Differential regulation of G protein expression in rat hearts exposed to chronic hypoxia

Cardiovascular physiology and pathophysiology at high altitude

Oxygen is vital for cellular metabolism; therefore, the hypoxic conditions encountered at high altitude affect all physiological functions. Acute hypoxia activates the adrenergic system and induces tachycardia, whereas hypoxic pulmonary vasoconstriction increases pulmonary artery pressure. After a few days of exposure to low oxygen concentrations, the autonomic nervous system adapts and tachycardia decreases, thereby protecting the myocardium against high energy consumption. Permanent exposure to high altitude induces erythropoiesis, which if excessive can be deleterious and lead to chronic mountain sickness, often associated with pulmonary hypertension and heart failure. Genetic factors might account for the variable prevalence of chronic mountain sickness, depending on the population and geographical region. Cardiovascular adaptations to hypoxia provide a remarkable model of the regulation of oxygen availability at the cellular and systemic levels. Rapid exposure to high altitude can have adverse effects in patients with cardiovascular diseases. However, intermittent, moderate hypoxia might be useful in the management of some cardiovascular disorders, such as coronary heart disease and heart failure. The aim of this Review is to help physicians to understand the cardiovascular responses to hypoxia and to outline some recommendations that they can give to patients with cardiovascular disease who wish to travel to high-altitude destinations.

Modeling the oxygen transport to the myocardium at maximal exercise at high altitude

Exposure to high altitude induces a decrease in oxygen pressure and saturation in the arterial blood, which is aggravated by exercise. Heart rate (HR) at maximal exercise decreases when altitude increases in prolonged exposure to hypoxia. We developed a simple model of myocardial oxygenation in order to demonstrate that the observed blunting of maximal HR at high altitude is necessary for the maintenance of a normal myocardial oxygenation. Using data from the available scientific literature, we estimated the myocardial venous oxygen pressure and saturation at maximal exercise in two conditions: (1) with actual values of maximal HR (decreasing with altitude); (2) with sea-level values of maximal heart rate, whatever the altitude (no change in HR). We demonstrated that, in the absence of autoregulation of maximal HR, myocardial tissue oxygenation would be incompatible with life above 6200 m-7600 m, depending on the hypothesis concerning a possible increase in coronary reserve (increase in coronary blood flow at exercise). The decrease in maximal HR at high altitude could be explained by several biological mechanisms involving the autonomic nervous system and its receptors on myocytes. These experimental and clinical observations support the hypothesis that there exists an integrated system at the cellular level, which protects the myocardium from a hazardous disequilibrium between O₂  supply and O₂ consumption at high altitude.

Prenatal hypoxia programs changes in β-adrenergic signaling and postnatal cardiac contractile dysfunction

Prenatal hypoxia leads to an increased risk of adult cardiovascular disease. We have previously demonstrated a programming effect of prenatal hypoxia on the cardiac β-adrenergic (βAR) response. The aim of this study was to determine 1) whether the decrease in βAR sensitivity in prenatally hypoxic 5-wk old chicken hearts is linked to changes in β1AR/β2ARs, Gαi expression and cAMP accumulation and 2) whether prenatal hypoxia has an effect on heart function in vivo. We incubated eggs in normoxia (N, 21% O2) or hypoxia from day 0 (H, 14% O2) and raised the posthatchlings to 5 wk of age. Cardiac β1AR/β2ARs were assessed through competitive binding of [(3)H]CGP-12177 with specific β1AR or β2AR blockers. Gαs and Gαi proteins were assessed by Western blot and cAMP accumulation by ELISA. Echocardiograms were recorded in anesthetized birds to evaluate diastolic/systolic diameter and heart rate and tissue sections were stained for collagen. We found an increase in relative heart mass, β1ARs, and Gαs in prenatally hypoxic hearts. cAMP levels after isoproterenol stimulation and collagen content was not changed in H compared with N, but in vivo echocardiograms showed systolic contractile dysfunction. The changes in βAR and G protein subtypes may be indicative of an early compensatory stage in the progression of cardiac dysfunction, further supported by the cardiac hypertrophy and systolic contractile dysfunction. We suggest that it is not the changes in the proximal part of the βAR system that causes the decreased cardiac contractility, but Ca(2+) handling mechanisms further downstream in the βAR signaling cascade.

