Chronic hypoxia opposes pregnancy-induced increase in uterine artery vasodilator response to flow

We tested the hypotheses that pregnancy increases the uterine artery (UA) vasodilator response to flow and that this increase is impaired under conditions of chronic hypoxia (30 days, simulated elevation 3,960 m). UA were isolated from 24 normoxic or chronically hypoxic midpregnant guinea pigs and studied with the use of pressure myography. Normoxic pregnancy increased UA flow vasodilator response and protected against a rise in wall shear stress (WSS). Chronic hypoxia opposed these effects, prompting vasoconstriction at high flow and increasing WSS above levels seen in normoxic pregnant UA. The nitric oxide synthase inhibitor N(G)-nitro-l-arginine (l-NNA) eliminated the pregnancy-associated increase in flow vasodilation in normoxic UA, suggesting that increased nitric oxide production was responsible. The considerable residual vasodilation after nitric oxide synthase and cyclooxygenase inhibition implicated endothelial-derived hyperpolarizing factor (EDHF) as an additional contributor to flow vasodilation. l-NNA increased flow vasodilation in UA from chronically hypoxic animals, suggesting that chronic hypoxia may have lowered EDHF or elevated peroxynitrite production. In conclusion, flow is an important physiological vasodilator for the acute and more chronic UA dimensional changes required to increase uteroplacental blood flow during normal pregnancy. Chronic hypoxia may be a mechanism that opposes the pregnancy-associated rise in UA flow vasodilation, thereby increasing the incidence of preeclampsia and intrauterine growth restriction at a high altitude.     

Cortisol-mediated regulation of uterine artery contractility: effect of chronic hypoxia

We previously demonstrated that cortisol regulated alpha(1)-adrenoceptor-mediated contractions differentially in nonpregnant and pregnant uterine arteries. Given that chronic hypoxia during pregnancy has profound effects on maternal uterine artery reactivity, the present study investigated the effects of chronic hypoxia on cortisol-mediated regulation of uterine artery contractions. Pregnant (day 30) and nonpregnant ewes were divided between normoxic control and chronically hypoxic [maintained at high altitude (3,820 m), arterial Po(2): 60 mmHg for 110 days] groups. Uterine arteries were isolated and contractions measured. In hypoxic animals, cortisol (10 ng/ml for 24 h) increased norepinephrine-induced contractions in pregnant, but not in nonpregnant, uterine arteries. The 11beta-hydroxysteroid dehydrogenase inhibitor carbenoxolone did not change cortisol effects in nonpregnant uterine arteries, but abolished it in pregnant uterine arteries by increasing norepinephrine pD(2) (-log EC(50)) in control tissues. The dissociation constant of norepinephrine-alpha(1)-adrenoceptors was not changed by cortisol in nonpregnant, but decreased in pregnant uterine arteries. There were no differences in the density of glucocorticoid receptors between normoxic and hypoxic tissues. Cortisol inhibited the norepinephrine-induced increase in Ca(2+) concentrations in nonpregnant arteries, but potentiated it in pregnant arteries. In addition, cortisol attenuated phorbol 12,13-dibutyrate-induced contractions in normoxic nonpregnant and pregnant uterine arteries, but had no effect on the contractions in hypoxic arteries. The results suggest that cortisol differentially regulates alpha(1)-adrenoceptor- and PKC-mediated contractions in uterine arteries. Chronic hypoxia suppresses uterine artery sensitivity to cortisol, which may play an important role in the adaptation of uterine vascular tone and blood flow in response to chronic stress of hypoxia during pregnancy.     

