Mechanism of the attenuated cardiac response to beta-adrenergic stimulation in chronic hypoxia

β-Adrenergic signaling, monoamine oxidase A and antioxidant defence in the myocardium of SHR and SHR-mtBN conplastic rat strains: the effect of chronic hypoxia

The β-adrenergic signaling pathways and antioxidant defence mechanisms play important roles in maintaining proper heart function. Here, we examined the effect of chronic normobaric hypoxia (CNH, 10% O2, 3 weeks) on myocardial β-adrenergic signaling and selected components of the antioxidant system in spontaneously hypertensive rats (SHR) and in a conplastic SHR-mtBN strain characterized by the selective replacement of the mitochondrial genome of SHR with that of the more ischemia-resistant Brown Norway strain. Our investigations revealed some intriguing differences between the two strains at the level of β-adrenergic receptors (β-ARs), activity of adenylyl cyclase (AC) and monoamine oxidase A (MAO-A), as well as distinct changes after CNH exposure. The β2-AR/β1-AR ratio was significantly higher in SHR-mtBN than in SHR, apparently due to increased expression of β2-ARs. Adaptation to hypoxia elevated β2-ARs in SHR and decreased the total number of β-ARs in SHR-mtBN. In parallel, the ability of isoprenaline to stimulate AC activity was found to be higher in SHR-mtBN than that in SHR. Interestingly, the activity of MAO-A was notably lower in SHR-mtBN than in SHR, and it was markedly elevated in both strains after exposure to hypoxia. In addition to that, CNH markedly enhanced the expression of catalase and aldehyde dehydrogenase-2 in both strains, and decreased the expression of Cu/Zn superoxide dismutase in SHR. Adaptation to CNH intensified oxidative stress to a similar extent in both strains and elevated the IL-10/TNF-α ratio in SHR-mtBN only. These data indicate that alterations in the mitochondrial genome can result in peculiar changes in myocardial β-adrenergic signaling, MAO-A activity and antioxidant defence and may, thus, affect the adaptive responses to hypoxia.

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.

Cardiovascular and autonomic nervous functions during acclimatization to hypoxia in conscious rats

The time courses of changes in cardiovascular and autonomic nervous functions during acclimatization to hypoxia were studied in conscious Sprague-Dawley rats. The animals were kept under a 12:12-h light-dark cycle and exposed to hypoxia (1 atm, 10% O2). Implanted telemetry transmitters were used to record blood pressure (BP). Changes in heart rate (HR) and BP were monitored over a 21-day period, and variations before and during hypoxia were analyzed using the wavelet transform method. The HR, high-frequency power of HR variability (HR-HF) and low-frequency power of BP variability (BP-LF) were all significantly increased after 1 h of hypoxia, whereas the LF/HF ratio of HR variability did not change. After this initial increase, both HR and the BP-LF were found to decrease. On the first day of hypoxia, HR and BP-LF values were significantly lower than those of the control rats, whereas the HR-HF was higher. Subsequently, these values altered so that they were similar to the control after 14 days of hypoxia. In addition, the amplitude of diurnal variation in HR was reduced during hypoxia. These results suggest that a sequence of dynamic interactions between sympathetic and parasympathetic nervous activities might have important roles in the regulation of cardiovascular function during acclimatization to hypoxia.

Exposure to intermittent high altitude induces different changes in adenylyl cyclase activity in hearts of young and adult Wistar rats

