Influences on the cardiovascular response to graded levels of systemic hypoxia of the accompanying hypocapnia in the rat

Time-dependent changes in cardiorespiratory functions of anesthetized rats exposed to sustained hypoxia

Although cardiovascular responses may be altered by respiratory changes under prolonged hypoxia, the relationship between respiratory and cardiovascular changes remains unknown. The aim of the present study is to clarify cardiorespiratory changes in anesthetized rats during and after hypoxic conditions using simultaneous recordings of cardiorespiratory variables with 20-sec recording intervals. After air breathing for 20 min (pre-exposure period), rats were subjected to 10% O₂ for 2 h (hypoxic exposure period) and then air for 30 min (recovery period). Minute ventilation (V_E), respiratory frequency, tidal volume, arterial blood pressure (BP), and heart rate (HR) were continuously monitored during the experimental period. Just after hypoxic exposure, V_E, BP, and HR exhibited an overshoot, undershoot, and overshoot followed by a decrease, respectively. During the remaining hypoxic exposure period, continuous high V_E and low BP were observed, whereas HR re-increased. In the recovery period, V_E, BP, and HR showed an undershoot, increase, and decrease followed by an increase, respectively. These results suggest that the continuation of enhanced V_E and re-increased HR, probably, due to carotid body excitation and accompanying sympathetic activation, during the late period of hypoxic exposure are protective responses to avoid worsening hypoxemia and further circulatory insufficiencies under sustained hypoxia.

Organs Blood Flow during Elevation of Body Temperature in Sevoflurane Anesthetized Rats

The aim of this study is to investigate how elevation of body temperature changes organs blood flow during sevoflurane anesthesia. We conducted in vivo research on 14 male Wistar rats to monitor pulse rate and arterial blood pressure and measure hepatic, small intestinal, renal, and descending aortic blood flow using a laser Doppler blood flowmeter. We assessed the changes in organ blood flow, pulse rate, and arterial blood pressure during elevation of the rats' body temperatures up to 41.5°C under anesthesia with 2.0% or 3.0% sevoflurane. We concluded that elevation of body temperature up to 39.5°C does not change hepatic, small intestinal, and renal blood flow during 2.0 and 3.0% sevoflurane anesthesia.

Acute effects of ferumoxytol on regulation of renal hemodynamics and oxygenation

The superparamagnetic iron oxide nanoparticle ferumoxytol is increasingly used as intravascular contrast agent in magnetic resonance imaging (MRI). This study details the impact of ferumoxytol on regulation of renal hemodynamics and oxygenation. In 10 anesthetized rats, a single intravenous injection of isotonic saline (used as volume control) was followed by three consecutive injections of ferumoxytol to achieve cumulative doses of 6, 10, and 41 mg Fe/kg body mass. Arterial blood pressure, renal blood flow, renal cortical and medullary perfusion and oxygen tension were continuously measured. Regulation of renal hemodynamics and oxygenation was characterized by dedicated interventions: brief periods of suprarenal aortic occlusion, hypoxia, and hyperoxia. None of the three doses of ferumoxytol resulted in significant changes in any of the measured parameters as compared to saline. Ferumoxytol did not significantly alter regulation of renal hemodynamics and oxygenation as studied by aortic occlusion and hypoxia. The only significant effect of ferumoxytol at the highest dose was a blunting of the hyperoxia-induced increase in arterial pressure. Taken together, ferumoxytol has only marginal effects on the regulation of renal hemodynamics and oxygenation. This makes ferumoxytol a prime candidate as contrast agent for renal MRI including the assessment of renal blood volume fraction.

Plasticity in breathing and arterial blood pressure following acute intermittent hypercapnic hypoxia in infant rat pups with a partial loss of 5-HT neurons

The role of serotonin (5-HT) neurons in cardiovascular responses to acute intermittent hypoxia (AIH) has not been studied in the neonatal period. We hypothesized that a partial loss of 5-HT neurons would reduce arterial blood pressure (BP) at rest, increase the fall in BP during hypoxia, and reduce the long-term facilitation of breathing (vLTF) and BP following AIH. We exposed 2-wk-old, 5,7-dihydroxytryptamine-treated and controls to AIH (10% O2; n = 13 control, 14 treated), acute intermittent hypercapnia (5% CO2; n = 12 and 11), or acute intermittent hypercapnic hypoxia (AIHH; 10% O2, 5% CO2; n = 15 and 17). We gave five 5-min challenges of AIH and acute intermittent hypercapnia, and twenty ∼20-s challenges of AIHH to mimic sleep apnea. Systolic BP (sBP), diastolic BP, mean arterial pressure, heart rate (HR), ventilation (V̇e), and metabolic rate (V̇o2) were continuously monitored. 5,7-Dihydroxytryptamine induced an ∼35% loss of 5-HT neurons from the medullary raphe. Compared with controls, pups deficient in 5-HT neurons had reduced resting sBP (∼6 mmHg), mean arterial pressure (∼5 mmHg), and HR (56 beats/min), and experienced a reduced drop in BP during hypoxia. AIHH induced vLTF in both groups, reflected in increased V̇e and V̇e/V̇o2, and decreased arterial Pco2. The sBP of pups deficient in 5-HT neurons, but not controls, was increased 1 h following AIHH. Our data suggest that a relatively small loss of 5-HT neurons compromises resting BP and HR, but has no influence on ventilatory plasticity induced by AIHH. AIHH may be useful for reversing cardiorespiratory defects related to partial 5-HT system dysfunction.