Calcium homeostasis and contraction of the uterine artery: effect of pregnancy and chronic hypoxia

The present study tested the hypothesis that chronic hypoxia alters pregnancy-mediated adaptation of Ca2+ homeostasis and contractility in the uterine artery. Uterine arteries were isolated from nonpregnant and near-term pregnant ewes of normoxic control or high-altitude (3820 m) hypoxic (oxygen pressure in the blood [PaO2], 60 mm Hg) treatment for 110 days. Contractions and intracellular-free Ca2+ concentration ([Ca2+]i) were measured simultaneously in the same tissue. In normoxic animals, pregnancy increased norepinephrine (NE), but not 5-hydroxy-thymide (5-HT) or KCl, contractile sensitivity in the uterine artery. Chronic hypoxia significantly attenuated NE-induced contractions in the pregnant, but not nonpregnant, uterine arteries. Similarly, 5-HT-mediated contractions of nonpregnant arteries were not changed. In the pregnant uterine artery, chronic hypoxia significantly increased NE-mediated Ca2+ mobilization, but decreased the Ca2+ sensitivity. In addition, hypoxia increased the calcium ionophore A23187-induced relaxation in pregnant, but not nonpregnant, uterine arteries. However, the A23187-mediated reduction of [Ca2+]i was significantly impaired in hypoxic arteries. In contrast, hypoxia significantly increased the slope of the [Ca2+]i-tension relationship of A23187-induced reductions in [Ca2+]i and tension in the pregnant uterine artery. The results suggest that the contractility of nonpregnant uterine artery is insensitive to moderate chronic hypoxia, but the adaptation of sympathetic tone that normally occurs in the uterine artery during pregnancy is inhibited by chronic hypoxia. In addition, changes in Ca2+ sensitivity of myofilaments play a predominant role in the adaptation of uterine artery contractility to pregnancy and chronic hypoxia.     

Pregnancy and estradiol decrease GTPase activity in the guinea pig uterine artery

The mechanisms by which pregnancy redistributes cardiac output in an organ-specific manner are poorly understood. We propose that it is consequential to estrogen-mediated alterations in G protein-mediated signal transduction. Aortas and uterine (UAs) and mesenteric arteries (MAs) were obtained from late-pregnant, nonpregnant, or ovariectomized guinea pigs chronically treated with 17beta-estradiol. High-affinity GTPase activity was assayed enzymatically. The cGMP generated in response to the endothelium-dependent agonist ACh was measured in UAs incubated with or without cholera toxin (CTX, which inhibits G(s)alpha). Pregnancy significantly decreased UA but not aorta or MA GTPase activity. 17beta-Estradiol decreased UA GTPase activity compared with untreated ovariectomized animals. ACh increased cGMP in pregnant but not nonpregnant UAs. Pretreatment of nonpregnant UAs with CTX increased ACh-induced cGMP levels similar to pregnancy. Thus pregnancy and estradiol decrease the GTPase activity of a CTX-sensitive G protein in UAs, increasing receptor-dependent cGMP release. This alteration in receptor-mediated G protein coupling in UAs may contribute to the characteristic cardiovascular adaptation to pregnancy.     

Vascular adaptations to pregnancy in mice: effects on myogenic tone

The mechanisms underlying vascular adaptations in pregnancy remain to be fully elucidated. One of the contributory mechanisms for reduced vascular tone may be a reduction of myogenic tone. Myogenic tone was assessed as the difference between internal diameter in the presence and absence of external calcium at different intramural pressure steps (60-100 mmHg). Myogenic responses were reduced in resistance-sized mesenteric and main uterine arteries in late pregnant compared with nonpregnant C57BL/6J mice. In vessels from pregnant, but not nonpregnant mice, the myogenic response was enhanced by preincubation with nitric oxide (NO) synthase inhibitor N(G)-nitro-l-arginine methyl ester, was further elevated by the gap junction inhibitor 18-alpha glycyrrhetinic acid, but was unaltered by the prostaglandin H synthase inhibitor meclofenamate. Endothelium removal enhanced myogenic tone only in the vessels from pregnant animals, thus confirming the role of the endothelium in modulating myogenic tone in pregnancy. These results suggest that endothelium-derived NO as well as gap junction communications modulate myogenic tone in mouse pregnancy.     

