Effect of peripheral and central alpha-adrenoceptor blockade on cerebral microvascular and blood flow responses to hypoxia.

The purpose of this study was to evaluate the effects of vascular and central alpha-adrenoceptor blockade on cerebral blood flow (CBF) and utilization of brain arteriolar and capillary reserve in conscious rats during normoxia and hypoxia (8% O2 in N2). Animals were divided into three groups and administered either saline, N-methyl chlorpromazine (does not cross the blood-brain barrier), or phenoxybenzamine (crosses the blood-brain barrier) in equipotent doses. Neither agent affected regional CBF and the utilization of brain microvascular reserve during normoxia. CBF increased from 70.9 +/- 2.9 (SEM) ml/min/100 g in the control normoxic group to 123.8 +/- 4.2 ml/min/100 g in control hypoxic animals. In control, hypoxic flow to pons and medulla of the brain was higher than to cortex, hypothalamus or thalamus. The percent of arterioles/mm2 perfused increased from 49.6 +/- 2.0% during control normoxia to 65.6 +/- 3.0% during control hypoxia. The percentage of capillaries/mm2 perfused changed similarly. Hypoxic CBF was increased similarly after administration of N-methyl chlorpromazine or phenoxybenzamine. Administration of N-methyl chlorpromazine or phenoxybenzamine eliminated regional differences in hypoxic CBF and the utilization of arterioles, and did not affect capillary response. There was no difference between the effect of N-methyl chlorpromazine and phenoxybenzamine on cerebral microvascular and blood flow responses to hypoxia. It was concluded that peripheral alpha-adrenoceptors affect the distribution of regional microvascular and blood flow responses to hypoxia, and central alpha-adrenoceptors probably do not participate in this effect.     

Effect of hemorrhage on regional morphometric indexes of cerebral capillarity

This study was performed to determine whether the brain can increase the number of perfused capillaries and arterioles supplying it regionally during hemorrhage. This was done using a technique to simultaneously determine total and perfused regional arteriolar and capillary morphology. Conscious Long-Evans rats served as unbled controls or were bled 65 mmHg or to 40-45 mmHg and stabilized for 30 min. Regional cerebral blood flow was determined using [14C]iodoantipyrine in half of these animals and fluorescein isothiocyanate-dextran was injected in the other half for determination of perfused cerebral microvascular morphometric indexes. The total microvasculature was labeled postmortem via an alkaline phosphatase stain. Regional cerebral blood flow was significantly increased in animals bled to 65 mmHg. During hemorrhage to 40-45 mmHg, cerebral blood flow was reduced 50% (from 59 +/- 28 to 26 +/- 11 ml X min-1 X 100 g-1, mean +/- SD) with no regional redistribution. For all treatments, total capillary density ranged from 400 to 500 capillaries/mm2, and in controls 47% were perfused. Animals bled to 65 mmHg did not mobilize their unperfused microvascular reserve even though they showed a slight tendency to do so. During hemorrhage to 40-45 mmHg, this percent increased significantly to 57% with the largest increase occurring in the pons. Approximately 51% of arterioles were perfused in controls and this was not different compared with the percent perfused during hemorrhage. Despite the overall lack of mobilization of unperfused arterioles, some regions within the brain significantly mobilized their reserves with severe hemorrhage, e.g., hippocampus (78%), hypothalamus (67%), and medulla (73%).(ABSTRACT TRUNCATED AT 250 WORDS)     

Impaired pulmonary conversion of angiotensin I to angiotensin II in rats exposed to chronic hypoxia

