Behavioural state influences the cardiovascular response to haemorrhage in lambs

We investigated the effect of behavioural state on the cardiovascular response to an acute venous haemorrhage in 7 lambs aged 13 to 19 days. Each lamb had previously been anaesthetized and instrumented for measurements of electrocorticogram, electro-oculogram, nuchal and diaphragm electromyograms, pulmonary blood flow (electromagnetic flow transducer), aortic and right atrial blood pressures. The lambs were allowed to recover from surgery at least three days before they were studied. Measurements were made during a 1-minute control period and during a 1-minute experimental period that followed a 10 ml/kg body weight haemorrhage during quiet wakefulness, quiet sleep and active sleep; the haemorrhage took approximately 30s. Haemorrhage produced similar decreases in right atrial pressure and pulmonary blood flow during the three behavioural states. However, mean aortic pressure decreased more following haemorrhage during active sleep than during quiet sleep or quiet wakefulness. These results provide evidence that reflex control of the peripheral circulation is altered during active sleep compared to quiet sleep and quiet wakefulness in lambs.     

Sympathetic control of the cardiovascular response to acute hypoxemia in the chick embryo

In response to an acute hypoxemic insult, the mammalian fetus shows a redistribution of the cardiac output in favor of the heart and brain. Peripheral vasoconstriction contributes to this response and is partly mediated by the release of catecholamines. Two mechanisms of catecholamine release in the fetus are reported: 1) neurogenic sympathetic stimulation and 2) a nonneurogenic mechanism via a direct effect of hypoxemia on chromaffin tissues. In the present study, the effects of sympathetic blockade on plasma catecholamine release and cardiac output distribution in response to acute hypoxemia were studied in the chick embryo at different stages of incubation. Only at the end of the incubation period, sympathetic blockade markedly attenuated the increase in plasma catecholamine concentrations and resulted in a greater fraction of the cardiac output distributed to the carcass. However, these effects did not prevent a significant increase in cardiac output to the brain and heart during acute hypoxemia. These data imply that in the chick embryo the contribution of neurogenic mechanisms to the catecholaminergic response to acute hypoxemia becomes greater by the end of the incubation period.     

Laryngeal response to nasal ventilation in nonsedated newborn lambs.

Although endoscopic studies in adult humans have suggested that laryngeal closure can limit alveolar ventilation during nasal intermittent positive pressure ventilation (nIPPV), there are no available data regarding glottal muscle activity during nIPPV. In addition, laryngeal behavior during nIPPV has not been investigated in neonates. The aim of the present study was to assess laryngeal muscle response to nIPPV in nonsedated newborn lambs. Nine newborn lambs were instrumented for recording states of alertness, electrical activity [electromyograph (EMG)] of glottal constrictor (thyroarytenoid, TA) and dilator (cricothyroid, CT) muscles, EMG of the diaphragm (Dia), and mask and tracheal pressures. nIPPV in pressure support (PS) and volume control (VC) modes was delivered to the lambs via a nasal mask. Results show that increasing nIPPV during wakefulness and quiet sleep led to a progressive disappearance of Dia and CT EMG and to the appearance and subsequent increase in TA EMG during inspiration, together with an increase in trans-upper airway pressure (TUAP). On rare occasions, transmission of nIPPV through the glottis was prevented by complete, active glottal closure, a phenomenon more frequent during active sleep epochs, when irregular bursts of TA EMG were observed. In conclusion, results of the present study suggest that active glottal closure develops with nIPPV in nonsedated lambs, especially in the VC mode. Our observations further suggest that such closure can limit lung ventilation when raising nIPPV in neonates.     

Fever in young lambs: hypoxemia alters the febrile response to a small dose of bacterial pyrogen.