Pregnancy enhances G protein activation and nitric oxide release from uterine arteries

We hypothesized that pregnancy modulates receptor-mediated responses of the uterine artery (UA) by altering G protein activation or coupling. Relaxation and contraction to NaF (0.5-11.5 mM), acetylcholine (10(-9)-10(-5) M), and bradykinin (10(-12)-3 x 10(-5) M) were measured in isolated UA of pregnant and nonpregnant guinea pigs. Responses were measured in the presence and absence of either cholera toxin (2 microg/ml) or pertussis toxin (Galpha(s) and Galpha(i) inhibitors, respectively). NaF relaxation was endothelium dependent and nitro-L-arginine sensitive (a nitric oxide synthase inhibitor). Relaxation to NaF, acetylcholine, and bradykinin were potentiated by pregnancy. Cholera but not pertussis toxin increased relaxation to acetylcholine and bradykinin in UA from nonpregnant animals, had no effect in UA from pregnant animals, and abolished the pregnancy-induced differences in acetylcholine relaxation. Cholera toxin potentiated the bradykinin-induced contraction of UA of both pregnant and nonpregnant animals, whereas pertussis toxin inhibited contraction of UA from pregnant animals only. Therefore, pregnancy may enhance agonist-stimulated endothelium-dependent relaxation and bradykinin-induced contraction of UA by inhibiting GTPase activity or enhancing Galpha(s) but not Galpha(i) activation in pregnant animals. Thus the diverse effects of pregnancy on UA responsiveness may result from hormonal modulation of G proteins coupled to their specific receptors.     

Effects of pregnancy and chronic hypoxia on contractile responsiveness to alpha 1-adrenergic stimulation

Decreasedcontractile response to vasoconstrictors in uterine and nonuterinevessels contributes to increased blood flow to the uterine circulationduring normal pregnancy. Pregnancies complicated by preeclampsiaand/or chronic hypoxia show a reversal or diminution of thesepregnancy-associated changes. We sought to determine whether chronichypoxia opposes the reduction in contractile response in uterine andnonuterine vessels during normal pregnancy and, if so, whetherdecreased basal nitric oxide (NO) activity was involved. We examinedthe contractile response to phenylephrine (PE) in guinea pig uterineartery (UA), mesenteric artery (MA), and thoracic aorta (TA) ringsisolated from nonpregnant or pregnant guinea pigs that had been exposedthroughout gestation to either low (1,600 m,n = 47) or high (3,962 m,n = 43) altitude. In the UA, pregnancyreduced contractile sensitivity to PE and did so similarly at low andhigh altitude (EC₅₀: 4.0 × 10⁸ nonpregnant, 9.3 × 10⁸ pregnant at lowaltitude; 4.8 × 10⁸nonpregnant, 1.0 ×10⁸pregnant at high altitude; both P < 0.05). Addition of the NO synthase inhibitornitro-L-arginine (NLA; 200 mM)to the vessel bath increased contractile sensitivity in the pregnant UA(P < 0.05) and eliminated the effectof pregnancy at both altitutes. NLA also raised contractile sensitivityin the nonpregnant high-altitude UA, but contractile response withoutNLA did not differ in the high- and low-altitude animals. In the MA,pregnancy decreased contractile sensitivity to PE at high altitudeonly, and this shift was reversed by NO inhibition. In the TA, neitherpregnancy nor altitude affected contractile response, but NO inhibition raised contractile response in nonpregnant and pregnant TA at bothaltitudes. We concluded that pregnancy diminished contractile responseto PE in the UA, likely as a result of increased NO activity, and thatthese changes were similar at low and high altitude. Counter to ourhypothesis, chronic hypoxia did not diminish the pregnancy-associatedreduction in contractile sensitivity to PE or inhibit basal NO activityin the UA; rather it enhanced, not diminished, basal NO activity in thenonpregnant UA and the pregnant MA.