The effects of exposing rats to hypoxia at normal atmospheric pressure for periods of 21-24 days on intrapulmonary conversion of angiotensin I (ANG I) to angiotensin II (ANG II) were examined using an isolated rat lung preparation perfused at constant flow. 125I-ANG I (160 fmol) was injected alone and with graded doses (0.1, 1.0, and 100 nmol) of unlabeled ANG I into the pulmonary artery, and the effluent was collected for measurement of ANG I, ANG II, and metabolites. At low doses of injected ANG I (125I-ANG I alone or with 0.1 or 1.0 nmol unlabeled ANG I), the percent conversion of ANG I to ANG II was 67.5 +/- 2.1 (SE), 65.1 +/- 2.0, and 62.5 +/- 1.6 in 21-day hypoxia-exposed animals and 83.8 +/- 2.7, 81.4 +/- 3.9, and 79.6 +/- 2.3 (P less than 0.01) in control rats maintained under normoxic conditions. At the highest dose (100 nmol) of injected ANG I, percent conversion was reduced in both hypoxic and control groups to 46.8 +/- 5.0 and 64.0 +/- 6.0, respectively (P less than 0.05). Mean transit times of labeled material through the pulmonary circulation were not significantly different in hypoxic vs. normoxic lungs at any ANG I load, suggesting that the decreased conversion seen in hypoxic lungs was not related to altered kinetics of substrate exposure. Thus chronic hypoxia is associated with significant inhibition of transpulmonary ANG I conversion that is independent of perfusate flow. We postulate that this phenomenon is due to alterations at the endothelial membrane level.     

Plasma from conscious hypoxic rats stimulates leukocyte-endothelial interactions in normoxic cremaster venules.

Systemic hypoxia results in rapid increases in leukocyte-endothelial adherence (LEA) and emigration, vascular permeability, and mast cell activation in several microcirculations. Observations in cremaster muscle suggest that this response is initiated by a mediator released from a distant site (Dix R, Orth T, Allen JA, Wood JG, and Gonzalez NC. J Appl Physiol 95: 2495-2502, 2003). The present experiments in rat cremaster muscle tested the hypothesis that, if a circulating mediator triggers hypoxia-induced inflammation, then plasma from hypoxic rats should elicit LEA in normoxic cremaster venules. Plasma from conscious donor rats breathing 10% O2-90% N2 for 5 min was applied topically to the cremaster of normoxic anesthetized rats. In this and all other groups described below, the donor plasma had attained normoxic PO2 when applied to the cremaster. LEA (leukocytes/100-microm venule) increased from 2.7 +/- 0.8 to 12.3 +/- 2.4, and venular shear rate and arteriolar diameter decreased to 79 +/- 9% (P < 0.05, n = 6) and 77 +/- 5% of control (P < 0.05, n = 5), respectively, 10 min after application of plasma from hypoxic donors. The decrease in venular shear rate was exclusively due to a reduction of venular blood flow, secondary to the upstream arteriolar vasoconstriction. Plasma from normoxic donors had no effects. Plasma from blood equilibrated in vitro for 5 min with 5% CO2-95% N2 did not alter LEA or shear rate of normoxic cremasters, suggesting that the putative mediator does not originate in blood cells. The effects of plasma from hypoxic rats persisted when the donors were pretreated with the mast cell stabilizer cromolyn, which prevents hypoxia-induced LEA. This suggests that the effects of hypoxic plasma are not due to inflammatory mediators released by adherent leukocytes in the donor rat. There was a positive correlation between LEA and mast cell degranulation observed histologically. These results support the idea that systemic hypoxia produces the release of a substance transported by the circulation that initiates the microvascular inflammation.     

Effect of hypoxia on bronchial circulation

We studied the effect of systemic hypoxia on the bronchial vascular pressure-flow relationship in anesthetized ventilated sheep. The bronchial artery, a branch of the bronchoesophageal artery, was cannulated and perfused with a pump with blood from a femoral artery. Bronchial blood flow was set so bronchial arterial pressure approximated systemic arterial pressure. For the group of 25 sheep, control bronchial blood flow was 22 ml/min or 0.7 ml.min-1.kg-1. During the hypoxic exposure, animals were ventilated with a mixture of N2 and air to achieve an arterial PO2 (PaO2) of 30 or 45 Torr. For the more severe hypoxic challenge, bronchial vascular resistance (BVR), as determined by the slope of the linearized pressure-flow curve, decreased acutely from 3.8 +/- 0.4 mmHg.ml-1.min to 2.9 +/- 0.3 mmHg.ml-1.min after 5 min of hypoxia. However, this vasodilation was not sustained, and BVR measured at 30 min of hypoxia was 4.2 +/- 0.8 mmHg.ml-1.min. The zero flow intercept, an index of downstream pressure, remained unaltered during the hypoxic exposure. Under conditions of moderate hypoxia (PaO2 = 45 Torr), BVR decreased from 4.6 +/- 0.3 to 3.8 +/- 0.4 mmHg.ml-1.min at 5 min and remained dilated at 30 min (3.6 +/- 0.5 mmHg.ml-1.min). To determine whether dilator prostaglandins were responsible for the initial bronchial vascular dilation under conditions of severe hypoxia (PaO2 approximately equal to 30 Torr), we studied an additional group of animals with pretreatment with the cyclooxygenase inhibitors indomethacin (2 mg/kg) and ibuprofen (12.5 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of vasopressin on myocardial capillary recruitment and coronary blood flow in the anesthetized rabbit.