Effects of high-altitude hypoxia on the hormonal response to hypothalamic factors

Acute and chronic exposure to high altitude induces various physiological changes, including activation or inhibition of various hormonal systems. In response to activation processes, a desensitization of several pathways has been described, especially in the adrenergic system. In the present study, we aimed to assess whether the hypophyseal hormones are also subjected to a hypoxia-induced decrease in their response to hypothalamic factors. Basal levels of hormones and the responses of TSH, thyroid hormones, prolactin, sex hormones, and growth hormone to the injection of TRH, gonadotropin-releasing hormone, and growth hormone-releasing hormone (GHRH) were studied in eight men in normoxia and on prolonged exposure (3-4 days) to an altitude of 4,350 m. Thyroid hormones were elevated at altitude (+16 to +21%), while TSH levels were unchanged, and follicle-stimulating hormone and prolactin decreased, while leutinizing hormone was unchanged. Norepinephrine and cortisol levels were elevated, while no change was observed in levels of epinephrine, dopamine, growth hormone (GH), IGF-1, and IGFBP-3. The mean response to hypothalamic factors was similar in both altitudes for all studied hormones, although total T4 was lower in hypoxia during 45 to 60 min after injection. The effect of hypoxia on the hypophyseal response to hypothalamic factors was similar among subjects, except for the GH response to GHRH administration. We conclude that prolonged exposure to high-altitude hypoxia induces contrasted changes in hormonal levels, but the hypophyseal response to hypothalamic factors does not appear to be blunted.

Modulation of intracellular calcium transient in response to β-adrenoceptor stimulation in the hearts of 4-wk-old rats during simulated weightlessness

Modulation of intracellular calcium ([Ca2+]i) transient in response to β-adrenoceptor stimulation in the hearts of hindlimb unweighted (HLU) rats during simulated weightlessness has not been reported. In the present study, we adopted the rat tail suspension for 4 wk to simulate weightlessness. Effects of simulated microgravity on β-adrenoceptor responsiveness were then studied. Mean arterial blood pressure, left ventricular pressure (LVP), systolic function [maximum positive change in pressure over time (+dP/d tmax)], and diastolic function [maximum negative change in pressure over time (−dP/d tmax)] were monitored during the in vivo experiment. β-Adrenoceptor density was quantitated by radioactive ligand binding. Single rat ventricular myocyte was obtained by enzymatic dissociation method. ±dP/d tmax, myocyte contraction, intracellular [Ca2+]i transient, and L-type calcium current in response to β-adrenoceptor stimulation with isoproterenol were measured. Compared with the control group, no significant changes were found in heart weight, body weight, and mean arterial blood pressure, whereas LVP and ±dP/d tmax were significantly reduced. LVP and ±dP/d tmax were significantly attenuated in the HLU group in response to isoproterenol administration. In the in vitro study, the β-adrenoceptor density was unchanged. Effects of isoproterenol on electrically induced single-cell contraction and [Ca2+]i transient in myocytes of ventricles in HLU rats were significantly attenuated. The enhanced L-type Ca2+ current elicited by isoproterenol in cardiomyocytes was significantly decreased in the HLU group. The above results indicate that impaired function of L-type Ca2+ current and decreased [Ca2+]i transient cause the depressed responsiveness of the β-adrenoceptor stimulation, which may be partially responsible for the depression of cardiac function.