     

Endogenous estrogen mediates vascular reactivity and distensibility in pregnant rat mesenteric arteries

The role of estrogen in the maternal systemic cardiovascular adaptations during pregnancy is still controversial. Female Sprague-Dawley rats were implanted at day 14 of pregnancy with either a 50-mg tamoxifen pellet (estrogen receptor blocker, n = 10) or placebo pellet (n = 10). Virgin female rats were a nonpregnant control (n = 7). At days 20-22 of pregnancy, resistance-sized mesenteric arteries were mounted onto a dual-chamber arteriograph system. Pregnancy significantly blunted the pressor response to phenylephrine [measurement of the effective concentration that yielded 50% maximum response (EC(50)) values were 1.5 +/- 0.22 vs. 0.69 +/- 0.16 microM (P < 0.05)] and enhanced vasodilation to ACh [EC(50) = 1.13 +/- 2.53 vs. 3.13 +/- 6.04 nM (P < 0.05)] compared with nonpregnant rats. However, tamoxifen treatment during pregnancy reversed these effects. Inhibition of nitric oxide (NO) synthase with N(G)-monomethyl-L-arginine (250 microM) shifted only the responses of the placebo-treated pregnant group to both phenylephrine and ACh. Arterial distensibility in the placebo-treated pregnant group was also significantly increased (P < 0.05) compared with nonpregnant and tamoxifen-treated pregnant animals. In summary, endogenous estrogen during pregnancy increases NO-dependent modulation of vessel tone and arterial distensibility.     

NMDA receptor modification in the fetal guinea pig brain during hypoxia

The effect of maternal hypoxia on the modification of the fetal brain cell membrane N-methyl-d-aspartate (NMDA) receptor and its modulatory sites was investigated. Experiments were conducted in pregnant guinea pigs of 60 days of gestation. Guinea pig fetuses were exposed to maternai hypoxia (FiO₂=7%) for 60 minutes. Tissue hypoxia in the fetal brain was documented biochemically by decreased levels of ATP and phosphocreatine (91.3% and 88.6% lower than normoxia, respectively). MK-801 binding characteristics (Bmax = number of receptors, Kd = affinity of receptor) were used as an index of NMDA receptor modification. P2 membrane fraction was prepared from the cortex of normoxic and hypoxic fetal brain and washed thoroughly before carrying out the binding assay. In hypoxic brains, Bmax decreased from the normoxic control level 0.79±0.03 pmol/mg protein to 0.58±0.03 pmol/mg protein (P<0.005) and Kd value decreased (increased affinity) from 8.54±0.27 nM to 4.01±0.23 nM (P<0.005) respectively. The MK-801 binding in the absence of added glutamate and glycine in hypoxic brain was 100% higher as compared to controls, indicating an increased sensitivity of the NMDA receptor to activation. The spermine dependent maximum activation of the NMDA receptor increased to 44% in the hypoxic animals as compared to 25% in controls. The Mg²⁺ response of the NMDA receptor was not affected by hypoxia. The increased affinity and increased basal activation (tone) of the NMDA receptor during hypoxia, as well as its increased activation by spermine, would hyperstimulate the NMDA receptor-ion channel complex function which could increase the susceptibility of the fetal brain to hypoxia. The results of this study indicate that hypoxia causes differential and selective modification of specific sites (recognition, co-activator, and modulatory) of the NMDA receptor ion channel complex. The hypoxia-induced modification of the NMDA receptor modulatory sites appears to be the potential mechanism of neuroexcitotoxicity.     

Influence of pregnancy on ventilatory and carotid body neural output responsiveness to hypoxia in cats

Pregnancy increases ventilation and ventilatory sensitivity to hypoxia and hypercapnia. To determine the role of the carotid body in the increased hypoxic ventilatory response, we measured ventilation and carotid body neural output (CBNO) during progressive isocapnic hypoxia in 15 anesthetized near-term pregnant cats and 15 nonpregnant females. The pregnant compared with nonpregnant cats had greater room-air ventilation [1.48 +/- 0.24 vs. 0.45 +/- 0.05 (SE) l/min BTPS, P less than 0.01], O2 consumption (29 +/- 2 vs. 19 +/- 1 ml/min STPD, P less than 0.01), and lower end-tidal PCO2 (30 +/- 1 vs. 35 +/- 1 Torr, P less than 0.01). Lower end-tidal CO2 tensions were also observed in seven awake pregnant compared with seven awake nonpregnant cats (28 +/- 1 vs. 31 +/- 1 Torr, P less than 0.05). The ventilatory response to hypoxia as measured by the shape of parameter A was twofold greater (38 +/- 5 vs. 17 +/- 3, P less than 0.01) in the anesthetized pregnant compared with nonpregnant cats, and the CBNO response to hypoxia was also increased twofold (58 +/- 11 vs. 29 +/- 5, P less than 0.05). The increased CBNO response to hypoxia in the pregnant compared with the nonpregnant cats persisted after cutting the carotid sinus nerve while recording from the distal end, indicating that the increased hypoxic sensitivity was not due to descending central neural influences. We concluded that greater carotid body sensitivity to hypoxia contributed to the increased hypoxic ventilatory responsiveness observed in pregnant cats.     