This study investigates changes of adenylyl cyclase activity in the heart of young and adult Wistar rats exposed to experimental conditions simulating high altitude hypoxia as a model for interpretation of some adaptive changes of adenylyl cyclase observed in human. The exposure of rats to intermittent high altitude (IHA) hypoxia (5000 m) showed significant adaptive changes. The right ventricular weight and the ratio of right/left ventricular weights of adult rats exposed to IHA were significantly increased when compared to appropriate controls; adaptive changes of cardiac adenylyl cyclase being dependent on the age of the animals. The isoprenaline-stimulated activity was higher in the left than in the right ventricle, and in both ventricles it was higher in young rats than in adult rats. When compared to controls, isoprenaline stimulation was decreased in the right ventricles of adapted young rats and, by contrast, it was increased in the left ventricles of adapted adult rats. This decrease and increase of adenylyl cyclase activity evoked by isoprenaline was paralleled by forskolin-induced adenylyl cyclase activity in these experimental groups. It seems therefore that the changes in the pattern of total adenylyl cyclase activity observed under IHA hypoxia may at least be partially explained by the changes of beta-adrenergic receptor susceptibility following IHA hypoxia.

Adrenergic contribution during acclimatization to high altitude: perspectives from Pikes Peak

We have examined the sympathoadrenal responses to both acute and chronic high-altitude exposure at the summit of Pikes Peak, CO, in both men and women. A dissociation between the adrenal medullary response (acute) with that of the sympathetic nervous system (chronic) is observed. Both alpha- and beta-adrenergic contributions to key metabolic and physiologic adjustments to high-altitude exposure are evident.

Human autonomic activity and its response to acute oxygen supplement after high altitude acclimatization

It is well established that after acclimatization at high altitude, many sympathetic pathways are hyperactive yet heart rate (HR) remains unchanged. In this study, we attempted to determine if this unchanged heart rate is due to compensatory mechanisms such as changes in parasympathetic activity or levels of receptors for autonomic neurotransmitters. We also examined the role played by hypoxia in these autonomic adaptations to high altitude. Three experiments were carried out on five healthy lowlanders both at sea level (SL) and after 2 weeks of acclimatization at 3800 m (Post-Ac) with: (a) placebo (control); (b) acute beta-adrenergic receptor blockade by propranolol (PRO), or (c) acute parasympathetic receptor blockade by glycopyrrolate (GLY). Compared with SL control values, post-Ac venous norepinephrine (NE) and dopamine increased by 96% (p < 0.001) and 55% (p < 0.05), but epinephrine and HR did not change. PRO resulted in a smaller decrease in HR (bpm) Post-Ac than at SL (15 +/- 6 vs. 21 +/- 6, p < 0.05), while GLY caused a greater increase in HR Post-Ac than at SL (59 +/- 8 vs. 45 +/- 6, p < 0.05). Breathing oxygen at SL concentration while at altitude did not decrease NE, or alter the effect of PRO on HR, but reduced the chronotropic effect of GLY by 14% (p < 0.05). These results suggest that after acclimatization to altitude, increased parasympathetic neurotransmitter release and decreased beta-adenoreceptor activity account for the unchanged HR despite enhanced sympathetic activity. Acute oxygen replacement rapidly counteracted the parasympathetic, but not sympathetic hyperactivity that occurs at high altitude.

Intermittent hypoxia modulates nNOS expression and heart rate response to sympathetic nerve stimulation

Nitric oxide (NO) decreases norepinephrine (NE) release and the heart rate (HR) response to sympathetic nerve stimulation (SNS). We tested the hypothesis that the enhanced HR response to sympathetic activation following chronic intermittent hypoxia (IH) results from a peripheral modulation of pacemaking by NO. Isolated guinea pig double atrial/right stellate ganglion preparations were studied from animals that had been exposed to IH (n = 20) and control animals (n = 22). The HR response to SNS was significantly enhanced in the IH group compared with the controls. However, the increase in HR with cumulative doses (0.1--10 microM) of bath-applied NE was similar in both groups. Western blot analysis showed less neuronal NO synthase in the right atria from the IH group. In IH animals, the NO synthase inhibitor, N(omega)-nitro-L-arginine (L-NNA; 100 microM) did not alter the increased HR response to SNS, whereas in control animals L-NNA significantly increased the HR response to SNS; an effect that was reversed with excess L-arginine. In conclusion, the enhanced HR response to SNS after IH may be related to a decreased inhibitory action of NO on presynaptic NE release.