The effects of infusion of arginine vasopressin (20 mU.kg-1.min-1) on coronary blood flow and the proportion of the coronary microvasculature perfused was studied in rabbit myocardium. Fluorescein isothiocyanate--dextran was injected into anesthetized open-chest rabbits to identify the perfused vessels and an alkaline phosphatase stain was employed to locate the total microvasculature. Coronary blood flow (radioactive microspheres) was studied in separate groups of rabbits. Vasopressin infusion caused bradycardia (243 +/- 19 to 165 +/- 22 beats/min, mean +/- SD) and an increase in mean blood pressure (92 +/- 18 to 104 +/- 12 mmHg) (1 mmHg = 133.32 Pa). Coronary blood flow decreased significantly with vasopressin from 209 +/- 68 to 97 +/- 36 mL.min-1.100 g-1. The proportion of the arteriolar bed per millimeter squared perfused decreased significantly after vasopressin from 54 +/- 13 to 44 +/- 21%, while the percentage of capillaries per millimeter squared increased significantly from 57 +/- 6 to 67 +/- 11%. There were no subepicardial versus subendocardial differences in any measured parameter. Thus, both coronary blood flow and the proportion of the arteriolar bed perfused decreased with vasopressin. However, compensation occurred in that the proportion of capillaries perfused increased. This indicated an independent level of control of the coronary arteriolar and capillary beds. These microvascular changes may help to maintain oxygen supply-demand balance with vasopressin in the heart.     

Cerebral regional capillary perfusion and blood flow after carbon monoxide exposure.

Alterations in regional cerebral blood flow (rCBF) and percent perfused capillaries (indicative of functional intercapillary distance) were determined in conscious male Long-Evans rats after reducing their blood O2-carrying capacity by exposing them to 1% CO for 12 min. rCBF was determined by the iodoantipyrine method. rCBF increased from a mean of 106 +/- 8 (SE) ml.min-1.100 g-1 before CO exposure to 173 +/- 14 ml.min-1.100 g-1 after CO exposure. There was a greater flow increase (126%) in the cerebral cortex than in the lower brain stem [pons (45%), medulla (39%)]. Presence of fluorescein isothiocyanate-labeled dextran identified the perfused capillaries before and after CO exposure. The volume fraction (Vv) and number/mm2 (Na) of all capillaries (perfused and nonperfused) in a given area of brain were determined after staining for alkaline phosphatase. The percent Vv and percent Na of perfused capillaries increased uniformly (from approximately 50% to approximately 80%) in all parts of the brain after CO exposure. In the presence of tissue hypoxia with undiminished plasma PO2, the brain vasculature allowed greater flow of blood while the microvasculature adjusted to reduce the diffusion distance for O2.     