Detailing the relation between renal T2* and renal tissue pO2 using an integrated approach of parametric magnetic resonance imaging and invasive physiological measurements

OBJECTIVES

This study was designed to detail the relation between renal T2* and renal tissue pO2 using an integrated approach that combines parametric magnetic resonance imaging (MRI) and quantitative physiological measurements (MR-PHYSIOL).

MATERIALS AND METHODS

Experiments were performed in 21 male Wistar rats. In vivo modulation of renal hemodynamics and oxygenation was achieved by brief periods of aortic occlusion, hypoxia, and hyperoxia. Renal perfusion pressure (RPP), renal blood flow (RBF), local cortical and medullary tissue pO2, and blood flux were simultaneously recorded together with T2*, T2 mapping, and magnetic resonance-based kidney size measurements (MR-PHYSIOL). Magnetic resonance imaging was carried out on a 9.4-T small-animal magnetic resonance system. Relative changes in the invasive quantitative parameters were correlated with relative changes in the parameters derived from MRI using Spearman analysis and Pearson analysis.

RESULTS

Changes in T2* qualitatively reflected tissue pO2 changes induced by the interventions. T2* versus pO2 Spearman rank correlations were significant for all interventions, yet quantitative translation of T2*/pO2 correlations obtained for one intervention to another intervention proved not appropriate. The closest T2*/pO2 correlation was found for hypoxia and recovery. The interlayer comparison revealed closest T2*/pO2 correlations for the outer medulla and showed that extrapolation of results obtained for one renal layer to other renal layers must be made with due caution. For T2* to RBF relation, significant Spearman correlations were deduced for all renal layers and for all interventions. T2*/RBF correlations for the cortex and outer medulla were even superior to those between T2* and tissue pO2. The closest T2*/RBF correlation occurred during hypoxia and recovery. Close correlations were observed between T2* and kidney size during hypoxia and recovery and for occlusion and recovery. In both cases, kidney size correlated well with renal vascular conductance, as did renal vascular conductance with T2*. Our findings indicate that changes in T2* qualitatively mirror changes in renal tissue pO2 but are also associated with confounding factors including vascular volume fraction and tubular volume fraction.

CONCLUSIONS

Our results demonstrate that MR-PHYSIOL is instrumental to detail the link between renal tissue pO2 and T2* in vivo. Unravelling the link between regional renal T2* and tissue pO2, including the role of the T2* confounding parameters vascular and tubular volume fraction and oxy-hemoglobin dissociation curve, requires further research. These explorations are essential before the quantitative capabilities of parametric MRI can be translated from experimental research to improved clinical understanding of hemodynamics/oxygenation in kidney disorders.

How bold is blood oxygenation level-dependent (BOLD) magnetic resonance imaging of the kidney? Opportunities, challenges and future directions

Renal tissue hypoperfusion and hypoxia are key elements in the pathophysiology of acute kidney injury and its progression to chronic kidney disease. Yet, in vivo assessment of renal haemodynamics and tissue oxygenation remains a challenge. Many of the established approaches are invasive, hence not applicable in humans. Blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) offers an alternative. BOLD-MRI is non-invasive and indicative of renal tissue oxygenation. Nonetheless, recent (pre-) clinical studies revived the question as to how bold renal BOLD-MRI really is. This review aimed to deliver some answers. It is designed to inspire the renal physiology, nephrology and imaging communities to foster explorations into the assessment of renal oxygenation and haemodynamics by exploiting the powers of MRI. For this purpose, the specifics of renal oxygenation and perfusion are outlined. The fundamentals of BOLD-MRI are summarized. The link between tissue oxygenation and the oxygenation-sensitive MR biomarker T2∗ is outlined. The merits and limitations of renal BOLD-MRI in animal and human studies are surveyed together with their clinical implications. Explorations into detailing the relation between renal T2∗ and renal tissue partial pressure of oxygen (pO2 ) are discussed with a focus on factors confounding the T2∗ vs. tissue pO2 relation. Multi-modality in vivo approaches suitable for detailing the role of the confounding factors that govern T2∗ are considered. A schematic approach describing the link between renal perfusion, oxygenation, tissue compartments and renal T2∗ is proposed. Future directions of MRI assessment of renal oxygenation and perfusion are explored.

Hemodynamic and ventilatory response to different levels of hypoxia and hypercapnia in carotid body-denervated rats

OBJECTIVE

Chemoreceptors play an important role in the autonomic modulation of circulatory and ventilatory responses to changes in arterial O(2) and/or CO(2). However, studies evaluating hemodynamic responses to hypoxia and hypercapnia in rats have shown inconsistent results. Our aim was to evaluate hemodynamic and respiratory responses to different levels of hypoxia and hypercapnia in conscious intact or carotid body-denervated rats.

METHODS

Male Wistar rats were submitted to bilateral ligature of carotid body arteries (or sham-operation) and received catheters into the left femoral artery and vein. After two days, each animal was placed into a plethysmographic chamber and, after baseline measurements of respiratory parameters and arterial pressure, each animal was subjected to three levels of hypoxia (15, 10 and 6% O(2)) and hypercapnia (10% CO(2)).

