1. Amperometric recordings were made from individual phaeochromocytoma (PC12) cells using carbon fibre microelectrodes to investigate the effects of chronic hypoxia (10% O2) on the secretory responses evoked by acute hypoxia. 2. Exposure to chronic hypoxia for 21-26 h increased the frequency of exocytotic events evoked in response to acute hypoxia (PO2 ca 10-60 mmHg). 3. Chronic hypoxia increased the value of Q1/3, determined by the integration of amperometric events, indicating an increase in quantal size: this reflects either an increase in vesicular dimensions or vesicular catecholamine concentration. 4. Exocytotic frequency evoked by bath application of tetraethylammonium (1-10 mM) was significantly enhanced following chronic hypoxia. 5. In both control and chronically hypoxic PC12 cells, exocytosis in response to acute hypoxia was completely abolished in Ca2+-free solutions. Cd2+ (200 microM) completely inhibited exocytosis from control cells, but left a significant residual release in chronically hypoxic PC12 cells. 6. The Cd2+-resistant release evoked by acute hypoxia in chronically hypoxic PC12 cells was inhibited by inorganic ions (0.01-10 mM) in a potency order of La3+ > Gd3+ > Zn2+. Ni2+ (10 mM) was without effect. 7. Our results suggest that chronic hypoxia enhances the secretory response of PC12 cells in part by increasing the depolarization mediated by an oxygen-sensitive K+ channel. In addition, acute hypoxia activates a Cd2+-resistant Ca2+ influx pathway in chronically hypoxic PC12 cells.

Adaptation to hypoxia makes the heart more oxygen efficient, by metabolising more glucose. In contrast, type 2 diabetes makes the heart metabolise more fatty acids. Diabetes increases the chances of the heart being exposed to hypoxia, but whether the diabetic heart can adapt and respond is unknown. In this study we show that diabetic hearts retain the ability to adapt their metabolism in response to hypoxia, with functional hypoxia signalling pathways. However, the hypoxia-induced changes in metabolism are additive to abnormal baseline metabolism, resulting in hypoxic diabetic hearts metabolising more fat and less glucose than controls. This stops the diabetic heart being able to recover its function when stressed. These results demonstrate that the diabetic heart retains metabolic flexibility to adapt to hypoxia, but is hindered by the baseline effects of the disease. This increases our understanding of how the diabetic heart is affected by hypoxia-associated complications of the disease.

Genes near adenosine monophosphate-activated protein kinase-α1 (PRKAA1) have been implicated in the greater uterine artery (UtA) blood flow and relative protection from fetal growth restriction seen in altitude-adapted Andean populations. Adenosine monophosphate-activated protein kinase (AMPK) activation vasodilates multiple vessels but whether AMPK is present in UtA or placental tissue and influences UtA vasoreactivity during normal or hypoxic pregnancy remains unknown. We studied isolated UtA and placenta from near-term C57BL/6J mice housed in normoxia (n = 8) or hypoxia (10% oxygen, n = 7-9) from day 14 to day 19, and placentas from non-labouring sea level (n = 3) or 3100 m (n = 3) women. Hypoxia increased AMPK immunostaining in near-term murine UtA and placental tissue. RT-PCR products for AMPK-α1 and -α2 isoforms and liver kinase B1 (LKB1; the upstream kinase activating AMPK) were present in murine and human placenta, and hypoxia increased LKB1 and AMPK-α1 and -α2 expression in the high- compared with low-altitude human placentas. Pharmacological AMPK activation by A769662 caused phenylephrine pre-constricted UtA from normoxic or hypoxic pregnant mice to dilate and this dilatation was partially reversed by the NOS inhibitor l-NAME. Hypoxic pregnancy sufficient to restrict fetal growth markedly augmented the UtA vasodilator effect of AMPK activation in opposition to PE constriction as the result of both NO-dependent and NO-independent mechanisms. We conclude that AMPK is activated during hypoxic pregnancy and that AMPK activation vasodilates the UtA, especially in hypoxic pregnancy. AMPK activation may be playing an adaptive role by limiting cellular energy depletion and helping to maintain utero-placental blood flow in hypoxic pregnancy.