Cortisol-mediated potentiation of uterine artery contractility: effect of pregnancy

During pregnancy, maternal plasma cortisol concentrations approximately double. Given that cortisol plays an important role in the regulation of vascular reactivity, the present study investigated the potential role of cortisol in potentiation of uterine artery (UA) contractility and tested the hypothesis that pregnancy downregulated the cortisol-mediated potentiation. In vitro cortisol treatment (3, 10, or 30 ng/ml for 24 h) produced a dose-dependent increase in norepinephrine (NE)-induced contractions in both nonpregnant and pregnant (138-143 days gestation) sheep UA. However, this cortisol-mediated response was significantly attenuated by approximately 50% in pregnant UA. The 11 beta-hydroxysteroid dehydrogenase (11-beta HSD) inhibitor carbenoxolone did not change the effect of cortisol in nonpregnant UA but abolished its effect in pregnant UA by increasing the NE pD(2) in control tissues from 6.20 +/- 0.05 to 6.59 +/- 0.11. The apparent dissociation constant value of NE alpha(1)-adrenoceptors was not changed by cortisol in pregnant UA but was decreased in nonpregnant UA. There was no difference in glucocorticoid receptor density between nonpregnant and pregnant UA. Cortisol significantly decreased endothelial nitric oxide (NO) synthase protein levels and NO release in both nonpregnant and pregnant UA, but the effect of cortisol was attenuated in pregnant UA by approximately 50%. Carbenoxolone alone had no effects on NO release in nonpregnant UA but was decreased in pregnant UA. These results suggest that cortisol potentiates NE-mediated contractions by decreasing NO release and increasing NE-binding affinity to alpha(1)-adrenoceptors in nonpregnant UA. Pregnancy attenuates UA sensitivity to cortisol, which may be mediated by increasing type-2 11-beta HSD activity in UA.     

Endothelium-dependent blunting of myogenic responsiveness after chronic hypoxia

Blunted agonist-induced vasoconstriction after chronic hypoxia is associated with endothelium-dependent vascular smooth muscle (VSM) cell hyperpolarization and decreased vessel-wall Ca(2+) concentration ([Ca(2+)]). We hypothesized that myogenic vasoconstriction and pressure-induced Ca(2+) influx would also be attenuated in vessels from chronically hypoxic (CH) rats. Mesenteric resistance arteries isolated from CH [barometric pressure (BP), 380 Torr for 48 h] or normoxic control (BP, 630 Torr) rats were cannulated and pressurized. VSM cell resting membrane potential was recorded at intraluminal pressures of 40-120 Torr under normoxic conditions. VSM cells in vessels from CH rats were hyperpolarized compared with control rats at all pressures. Inner diameter was maintained for vessels from control rats, whereas vessels from CH rats developed less tone as pressure was increased. Pressure-induced increases in vessel-wall [Ca(2+)] were also attenuated for arteries from CH rats. Endothelium removal restored myogenic constriction to vessels from CH rats and normalized VSM cell resting membrane potential and pressure-induced Ca(2+) responses to control levels. Myogenic constriction and pressure-induced vessel-wall [Ca(2+)] increases remained blunted in the presence of nitric oxide (NO) synthase inhibition for arteries from CH rats. We conclude that blunted myogenic reactivity after chronic hypoxia results from a non-NO, endothelium-dependent VSM cell hyperpolarizing influence.     

Chronic hypoxia increases MCA contractile response to U-46619 by reducing NO production and/or activity.