Experiments were done on eight young lambs to investigate the effects of hypoxemia on the body temperature, metabolic and cardiovascular responses to intravenous administration of a small dose of bacterial pyrogen (0.3 micrograms lipopolysaccharide extracted from Salmonella Abortus Equi; SAE). Each lamb was anaesthetized with halothane and prepared for sleep staging and measurements of cardiac output, arterial and mixed-venous haemoglobin oxygen saturations, body-core and ear-skin temperatures. Three experiments were done on each lamb, the first being done no sooner than three days after surgery. The first experiment consisted of establishing the thermal neutral environment during normoxemia (ie, environmental temperature at which total body oxygen consumption was minimal while body temperature was maintained) for each lamb. The second and third experiments were done at the lamb's thermoneutral environment as determined on day 1. One experiment was done during normoxemia (ie, control condition, SaO2 approximately 90%) and one experiment was done during hypoxemia (ie, experimental condition, SaO2 approximately 50%). Measurements were made during a control period and during one-minute experimental periods at 10 minute intervals for 120 minutes following administration of 0.3 micrograms of bacterial pyrogen in sterile saline. Administration of SAE produced a short-lived fever of about 0.8 degrees C in the normoxemic lambs, whereas no change in body-core temperature was observed in the hypoxemic lambs. During normoxemia, the increase in body-core temperature was preceded by peripheral vasoconstriction, the onset of shivering, and a surge in total body oxygen consumption. The increase in total body oxygen consumption was met primarily by an increase in total body oxygen extraction during the development of fever. Cardiac index, heart rate, and systemic oxygen transport increased during the peak body-core temperature response. Systemic arterial blood pressure did not change significantly during the febrile response; however, pulmonic arterial blood pressure increased. During hypoxemia, peripheral vasoconstriction and shivering occurred following administration of SAE, but there was no change in total body oxygen consumption or body-core temperature. Thus, our data provide evidence that hypoxemia alters the febrile response of young lambs to bacterial pyrogen. The precise mechanism remains to be determined.     

Meclofenamate increases ventilation in lambs

To investigate the effects of the prostaglandin synthetase inhibitor, meclofenamate, on postnatal ventilation, we studied 11 unanaesthetised, spontaneously-breathing lambs at an average age of 7.9 +/- 1.1 days (SEM; range 5-14 days) and an average weight of 4.9 +/- 0.5 kg (range 3.0-7.0 kg). After a 30-min control period we infused 4.23 mg/kg meclofenamate over 10 min and then gave 0.23 mg/h per kg for the remainder of the 4 h. Ventilation increased progressively from a control value of 515 +/- 72 ml/min per kg to a maximum of 753 +/- 100 ml/min per kg after 3h of infusion (P less than 0.05) due to an increased breathing rate; the effects were similar during both high- and low-voltage electrocortical activity. There were no significant changes in tidal volume, heart rate, blood pressure, arterial pH or PaCO2, the increased ventilation resulted from either an increase in dead space ventilation or an increase in CO2 production. This study indicates that meclofenamate causes an increase in ventilation in lambs but no changes in pH of PaCO2. The mechanism and site of action remain to be defined.     

Selenium supplementation influences growth performance, antioxidant status and immune response in lambs

To investigate and compare the effect of inorganic and organic Se supplementation, 18 male lambs (24.68 ± 2.89 kg mean body weight, about 8–9 months of age) were divided into three groups of six animals in each, following randomized block design. While animals in the control group (Gr I) were fed a standard TMR containing 195 g/kg crushed maize grain, 175.5 g/kg soybean meal, 260 g/kg wheat bran, 13 g/kg mineral mixture (without Se), 6.5 g/kg common salt and 350 g/kg wheat straw, animals in Gr II and Gr III were additionally supplemented with 0.15 mg Se/kg of diet through sodium selenite (inorganic Se) and Jevsel-101 (organic Se), respectively. Experimental feeding was done for a period of 90 days. To assess the humoral immune response, all the lambs were intramuscularly inoculated with a single dose (2 mL) of Haemorrhagic septicaemia oil adjuvant vaccine on day 0; and blood samples were collected on day 0, 30, 60 and 90. Supplementation of Se had no effect on serum total cholesterol, total protein, albumin, globulin, albumin:globulin ratio, T₃, T₄, T₄:T₃ ratio; serum Ca and P levels and SGOT and SGPT activity. However, there was a significant increase in the serum Se level, RBC GSH-Px activity and humoral immune response in both the Se supplemented groups as compared to control group. Average daily gain (g) was highest (110) in Gr III, followed by Gr II (98.2) and lowest in Gr I (89.1). Thus, supplementation of organic as well as inorganic Se was found to improve the growth rate, humoral immune response and antioxidant status of the lambs; and between two sources, organic Se was more effective than inorganic Se.     