RESULTS

The results indicated that 15% O(2) decreased the mean arterial pressure and increased the heart rate (HR) in both intact (n = 8) and carotid body-denervated (n = 7) rats. In contrast, 10% O(2) did not change the mean arterial pressure but still increased the HR in intact rats, and it decreased the mean arterial pressure and increased the heart rate in carotid body-denervated rats. Furthermore, 6% O(2) increased the mean arterial pressure and decreased the HR in intact rats, but it decreased the mean arterial pressure and did not change the HR in carotid body-denervated rats. The 3 levels of hypoxia increased pulmonary ventilation in both groups, with attenuated responses in carotid body-denervated rats. Hypercapnia with 10% CO(2) increased the mean arterial pressure and decreased HR similarly in both groups. Hypercapnia also increased pulmonary ventilation in both groups to the same extent.

CONCLUSION

This study demonstrates that the hemodynamic and ventilatory responses varied according to the level of hypoxia. Nevertheless, the hemodynamic and ventilatory responses to hypercapnia did not depend on the activation of the peripheral carotid chemoreceptors.

Changes in muscle sympathetic nerve activity and vascular responses evoked in the spinotrapezius muscle of the rat by systemic hypoxia

Responses evoked in muscle sympathetic nerve activity (MSNA) by systemic hypoxia have received relatively little attention. Moreover, MSNA is generally identified from firing characteristics in fibres supplying whole limbs: their actual destination is not determined. We aimed to address these limitations by using a novel preparation of spinotrapezius muscle in anaesthetised rats. By using focal recording electrodes, multi-unit and discriminated single unit activity were recorded from the surface of arterial vessels.This had cardiac- and respiratory-related activities expected of MSNA, and was increased by baroreceptor unloading, decreased by baroreceptor stimulation and abolished by autonomic ganglion blockade. Progressive, graded hypoxia (breathing sequentially 12, 10, 8% O2 for 2min each) evoked graded increases in MSNA.In single units, mean firing frequency increased from 0.2±0.04 in 21% O2 to 0.62 ± 0.14 Hz in8% O2, while instantaneous frequencies ranged from 0.04–6Hz in 21% O2 to 0.09–20 Hz in 8%O2. Concomitantly, arterial pressure (ABP), fell and heart rate (HR) and respiratory frequency(RF) increased progressively, while spinotrapezius vascular resistance (SVR) decreased (Spinotrapezius blood flow/ABP), indicating muscle vasodilatation. During 8% O2 for 10 min, the falls in ABP and SVR were maintained, but RF, HR and MSNA waned towards baselines from the second to the tenth minute. Thus, we directly show that MSNA increases during systemic hypoxia to an extent that is mainly determined by the increases in peripheral chemoreceptor stimulation and respiratory drive, but its vasoconstrictor effects on muscle vasculature are largely blunted by local dilator influences, despite high instantaneous frequencies in single fibres.

Responses evoked in single sympathetic nerve fibres of the rat tail artery by systemic hypoxia are dependent on core temperature

No direct evidence exists of the changes evoked by systemic hypoxia in sympathetic nerves to the rat cutaneous circulation, and of the concomitant changes in cutaneous blood flow. Here we investigated responses evoked by two levels of systemic hypoxia (12% and 8% inspired O(2)) in single sympathetic units supplying tail caudal ventral artery (CVA) in spontaneously breathing anaesthetized rats, whilst simultaneously recording tail blood flow and vascular resistance (TVR) from the CVA, under conditions of modest hypothermia and hyperthermia. During modest hypothermia and normoxia, TVR was high and CVA unit activity was present, with marked respiratory modulation and a rhythmictiy (T-rhythm) that was often independent of respiration. Hypoxia evoked a graded fall in TVR indicating vasodilatation, but there were no consistent changes in CVA unit firing rate or T-rhythm frequency, although respiratory modulation increased. By contrast, during hyperthermia, TVR was low and CVA unit activity was absent. Systemic hypoxia evoked graded increases in TVR, indicating vasoconstriction, and in 8% O(2) there was recommencement of firing in some CVA units, at low discharge rate, with respiratory modulation but no T-rhythm. These results indicate that the changes evoked by systemic hypoxia in TVR and sympathetic nerve activity to CVA are dependent on core temperature. During modest hypothermia, hypoxia-induced cutaneous vasodilatation in the tail is independent of sympathetic activity, whereas during hyperthermia, when sympathetic activity is 'switched off', severe hypoxia initiates respiratory-related low level activity, causing cutaneous vasoconstriction.

Cerebral blood flow and BOLD fMRI responses to hypoxia in awake and anesthetized rats

This study investigated the functional MRI responses to graded hypoxia in awake/restrained and anesthetized animals by measuring cerebral blood flow (CBF) and blood oxygenation (BOLD) changes and estimating changes in cerebral metabolic rate of oxygen (CMRO2). Hypoxia in isoflurane anesthetized rats reduced blood pressure but did not change heart rate and respiration rate. In contrast, hypoxia in awake animals showed compensatory responses by sustaining blood pressure, increasing heart rate and respiration rate. Basal CBF was higher under isoflurane anesthesia than awake state because isoflurane is a vasodilator. Graded hypoxia decreased BOLD signals. Surprisingly, hypoxia also decreased CBF likely because hypoxia induced hypocapnia. Hypoxia-induced CBF and BOLD decreases were smaller in awake, relative to anesthetized, rats at low pO2, but similar at high pO2. CBF leveled off with decreasing hypoxia-induced pCO2 in awake rats, but monotonically decreased in anesthetized rats. CMRO2 estimated using a biophysical BOLD model did not change under mild hypoxia but was reduced under severe hypoxia relative to baseline. These results showed that isoflurane attenuated autonomic responses to hypoxia, hypoxia-induced hypocapnia dominated CBF changes, tissues in awake conditions appeared better oxygenated, and severe hypoxia reduced oxygen metabolism. This study underscored the marked differences in BOLD and CBF MRI responses to hypoxia in vivo between awake and anesthetized conditions and has implications for functional MRI studies of hypoxia in anesthetized animal models.