Chronic hypoxia alters contractile sensitivity of isolated arteries to alpha-adrenergic stimulation and other agonists. However, most studies have been performed in thoracic aortas or other large vessels making little contribution to vascular resistance in their respective circulations. To determine the effect of chronic hypoxia on the vasoconstrictor response in a small, resistance-sized vessel, we studied second and third generation middle cerebral arteries (MCA; approximately 75-microm internal diameter before mounting). MCA were isolated from normoxic (inspired oxygen = 125 Torr) and hypoxic (8 wk at 3,960 m; inspired oxygen = 90 Torr) guinea pigs, and their vasoconstrictor responses were determined to the thromboxane mimetic U-46619 by using dual-pipette video microscopy. Arteries from hypoxic animals had greater contractile sensitivity to U-46619 compared with those of the normoxic animals (-log EC50 = 7.86 +/- 0.11 vs. 7.62 +/- 0.06, respectively, P < 0.05). Addition of the nitric oxide (NO) inhibitor nitro-L-arginine (200 microM) to the vessel bath eliminated the differences in contractile sensitivity between the MCA from the normoxic and chronically hypoxic groups. Supplementation with L-arginine in the drinking water sufficient to raise plasma L-arginine levels 41% reduced MCA contractile sensitivity to U-46619 in the normoxic group (-log EC50 = 7.22 +/- 0.31, P < 0.05 compared with the nonsupplemented normoxic group) but not in the chronically hypoxic group. These results show that chronic hypoxia increases the sensitivity of the MCA to the vasoconstrictor U-46619, likely because of a reduction in NO production and/or activity.     

Influence of pregnancy on mean systemic filling pressure and the cardiac function curve in guinea pigs.

To assess the degree of circulatory fullness and to evaluate the influence of peripheral and cardiac factors in the regulation of cardiac output during pregnancy, the following studies were conducted using pentobarbital-anesthetized, open-chest nonpregnant and late term pregnant guinea pigs. Mean circulatory filling pressure was taken as the equilibrium pressure when the pulmonary artery was constricted. Total vascular compliance was assessed by +/- 5-mL changes in blood volume performed while this constriction was maintained. A separate group of guinea pigs was prepared with a pulmonary artery electromagnetic flow probe and right atrial catheter. Rapid infusion of saline was used to increase right atrial pressure while the cardiac output was determined. Pregnancy was characterized by the following changes relative to nonpregnant controls: 51Cr-labelled RBC blood volume increased from 55 +/- 3 to 67 +/- 3 mL/kg; mean circulatory filling pressure increased from 7.1 +/- 0.2 to 8.0 +/- 0.5 mmHg (1 mmHg = 133.322 Pa); right atrial pressure decreased from 3.4 +/- 0.2 to 2.1 +/- 0.3 mmHg; and cardiac output increased from 71.8 +/- 3.9 to 96.8 +/- 3.3 mL.min-1.kg-1. Total vascular compliance was not changed (2.1 +/- 0.1 mL.kg-1.mmHg-1) and most of the expanded blood volume was accommodated as unstressed volume. The cardiac function curve was shifted upwards in pregnant animals. The resistance to venous return, as determined from the slope of the venous return curves, was not changed. These data suggest that the circulation of the pregnant guinea pig is slightly overfilled.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chronic hypoxia abolishes posthypoxia frequency decline in the anesthetized rat