Chronic intermittent hypobaric hypoxia decreases β-adrenoceptor activity in right ventricular papillary muscle

Chronic intermittent hypobaric hypoxia (CIHH) has an effective cardiac protection against ischemia-reperfusion injury. However, the underlying mechanisms are not fully known. It has been shown that blockade of β-adrenergic receptor exerts anti-arrhythmic action and improves cardiac remodeling in ischemic myocardium. Thus we determined the influence of CIHH on β-adrenergic receptor activity in right ventricular papillary muscle of rats. We found that the action potential duration in right ventricular papillary muscle was significantly longer in CIHH rats than in control rats. Activation of β-adrenergic receptor with dl-isoproterenol dose-dependently increased action potential duration and the contractility in right ventricular papillary muscle. In CIHH rats, the prolonged effect of dl-isoproterenol on action potential duration and the positive inotropic effect were significantly decreased compared with that in control rats. Furthermore, radioligand-binding experiments revealed that the density and affinity of β-adrenergic receptor in right ventricular myocardium was significantly lower in CIHH rats. In addition, Western blot analysis revealed that the membrane-bound G protein Gsα expression level in cardiac myocardium was significantly lower in CIHH rats than that in control rats. Collectively, these data suggest that CIHH suppresses β-adrenergic receptor action in right ventricular papillary muscle through decreasing receptor density and affinity, as well as membrane-bound Gsα. This mechanism may be involved in the cardiac protective effect of CIHH.

Modulation of {beta}-adrenoceptor signaling in the hearts of 4-wk simulated weightlessness rats

The modulation of beta-adrenoceptor signaling in the hearts of hindlimb unweighting (HU) simulated weightlessness rats has not been reported. In the present study, we adopted the rat tail suspension for 4 wk to simulate weightlessness; then the effects of simulated microgravity on beta-adrenoceptor signaling were studied. Mean arterial blood pressure (ABP), left ventricular pressure (LVP), systolic function (+dP/dtmax), and diastolic function (-dP/dtmax) were monitored in the course of the in vivo experiment. Single rat ventricular myocyte was obtained by the enzymatic dissociation method. Hemodynamics, myocyte contraction, and cAMP production in response to beta-adrenoceptor stimulation with isoproterenol or adenylyl cyclase stimulation with forskolin were measured, and Gs protein was also determined. Compared with the control group, no significant changes were found in heart weight, body weight and ABP, while LVP and +/-dP/dtmax were significantly reduced. The ABP decrease, LVP increase, and +/-dP/dtmax in response to isoproterenol administration were significantly attenuated in the HU group. The effects of isoproterenol on electrically induced single-cell contraction and cAMP production in myocytes of ventricles in the HU rats were significantly attenuated. The biologically active isoform, Gsalpha (45 kDa) in the heart, was unchanged. Both the increased electrically induced contraction and cAMP production in response to forskolin were also significantly attenuated in the simulated weightlessness rats. Above results indicated that impaired function of adenylyl cyclase causes beta-adrenoceptor desensitization, which may be partly responsible for the depression of cardiac function.

Exercise and hypoxia: The role of the autonomic nervous system

The reduction in maximal oxygen consumption in hypoxia can be due to physiological factors, the relative importance of which depends on the degree of hypoxia: the reduction in inspired PO2, the impairment of lung gas exchange contributing to an exercise-induced decrease in arterial O(2) saturation, the reduction in maximal cardiac output and the limitation in tissue diffusion. This paper focuses on two aspects of this oxygen cascade. First, the decrease in heart rate at maximal exercise in prolonged exposure to hypoxia is discussed and the role of changes in the autonomous nervous system is emphasised. The desensitization of the beta-adrenergic pathway and the upregulation of the muscarinic pathway, both using G-protein systems, contribute to limit the myocardial O(2) consumption in face of reduced O(2) availability during maximal exercise in hypoxia. The changes in O(2) diffusion to the tissues are discussed in relation to the expression of hypoxia inducible factor (HIF-1alpha) and vascular endothelial growth factor (VEGF) and their possible changes induced by training and/or hypoxic exposure.

Effect of acute hypoxia on maximal exercise in trained and sedentary women

PURPOSE

The purpose of this study was to determine the physiological responses of sedentary and endurance-trained female subjects during maximal exercise at different levels of acute hypoxia.

METHODS

Fourteen women who were sea level residents were divided into two groups according to their level of fitness: 1) endurance-trained women (TW) (N = 7), VO(2max) = 56.3 +/- 4.7 mL.kg(-1).min(-1); and 2) sedentary women (SW) (N = 7), VO(2max) = 34.8 +/- 5.6 mL.kg(-1).min(-1). Subjects performed four maximal cycle ergometer tests in normoxia and under hypoxic conditions (F(I)O(2) = 0.187, 0.154, and 0.117, corresponding to altitudes of 1000, 2500, and 4500 m, respectively).