Cognitive performance is impaired by hypoxia despite global cerebral oxygen delivery and metabolism being maintained. Using arterial spin labelled (ASL) magnetic resonance imaging, this is the first study to show regional reductions in cerebral blood flow (CBF) in response to decreased oxygen supply (hypoxia) at 2 h that increased in area and became more pronounced at 10 h. Reductions in CBF were seen in brain regions typically associated with the 'default mode' or 'task negative' network. Regional reductions in CBF, and associated vasoconstriction, within the default mode network in hypoxia is supported by increased vasodilatation in these regions to a subsequent hypercapnic (5% CO2 ) challenge. These results suggest an anatomical mechanism through which hypoxia may cause previously reported deficits in cognitive performance.

Obstructive sleep apnoea (OSA) is a chronic condition characterized by intermittent hypoxia that induces oxidative stress and inflammation leading to cardiovascular disease. Women can develop OSA during late pregnancy, which is associated with adverse maternal and fetal outcomes. However, the effects of OSA throughout pregnancy on fetoplacental outcomes are unknown. Using a mouse model of intermittent hypoxia, we evaluated main uterine artery function, spiral artery remodelling, circulating angiogenic and anti-angiogenic factors, and placental hypoxia and oxidative stress at gestational day 14.5 in pregnant mice. Gestational intermittent hypoxia increased placental weight but decreased fetal weight, impaired uterine artery function, increased circulating angiogenic and anti-angiogenic factors, and induced placental hypoxia and oxidative stress, but had no impact on spiral artery remodelling. Our results suggest that pregnant women experiencing OSA during pregnancy could be at risk of maternal and fetal complications.

Obstructive sleep apnoea, a syndrome that leads to recurrent intermittent hypoxia, is associated with insulin resistance in obese individuals, but the mechanisms underlying this association remain unknown. We utilized a mouse model to examine the effects of intermittent hypoxia on insulin resistance in lean C57BL/6J mice and leptin-deficient obese (C57BL/6J-Lepob) mice. In lean mice, exposure to intermittent hypoxia for 5 days (short term) resulted in a decrease in fasting blood glucose levels (from 173 +/- 11 mg dl-1 on day 0 to 138 +/- 10 mg dl-1 on day 5, P < 0.01), improvement in glucose tolerance without a change in serum insulin levels and an increase in serum leptin levels in comparison with control (2.6 +/- 0.3 vs. 1.7 +/- 0.2 ng ml-1, P < 0.05). Microarray mRNA analysis of adipose tissue revealed that leptin was the only upregulated gene affecting glucose uptake. In obese mice, short-term intermittent hypoxia led to a decrease in blood glucose levels accompanied by a 607 +/- 136 % (P < 0.01) increase in serum insulin levels. This increase in insulin secretion after 5 days of intermittent hypoxia was completely abolished by prior leptin infusion. Obese mice exposed to intermittent hypoxia for 12 weeks (long term) developed a time-dependent increase in fasting serum insulin levels (from 3.6 +/- 1.1 ng ml-1 at baseline to 9.8 +/- 1.8 ng ml-1 at week 12, P < 0.001) and worsening glucose tolerance, consistent with an increase in insulin resistance. We conclude that the increase in insulin resistance in response to intermittent hypoxia is dependent on the disruption of leptin pathways.

Balloon-assisted enteroscopy (BAE) is an emerging standard procedure by utilizing distensible balloons to facilitate deep endoscopy in the small and large intestine. Sporadic cases of bacteraemia were found after BAE. Balloon distension by BAE caused gut tissue hypoxia. The impact of balloon distension-induced hypoxia on intestinal barriers remains unclear. Murine models of BAE by colonic balloon distension showed that short- and long-term hypoxia evoked opposite effects on epithelial tight junctions (TJs). Short-term hypoxia fortified TJ integrity, whereas long-term hypoxia caused damage to barrier function. Our data showed for the first time the molecular mechanisms and signalling pathways of epithelial barrier fortification and TJ reorganization by short-term hypoxia for the maintenance of gut homeostasis. The findings suggest avoiding prolonged balloon distension during BAE to reduce the risk of hypoxia-induced gut barrier dysfunction.