Microenvironmental adaptation of experimental tumours to chronic vs acute hypoxia

This study investigated long-term microenvironmental responses (oxygenation, perfusion, metabolic status, proliferation, vascular endothelial growth factor (VEGF) expression and vascularisation) to chronic hypoxia in experimental tumours. Experiments were performed using s.c.-implanted DS-sarcomas in rats. In order to induce more pronounced tumour hypoxia, one group of animals was housed in a hypoxic atmosphere (8% O₂) for the whole period of tumour growth (chronic hypoxia). A second group was acutely exposed to inspiratory hypoxia for only 20 min prior to the measurements (acute hypoxia), whereas animals housed under normal atmospheric conditions served as controls. Acute hypoxia reduced the median oxygen partial pressure (pO₂) dramatically (1 vs 10 mmHg in controls), whereas in chronically hypoxic tumours the pO₂ was significantly improved (median pO₂=4 mmHg), however not reaching the control level. These findings reflect the changes in tumour perfusion where acutely hypoxic tumours show a dramatic reduction of perfused tumour vessels (maybe the result of a simultaneous reduction in arterial blood pressure). In animals under chronic inspiratory hypoxia, the number of perfused vessels increased (compared to acute hypoxia), although the perfusion pattern found in control tumours was not reached. In the chronically hypoxic animals, tumour cell proliferation and tumour growth were significantly reduced, whereas no differences in VEGF expression and vascular density between these groups were observed. These results suggest that long-term adaptation of tumours to chronic hypoxia in vivo, while not affecting vascularity, does influence the functional status of the microvessels in favour of a more homogeneous perfusion.

Roles of adenosine in skeletal muscle during systemic hypoxia

1. The present review is concerned with the effects of acute systemic hypoxia on the gross vascular conductance of skeletal muscle (MVC) and on the behaviour of muscle microcirculation. 2. On the basis of experiments performed in the rat, it is argued that adenosine released from the vascular endothelium plays a major role in dilating muscle vasculature by acting on adenosine A1 receptors. 3. The dilatation of the proximal arterioles is primarily important in increasing MVC and in limiting the fall in O(2) delivery to muscle. It is suggested that the action of adenosine on proximal arterioles is dependent on nitric oxide (NO) rather than mediated by NO, such that adenosine dilates the proximal arterioles via other mechanisms when synthesis of NO is blocked. 4. In contrast, dilatation of terminal arterioles, particularly in regions within muscle where the hypoxia is most severe, helps to improve the distribution of available O(2), allowing muscle O(2) consumption to be maintained by increased O(2) extraction. It is concluded that the action of adenosine on terminal arterioles is mainly mediated by NO arising from stimulation of endothelial A1 receptors. 5. Therefore, adenosine plays a major role in coordinating the behaviour of muscle vasculature such that the relationship between O(2) supply and O(2) demand can be optimized even when the O(2) content of the arterial blood is greatly reduced.

Nicotine delays arousal during hypoxemia in lambs

A decreased ability to arouse from sleep in response to arterial hypoxemia may lead to severe asphyxia and has been proposed as a mechanism of sudden infant death syndrome. Based on previous observations that nicotine exposure, a major environmental risk factor for sudden infant death syndrome, may impair hypoxic defense in neonates, we hypothesized that a short-term infusion of nicotine could impair hypoxic arousal through interference with oxygen-sensing mechanisms. Seven chronically instrumented unanesthetized lambs were studied at the age of 4.6 +/- 1.3 d during normoxia and acute hypoxia (0.1 fraction of inspired oxygen) for 5 min. Ventilation, transcutaneous Hb oxygen saturation, blood pressure, heart rate, and time to arousal were compared during a control saline infusion and during a 0.5 microg x kg(-1) x min(-1) nicotine infusion. Activity states, i.e. wakefulness and quiet sleep as well as arousal, were defined by EEG, nuchal electromyogram, and electrooculogram. Each lamb acted as its own control. Arousal from quiet sleep occurred significantly later during nicotine infusion compared with control (177 +/- 93 versus 57 +/- 41 s, p < 0.01) and at a lower transcutaneous Hb oxygen saturation (60 +/- 12 versus 79 +/- 12%, p < 0.01) (paired t test). The ventilatory response to hypoxia in wakefulness was similar during both conditions but was significantly attenuated in quiet sleep during nicotine infusion (p < 0.001, 2-way ANOVA repeated-measures design). Blood pressure and heart rate responses were similar during both conditions. These results suggest that a brief nicotine exposure blunts oxygen sensitivity in young lambs, a finding of potential relevance for sudden infant death syndrome.