Ventilatory acclimatization to hypoxia is not dependent on arterial hypoxemia

Goats were prepared so that one carotid body (CB) could be perfused with blood in which the gas tensions could be controlled independently from the blood perfusing the systemic arterial system, including the brain. Since one CB is functionally adequate, the nonperfused CB was excised. To determine whether systemic arterial hypoxemia is necessary for ventilatory acclimatization to hypoxia (VAH), the CB was perfused with hypoxic normocapnic blood for 6 h [means +/- SE: partial pressure of carotid body O2 (PcbO2), 40.6 +/- 0.3 Torr; partial pressure of carotid body CO2 (PcbCO2), 38.8 +/- 0.2 Torr] while the awake goat breathed room air to maintain systemic arterial normoxia. In control periods before and after CB hypoxia the CB was perfused with hyperoxic normocapnic blood. Changes in arterial PCO2 (PaCO2) were used as an index of changes in ventilation. Acute hypoxia (0.5 h of hypoxic perfusion) resulted in hyperventilation sufficient to reduce average PaCO2 by 6.7 Torr from control (P less than 0.05). Over the subsequent 5.5 h of hypoxic perfusion, average PaCO2 decreased further, reaching 4.8 Torr below that observed acutely (P less than 0.05). Acute CB hyperoxic perfusion (20 min) following 6 h of hypoxia resulted in only partial restoration of PaCO2 toward control values; PaCO2 remained 7.9 Torr below control (P less than 0.05). The progressive hyperventilation that occurred during and after 6 h of CB hypoxia with concomitant systemic normoxia is similar to that occurring with total body hypoxia. We conclude that systemic (and probably brain) hypoxia is not a necessary requisite for VAH.     

Brain hypoxia and control of breathing: neuromechanical control

The effects of graded brain hypoxia on respiratory cycle timing, the lung inflation reflex, and respiratory compensation for an inspiratory flow-resistive load were studied in unanesthetized goats. Two models, inhalation and CO and acute reduction of brain blood flow (BBF) were used to produce comparable levels of brain hypoxia. The lung inflation reflex was assessed as the ratio of inspiratory time of an occluded breath to that of the preceding spontaneous breath (TIoccl/TIspont). Compensation for flow-resistive loading was assessed as the effect of the load upon the airway occlusion pressure response to rebreathing CO2 (delta P 0.1/delta PCO2). Major findings were 1) severe brain hypoxia (HbCO of 60% or BBF of 42%) caused tachypnea due to a 50% or more reduction of expiratory time but only a 20% or less reduction of inspiratory time; 2) moderate carboxyhemoglobinemia (HbCO of 25-30%) enhanced TIoccl/TIspont from 1.5 +/- 0.1 at control to 2.1 +/- 0.1, while severe brain hypoxia (HbCO of 60% and BBF of 42%) reduced the ratio to 1.0 +/- 0.2; and 3) compensation for a flow-resistive load, manifested by increases of delta P 0.1/delta PCO2 of 75-300% in the control state, was abolished at HbCO of 45-50% and BBF of 60%. The data suggest that in unanesthetized animals brain hypoxia elicits tachypnea largely by an effect on the expiratory phase of the bulbopontine timing mechanism. The observed enhancement of the lung inflation reflex and abolition of flow-resistive load compensation are best explained by hypoxic depression of higher than brain stem neural function.     

Formation of activated oxygen in the hypoxic rat liver

The biliary GSSG efflux rate of normoxic perfused rat liver was 1.5 +/- 0.2 nmol/min/g liver wet weight. The GSSG efflux rate as indicator for the flux through the glutathione peroxidase reaction and, therefore, for an oxidative loading increased with the extent of hypoxia. 2.6 +/- 0.5 nmol/min/g were released from the severely hypoxic liver. The hydroxyl radical scavenger formate as well as the xanthine oxidase inhibitor allopurinol reduced the efflux rate of GSSG. GSH was released from the perfused liver at a rate of 15.5 nmol/min/g which was nearly unchanged in severe hypoxia. The high rate of glucose liberation from the hypoxic liver declined to almost that of the normoxic organ in the presence of formate. There is an 'oxidative stress' during hypoxic liver perfusion which probably originates from increased generation of activated oxygen species in the degradation of purine nucleotides.     

Ultrastructural characteristics of the cerebral capillaries of aging animals subjected to hypoxic hypoxia

The ultrastructure of capillary walls of the mamillary bodies was studied in rats distributed in two age groups: adult (6-8-month-old) and old (28-30-month-old) animals under hypoxic hypoxia. It was found that age-related differences in the response of brain capillary walls to the injuring agent were reduced to a rapid increase in dystrophic phenomena and less conspicuous compensatory processes seen in the old animals. More profound injuries to other brain tissues adjacent to the vessels were also indicative of less efficacy of the adaptive reactions in the old animals.     