Under chronic hypoxia, tumour cells undergo adaptive changes involving hypoxia-inducible factors (HIFs). Here we report that ion currents mediated by Ca2+-activated K+ (K(Ca)) channels in human melanoma IGR1 cells are increased by chronic hypoxia (3% O2), as well as by hypoxia mimetics. This increase involves the HIF system as confirmed by overexpression of HIF-1alpha or the von Hippel-Lindau tumour suppressor gene. Under normoxic conditions the K(Ca) channels in IGR1 cells showed pharmacological characteristics of intermediate conductance K(Ca) subtype IK channels, whereas the subtype SK2 channels were up-regulated under hypoxia, shown with pharmacological tools and with mRNA analysis. Hypoxia increased cell proliferation, but the K(Ca) channel blockers apamin and charybdotoxin slowed down cell growth, particularly under hypoxic conditions. Similar results were obtained for the cell line IGR39 and for acutely isolated cells from a biopsy of a melanoma metastasis. Thus, up-regulation of K(Ca) channels may be a novel mechanism by which HIFs can contribute to the malignant phenotype of human tumour cells.

Chronic intermittent hypoxia (CIH, a model for sleep apnoea) is a major risk factor for several cardiovascular diseases. Autonomic imbalance (sympathetic overactivity and parasympathetic withdrawal) has emerged as a causal contributor of CIH-induced cardiovascular disease. Previously, we showed that CIH remodels the parasympathetic pathway. However, whether CIH induces remodelling of the cardiac sympathetic innervation remains unknown. Mice (male, C57BL/6J, 2-3 months) were exposed to either room air (RA, 21% O2 ) or CIH (alternating 21% and 5.7% O2 , every 6 min, 10 h day-1 ) for 8-10 weeks. Flat-mounts of their left and right atria were immunohistochemically labelled for tyrosine hydroxylase (TH, a sympathetic marker). Using a confocal microscope (or fluorescence microscope) and Neurlocudia 360 digitization and tracing system, we scanned both the left and right atria and quantitatively analysed the sympathetic axon density in both groups. The segmentation data was mapped onto a 3D mouse heart scaffold. Our findings indicated that CIH significantly remodelled the TH immunoreactive (-IR) innervation of the atria by increasing its density at the sinoatrial node, the auricles and the major veins attached to the atria (P < 0.05, n = 7). Additionally, CIH increased the branching points of TH-IR axons and decreased the distance between varicosities. Abnormal patterns of TH-IR axons around intrinsic cardiac ganglia were also found following CIH. We postulate that the increased sympathetic innervation may further amplify the effects of enhanced CIH-induced central sympathetic drive to the heart. Our work provides an anatomical foundation for the understanding of CIH-induced autonomic imbalance. KEY POINTS: Chronic intermittent hypoxia (CIH, a model for sleep apnoea) causes sympathetic overactivity, cardiovascular remodelling and hypertension. We determined the effect of CIH on sympathetic innervation of the mouse atria. In vivo CIH for 8-10 weeks resulted in an aberrant axonal pattern around the principal neurons within intrinsic cardiac ganglia and an increase in the density, branching point, tortuosity of catecholaminergic axons and atrial wall thickness. Utilizing mapping tool available from NIH (SPARC) Program, the topographical distribution of the catecholaminergic innervation of the atria were integrated into a novel 3D heart scaffold for precise anatomical distribution and holistic quantitative comparison between normal and CIH mice. This work provides a unique neuroanatomical understanding of the pathophysiology of CIH-induced autonomic remodelling.

The respiratory control of the glottis by laryngeal motoneurones is characterized by inspiratory abduction and post-inspiratory adduction causing decreases and increases in upper airway resistance, respectively. Chronic intermittent hypoxia (CIH), an important component of obstructive sleep apnoea, exaggerated glottal abduction (before inspiration), associated with active expiration and decreased glottal adduction during post-inspiration. CIH increased the inspiratory and decreased the post-inspiratory laryngeal motoneurone activities, which is not associated to changes in their intrinsic electrophysiological properties. We conclude that the changes in the respiratory network after CIH seem to be an adaptive process required for an appropriated pulmonary ventilation and control of upper airway resistance under intermittent episodes of hypoxia.