RESULTS

VO(2max) decreased significantly by 3.6 +/- 2.1, 14 +/- 2.5, and 27.4 +/- 3.6% in TW, and by 5 +/- 4, 9.4 +/- 6.4, and 18.7 +/- 7% in SW at 1000, 2500, and 4500 m, respectively. The drop of VO(2max) (DeltaVO(2max)) was greater in TW at and above 2500 m. Arterial O2 saturation (SpO(2)) at maximal exercise was lower in TW at every altitude (1000 m: 90.9 +/- 1.9 vs 94.6 +/- 1.4%; 2500 m: 82.8 +/- 2.8 vs 90.0 +/- 2.1%; 4500 m: 65.0 +/- 4.7 vs 73.6 +/- 4.5%). Maximal heart rate decreased significantly from 1000 m in the two groups. SpO(2) was correlated to DeltaVO(2max) at 4500 m (r = -0.81, P < 0.01) and 2500 m (r = -0.81, P < 0.01), but not below. Furthermore, we noted a relationship between SpO(2) and O2 pulse (VO(2)/HR) at every F(I)O(2).

CONCLUSION

These results demonstrate that endurance-trained women show a greater decrement in VO(2max) at high altitudes. This could be explained mainly by a higher arterial desaturation, which is largely caused, according to our results, by diffusion limitation.

Analysis of expression and posttranslational modification of proteins during hypoxia

The cellular responses to hypoxia are complex and characterized by alterations in the expression of a number of genes, including stress-related genes and corresponding proteins that are necessary to maintain homeostasis. The purpose of this article is to review previous and recent studies that have examined the changes in the expression and posttranslational modification of proteins in response to chronic sustained and intermittent forms of hypoxia. A large number of studies focused on the analysis of either the single protein or a subset of related proteins using one-dimensional gel electrophoresis to separate a complex set of proteins from solubilized tissues or cell extracts, followed by immunostaining of proteins using antibodies that are specific to either native or posttranslationally modified forms. On the other hand, only a limited number of studies have examined the global perturbations on protein expression by hypoxia using proteomics approach involving two-dimensional electrophoresis coupled with mass spectrometry. Results derived from specific protein analysis of a variety of tissues and cells showed that hypoxia, depending on the duration and severity of the stimulus, affects the level and the state of posttranslational modification of a subset of proteins that are associated with energy metabolism, stress response, cell injury, development, and apoptosis. Some of these earlier findings are further corroborated by recent studies that utilize a global proteomics approach, and, more importantly, results from these proteomics investigations on the effects of hypoxia provide new protein targets for further functional analysis. The anticipated new information stems from the analysis of expression, and posttranslational modification of these novel protein targets, along with gene expression profiles, offers exciting new opportunities to further define the mechanisms of cellular responses to hypoxia and to control more effectively the clinical consequences of prolonged or periodic lack of oxygen.

Calcium homeostasis in rat cardiomyocytes during chronic hypoxia: a time course study

The present study determined Ca2+ handling in the hearts of rats subjected to chronic hypoxia (CH). Spectrofluorometry was used to measure intracellular Ca2+ concentration ([Ca2+]i) and its responses to electrical stimulation, caffeine, and isoproterenol in myocytes from the right ventricle of rats breathing 10% oxygen for 1, 3, 7, 14, 21, 28, and 56 days and age-matched controls. The protein expression of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) and its ryanodine receptor (RyR) were measured. The uptake of 45Ca2+ by SERCA, release by RyR, and extrusion by Na+/Ca2+ exchange (NCX) were determined. It was found that Ca2+ homeostasis and Ca2+ responses to beta-adrenoceptor stimulation reached a new equilibrium after 4 wk of CH. Ca2+ content in the sarcoplasmic reticulum (SR) was reduced, but cytosolic Ca2+ remained unchanged after CH. Expression of SERCA and its Ca2+ uptake, Ca2+ release via RyR, and NCX activity were suppressed by CH. The results indicate impaired Ca2+ handling, which may be responsible for the attenuated Ca2+ responses to beta-adrenoceptor stimulation in CH.