Role of hypoxemia for the cardiovascular responses to apnea during exercise

We sought to define the role of hypoxemia in eliciting the cardiovascular responses to apnea during exercise. Eleven men performed repeated apneas during 100-W steady-state exercise, either with normoxic gas (air) or 95% oxygen (oxygen). Beat-by-beat arterial blood pressure, arterial oxygen saturation, and heart rate (HR) were determined, and stroke volume (SV) was estimated from impedance cardiography calibrated with soluble gas rebreathing. There were large interindividual variabilities of HR, mean arterial pressure (MAP), and total peripheral resistance (TPR) at end-apnea (ea). However, for each individual, HR(ea), MAP(ea), and TPR(ea) were highly correlated between air and oxygen (R = 0.94, 0.78, and 0.93). HR decreased and MAP increased faster during apnea with air than with oxygen (ANOVA, P < 0.05), but MAP(ea) was not different between conditions. Cardiac output was reduced by 33% with air and by 11% with oxygen (P < 0.001 for air vs. oxygen). We conclude that the hypoxemia component cannot account for the wide interindividual differences of HR and TPR responses to apnea. However, hypoxemia augments the HR and TPR responses and may limit the MAP response to apnea by preventing a bradycardia-associated increase of SV.     

Aldosterone response to angiotensin II during hypoxemia

Exercise in humans causes increases in plasma renin activity (PRA) and plasma aldosterone concentrations (PAC) except when performed at high altitude or while the subjects breathe hypoxic gas. Under those conditions, PRA increases with exercise but PAC does not. We speculated that the PAC suppression during hypoxemic exercise was due to hypoxemia-induced release of a circulating inhibitor of angiotensin II-mediated aldosterone secretion. To test this hypothesis, we measured the PAC response to graded infusions of angiotensin II during hypoxemia and normoxemia. Eight normal volunteers were given increasing doses of angiotensin II (first 2 ng X kg-1 X min-1 and then 4, 8, and finally 12 ng X kg-1 X min-1, each for 20-min periods) on 2 separate days, once while breathing room air and the other day while breathing hypoxic gas adjusted to maintain the subjects' hemoglobin saturation at 90%. The PAC response to different doses of angiotensin II did not significantly differ during hypoxemia from normoxemia. We conclude that our model of hypoxemia does not cause release of an inhibitor of angiotensin II-mediated aldosterone release.     

Effects of acute acidemia on the fetal cardiovascular defense to acute hypoxemia

In complicated pregnancy, fetal hypoxemia rarely occurs in isolation but is often accompanied by fetal acidemia. There is growing clinical concern about the combined effects of fetal hypoxemia and fetal acidemia on neonatal outcome. However, the effects on the fetal defense responses to acute hypoxemia during fetal acidemia are not well understood. This study tested the hypothesis that fetal acidemia affects the fetal defense responses to acute hypoxemia. The hypothesis was tested by investigating, in the late-gestation sheep fetus surgically prepared for long-term recording, the in vivo effects of acute fetal acidemia on 1) the fetal cardiovascular responses to acute hypoxemia and 2) the neural and endocrine mechanisms mediating these responses. Under general anesthesia, five sheep fetuses at 0.8 gestation were instrumented with catheters and Transonic flow probes around the femoral and umbilical arteries. After 5 days, animals were subjected to an acute hypoxemia protocol during intravenous infusion of saline or treatment with acidified saline. Treatment with acidified saline reduced fetal basal pH from 7.35 +/- 0.01 to 7.29 +/- 0.01 but did not alter basal cardiovascular variables, blood glucose, or plasma concentrations of catecholamines, ACTH, and cortisol. During hypoxemia, treatment with acidified saline increased the magnitude of the fetal bradycardia and femoral vasoconstriction and concomitantly increased chemoreflex function and enhanced the increments in plasma concentrations of catecholamines, ACTH, and cortisol. Acidemia also reversed the increase in umbilical vascular conductance during hypoxemia to vasoconstriction. In conclusion, the data support our hypothesis and show that acute acidemia markedly alters fetal hemodynamic, metabolic, and endocrine responses to acute hypoxemia.     

Development of the ovine fetal cardiovascular defense to hypoxemia towards full term

We tested the hypothesis that fetal cardiovascular responses to hypoxemia change close to full term in relation to the prepartum increase in fetal basal cortisol and investigated, in vivo, the neural and endocrine mechanisms underlying these changes. Fetal heart rate and peripheral hemodynamic responses to 1 h of hypoxemia were studied in 25 chronically instrumented sheep within three narrow gestational age ranges: 125-130 (n = 13), 135-140 (n = 6), and >140 (n = 6) days (full term approximately 145 days). Chemoreflex function and plasma concentrations of vasoconstrictor hormones were measured. Reductions in fetal arterial Po(2) during hypoxemia were similar at all ages. At 125-130 days, hypoxemia elicited transient bradycardia, femoral vasoconstriction, and increases in plasma concentrations of catecholamines, neuropeptide Y (NPY), AVP, ACTH, and cortisol. Close to full term, in association with the prepartum increase in fetal basal cortisol, there was a developmental increase in the magnitude and persistence of fetal bradycardia and in the magnitude of the femoral constrictor response to hypoxemia. The mechanisms mediating these changes close to full term included increases in the gain of chemoreflex function and in the magnitudes of the fetal NPY and AVP responses to hypoxemia. Data combined irrespective of gestational age revealed significant correlations between fetal basal cortisol and fetal bradycardia, femoral resistance, chemoreflex function, and plasma AVP concentrations. The data show that the fetal cardiovascular defense to hypoxemia changes in pattern and magnitude just before full term because of alterations in the gain of the neural and endocrine mechanisms mediating them, in parallel with the prepartum increase in fetal basal cortisol.     

Pulmonary mechanics during the ventilatory response to hypoxemia in the newborn monkey

Dynamic lung compliance (CL), inspiratory pulmonary resistance (RL), and functional residual capacity (FRC) were measured in 10 unanesthetized 48 h-old newborn monkeys and seven 21-day-old infant monkeys during acute exposures to an equivalent level of hypoxemia. End-expiratory airway occlusions were performed and the pressure developed by 200 ms (P0.2) was utilized as an index of central respiratory drive. P0.2 demonstrated a sustained increase throughout the period of hypoxemia on day 2 despite the fact that minute ventilation (VI) initially increased but then fell back to base-line levels. Dynamic lung compliance fell and FRC increased by 5 min of hypoxemia in the newborns. The 21-day-old monkeys exhibited a sustained increase in both VI and P0.2 throughout the hypoxic period with no change in CL and FRC. RL did not change at either postnatal age during hypoxemia. These data indicate that the neonatal monkey is subject to changes in pulmonary mechanics (decreased CL and increased FRC) during hypoxemia and that these changes are eliminated with maturation.     

Diphasic ventilatory response to hypoxia in newborn lambs

The ventilatory response of newborn lambs to hypoxemia was evaluated in two groups of seven awake lambs studied at 2 and 7 days of life. Minute ventilation (VE) and airway occlusion pressure (P0.1) were monitored as the animals were exposed in sequence to room air, 12% O2 (15 min), 7% O2 (15 min), and room air. On 12 and 7% O2, 2-day-old lambs experienced a brisk hyperventilation followed by a VE depression, previously described in newborns of other species (diphasic response). The 7-day-old lambs had a clear diphasic VE response only on 7% O2 breathing. In the 2-day-old lambs, at the time of the relative VE depression to 12% O2, the respiratory centers showed a persisting responsiveness to further hypoxia; switching to 7% O2 caused a brisk increase in VE and P0.1 of 70 and 130%, respectively, which was followed again by a VE depression. The magnitude of the immediate VE response to hypoxia, taken as an index of the chemoreceptor strength, was inversely related to the magnitude of the VE depression (R = 0.81, P less than 0.001). It was concluded that 1) lambs as well as other neonates have an age-related diphasic VE response to hypoxia; 2) at the time of the VE depression, the respiratory centers maintain their responsiveness to further acute hypoxia; and 3) the weakness of the chemoreceptors in the newborn is a major determinant of the diphasic response.     

Pulmonary vascular response to prostacyclin in fetal lambs

Eighteen prostacyclin injections (19.4±1.5 μg/kg) were performed in five chronically instrumented, intact fetal lambs in order to study the effects on pulmonary blood flow. These resulted in a brief period of bradycardia followed by a more prolonged period of increased pulmonary blood flow. In this latter phase, pulmonary blood flow increased from a baseline value of 49±4 ml/(kg min) to 122±10 ml/(kg min). Systolic/diastolic pulmonary arterial pressure simultaneously fell from to mm Hg. Flow through the ductus arteriosus was unchanged and right ventricular output increased to account for the increased pulmonary blood flow. Thus, prostacyclin causes pulmonary vasodilation in intact fetal lambs and may participate in the control of fetal pulmonary blood flow and the circulatory adjustments to extra-uterine life.     

Abolition of ventilatory response to caffeine in chemodenervated lambs

In this study we have evaluated the role of the peripheral chemoreceptors in the ventilatory response to caffeine at a dose currently used in human infants for treatment of central apneas (10 mg/kg). Twelve lambs were studied; six had carotid body denervation (CBD) and six had a sham denervation (intact). The denervation was done the 2nd wk of life, and the study of the response to caffeine infusion was carried out at a mean age of 82 days. The awake and nonsedated animals received 10 mg/kg of caffeine, and caffeine blood levels were, respectively, 8.8 and 9.0 mg/l in the intact and in the CBD lambs. The intact lambs responded to caffeine by a significant immediate increase in minute ventilation (VE) of 46% from 274 to 400 ml X min-1 X kg-1 (P less than 0.001), 1 min after caffeine infusion. This response rapidly faded, but VE was still increased at 2 h, 314 ml X min-1 X kg-1. The increase in ventilation was brought about by a change in mean inspiratory flow (VT/TI), which increased from 9.9 to 14.0 ml X s-1 X kg-1 within 1 min (P less than 0.01); VT/TI was still increased at 11.2 ml X s-1 X kg-1 2 h later. In contrast, for the CBD lambs there was no response to caffeine infusion as measured by VE or VT/TI. We conclude that bolus caffeine infusion produces a rapid response in VE followed by a fall in VE that remained above base line until at least 2 h postinfusion, and the intact chemoreceptor function appears as an essential mediator for these increases in ventilation, since the peripheral chemodenervation has completely abolished the VE response to this particular dose of caffeine.     

Growth hormone response to long-term GH-RH administration in lambs

The pattern of long-term GHRH administration capable of stimulating GH release without depleting pituitary GH content has been investigated using two experimental approaches. In experiment 1, recently weaned male lambs were treated for 3 weeks as follows: Group A) control; B) subcutaneous (sc) continuous infusion of GHRH (1200 mg/day) using a slow release pellet; C) the same as B plus 1 daily sc injection of long acting somatostatin (SS) (octreotide, 20 mg) ; D) 3 daily sc GHRH (250 mg) injections ; E) 2 daily sc injections of GHRH (250 mg) and 2 of natural SS (250 mg). In experiment 2, recently weaned male lambs were continuously GHRH-treated using sc osmotic minipumps (900 mg/day) alone or combined with a daily sc injection of octreotide (20 mg) for 4 weeks. Basal plasma GH levels were increased after chronic pulsatile GHRH treatment but not after any kind of continuous GHRH administration. This increment was maintained during the 3 weeks of experimentation and appeared accompanied by a pituitary GH content similar to controls. A marked GH response to the iv GHRH challenge was observed in controls and in lambs receiving both types of continuous sc GHRH infusions, whereas pulsatile sc GHRH-treated animals did not respond to the iv GHRH challenge in the first and second weeks of the study but did so in the third week of treatment. These data demonstrate that long-term pulsatile GHRH administration is capable of stimulating GH release in growing male lambs, without producing pituitary desensitization.     

Ovarian response to exogenous hormones in six-week-old lambs.

Crossbred lambs 5--6 weeks old were treated with human chorionic gonadotrophin (hCG) (500 or 1500 i.u.) alone, hCG plus pregnant mare serum gonadotrophin (PMSG) (1000 or 2000 i.u.), 1000 i.u. PMSG alone, or were untreated. PMSG alone and PMSG + hCG increased ovarian weight and uterine weight. PMSG alone stimulated growth and luteinization of follicles but PMSG + hCG induced ovulations and formation of corpora lutea. hCG alone did not change any of the characteristics which were measured. PMSG had a significant effect on the number of vesicular follicles but none of the treatments affected the number of growing follicles.