Neonatal hypoxia alters the development of the hypoxic ventilatory response in rats and other mammals. Here we demonstrate that neonatal hypoxia impairs the hypoxic ventilatory response in adult male, but not adult female, rats. Rats were raised in 10% O(2) for the first postnatal week, beginning within 12 h after birth. Subsequently, ventilatory responses were assessed in 7- to 9-week-old unanaesthetized rats via whole-body plethysmography. In response to 12% O(2), male rats exposed to neonatal hypoxia increased ventilation less than untreated control rats (mean +/-s.e.m. 35.2 +/- 7.7%versus 67.4 +/- 9.1%, respectively; P= 0.01). In contrast, neonatal hypoxia had no lasting effect on hypoxic ventilatory responses in female rats (67.9 +/- 12.6%versus 61.2 +/- 11.7% increase in hypoxia-treated and control rats, respectively; P > 0.05). Normoxic ventilation was unaffected by neonatal hypoxia in either sex at 7-9 weeks of age (P > 0.05). Since we hypothesized that neonatal hypoxia alters the hypoxic ventilatory response at the level of peripheral chemoreceptors or the central neural integration of chemoafferent activity, integrated phrenic responses to isocapnic hypoxia were investigated in urethane-anaesthetized, paralysed and ventilated rats. Phrenic responses were unaffected by neonatal hypoxia in rats of either sex (P > 0.05), suggesting that neonatal hypoxia-induced plasticity occurs between the phrenic nerve and the generation of airflow (e.g. neuromuscular junction, respiratory muscles or respiratory mechanics) and is not due to persistent changes in hypoxic chemosensitivity or central neural integration. The basis of sex differences in this developmental plasticity is unknown.

We examined the effects of hypoxia on the release of serotonin (5-HT) from intact neuroepithelial body cells (NEB), presumed airway chemoreceptors, in rabbit lung slices, using amperometry with carbon fibre microelectrodes. Under normoxia (P(O2) ~155 mmHg; 1 mmHg approximately 133 Pa), most NEB cells did not exhibit detectable secretory activity; however, hypoxia elicited a dose-dependent (P(O2) range 95-18 mmHg), tetrodotoxin (TTX)-sensitive stimulation of spike-like exocytotic events, indicative of vesicular amine release. High extracellular K(+) (50 mM) induced a secretory response similar to that elicited by severe hypoxia. Exocytosis was stimulated in normoxic NEB cells after exposure to tetraethylammonium (20 mM) or 4-aminopyridine (2 mM). Hypoxia-induced secretion was abolished by the non-specific Ca(2+) channel blocker Cd(2+) (100 microM). Secretion was also largely inhibited by the L-type Ca(2+) channel blocker nifedipine (2 microM), but not by the N-type Ca(2+) channel blocker omega-conotoxin GVIA (1 microM). The 5-HT(3) receptor blocker ICS 205 930 also inhibited secretion from NEB cells under hypoxia. These results suggest that hypoxia stimulates 5-HT secretion from intact NEBs via inhibition of K(+) channels, augmentation of Na(+)-dependent action potentials and calcium entry through L-type Ca(2+) channels, as well as by positive feedback activation of 5-HT(3) autoreceptors.

Chronic fetal hypoxia is one of the most common complications of pregnancy and is known to cause fetal growth restriction. The structural adaptations of the placental vasculature responsible for growth restriction with chronic hypoxia are not well elucidated. Using a mouse model of chronic maternal hypoxia in combination with micro-computed tomography and scanning electron microscopy, we found several placental adaptations that were beneficial to fetal growth including capillary expansion, thinning of the interhaemal membrane and increased radial artery diameters, resulting in a large drop in total utero-placental vascular resistance. One of the mechanisms used to achieve the rapid increase in capillaries was intussusceptive angiogenesis, a strategy used in human placental development to form terminal gas-exchanging villi. These results contribute to our understanding of the structural mechanisms of the placental vasculature responsible for fetal growth restriction and provide a baseline for understanding adaptive physiological responses of the placenta to chronic hypoxia.

In the present study, we provide evidence for divergent physiological responses to moderate compared to severe hypoxia, addressing an important knowledge gap related to severity, duration and after-effects of hypoxia encountered in cardiopulmonary situations. The physiological responses to moderate and severe hypoxia were not proportional, linear or concurrent with the time-of-day. Hypoxia elicited severity-dependent physiological responses that either persisted or fluctuated throughout normoxic recovery. The physiological basis for these distinct cardiovascular responses implicates a shift in the sympathovagal set point and probably not molecular changes at the artery resulting from hypoxic stress.

Carotid body dysfunction is recognized as a cause of hypertension in a number of cardiorespiratory diseases states and has therefore been identified as a potential therapeutic target. Purinergic transmission is an important element of the carotid body chemotransduction pathway. We show that inhibition of ecto-5'-nucleotidase (CD73) in vitro reduces carotid body basal discharge and responses to hypoxia and mitochondrial inhibition. Additionally, inhibition of CD73 in vivo decreased the hypoxic ventilatory response, reduced the hypoxia-induced heart rate elevation and exaggerated the blood pressure decrease in response to hypoxia. Our data show CD73 to be a novel regulator of carotid body sensory function and therefore suggest that this enzyme may offer a new target for reducing carotid body activity in selected cardiovascular diseases.