Modulation of sympathetic actions on the heart by opioid receptor stimulation

The sympathetic nervous system, the most important extrinsic regulatory mechanism of the heart, is inhibited postsynaptically and presynaptically by opioid peptides produced in the heart via their respective receptors. The cardiac actions of beta-adrenergic receptor (beta-AR) stimulation are attenuated by activation of the opioid receptor (OR) with OR agonist at ineffective concentrations, implying cross-talk between the OR and beta-AR. This cross-talk results from inhibition of the Gs protein and adenylyl cyclase of the beta-AR pathway by the pertussis toxin-sensitive G protein of the opioid pathway. Alterations in cross-talk between these two receptors occur in pathological situations to meet bodily needs. In myocardial ischemia, when the sympathetic activity is increased, the inhibition of beta-AR stimulation by kappa-opioid stimulation is also enhanced, thus reducing the workload, oxygen consumption and cardiac injury. Whereas cardiac responsiveness to sympathetic discharges is also reduced after chronic hypoxia, the cross-talk between kappa-OR and beta-AR is reduced to prevent undue suppression of the sympathetic influence on the heart. On the other hand, impairment of the cross-talk may result in abnormality. A lack or a significant reduction in the inhibition of beta-AR stimulation by kappa-OR stimulation may lead to an excessive increase in cardiac activities, which contribute to the maintenance of high arterial blood pressure in spontaneously hypertensive rats. Other than opioid peptides, female sex hormone and adenosine also inhibit the sympathetic actions on the heart. In addition, sympathetic action is also inhibited presynaptically by kappa-opioid peptides via their receptor.

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.

Hypoxic pressor response, cardiac size, and natriuretic peptides are modified by long-term intermittent hypoxia

We investigated whether the effect of long-term intermittent hypoxia (LTIH) on cardiovascular function may be modified by preexisting genetic traits. To induce LTIH experimentally, cycles of 90-s hypoxia (nadir 6%) followed by 90-s normoxia were applied to six Wistar-Kyoto and six spontaneously hypertensive rats during 8 h daily. Comparison with the same number of control animals after 70 days revealed no alteration of intra-arterial blood pressure or heart rate. Blood pressure responsiveness to a brief hypoxic stimulus was enhanced in the LTIH animals, regardless of strain, whereas the hypoxia-induced increase in heart rate was abolished. In the spontaneously hypertensive but not the Wistar-Kyoto rats, LTIH increased left ventricular weight-to-body weight ratio and content of atrial natriuretic peptide mRNA. Expression of B-type natriuretic peptide was unchanged (Northern blot). Slightly increased right ventricular weight-to-body weight ratios in the LTIH animals were associated with higher right ventricular atrial natriuretic peptide and B-type natriuretic peptide mRNA amounts. Consequently, the effects of LTIH on different components of cardiovascular function appear incompletely related to each other and differentially influenced by constitutional traits.

Development of cardiac sensitivity to oxygen deficiency: comparative and ontogenetic aspects