Chemoreceptors and cardiovascular control in acute and chronic systemic hypoxia

This review describes the ways in which the primary bradycardia and peripheral vasoconstriction evoked by selective stimulation of peripheral chemoreceptors can be modified by the secondary effects of a chemoreceptor-induced increase in ventilation. The evidence that strong stimulation of peripheral chemoreceptors can evoke the behavioural and cardiovascular components of the alerting or defence response which is characteristically evoked by novel or noxious stimuli is considered. The functional significance of all these influences in systemic hypoxia is then discussed with emphasis on the fact that these reflex changes can be overcome by the local effects of hypoxia: central neural hypoxia depresses ventilation, hypoxia acting on the heart causes bradycardia and local hypoxia of skeletal muscle and brain induces vasodilatation. Further, it is proposed that these local influences can become interdependent, so generating a positive feedback loop that may explain sudden infant death syndrome (SIDS). It is also argued that a major contributor to these local influences is adenosine. The role of adenosine in determining the distribution of O2 in skeletal muscle microcirculation in hypoxia is discussed, together with its possible cellular mechanisms of action. Finally, evidence is presented that in chronic systemic hypoxia, the reflex vasoconstrictor influences of the sympathetic nervous system are reduced and/or the local dilator influences of hypoxia are enhanced. In vitro and in vivo findings suggest this is partly explained by upregulation of nitric oxide (NO) synthesis by the vascular endothelium which facilitates vasodilatation induced by adenosine and other NO-dependent dilators and attenuates noradrenaline-evoked vasoconstriction.

Acute blood pressure elevation during repetitive hypocapnic and eucapnic hypoxia in rats

Using a rat model, we investigated whether episodic eucapnic hypoxia was a more potent stimulus to acute blood pressure (BP) elevation and bradycardia than episodic hypocapnic hypoxia. We also investigated the role of sympathetic and parasympathetic nervous system in this cardiovascular response. Sprague-Dawley (SD) and Wistar Kyoto (WKY) rats were exposed to repetitive 30-s cycles of hypocapnic or eucapnic hypoxia before and after intravenous injection of the alpha1-adrenergic blocker prazosin, alpha2-adrenergic blocker yohimbine, or atropine. Eucapnic hypoxia caused a threefold elevation in systolic BP from baseline (83.5 +/- 3.5 mmHg in WKY, 70.6 +/- 4.6 mmHg in SD) and greater bradycardia (-178 +/- 20 beats/min in WKY, -178 +/- 21 beats/min in SD) compared with hypocapnic hypoxia (29.8 +/- 3.6 mmHg and -43 +/- 15 beats/min in WKY, 19.0 +/- 4.1 mmHg and -45 +/- 12 beats/min in SD). After prazosin, the BP increase from eucapnic hypoxia was blunted, yohimbine showed no effect, and atropine blocked the bradycardia. Direct measurement of sympathetic nerve activity confirmed that adding CO2 to the hypoxic gas mixture caused a 61% increase in sympathetic nerve activity. WKY rats seem more vulnerable than SD rats to both hypoxia exposures in terms of the elevation in BP. We conclude that, in the rat, eucapnic hypoxia is a more potent stimulus to acute BP elevation and bradycardia than is hypocapnic hypoxia. An increased sympathetic tone appears to be involved in the BP response to acute episodic hypoxia.

The behaviour of muscle microcirculation in chronically hypoxic rats: the role of adenosine

1. In rats housed in a hypoxic chamber at 12% O2 for 3-5 weeks (CH) and in normal rats housed in air (N), we directly observed responses of arterial and venous vessels of the spinotrapezius muscle to changes in O2 concentration in the inspirate. Both CH and N rats were anaesthetized with Saffan. They had haematocrits of 55.0 +/- 0.9% (mean +/- S.E.M.) and 41.9 +/- 0.5%, respectively. 2. In CH rats breathing 12% O2 and N rats breathing air, arterial and venous vessels from comparable anatomical positions in the vascular tree were of similar internal diameter. They also showed similar maximum dilator responses to topical adenosine (10(-3) M); 14.1 +/- 1.1 and 16.3 +/- 1.7% in all arterioles, 15.5 +/- 1.2 and 11.5 +/- 0.6% in all venules in CH and N rats, respectively. 3. In CH rats, the change from 12% O2 to air for 3 min induced constriction in all arterioles and venules (-12.9 +/- 1.0 and -14.3 +/- 1.7%, respectively), whereas in N rats, the change from air to 12% O2 for 3 min induced net dilatation (3.9 +/- 1.8% in arterioles and 4.7 +/- 0.8% in venules). Topical application of the adenosine receptor antagonist 8-sulphophenyltheophylline (8-SPT, 10(-3) M) had no effect on control diameters in CH or N rats, nor on constrictor responses to air in CH, but reversed or reduced dilator responses to 12% O2 in N rats (to -2.4 +/- 1.3% in arterioles and 2.0 +/- 0.9% in venules). 4. In CH rats, the change from 12 to 8% O2 produced net dilatation as great as that induced in N rats by the larger change from air to 8% O2: 8.5 +/- 2.6 and 5.0 +/- 3.7% in arterioles and 10.3 +/- 1.8 and 6.4 +/- 1.9% in venules, respectively. These responses were similarly reduced by 8-SPT to -4.3 +/- 1.9 and -5.2 +/- 2.7% in arterioles and to -6.9 +/- 2.0 and -1.5 +/- 2.0% in venules, respectively. 5. These results indicate that CH rats were acclimated to 12% O2 such that the resting tone of arterial and venous vessels of muscle was comparable to that of N rats breathing air. They also suggest that adenosine had little tonic dilator influence in CH rats breathing 12% O2 despite its contribution to the dilatation induced in N rats by acute exposure to 12% O2. This may reflect the greater haematocrit in CH rats which normalized the O2 supply to muscle. However, CH rats were more sensitive than N rats to the dilator influence of acute systemic hypoxia and this was largely mediated by adenosine.