慢性间歇性低氧降低急性缺氧对大鼠肺动脉平滑肌细胞膜上电压门控性钾通道的抑制

为了从离子通道水平上探讨机体低氧适应的离子机制,本实验将雄性 SD 大鼠随机分为常氧对照组和慢性间歇性低氧组[氧浓度(10 ± 0.5) %, 间断缺氧每天 8 h]。用酶解法急性分离单个大鼠肺内动脉平滑肌细胞(pulmonary artery smoothmuscle cells, PASMCs),以全细胞膜片钳技术记录 PASMCs 膜上的电压门控性钾通道 (voltage-gated potassium channel, KV) 电流,观察急性缺氧对慢性间歇性低氧大鼠 PASMCs 的 KV 的影响, 为机体适应低氧能力提供实验依据。结果显示:⑴常氧对照组在电流钳下,急性缺氧可使膜电位明显去极化(由-47.2 ±2.6 mV 去极到 -26.7 ±1.2 mV ); 在电压钳下, 急性缺氧可显著抑制 KV电流( 60 mV 时, KV电流密度从 153.4 ± 9.5 pA/pF降到 70.1 ± 10.6 pA/pF), 峰电流的抑制率为(57.6 ± 3.3) %, 电流-电压关系曲线向右下移。⑵慢性间歇性低氧组KV电流密度随低氧时间延长而逐渐减少(慢性低氧10 d后就有显著性意义),电流- 电压关系曲线逐渐右下移。⑶急性缺氧对慢性间歇性低氧大鼠PASMCs KV电流的抑制作用随慢性间歇性低氧时间延长而逐渐减弱。上述观察结果提示慢性间歇性低氧减弱急性缺氧对 KV 的抑制, 这可能是机体低氧适应的一种重要机制。     

Effects of adaptation to intermittent high altitude hypoxia on ischemic ventricular arrhythmias in rats

We compared the effects of adaptation to intermittent high altitude (IHA) hypoxia of various degree and duration on ischemia-induced ventricular arrhythmias in rats. The animals were exposed to either relatively moderate hypoxia of 5000 m (4 or 8 h/day, 2-3 or 5-6 weeks) or severe hypoxia of 7000 m (8 h/day, 5-6 weeks). Ventricular arrhythmias induced by coronary artery occlusion were assessed in isolated buffer-perfused hearts or open-chest animals. In the isolated hearts, both antiarrhythmic and proarrhythmic effects were demonstrated depending on the degree and duration of hypoxic exposure. Whereas the adaptation to 5000 m for 4 h/day decreased the total number of premature ventricular complexes (PVCs), extending the daily exposure to 8 h and/or increasing the altitude to 7000 m led to opposite effects. On the contrary, the open-chest rats adapted to IHA hypoxia exhibited an increased tolerance to arrhythmias that was even more pronounced at the higher altitude. The distribution of PVCs over the ischemic period was not altered by any protocol of adaptation. It may be concluded that adaptation to IHA hypoxia is associated with enhanced tolerance of the rat heart to ischemic arrhythmias unless its severity exceeds a certain upper limit. The opposite effects of moderate and severe hypoxia on the isolated hearts cannot be explained by differences in the occluded zone size, heart rate or degree of myocardial fibrosis. The proarrhythmic effect of severe hypoxia may be related to a moderate left ventricular hypertrophy (27 %), which was present in rats adapted to 7000 m but not in those adapted to 5000 m. This adverse effect can be overcome by an unknown protective mechanism(s) that is absent in the isolated hearts.     