Hypoxic states of the cardiovascular system are undoubtedly associated with the most frequent diseases of modern times. They originate as a result of disproportion between the amount of oxygen supplied to the cardiac cell and the amount actually required by the cell. The degree of hypoxic injury depends not only on the intensity and duration of the hypoxic stimulus, but also on the level of cardiac tolerance to oxygen deprivation. This variable changes significantly during phylogenetic and ontogenetic development. The heart of an adult poikilotherm is significantly more resistant as compared with that of the homeotherms. Similarly, the immature homeothermic heart is more resistant than the adult, possibly as a consequence of its greater capability for anaerobic glycolysis. Tolerance of the adult myocardium to oxygen deprivation may be increased by pharmacological intervention, adaptation to chronic hypoxia, or preconditioning. Because the immature heart is significantly more dependent on transsarcolemmal calcium entry to support contraction, the pharmacological protection achieved with drugs that interfere with calcium handling is markedly altered. Developing hearts demonstrated a greater sensitivity to calcium channel antagonists; a dose that induces only a small negative inotropic effect in adult rats stops the neonatal heart completely. Adaptation to chronic hypoxia results in similarly enhanced cardiac resistance in animals exposed to hypoxia either immediately after birth or in adulthood. Moreover, decreasing tolerance to ischemia during early postnatal life is counteracted by the development of endogenous protection; preconditioning failed to improve ischemic tolerance just after birth, but it developed during the early postnatal period. Basic knowledge of the possible improvements of immature heart tolerance to oxygen deprivation may contribute to the design of therapeutic strategies for both pediatric cardiology and cardiac surgery.

Altered gene expression during hypoxia and reoxygenation of the heart

The heart is exposed to alterations in oxygen tension under different pathophysiological conditions. In order to maintain function, changes in the pattern of cardiac gene expression arise. Through the activity of multiple transcription factors, which include activating protein-1, hypoxia-inducible factor-1, and nuclear factor kappaB, there is up-regulation of mRNA encoding factors that enable the cardiomyocyte to adapt to the new environment. In the case of hypoxia or anoxia, there is an increased expression of growth factors, glucose transporters, enzymes associated with anaerobic glycolysis, and stress proteins. When the cardiomyocyte is reoxygenated after hypoxia, there is a rapid increase in antioxidants, pro-inflammatory cytokines, and stress proteins.

Effects of 5-day hypoxia on cardiac adrenergic neurotransmission in rats

Chronic hypoxia induces an overall sympathetic hyperactivation associated with a myocardial beta-receptor desensitization. The mechanisms involved in this desensitization were evaluated in 32 male Wistar rats kept in a hypobaric pressure chamber (PO2 = 40 Torr, atmospheric pressure = 450 Torr) for 5 days. In hypoxic compared with normoxic conditions, plasma norepinephrine (NE) levels were higher (2.1 +/- 0.7 vs. 0.6 +/- 0.2 ng/ml) with no difference in the plasma epinephrine levels (2.2 +/- 0.7 vs. 1.8 +/- 0.3 ng/ml). In hypoxia neuronal NE uptake measured by [3H]NE was decreased by 32% in the right ventricle (RV) and by 35% in the left ventricle (LV), and [3H]mazindol in vitro binding showed a decrease in uptake-1 carrier protein density by 38% in the RV and by 41% in the LV. In vitro binding assays with [3H]CGP-12177 indicate beta-adrenoceptor density reduced by 40% in the RV and by 32% in the LV, and this was due to reduced beta1-subtype fraction (competition binding experiments with practolol). Hypoxia reduced the production of cAMP induced by isoproterenol (36% decrease in the RV and 41% decrease in the LV), 5'-guanylylimododiphosphate (40% decrease in the RV and 42% decrease in the LV), and forskolin (39% decrease in the RV and 41% decrease in the LV) but did not alter the effect of MnCl2 and NaF. Quantitation of inhibitory G-protein alpha-subunit by immunochemical analysis showed a 46% increase in the cardiac-specific isoform Gialpha2 in hypoxic hearts. The present data demonstrate that in rats 5-day hypoxia leads to changes in pre- and postsynaptic myocardial adrenergic function. The myocardial desensitization associated with both a reduction in externalized beta1-adrenoceptor and an increase in inhibitory G-protein subunit may be caused by increased synaptic NE levels due to impaired uptake-1 system.