In anesthetized rats, increases in phrenic nerve amplitude and frequency during brief periods of hypoxia are followed by a reduction in phrenic nerve burst frequency [posthypoxia frequency decline (PHFD)]. We investigated the effects of chronic exposure to hypoxia on PHFD and on peripheral and central O2-sensing mechanisms. In Inactin-anesthetized (100 mg/kg) Sprague-Dawley rats, phrenic nerve discharge and arterial pressure responses to 10 s N2 inhalation were recorded after exposure to hypoxia (10 +/- 0.5% O2) for 6-14 days. Compared with rats maintained at normoxia, PHFD was abolished in chronic hypoxic rats. Because of inhibition of PHFD, the increased phrenic burst frequency and amplitude after N2 inhalation persisted for 1.8-2.8 times longer in chronic hypoxic (70 s) compared with normoxic (25-40 s) rats (P < 0.05). After acute bilateral carotid body denervation, N2 inhalation produced a short depression of phrenic nerve discharge in both chronic hypoxic and normoxic rats. However, the degree and duration of depression of phrenic nerve discharge was smaller in chronic hypoxic compared with normoxic rats (P < 0.05). We conclude that after exposure to chronic hypoxia, a reduction in PHFD contributes to an increased duration of the acute hypoxic ventilatory response in anesthetized rats. Furthermore, after exposure to chronic hypoxia, the central network responsible for respiration is more resistant to the depressant effects of acute hypoxia in anesthetized rats.     

Chronic hypoxia, pregnancy, and endothelium-mediated relaxation in guinea pig uterine and thoracic arteries

Vasodilation that occurs during normal pregnancy is associated with enhanced relaxation and decreased contractile response to agonists, which are in part due to increased stimulated and basal nitric oxide (NO). In preeclampsia and/or pregnancies carried at high altitude (HA), this normal vascular adjustment is reversed or diminished. We previously reported that HA exposure did not inhibit the pregnancy-associated decrease in contractile response to agonist or basal NO in guinea pig uterine arteries (UA). We therefore sought to determine whether altitude interfered with effects of pregnancy on endothelium-dependent relaxation through a reduction in stimulated NO. We examined the relaxation response to ACh in UA and bradykinin in thoracic arteries (TA) and effects of NO inhibition with 200 microM N(G)-nitro-L-arginine (L-NNA) in arterial rings isolated from nonpregnant and pregnant guinea pigs exposed throughout gestation to low altitude (LA, 1,600 m, n = 26) or HA (3,962 m, n = 22). In pregnant UA, relaxation to ACh was enhanced (P < 0.05) at both altitudes and NO inhibition diminished, but did not reverse, ACh relaxation. The effect of L-NNA on the relaxation response to ACh was less in HA than in LA animals (P = 0.0021). In nonpregnant UA, relaxation to ACh was similar in LA and HA animals. L-NNA reversed the relaxation response to ACh at HA but not at LA. In TA, relaxation to bradykinin was unaltered by pregnancy or altitude and was completely reversed by NO inhibition. These data suggest that effects of NO inhibition are diminished in UA during pregnancy at HA. Additional studies are needed to confirm whether these effects are mediated through inhibition of stimulated NO. HA exposure did not inhibit relaxation to ACh, perhaps because of stimulation of other vasodilators.     

Upregulation of eNOS in pregnant ovine uterine arteries by chronic hypoxia

We tested the hypothesis that chronic high-altitude (3,820 m) hypoxia during pregnancy was associated with the upregulation of endothelial nitric oxide (NO) synthase (eNOS) protein and mRNA in ovine uterine artery endothelium and enhanced endothelium-dependent relaxation. In pregnant sheep, norepinephrine-induced dose-dependent contractions were increased by removal of the endothelium in both control and hypoxic uterine arteries. The increment was significantly higher in hypoxic tissues. The calcium ionophore A23187-induced relaxation of the uterine artery was significantly enhanced in hypoxic compared with control tissues. However, sodium nitroprusside- and 8-bromoguanosine 3',5'-cyclic monophosphate-induced relaxations were not changed. Accordingly, chronic hypoxia significantly increased basal and A23187-induced NO release. Chronic hypoxia increased eNOS protein and mRNA levels in the endothelium from uterine but not femoral or renal arteries. In nonpregnant animals, chronic hypoxia increased eNOS mRNA in uterine artery endothelium but had no effects on eNOS protein, NO release, or endothelium-dependent relaxation. Chronic hypoxia selectively augments pregnancy-associated upregulation of eNOS gene expression and endothelium-dependent relaxation of the uterine artery.     