The effects of systemic hypoxia on renal function in the anaesthetized rat

1. In rats anaesthetized with Saffan, renal function was monitored from the left kidney from the 5th minute of spontaneous breathing of 12% O2 for two 20 min periods and during air breathing before, between and after the hypoxic periods. Two groups of animals (I and II) were used, each group comprising two subgroups in which the left kidney was innervated or denervated, respectively; in Group II, renal perfusion pressure (RPP) was maintained during the 2nd hypoxic period by occl97uding the distal aorta. 2. In both subgroups of Group I, both hypoxic periods produced hyperventilation, arterial PO2 falling to approximately 50 mmHg. Concomitantly, mean arterial pressure (MABP) fell by similar extents (approximately 23%, from a baseline level of 140 mmHg during the 2nd hypoxic period). In the innervated subgroup, renal vascular conductance (RVC) increased, but glomerular filtration rate (GFR) fell (by 48 and 6%, respectively, during the 2nd hypoxic period), while urine flow, absolute sodium excretion (UNaV) and fractional sodium excretion (FENa) fell (by 52, 63 and 61%, respectively). Baseline urine flow, UNaV and FENa were higher in the denervated subgroup, but hypoxia produced similar percentage changes from baseline in all variables. 3. In Group II, both subgroups showed similar changes during the 1st hypoxic period as the corresponding subgroups of Group I. However, during the 2nd hypoxic period when the fall in MABP was reduced to approximately 7%, the increase in RVC persisted only in the denervated subgroup; there was no significant change in GFR, urine flow, UNaV or FENa in either subgroup. 4. These results indicate that, in the rat, moderate hypoxia produces antidiuresis and antinatriuresis that are not dependent on the renal nerves, but are dependent on the hypoxia-induced fall in MABP. The fall in renal perfusion pressure (RPP) may directly determine renal function, but reflex influences upon the kidney initiated by, for example, arterial baroreceptor unloading, may play a role. The fall in GFR and increase in RVC, which persisted after denervation or when renal perfusion was controlled, implies a local dilatatory influence acting preferentially on the efferent arterioles.

Interdependence of respiratory and cardiovascular changes induced by systemic hypoxia in the rat: the roles of adenosine

1. In ten spontaneously breathing, Saffan-anaesthetized rats (group I), respiratory and cardiovascular responses evoked by 10 min periods of hypoxia (arterial partial pressure of O2, Pa,O2, 33 mmHg) were recorded before and after the administration of the adenosine receptor antagonist 8-phenyltheophylline (8-PT, 10 mg kg-1 i.v.). Similar experiments were performed on nine constantly ventilated rats (group II; Pa,O2, 29 mmHg) with arterial partial pressure of CO2 (Pa,CO2) held constant. 2. In group I, hypoxia induced an initial increase and a secondary fall in ventilation (VE) with an accompanying secondary fall in heart rate (HR), arterial pressure (ABP) fell and cerebral vascular conductance (CVC) increased progressively. Cerebral blood flow (CBF) tended to fall with time during hypoxia. 8-PT abolished the secondary falls in VE and HR and reduced the fall in ABP and increase in CVC, while CBF was better maintained. 3. In group II, hypoxia induced a similar cardiovascular response to that in group I, but at the 1st minute of hypoxia, the HR was lower and the increase in CVC was greater. 8-PT did not affect the hypoxia-induced changes in HR, ABP, CVC or CBF. 4. These results indicate specific ways in which the ventilatory and cardiovascular responses induced by hypoxia in the spontaneously breathing rat are interdependent. They also indicate that the influences of 8-PT on the cardiovascular changes induced by hypoxia during spontaneous ventilation are mainly a consequence of its ability to block the centrally mediated contribution of adenosine to the secondary fall in ventilation.

The role of vasopressin in the regional vascular responses evoked in the spontaneously breathing rat by systemic hypoxia