Influence of Prenatal Hypoxia on Brain Development: Effects on Body Weight, Brain Weight, DNA, Protein, Acetylcholinesterase, 3-Quinuclidinyl Benzilate Binding, and In Vivo Incorporation of [14C]Lysine into Subcellular Fractions

Abstract: The influence of prenatal hypoxia on subsequent brain development in the young rat was investigated by examining body and brain weight, cerebral cortex wet weight, protein and DNA concentrations, acetylcholinesterase (AChE) activity, 3-quinuclidinyl benzilate (QNB)-binding levels, the relative amounts of protein in various subcellular fractions, and the in vivo incorporation of [¹⁴C]lysine into the protein of homogenate and subcellular fractions. Exposure of pregnant females to a mild hypoxia (9.1% Os, 10 h per day for the 9-11 days preceding birth) resulted in a reduced body weight in the pups at days 1 and 5 after birth; total cortical DNA was reduced but brain weight and protein content were unaffected, leading to a higher protein/DNA ratio in prenatally hypoxic pups. By 10 days of age these differences between prenatally hypoxic and control animals were no longer apparent. There were no differences between prenatally hypoxic and control animals in AChE and QNB binding per milligram cortex protein. The relative amount of synaptic membrane protein from the cerebral cortex was reduced at day 1 in prenatally hypoxic animals and the synaptic membrane fraction showed a higher level of incorporation of [¹⁴C]lysine on days 1, 5, and 10. The developmental profile of [¹⁴C]lysine incorporation showed a peak on day 10 which was higher in prenatally hypoxic rats. By 46 days after birth little difference could be found between prenatally hypoxic and control animals.     

Exercise delays the hypoxic thermal response in rats.

Exercise exacerbates acute mountain sickness. In infants and small mammals, hypoxia elicits a decrease in body temperature (Tb) [hypoxic thermal response (HTR)], which may protect against hypoxic tissue damage. We postulated that exercise would counteract the HTR and promote hypoxic tissue damage. Tb was measured by telemetry in rats (n = 28) exercising or sedentary in either normoxia or hypoxia (10% O2, 24 h) at 25 degrees C ambient temperature (Ta). After 24 h of normoxia, rats walked at 10 m/min on a treadmill (30 min exercise, 30 min rest) for 6 h followed by 18 h of rest in either hypoxia or normoxia. Exercising normoxic rats increased Tb ( degrees C) vs. baseline (39.68 +/- 0.99 vs. 38.90 +/- 0.95, mean +/- SD, P < 0.05) and vs. sedentary normoxic rats (38.0 +/- 0.09, P < 0.05). Sedentary hypoxic rats decreased Tb (36.15 +/- 0.97 vs. 38.0 +/- 0.36, P < 0.05) whereas Tb was maintained in the exercising hypoxic rats during the initial 6 h of exercise (37.61 +/- 0.55 vs. 37.72 +/- 1.25, not significant). After exercise, Tb in hypoxic rats reached a nadir similar to that in sedentary hypoxic rats (35.05 +/- 1.69 vs. 35.03 +/- 1.32, respectively). Tb reached its nadir significantly later in exercising hypoxic vs. sedentary hypoxic rats (10.51 +/- 1.61 vs. 5.36 +/- 1.83 h, respectively; P = 0.002). Significantly greater histopathological damage and water contents were observed in brain and lungs in the exercising hypoxic vs. sedentary hypoxic and normoxic rats. Thus exercise early in hypoxia delays but does not prevent the HTR. Counteracting the HTR early in hypoxia by exercise exacerbates brain and lung damage and edema in the absence of ischemia.     