Cardioprotection in chronically hypoxic rabbits persists on exposure to normoxia: role of NOS and KATP channels

Hypoxia from birth increases resistance to myocardial ischemia in infant rabbits. We hypothesized that increased cardioprotection in hearts chronically hypoxic from birth persists following development in a normoxic environment and involves increased activation of nitric oxide synthase (NOS) and ATP-dependent K (K(ATP)) channels. Resistance to myocardial ischemia was determined in rabbits raised from birth to 10 days of age in a normoxic (Fi(O(2)) = 0.21) or hypoxic (Fi(O(2)) = 0.12) environment and subsequently exposed to normoxia for up to 60 days of age. Isolated hearts (n = 8/group) were subjected to 30 min of global ischemia followed by 35 min of reperfusion. At 10 days of age, resistance to myocardial ischemia (percent recovery postischemic recovery left ventricular developed pressure) was higher in chronically hypoxic hearts (68 +/- 4%) than normoxic controls (43 +/- 4%). At 10 days of age, N(G)-nitro-L-arginine methyl ester (200 microM) and glibenclamide (3 microM) abolished the cardioprotective effects of chronic hypoxia (45 +/- 4% and 46 +/- 5%, respectively) but had no effect on normoxic hearts. At 30 days of age resistance to ischemia in normoxic hearts declined (36 +/- 5%). However, in hearts subjected to chronic hypoxia from birth to 10 days and then exposed to normoxia until 30 days of age, resistance to ischemia persisted (63 +/- 4%). L-NAME or glibenclamide abolished cardioprotection in previously hypoxic hearts (37 +/- 4% and 39 +/- 5%, respectively) but had no effect on normoxic hearts. Increased cardioprotection was lost by 60 days. We conclude that cardioprotection conferred by adaptation to hypoxia from birth persists on subsequent exposure to normoxia and is associated with enhanced NOS activity and activation of K(ATP) channels.     

Simultaneous measurement of O2 radicals and pulmonary vascular reactivity in rat lung

The role of endogenous radicals in the regulation of pulmonary vascular tone was evaluated by simultaneous measurement of pulmonary artery pressure and lung radical levels during exposure of isolated rat lungs to varying inspired O2 concentrations (0-95%) and angiotensin II. Lung radical levels, measured "on-line" using luminol and lucigenin-enhanced chemiluminescence, decreased in proportion to the degree of alveolar hypoxia. Radical levels fell during hypoxia before the onset of pulmonary vasoconstriction and promptly returned to basal levels with restoration of normoxic ventilation. Mild alveolar hypoxia (10% O2), which failed to decrease chemiluminescence, did not trigger pulmonary vasoconstriction. Although chemiluminescence tended to decrease more as the hypoxic response strengthened, there was not a simple correlation between the magnitude of the change in chemiluminescence induced by hypoxia and the strength of the hypoxic pressor response. Normoxic chemiluminescence was largely inhibited by superoxide dismutase but not catalase. Superoxide dismutase also increased normoxic pulmonary vascular tone and the strength of the pressor response to hypoxia and angiotensin II. Thus the predominant activated O2 species in the lung, during normoxia, was the superoxide anion or a closely related substance. Alteration of endogenous radical levels can result in changes in vascular tone. It remains uncertain whether the decrease in lung radical production during hypoxia caused pulmonary vasoconstriction or was merely associated with hypoxic ventilation.     

True pregnancy toxemia (preeclampsia) in the guinea pig (Cavia porcellus)

In this study, normal nonpregnant, normal full-term pregnant, fasting ketotic and spontaneous pregnancy toxemic guinea pigs were compared to define the mechanism of this disease. In addition to conventional clinical, laboratory and pathologic studies, arterial blood pressure (thoracic and abdominal aortic) measurements and angiography were used. The results showed that in spontaneous cases of pregnancy toxemia, there is an aortic compression just caudal to the renal arteries. This compression reduced the aortic diameter by 22% of prerenal level as compared to 10% for fasting ketotic and normal pregnant guinea pigs. The aortic compression also resulted in a 30% postcompression reduction in blood pressure. No pressure differences were seen in the other groups. The postulated etiology for true toxemia of pregnancy in guinea pigs is, therefore, similar to that of man where aortic compression produces uterine ischemia and the resultant syndrome.