr was greater during the transition to and at LV (16.98 +/- 1.98 ml/h, mean +/- SE, n = 23) than values during the transition to and at HV (8.69 +/- 0.8 ml/h). A pronounced peak in Vtr of 22.3 +/- 1.8 ml/h (n = 197) occurred at the transition to LV and early in the LV state. Ten minutes or more into LV, Vtr had declined to 10.3 +/- 1.8 ml/h (n = 235). Vtr remained low throughout the HV state. Vs values were not significantly different throughout the LV (11.83 +/- 1.34 ml/h, n = 216) and the HV (13.61 +/- 2.34 ml/h, n = 174) states. Diaphragmatic burst rate during LV (146.9 +/- 6.7 bursts/5 min, n = 432) was greater than during HV (26.5 +/- 4.6 bursts/5 min, n = 348), but burst rate was not correlated with Vtr. In summary, Vtr reaches a peak during the early part of LV when breathing commences and Vs remains constant throughout the behavioral cycle. As a result, lung liquid volume increases slightly during HV and decreases by a similar amount in the early part of LV.

Decline in lung liquid volume before labor in fetal lambs.

The volume of liquid in the fetal lung depends on the amount of liquid secreted across the pulmonary epithelium and the amount flowing through the trachea. Lung liquid volume (V1) and secretion rate Vs) were determined using an indicator-dilution technique, while tracheal flow rate (Vtr) was measured simultaneously with a bubble flowmeter. Least-squares regression analysis showed that in 10 chronically instrumented fetal lambs, V1 increased from 51.0 ml at 119 days to 104.6 ml at 135 days (V1 = -347.65 + 3.35 X days; 95% confidence limits on slope: 1.89-4.81) before declining to 70.2 ml at 142 days gestation (V1 = 768.8 - 4.92 X days; 95% confidence limits on slope: -2.55 to -7.30). Similarly Vs increased from 7.4 ml/h at 119 days to 16.8 ml/h at 133 days (Vs = -72.35 + 0.67 X days; 95% confidence limits on slope: 0.21-1.14), before declining to 7.1 ml/h at 142 days (Vs = 159.07 - 1.07 X days; 95% confidence limits on slope: -0.56 to -1.57). Vtr did not change significantly with gestation. We conclude that V1 increases until 135 days gestation, after which it falls substantially. This fall in volume, which occurs well before the onset of labor, results predominantly from the decline in Vs.