1. In spontaneously breathing rats anaesthetized with Saffan, we have investigated the role of vasopressin in the cardiovascular responses evoked by systemic hypoxia (breathing 8 or 6% O2 for 5 min). 2. Breathing 8% O2 evoked an increase in respiratory frequency and tidal volume; arterial O2 pressure (Pa,O2) fell to 37 mmHg and arterial CO2 pressure (Pa,CO2) fell to 30 mmHg. Concomitantly, there was a fall in arterial pressure, tachycardia and increases in femoral and renal vascular conductances indicating net vasodilatation in skeletal muscle and kidney. The vasopressin V1-receptor antagonist, d(CH2)5Tyr(Me)-arginine vasopressin (20 micrograms kg-1 i.v.), had no significant effect on the baseline values of any recorded variables, nor on the respiratory or blood gas changes evoked by 8% O2. However, it accentuated the fall in arterial pressure and the increase in femoral vascular conductance (+22 vs. +77% at the 5th minute) produced by 8% O2, but had no significant effect on the increase in renal vascular conductance. 3. Breathing 6% O2 evoked qualitatively similar responses as 8% O2 but Pa,O2 fell to 33 mmHg and Pa,CO2 fell to 28 mmHg and the respiratory and cardiovascular changes tended to be larger than those evoked by 8% O2. Again the V1-receptor antagonist accentuated the hypoxia-induced fall in arterial pressure and increase in femoral vascular conductance (+5 vs. +76% at the 5th minute). 4. Infusion of vasopressin (1.5 ng min-1 kg-1 i.v.) for 5 min with the aim of producing a plasma concentration comparable to that reached during 8% O2, induced a rise in arterial pressure (9%), bradycardia (-5%) and a decrease in femoral (-11%) and renal vascular conductance (-4%). 5. These results suggest that vasopressin released during hypocapnic hypoxia helps to limit the evoked fall in arterial pressure by exerting a vasoconstrictor influence on skeletal muscle.

Effect of chronic hypoxia on hemodynamics, organ blood flow and O2 supply in rats

Aortic blood flow, heart rate, blood pressure and blood flow distribution were measured in 10 chronically hypoxic rats (3 weeks, PB 370-380 Torr) breathing 10% O2 (chronic hypoxia) and after 30 min of breathing air (acute normoxia). Controls were 10 normoxic littermates breathing air (normoxia) and 10% O2 for 30 min (acute hypoxia). Acute hypoxia resulted in increased aortic blood flow and heart rate, and decreased total peripheral resistance. Blood flow and oxygen supply to vital organs increased, indicating that blood flow redistribution plays an important role in oxygen supply. In chronic hypoxia, aortic blood flow and heart rate remained elevated, and total peripheral resistance remained decreased. Blood flow distribution returned towards normoxia levels. Oxygen supply was maintained via increased arterial oxygen concentration. Acute normoxia resulted in decreased aortic blood flow and heart rate, and increased blood pressure and total peripheral resistance. Blood flow distribution was similar to that of chronic hypoxia except skeletal muscles, in which blood flow decreased markedly. Oxygen supply remained unchanged or increased.

The roles of catecholamines in responses evoked in arterioles and venules of rat skeletal muscle by systemic hypoxia

1. Studies have been made in the anaesthetized rat of the roles played by alpha- and beta-adrenoreceptor stimulation in determining diameter changes induced in individual arterioles and venules of the spinotrapezius muscle during systemic hypoxia (breathing 6% O2 for 3 min). 2. Topical application to the spinotrapezius of phentolamine, the alpha-adrenoreceptor antagonist, or sotalol, the beta-adrenoreceptor antagonist, had no effect on the fall in systemic arterial pressure and tachycardia induced by hypoxia. 3. All arterioles and venules showed a decrease in diameter in response to topical application of noradrenaline (10(-6) g ml-1): these responses were abolished by topical application of phentolamine. Moreover, those arterioles and venules that showed a decrease in diameter during hypoxia before phentolamine, showed a significantly smaller decrease, or an increase in diameter after phentolamine. This effect was most marked in primary and secondary arterioles (13-50 microns diameter). 4. All arterioles and venules showed an increase in diameter in response to topical application of isoprenaline (10(-6) g ml-1); these responses were abolished by topical application of sotalol. Moreover, these arterioles and venules that showed an increase in diameter during hypoxia before sotalol, showed a significantly smaller increase or even a decrease in diameter after sotalol. 5. These results suggest that during hypoxia the arterioles of skeletal muscle, especially primary and secondary arterioles, are under the constrictor influence of a reflex increase in sympathetic nerve activity while the venules, which have no sympathetic innervation, are under the constrictor influence of circulating catecholamines. They also suggest that in individual arterioles and venules, these constrictor influences may be overcome by dilatation mediated by the beta-adrenoreceptor influence of circulating catecholamines. 6. Since some arterioles and venules still showed constriction during hypoxia after phentolamine and some still showed dilatation during hypoxia after sotalol, it seems that factors other than catecholamines contribute to the diameter changes. It is suggested that locally released metabolites exert a substantial dilator influence, particularly on terminal arterioles and collecting venules, those vessels nearest to the capillary bed.

Responses observed in individual arterioles and venules of rat skeletal muscle during systemic hypoxia

1. In rats anaesthetized with Saffan, responses induced in individual arterial and venous vessels of the spinotrapezius muscle by systemic hypoxia (breathing 12 or 6% O2 for 3 min) were directly observed by in vivo microscopy. 2. Both 12 and 6% O2 induced gradual tachycardia and a fall in arterial pressure. Concommitantly, in each section of the vascular tree, some vessels showed a gradual increase in diameter, others, a gradual decrease. 3. During 12% O2, mean diameter changes were graded from mean increases of approximately 2% in main arteries (resting diameter 40-90 microns) to approximately 20% in terminal arterioles (7-13 microns) and ranged from mean increases of 5-8% in collecting and secondary venules (9-18 microns, 18-30 microns), to a decrease of approximately 2% in main veins (65-130 microns). 4. During 6% O2, constrictor responses were more common in arterial vessels. Thus, mean changes amounted to diameter decreases of less than 5% in main arteries and secondary arterioles (13-18 microns), and increases of approximately 5% in primary arterioles (22-50 microns) and terminal arterioles. By contrast, diameter increases predominated in venous vessels being graded from approximately 20% in collecting venules to approximately 2% in main veins. 5. In seventeen rats, 6% O2 was administered for eight 3 min periods separated by 30 min control periods. The changes evoked in arterial pressure and heart rate were consistent throughout. Diameter changes evoked in individual arterial and venous vessels were consistent in the first two hypoxic periods. However, diameter changes in the third and successive periods were significantly different from those recorded in the first period: increases in diameter became more common and pronounced. 6. These changes in vessel diameter, especially their variability, are considered in relation to recordings made previously of changes in gross blood flow and vascular conductance of limb muscle during systemic hypoxia.