1. The present study was undertaken to determine the role of adenosine in mediating the cellular responses to hypoxia in rat phaeochromocytoma (PC12) cells, an oxygen-sensitive clonal cell line. 2. Reverse transcriptase polymerase chain reaction studies revealed that PC12 cells express adenosine deaminase (the first catalysing enzyme of adenosine degradation) and the A2A and A2B adenosine receptors, but not the A1 or A3 adenosine receptors. 3. Whole-cell current- and voltage-clamp experiments showed that adenosine attenuated the hypoxia-induced membrane depolarization. The hypoxia-induced suppression of the voltage-sensitive potassium current (IK(V)) was markedly reduced by adenosine. Furthermore, extracellularly applied adenosine increased the peak amplitudes of IK(V) in a concentration-dependent manner. This increase was blocked by pretreatment not only with a non-specific adenosine receptor antagonist, 8-phenyltheophylline (8-PT), but also with a selective A2A receptor antagonist, ZM241385. 4. Ca2+ imaging studies using fura-2 acetoxymethyl ester (fura-2 AM) revealed that the increase in intracellular free Ca2+ during hypoxic exposure was attenuated significantly by adenosine. Voltage-clamp studies showed that adenosine inhibited the voltage-dependent Ca2+ currents (ICa) in a concentration-dependent fashion. This inhibition was also abolished by both 8-PT and ZM241385. 5. The modulation of both IK(V) and ICa by adenosine was prevented by intracellular application of an inhibitor of protein kinase A (PKA), PKA inhibitor fragment (6-22) amide. In addition, the effect of adenosine on either IK(V) or ICa was absent in PKA-deficient PC12 cells. 6. These results indicate that the modulatory effects of adenosine on the hypoxia-induced membrane responses of PC12 cells are likely to be mediated via activation of the A2A receptor, and that the PKA pathway is required for these modulatory actions. We propose that this modulation serves to regulate membrane excitability in PC12 cells and possibly other oxygen-sensitive cells during hypoxia.

1. Acute exposure to hypoxia stimulates ventilation and induces hypocapnia. Long-term exposure to hypoxia generates changes in respiratory control known as ventilatory acclimatization to hypoxia. The object of this study was to investigate the degree to which the hyperventilation and hypocapnia can induce the changes known as ventilatory acclimatization to hypoxia, in the absence of the primary hypoxic stimulus itself. 2. Three 6 h protocols were each performed on twelve healthy volunteers: (1) passive hypocapnic hyperventilation, with end-tidal CO2 pressure (PET,CO2) held 10 Torr below the eupnoeic value; (2) passive eucapnic hyperventilation, with PET,CO2 maintained eucapnic; (3) control. 3. Ventilatory responses to acute hypercapnia and hypoxia were assessed before and half an hour after each protocol. 4. The presence of prior hypocapnia, but not prior hyperventilation, caused a reduction in air-breathing PET,CO2 (P < 0.05, ANOVA), and a leftwards shift of the ventilatory response to hypercapnia (P < 0.05). The presence of prior hyperventilation, but not prior hypocapnia, caused an increase in the ventilatory sensitivity to CO2 (P < 0.05). No significant effects of any protocol were detected on the ventilatory sensitivity to hypoxia. 5. We conclude that following 6 h of passive hyperventilation: (i) the left shift of the VE-PET,CO2 relationship is due to alkalosis and not to hyperventilation; (ii) the increase in slope of the VE-PET,CO2 relationship is due to the hyperventilation and not the alkalosis; and (iii) ventilatory sensitivity to hypoxia is unaltered.