Catecholamine response during 12 days of high-altitude exposure (4, 300 m) in women

We have previously demonstrated that acclimatization to high altitude elicits increased sympathetic nerve activity in men. The purpose of this investigation was to determine 1) whether women respond in a similar manner as found previously in men and 2) the extent to which menstrual cycle phase influences this response. Sixteen eumenorrheic women (age, 23.6 +/- 1.2 yr; weight, 56.2 +/- 4. 3 kg) were studied at sea level and during 12 days of high-altitude exposure (4,300 m) in either their follicular (F; n = 11) or luteal (L; n = 5) phase. Twenty-four-hour urine samples were collected at sea level and during each day at altitude. Catecholamines were determined by high-performance liquid chromatography with electrochemical detection. Compared with sea-level values, urinary norepinephrine excretion increased significantly during altitude exposure, peaking on days 4-6. Thereafter, levels remained constant throughout the duration of altitude exposure. The magnitude of this increase was similar between the F (138%) and L (93%) phase. Urinary epinephrine levels were elevated on day 2 of altitude exposure compared with sea-level values for both F and L subjects (93%). Thereafter, urinary epinephrine excretion returned to sea-level values, and no differences were found between F and L subjects. Plasma catecholamine content was consistent with urinary values and supports the concept of an elevation in sympathetic activity over time at altitude. Mean and diastolic blood pressure as well as heart rate adjustments to high altitude correlated significantly with urinary norepinephrine excretion rates. It was concluded that 1) urinary and plasma catecholamine responses to 12 days of high-altitude exposure in women are similar to those previously documented to occur for men; 2) whereas no differences in catecholamine levels were observed between F- and L-phase assignments, for a given urinary norepinephrine excretion rate, blood pressure and heart rates were lower for F vs. L subjects; and 3) several cardiovascular adaptations associated with high-altitude exposure correlated with 24-h urinary norepinephrine excretion rates and thus sympathetic nerve activity.

Inter and intra-species-related differences in the regulation of the cardiac autonomic system

The heart rate response to isoproterenol (HR-Iso), density and affinity (kd) of beta-adrenergic (beta-AR) and muscarinic (M2) receptors were compared among three rodents with different generation-life histories of confinement and of high altitude exposure. The European guinea pig (Cavia porcellus) (EGp), a laboratory animal that arrived in Europe after the Spanish Conquest of South America and the Peruvian guinea pig (C. porcellus) (PGp), a semi-wild animal that came from the altiplano to sea level at least 25 generations ago, were used for intra-species comparison. Wistar rats (WR) were used for inter-species comparison as representative of a typical sea level laboratory animal. The HR-Iso was lower in EGp than in the PGp. The PGp showed the highest beta-AR density (P < 0.0005) and the highest beta-AR kd values (P < 0.0005) when compared to both EGp and WR groups (beta-AR Bmax (fmol mg-1 prot), WR, 19 +/- 4; Egp, 34 +/- 10; PGp, 74 +/- 15. beta-AR kd (pM), WR, 24 +/- 10; Egp, 17 +/- 7; PGp, 39 +/- 14). In contrast, PGp showed lower M2 receptor density values than the EGp (P < 0.0005). The WR had the highest M2 receptor densities (M2 Bmax (fmol mg-1 prot), WR, 188 +/- 15; Egp, 147 +/- 9; PGp, 118 +/- 6 and M2 kd (pM), WR, 65 +/- 12; Egp, 67 +/- 6; PGp, 92 +/- 2). The inter and intra-species differences found may be related to their respective history of confinement rather than to their history of exposure to high altitude.

Basal muscarinic activity does not impede beta-adrenergic activation in rabbit hearts in controls or thyroxine-induced cardiac hypertrophy

We tested the hypothesis that basal myocardial muscarinic receptor activity acts as a "brake" on beta-adrenergic activation and that this effect would be greater in hearts subjected to thyroxine (T4)-induced (0.5 mg/kg for 16 days) hypertrophy due to an increase in muscarinic receptor density. Twenty control and 20 T4-treated open-chest anesthetized New Zealand white rabbits were given isoproterenol (0.5 microg/kg/min, 10 min i.v.) and/or atropine (3 mg/kg bolus). