Unloading of baroreceptors by carotid occlusion does not increase heart rate in fetal sheep.

UNLABELLED: Unloading of baroreceptors by carotid occlusion does not increase heart rate in fetal sheep; OBJECTIVES: To test the hypothesis that in fetal sheep reduction of carotid sinus pressure by carotid occlusion increases heart rate. STUDY DESIGN: Fetal sheep (gestational age 121-132 days) were chronically instrumented with bilateral carotid occluders, catheters and electrodes (ECG) to measure systemic arterial and carotid sinus (CSP) blood pressures, and fetal heart rate. RESULTS: Bilateral carotid occlusion (BCO) increased mean arterial blood pressure from 46+/-7 mmHg to 53+/-8 mmHg (mean+/-S.D.) while CSP decreased from 44+/-7 mmHg to 17+/-7 mmHg. Fetal heart rate fell during occlusion significantly from 186+/-34 bpm to 159+/-26 bpm (n=20 animals). Infusion of phenylephrine (8.5-20 microg min(-1)kg(-1)) or methoxamine (60-200 microg min(-1)kg(-1)) increased mean blood pressure from 44+/-6 to 61+/-9 mmHg, and fetal heart rate decreased from 186+/-30 to 132+/-31 bpm (n=12). BCO increased systemic arterial pressure further to 70+/-11 mmHg whereas carotid sinus pressure was reduced to 31+/-13 mmHg. However, average heart rate did not increase significantly (136+/-28 bpm). CONCLUSION: We conclude that in contrast to adult animals, in fetal sheep carotid occlusion with subsequent unloading of baroreceptors does not increase heart rate even when the baroreflex had been activated by arterial hypertension. It seems likely that stimulation of carotid chemoreceptors prevents the expected baroreceptor mediated heart-rate response.

Efficacy of carotid collateral perfusion in anaesthetized sheep fetuses.

The purpose of this study was to learn to what extent carotid collateral circulation is efficient in maintaining cephalic blood flow in the sheep fetus. Under halothane anaesthesia six fetal sheep at 124-135 days of gestation were instrumented with inflatable occluders around both common carotid arteries, an inductive flow probe around one external carotid artery, and arterial catheters to measure carotid sinus and aortic pressure. In acute experiments, the occluders were inflated on one side, or the other, or both sides simultaneously, while carotid blood flow, driving pressures and fetal heart rate were determined. Ipsi- and bilateral occlusion reduced carotid blood flow from about 42 ml/min to 10-0 ml/min and decreased sinus pressure from 39 mmHg to 29.1+/-2.9 (mean+/-s.d.) and 16.7+/-3.7 mmHg, respectively. Occlusion of the contralateral carotid artery increased ipsilateral carotid blood flow from 45+/-10 ml/min to 64+/-14 ml/min within 0.2 sec. Heart rate and aortic and sinus pressures did not change appreciably. Analysis of an analogue resistance network demonstrated that the observed carotid flow increases (less than twice normal) do not require changes of vascular resistances.It is concluded that the fetal sheep, as in the adult of many species, possesses an efficient carotid collateral system.

Doppler ultrasound evaluation of ductus venosus blood flow during acute hypoxemia in fetal lambs.

It has been demonstrated with invasive techniques in fetal lambs that the ratio of ductus venosus to umbilical vein blood flow rate (DV/UV ratio) increases during hypoxemia and infusion of catecholamines. Recently it was found in human fetuses using pulsed wave Doppler ultrasound equipment that the DV/UV ratio in fetuses with intrauterine growth restriction was significantly increased. The aim of the present study was to show in fetal lambs whether routine Doppler ultrasound devices were capable of determining the DV/UV ratio with sufficient reliability. The experiments were performed on seven near-term instrumented fetal lambs using pulsed wave Doppler ultrasound to measure flow rates (derived, in milliliters per min, from the intensity-weighted mean velocity (Vmean) and the vessel's cross-sectional area) in the ductus venosus and intra-abdominal umbilical vein. Fetal hypoxemia was induced by administering a low-oxygen gas to the ewe (5-7% oxygen, 2% carbon dioxide). Fetal arterial pO2 and heart rate decreased significantly during maternal hypoxia. The proportion of umbilical venous return passing through the ductus venous in controls was 36 +/- 5% (mean +/- SD). This increased to 53 +/- 6% (p < 0.001) because the umbilical venous blood flow fell during late hypoxemia when the heart rate had decreased by 20%. Severe hypoxemia tended to reduce the mean velocity (Vmean) and the minimum velocity (Vmin) (based on the maximum velocity envelope curve) in the ductus venosus, descending aorta and inferior vena cava. The pulsatility index of the umbilical artery significantly increased at the end of hypoxemia. We conclude that determination of the proportion of umbilical vein blood flow entering the ductus venosus by Doppler ultrasound in a clinical setting may contribute to the detection and evaluation of fetal distress.

Ethophysiological responses to overwinter food shortage in wild European rabbits.

Wild European rabbits studied under seminatural conditions turned out to have about 30% lower heart rates during periods of overwinter food shortage than during periods with access to green forage or pelleted rabbit chow. Concomitant changes in the animals' metabolic rate (ADMR), body weight, blood sugar level, and foraging activity clearly indicate that this heart rate reduction is a sign of starvation due to insufficient food quality. Telemetered body temperature, on the other hand, was amazingly stable in most subjects, even during overwinter food shortage. These data are discussed with respect to nutritional requirements and possible mechanisms of energy conservation in wild European rabbits.

Long-term heartrate responses to social stress in wild European rabbits: predominant effect of rank position.

The long-term mean heartrates of wild European rabbits living in a seminatural environment have been studied based on more than 1500 days of radiotelemetrical heartrate recordings. Significant differences between the heartrate levels of dominant and subordinate individuals have been found. Subordinate rabbits show chronically elevated heartrate that cannot be explained by limited access to burrow shelter or by the cumulative effect of heartrate reactions during aggressive encounters. If a subordinate rabbit achieves a dominant position, its heartrate is gradually adjusted to the lower level characteristic of dominant individuals. These results are discussed with respect to the consequences of dominance in species with different social systems.

Seasonal and environmental influences upon the diurnal heart-rate pattern in wild rabbits living under seminatural conditions.

Diurnal patterns in the heart-rates (HR) of 16 wild European rabbits have been studied, based on more than 1200 days of radiotelemetrical HR recordings. HR shows a distinct diurnal rhythm which is closely coupled to the diurnal pattern of activity for free ranging animals. The rabbits' activity phase, accompanied by high HRs extends from late afternoon to early morning. It is followed by diurnal rest with low HRs, irrespective of burrow utilization. This diurnal resting phase is often interrupted by short excursions outside the burrow. The duration of the activity phase and the times of onset and cessation of activity are very stable across different individuals and over three years of measurement. Disturbances of this stable seasonal pattern are by predation and by weather influences. The importance of light cycle and human disturbances as synchronizers for diurnal HR pattern in wild rabbits is discussed.

Diurnal patterns of scent-marking, serum testosterone concentration and heart rate in male Mongolian gerbils.

The diurnal rhythms of scent marking behavior in an open-field test as well as the diurnal rhythms of serum testosterone concentration and heart rate under home cage conditions were investigated in male Mongolian gerbils. The behavioral measures of scent marking activity, sniffing frequency and locomotor activity in the open field showed a clear rhythm during the 24 hour period with a significant maximum during the dark. In contrast to this, the mean testosterone concentration was lower during the dark as compared to the light phase. The heart rate was not different during light and dark phase of the diurnal cycle, instead, bursts of increased frequency occurred approximately once per hour, both during light and dark phase.