Dopamine not only is a precursor of the catecholamines noradrenaline and adrenaline but also serves as an independent neurotransmitter and paracrine hormone. It plays an important role in the pathogenesis of hypertension and is a potent vasodilator in many mammalian systemic arteries, strongly suggesting an endogenous source of dopamine in the vascular wall. Here we demonstrated dopamine, noradrenaline and adrenaline in rat aorta and superior mesenteric arteries (SMA) by radioimmunoassay. Chemical sympathectomy with 6-hydroxydopamine showed a significant reduction of noradrenaline and adrenaline, while dopamine levels remained unaffected. Isolated endothelial cells were able to synthesize and release dopamine upon cAMP stimulation. Consistent with these data, mRNAs coding for catecholamine synthesizing enzymes, i.e. tyrosine hydroxylase (TH), aromatic l-amino acid decarboxylase, and dopamine-β-hydroxylase were detected by RT-PCR in cultured endothelial cells from SMA. TH protein was detected by immunohistochemisty and Western blot. Exposure of endothelial cells to hypoxia (1% O2) increased TH mRNA. Vascular smooth muscle cells partially expressed catecholaminergic traits. A physiological role of endogenous vascular dopamine was shown in SMA, where D1 dopamine receptor blockade abrogated hypoxic vasodilatation. Experiments on SMA with endothelial denudation revealed a significant contribution of the endothelium, although subendothelial dopamine release dominated. From these results we conclude that endothelial cells and cells of the underlying vascular wall synthesize and release dopamine in an oxygen-regulated manner. In the splanchnic vasculature, this intrinsic non-neuronal dopamine is the dominating vasodilator released upon lowering of oxygen tension.

This study explored arterial remodelling in fetuses growth restricted by hypoxia. Chronically catheterized fetal sheep were made moderately or severely hypoxic by placental embolization for 15 days starting at gestational age 116-118 (term ∼147 days). Cross-sections of the aorta were analysed for collagen and elastin content using histological procedures, while immunofluorescence was applied to measure markers of vascular smooth muscle cell (VSMC) type. In frozen aortae quantitative PCR was used to measure mRNA levels of extracellular matrix (ECM) precursor proteins as well as molecular regulators of developmental and pathological remodelling. Relative to Control (n =6), aortic wall thickness was increased by 23% in the Moderate group (n =5) and 33% (P <0.01) in the Severe group (n =5). Relative to Control, the Severe group exhibited a 5-fold increase in total collagen content (P <0.01) that paralleled increases in mRNA levels of procollagen I (P <0.05) and III and transforming growth factor β (TGF-β1) (P <0.05). The percentage area stained for α-actin was inversely related to fetal arterial oxygen saturation (P <0.05) and total α-actin content was 45% higher in the Moderate group and 65% (P <0.05) higher in the Severe group, compared to Control. A 12% and 39% (P <0.05) reduction in relative elastic fibre content was observed in Moderate and Severe fetuses, respectively. mRNA levels of the elastolytic enzyme, matrix metalloproteinase-2 (MMP-2) were inversely correlated with fetal arterial oxygen saturation (P <0.05) (Fig. 7) and mRNA levels of its activator, membrane-type MMP (MTI-MMP), were elevated in the Severe group (P <0.05). Marked neointima formation was apparent in Severe fetuses (P <0.05) concomitant with an increase in E-selectin mRNA expression (P <0.05). Thus, aberrant aortic formation in utero mediated by molecular regulators of arterial growth occurs in response to chronic hypoxaemia.

1. Direct observations were made of responses to systemic hypoxia (breathing 12 or 6% O2 for 3 min) evoked in terminal arterioles (TA, 14-30 microns internal diameter), precapillary arterioles (PCA, 8-18 microns), collecting venules (CV, 12-30 microns) and small veins (SV, 20-50 microns) of the mesenteric circulation of the anaesthetized rat. Changes in vessel diameter were recorded before and after local blockade of alpha-adrenoreceptors with phentolamine when the mesentery was covered with Saran Wrap, which is impermeable to O2, and then after removal of the Saran Wrap, which would have kept local PO2 relatively high even during systemic hypoxia. 2. The majority of TA showed an initial decrease in diameter of 14 +/- 1% (mean +/- S.E.M.). These responses were reversed to increases in diameter (12 +/- 2%) after phentolamine, but virtually abolished after removal of the Saran Wrap (0.3 +/- 2%). 3. Some PCA showed similar behaviour to the TA; others showed an increase in diameter (11 +/- 1%). The increases in diameter were accentuated after phentolamine (16 +/- 1%), but were reduced after removal of the Saran Wrap (6 +/- 2%). 4. CV and SV showed either a decrease in diameter followed by relaxation towards control levels, or an increase in diameter that waned before hypoxia ceased (6 +/- 1% and 1 +/- 1%, respectively). The responses of CV were not altered by phentolamine (8 +/- 1%), but SV showed larger increases in diameter (5 +/- 1%).(ABSTRACT TRUNCATED AT 250 WORDS)