Age-dependent effects of chronic hypoxia on pulmonary vascular reactivity

Effects of age on the pulmonary vascular responses to histamine (HIST), norepinephrine (NE), 5-hydroxytryptamine (5-HT), and KCl were studied in isolated, perfused lungs from juvenile (7-wk-old), adult (14-wk-old), and mature adult (28-wk-old) normoxic rats and compared with age-matched rats exposed to chronic hypoxia for either 14 or 28 days. Chronic hypoxia changed vasoconstriction to HIST and NE to vasodilation in lungs from juvenile and adult rats. Mature adult lungs only vasoconstricted to these amines in both control and hypoxic animals. Pressor responses to 5-HT were not affected by chronic hypoxia regardless of age group. Pressor responses to KCl were also not altered by hypoxia, but lungs from older rats showed greater control responsiveness to KCl compared with lungs from juveniles. Only lungs from juvenile animals developed significant elevations of base-line resistance as a result of hypoxic exposure. To investigate the contribution of H1-, H2-, and beta-receptors in these changes, we employed chlorpheniramine, metiamide, and propranolol, respectively, as blocking agents in another series of experiments. Chlorpheniramine either reduced vasoconstriction or increased vasodilation to HIST in lungs from both control and hypoxic animals, whereas metiamide was without effect. Propranolol either increased vasoconstriction or reversed vasodilation to HIST and NE in all lungs studied. The present data demonstrate the important interaction between chronic hypoxia and age that can alter pulmonary vascular tone and reactivity. The inverse relationship between age and elevation of pulmonary vascular resistance after chronic hypoxic exposure may be the key element that changes pulmonary vascular reactivity observed during hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of maternal chronic hypoxic exposure during gestation on apoptosis in fetal rat heart

Chronic hypoxia during pregnancy is one of the most common insults to fetal development. We tested the hypothesis that maternal hypoxia induced apoptosis in the hearts of near-term fetal rats. Pregnant rats were divided into two groups, normoxic control and continuous hypoxic exposure (10.5% O2) from day 15 to 21 of gestation. Hearts were isolated from fetal rats of 21-day gestational age. Maternal hypoxia increased hypoxia-inducible factor-1alpha protein in fetal hearts. Chronic hypoxia significantly increased the percentage and size of binucleated myocytes and increased apoptotic cells from 1.4 +/- 0.14% to 2.7 +/- 0.3% in the fetal heart. In addition, the active cleaved form of caspase 3 was significantly increased in the hypoxic heart, which was associated with an increase in caspase 3 activity. There was a significant increase in Fas protein levels in the hypoxic heart. Chronic hypoxia did not change Bax protein levels but significantly decreased Bcl-2 proteins. In addition, chronic hypoxia significantly suppressed expression of heat shock protein 70. However, chronic hypoxia significantly increased expression of the anti-apoptotic protein 14-3-3, among other 14-3-3 isoforms. Chronic hypoxia differentially regulated beta-adrenoreceptor (beta-AR) subtypes with an increase in beta1-AR levels but no changes in beta2-AR. The results demonstrate that maternal hypoxia increases apoptosis in fetal rat heart, which may be mediated by an increase in Fas and a decrease in Bcl-2 proteins. Chronic hypoxia-mediated increase in beta1-AR and decrease in heat shock proteins may also play an important role in apoptosis in the fetal heart.     

Pulmonary vascular tone is increased by a voltage-dependent calcium channel potentiator

The mechanism of hypoxia-induced pulmonary vasoconstriction remains unknown. To explore the possible dependence of the hypoxic response on voltage-activated calcium (Ca2+) channels, the effects of BAY K 8644 (BAY), a voltage-dependent Ca2+ channel potentiator, were observed on the pulmonary vascular response to hypoxia of both the intact anesthetized dog and the perfused isolated rat lung. In six rat lungs given BAY (1 X 10(-6)M), hypoxia increased mean pulmonary arterial pressure (Ppa) to 30.5 +/- 1.7 (SEM) Torr compared with 14.8 +/- 1.2 Torr for six untreated rat lungs (P less than 0.01). After nifedipine, the maximum Ppa during hypoxia fell 14.1 +/- 2.4 Torr from the previous hypoxic challenge in the BAY-stimulated rats (P less than 0.01). BAY (1.2 X 10(-7) mol/kg) given during normoxia in seven dogs increased pulmonary vascular resistance 2.5 +/- 0.3 to 5.0 +/- 1.2 Torr X 1(-1) X min (P less than 0.05), and systemic vascular resistance 55 +/- 4.9 to 126 +/- 20.7 Torr X 1(-1) X min (P less than 0.05). Systemic mean arterial pressure rose 68 Torr, whereas Ppa remained unchanged. Administration of BAY during hypoxia produced an increase in Ppa: 28 +/- 1.5 to 33 +/- 1.9 Torr (P less than 0.05). Thus BAY, a Ca2+ channel potentiator, enhances the hypoxic pulmonary response in vitro and in vivo. This, together with the effect of nifedipine on BAY potentiation, suggests that increased Ca2+ channel activity may be important in the mechanism of hypoxic pulmonary vasoconstriction.     