Altered myocardial Gs protein and adenylyl cyclase signaling in rats exposed to chronic hypoxia and normoxic recovery

The present work has analyzed the consequences of chronic intermittent high-altitude hypoxia for functioning of the G protein-mediated adenylyl cyclase (AC) signaling system in the right (RV) and left ventricular (LV) myocardium in rats. Adaptation to hypoxia did not appreciably affect the number of beta-adrenoceptors and the content of predominantly membrane-bound alpha-subunit (G(s)alpha) of the stimulatory G protein, but it raised the amount of cytosolic G(s)alpha in RV. The levels of myocardial inhibitory Galpha protein were not altered. Activity of AC stimulated by GTP, fluoride, forskolin, or isoprotertenol was reduced by approximately 50% in RV from chronically hypoxic rats, and a weaker depression was also found in LV. In addition, hypoxia significantly diminished a functional activity of membrane-bound G(s)alpha in both RV and LV. The RV baseline contractile function was markedly increased in chronically hypoxic animals, and its sensitivity to beta-adrenergic stimulation was decreased. Animals recovering from hypoxia for 5 wk still exhibited markedly elevated levels of cytosolic G(s)alpha and significantly lower activity of AC in RV than did age-matched controls, but contractile responsiveness to beta-agonists was normal.

Influence of barometric pressure on interleukin-1β secretion

Monocytes and macrophages are activated by various environmental challenges, including microorganisms, radiation, and pollutants. These cells release cytokines, such as interleukin (IL)-1β, that mediate physiological adaptations to stress. This study sought to define further the role of IL-1β in general adaptation to environmental stress by testing the hypothesis that high altitude (20,000 ft, 6,096 m) would stimulate IL-1β secretion from isolated human blood mononuclear cells. Cells from six young men (aged 22–26 yr) were divided into separate cultures incubated in either standard ambient conditions or in one of three test conditions, hypobaric hypoxia (simulating 20,000 ft), hypobaric normoxia (20,000 ft, O2supplemented), and normobaric hypoxia (10% O2). This design allowed differentiation between pressure-related vs. oxygen-related effects. Each subject made multiple blood donations in order that cells from all subjects were tested in all conditions. Contrary to the hypothesis, IL-1β secretion was not induced at simulated altitude in basal cell cultures. In lipopolysaccharide-stimulated cell cultures, exposure to altitude inhibited IL-1β secretion by ∼40%, and the inhibition was due to the change in pressure ( P = 0.039) rather than the change in oxygen. Secretion of other factors (IL-1 receptor antagonist and soluble IL-1 receptor type II) was not inhibited. Although these results are in opposition to the original hypothesis, they provide insight regarding adaptations necessary for hematopoiesis in response to high altitude and also provide a cellular rationale for the mountain sanatoriums of the 19th and early 20th centuries.

Differential alterations in cardiac adrenergic signaling in chronic hypoxia or norepinephrine infusion

Norepinephrine (NE)-induced desensitization of the adrenergic receptor pathway may mimic the effects of hypoxia on cardiac adrenoceptors. The mechanisms involved in this desensitization were evaluated in male Wistar rats kept in a hypobaric chamber (380 Torr) and in rats infused with NE (0.3 mg. kg(-1). h(-1)) for 21 days. Because NE treatment resulted in left ventricular (LV) hypertrophy, whereas hypoxia resulted in right (RV) hypertrophy, the selective hypertrophic response of hypoxia and NE was also evaluated. In hypoxia, alpha(1)-adrenergic receptors (AR) density increased by 35%, only in the LV. In NE, alpha(1)-AR density decreased by 43% in the RV. Both hypoxia and NE decreased beta-AR density. No difference was found in receptor apparent affinity. Stimulated maximal activity of adenylate cyclase decreased in both ventricles with hypoxia (LV, 41%; RV, 36%) but only in LV with NE infusion (42%). The functional activities of G(i) and G(s) proteins in cardiac membranes were assessed by incubation with pertussis toxin (PT) and cholera toxin (CT). PT had an important effect in abolishing the decrease in isoproterenol-induced stimulation of adenylate cyclase in hypoxia; however, pretreatment of the NE ventricle cells with PT failed to restore this stimulation. Although CT attenuates the basal activity of adenylate cyclase in the RV and the isoproterenol-stimulated activity in the LV, pretreatment of NE or hypoxic cardiac membranes with CT has a less clear effect on the adenylate cyclase pathway. The present study has demonstrated that 1) NE does not mimic the effects of hypoxia at the cellular level, i.e., hypoxia has specific effects on cardiac adrenergic signaling, and 2) changes in alpha- and beta-adrenergic pathways are chamber specific and may depend on the type of stimulation (hypoxia or adrenergic).