Effects of systemic hypoxia on the distribution of cardiac output in the rat

1. Studies were made in unanaesthetized rats of cardiovascular responses induced during 3 min periods of systemic hypoxia (inspirate 8 or 6% O2). Arterial pressure and heart rate were recorded continuously; cardiac index and regional blood flows were measured in normoxia and at the 2nd min of hypoxia by injection of radiolabelled microspheres. Comparisons are made with changes recorded in Saffan-anaesthetized rats during 8% O2 using microspheres and in previous studies using electromagnetic transducers on renal, mesenteric and femoral arteries (Marshall & Metcalfe, 1988a). 2. In unanaesthetized rats, the initial 1-1.5 min of hypoxia evoked behavioural arousal associated with a short-lasting rise in arterial pressure and heart rate. This agrees with our previous proposal that hypoxia activates the brain stem defence areas by stimulating peripheral chemoreceptors. 3. In unanaesthetized rats, these changes were superimposed upon a gradual fall in arterial pressure and tachycardia, the responses being greater during 6 than 8% O2 (cf. Saffan-anaesthetized rats). Further, in all rats at the 2nd min of hypoxia, cardiac index and vascular conductance of most body tissues was increased. It is concluded that the fall in arterial pressure is due to peripheral vasodilatation. 4. In the unanaesthetized rats, the tendency for vascular conductance in kidney, intestine and gastrocnemius muscle to increase (more during 6 than 8% O2) allowed increases in blood flow in the last two regions. These changes accord with those recorded under Saffan anaesthesia. 5. In both unanaesthetized and anaesthetized rats, hypoxia induced pronounced increases in vascular conductance of diaphragm, adrenal gland, cerebral hemispheres, cerebellum and brain stem, the resultant increases in blood flow being larger in the unanaesthetized rats. 6. It is proposed that in unanaesthetized, as in anaesthetized, rats the regional dilator responses predominantly reflect the local dilator effects of tissue hypoxia. Possible dilator factors are considered.

Analysis of factors that contribute to cardiovascular changes induced in the cat by graded levels of systemic hypoxia

1. In cats anaesthetized with Saffan, which does not block afferent activation of the brain stem defence areas, we have analysed the cardiovascular changes induced by 3 min periods of graded systemic hypoxia (fraction of O2 in inspirate, Fi,O2, 0.15, 0.12, 0.08, 0.06). 2. At light levels of Saffan anaesthesia, hypoxia (particularly Fi, O2 0.08 and 0.06) or selective stimulation of carotid chemoreceptors evoked the pattern of tachycardia, decrease in renal and mesenteric vascular conductance (RVC, MVC), but increase in femoral vascular conductance (FVC) which is characteristic of the alerting-defence response. This supports our view that activation of the defence areas is an integral part of the response to systemic hypoxia. 3. Hypoxia also induced an increase in frequency of augmented breaths which was graded with the level of hypoxia: 0.6 min-1 at Fi, O2 0.21 to 1.1 min-1 at Fi, O2 0.06; in some cats each of these was accompanied by a transient fall in arterial pressure (ABP) and increase in FVC. It is proposed that these responses were all part of a reflex elicited by lung irritant receptors and facilitated by peripheral chemoreceptors. However, their low rate of occurrence and the liability of the vasodilatation suggests they do not make major contributions to the overall response. 4. The above short-lasting responses were superimposed upon gradual changes whose magnitudes were graded with the level of hypoxia: hyperventilation, slight tachycardia, but bradycardia at Fi, O2 0.6, small increases in ABP, FVC and MVC allowing femoral and mesenteric blood flow to increase, but decreases in RVC which maintained renal blood flow constant. 5. Vagotomy had no significant effect on these changes. Further, hyperinflation of the lungs with pressures of 10 mmHg evoked the Breuer-Hering reflex but had no noticeable cardiovascular effect. It is proposed that, in the cat, reflex tachycardia and vasodilatation elicited by lung stretch receptors play no significant part in the response to hypoxia. 6. By contrast, after pneumothorax, with ventilation and thereby arterial PCO2 (Pa, CO2) maintained constant, graded hypoxia produced graded bradycardia, decrease in MVC and RVC and no change in FVC. Taken together, these results suggest that in the spontaneously breathing cat, the response to hypoxia is dominated by the effects of hypocapnia secondary to hyperventilation, which by inhibiting peripheral and central chemoreceptor activity effectively counteracts the primary bradycardia and peripheral vasoconstriction elicited by hypoxic stimulation of peripheral chemoreceptors. 7. These proposals are compared with those drawn for other species.