Hyperoxia and recovery from hypoxia alter collagen in peripheral pulmonary arteries similarly

Chronic hypoxia causes pulmonary hypertension, the mechanism of which includes altered collagen metabolism in the pulmonary vascular wall. This chronic hypoxic pulmonary hypertension is gradually reversible upon reoxygenation. The return to air after the adjustment to chronic hypoxia resembles in some aspects a hyperoxic stimulus and we hypothesize that the changes of extracellular matrix proteins in peripheral pulmonary arteries may be similar. Therefore, we studied the exposure to moderate chronic hyperoxia (FiO2 = 0.35, 3 weeks) in rats and compared its effects on the rat pulmonary vasculature to the effects of recovery (3 weeks) from chronic hypoxia (FiO2 = 0.1, 3 weeks). Chronically hypoxic rats had pulmonary hypertension (Pap = 26 +/- 3 mm Hg, controls 16 +/- 1 mm Hg) and right ventricular hypertrophy. Pulmonary arterial blood pressure and right ventricle weight normalized after 3 weeks of recovery in air (Pap = 19 +/- 1 mm Hg). The rats exposed to moderate chronic hyperoxia also did not have pulmonary hypertension (Pap = 18 +/- 1 mm Hg, controls 17 +/- 1 mm Hg). Collagenous proteins isolated from the peripheral pulmonary arteries (100-300 microm) were studied using polyacrylamide gel electrophoresis. A dominant low molecular weight peptide (approx. 76 kD) was found in hypoxic rats. The proportion of this peptide decreases significantly in the course of recovery in air. In addition, another larger peptide doublet was found in rats recovering from chronic hypoxia. It was localized in polyacrylamide gels close to the zone of alpha2 chain of collagen type I. It was bound to anticollagen type I antibodies. An identically localized peptide was found in rats exposed to moderate chronic hyperoxia. The apparent molecular weight of this collagen fraction suggests that it is a product of collagen type I cleavage by a rodent-type interstitial collagenase (MMP-13). We conclude that chronic moderate hyperoxia and recovery from chronic hypoxia have a similar effect on collagenous proteins of the peripheral pulmonary arterial wall.     

Transient Hypoxia Alters Striatal Catecholamine Metabolism in Immature Brain: An In Vivo Microdialysis Study

Microdialysis probes were inserted bilaterally into the striatum of 7-day-old rat pups (n = 30) to examine extracellular fluid levels of dopamine, its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA). The dialysis samples were assayed by HPLC with electrochemical detection. Baseline levels, measured after a 2-h stabilization period, were as follows: dopamine, not detected; DOPAC, 617 +/- 33 fmol/min; HVA, 974 +/- 42 fmol/min; and 5-HIAA, 276 +/- 15 fmol/min. After a 40-min baseline sampling period, 12 animals were exposed to 8% oxygen for 120 min. Hypoxia produced marked reductions in the striatal extracellular fluid levels of both dopamine metabolites (p less than 0.001 by analysis of variance) and a more gradual and less prominent reduction in 5-HIAA levels (p less than 0.02 by analysis of variance), compared with controls (n = 12) sampled in room air. In the first hour after hypoxia, DOPAC and HVA levels rose quickly, whereas 5-HIAA levels remained suppressed. The magnitude of depolarization-evoked release of dopamine (elicited by infusion of potassium or veratrine through the microdialysis probes for 20 min) was evaluated in control and hypoxic animals. Depolarization-evoked dopamine efflux was considerably higher in hypoxic pups than in controls: hypoxic (n = 7), 257 +/- 32 fmol/min; control (n = 12), 75 +/- 14 fmol/min (p less than 0.001 by analysis of variance). These data demonstrate that a brief exposure to moderate hypoxia markedly disrupts striatal catecholamine metabolism in the immature rodent brain.