Impaired G(s)alpha and adenylyl cyclase cause beta-adrenoceptor desensitization in chronically hypoxic rat hearts

The effects of beta-adrenoceptor stimulation with isoproterenol on electrically induced contraction and intracellular calcium ([Ca(2+)](i)) transient, and cAMP in myocytes from both hypertrophied right and nonhypertrophied left ventricles of rats exposed to 10% oxygen for 4 wk, were significantly attenuated. The increased [Ca(2+)](i) transient in response to cholera toxin was abolished, whereas increased cAMP after NaF significantly attenuated. The biologically active isoform, G(s)alpha-small (45 kDa), was reduced while the biologically inactive isoform, G(s)alpha-large (52 kDa), increased. The increased electrically induced [Ca(2+)](i) transient and cAMP with 10-100 microM forskolin were significantly attenuated in chronically hypoxic rats. The content of G(i)alpha(2), the predominant isoform of G(i) protein in the heart, was unchanged. Results indicate that impaired functions of G(s) protein and adenylyl cyclase cause beta-adrenoceptor desensitization. The impaired function of the G(s) protein may be due to reduced G(s)alpha-small and/or increased G(s)alpha-large, which does not result from changes in G(i) protein. Responses to all treatments were the same for right and left ventricles, indicating that the impaired cardiac functions are not secondary to cardiac hypertrophy.

Role of endogenous cortisol in basal liquid clearance from distal air spaces in adult guinea-pigs

1. We investigated the role of endogenous cortisol in the modulation of distal air space liquid clearance in adult guinea-pigs. Cortisol synthesis was inhibited with the 11-beta-hydroxylase inhibitor metyrapone (0-7 days pretreatment). After cortisol synthesis inhibition, distal air space liquid clearance was measured by the increase in concentration of an instilled 5 % albumin solution after 1 h. 2. Two days of metyrapone pretreatment resulted in a 46+/-19 % decrease in plasma cortisol levels compared with control, which was paralleled by a 60+/-13 % decrease in distal air space liquid clearance. The Na+ channel inhibitor amiloride inhibited 40+/-22 % of distal air space liquid clearance in control animals but did not inhibit distal air space liquid clearance in the metyrapone-pretreated group. Co-injection of dexamethasone prevented the inhibition by metyrapone and the amiloride sensitivity of distal air space liquid clearance was greater than in control animals. After 7 days of metyrapone pretreatment, plasma cortisol levels and distal air space liquid clearance were not significantly different from normal, but amiloride sensitivity was greater than in control animals (91+/-37%). 3. Pretreatment with emetine, a protein synthesis inhibitor, reduced distal air space liquid clearance in control animals and in dexamethasone-co-injected animals, but failed to inhibit distal air space liquid clearance after metyrapone pretreatment. Expression of the epithelial sodium channel alpha-subunit (alphaENaC) mRNA in lung tissue was decreased after 2 days of metyrapone pretreatment and after 7 days pretreatment or after co-injection with dexamethasone, alphaENaC mRNA expression was restored towards control levels. 4. Thus, endogenous cortisol is important for maintaining normal liquid balance in the adult guinea-pig lung and a critical regulatory pathway is by modulation of ENaC expression and/or function.

Chronic hypoxia reduces adenosine A2A receptor-mediated inhibition of calcium current in rat PC12 cells via downregulation of protein kinase A

1. Adenosine has been shown to decrease Ca2+ current (ICa) and attenuate the hypoxia-induced enhancement of intracellular free Ca2+ ([Ca2+]i) in oxygen-sensitive rat phaeochromocytoma (PC12) cells. These effects are mediated via the adenosine A2A receptor and protein kinase A (PKA). The current study was undertaken to determine the effects of adenosine on Ca2+ current and hypoxia-induced change in [Ca2+]i during chronic hypoxia. 2. Whole cell patch-clamp studies revealed that the effect of adenosine on ICa was significantly reduced when PC12 cells were exposed to hypoxia (10 % O2) for 24 and 48 h. 3. Ca2+ imaging studies using fura-2 revealed that the anoxia-induced increase in [Ca2+]i was significantly enhanced when PC12 cells were exposed to 10 % O2 for up to 48 h. In contrast, the inhibitory effects of adenosine on anoxia-induced elevation of [Ca2+]i was significantly blunted in PC12 cells exposed to hypoxia for 48 h. 4. Northern blot analysis revealed that mRNA for the A2A receptor, which is the only adenosine receptor subtype expressed in PC12 cells, was significantly upregulated by hypoxia. Radioligand binding analysis with [3H]CGS21680, a selective A2A receptor ligand, showed that the number of adenosine A2A receptor binding sites was similarly increased during exposure to 10% O2 for 48 h. 5. PKA enzyme activity was significantly inhibited when PC12 cells were exposed to 10% O2 for 24 and 48 h. However, we found that hypoxia failed to induce change in adenosine- and forskolin-stimulated adenylate cyclase enzyme activity. Chronic hypoxia also did not alter the immunoreactivity level of the G protein Gsalpha, an effector of the A2 signalling pathway. 6. Whole cell patch-clamp analysis showed that the effect of 8-bromo-cAMP, an activator of PKA, on ICa was significantly attenuated during 48 h exposure to 10% O2.7. We conclude therefore that the reduced effect of adenosine on ICa and [Ca2+]i in PC12 cells exposed to chronic hypoxia is due to hypoxia-induced downregulation of PKA. This mechanism may serve to reduce the negative feedback on ICa and [Ca2+]i by adenosine and therefore maintain enhanced membrane excitability of PC12 cells during long-term hypoxia.

Alveolar epithelial fluid clearance is mediated by endogenous catecholamines at birth in guinea pigs

Transition from placental to pulmonary oxygenation at birth depends on a rapid removal of fetal lung fluid from the developing alveoli. Alveolar fluid clearance was examined in ventilated, anesthetized developing guinea pigs of the ages newborn, 2-d-old, 5-d-old, 30-d-old, and 60-d-old (adult). An isosmolar 5% albumin solution was instilled into the lungs of the guinea pigs; the guinea pigs were then studied for 1 h. Alveolar fluid clearance was measured from the increase in alveolar protein concentration as water was reabsorbed. Newborn guinea pigs had a very high alveolar fluid clearance rate that declined rapidly within the first 5 postnatal days towards adult levels. The high alveolar fluid clearance at birth was apparently mediated by the beta-adrenergic system as demonstrated by the elevated plasma epinephrine levels and the increased sensitivity to inhibition by the beta-adrenergic antagonist propranolol immediately after birth. Surprisingly, exogenous addition of epinephrine was not able to stimulate alveolar fluid clearance in the newborn lung, but exogenous epinephrine stimulation increased over time to adult levels. The elevated alveolar fluid clearance at birth was associated with a significantly greater amiloride sensitivity in the newborn guinea pig lung. Northern blot analysis of distal lung tissue as well as isolated alveolar epithelial type II cells showed and confirmed higher levels of the alpha-subunit of the epithelial sodium channel mRNA in the newborn lung that rapidly tapered off toward adult levels. In conclusion, these data demonstrate the importance of the beta-adrenergic system and amiloride-sensitive sodium transporting pathways for clearance of fetal lung fluid at birth.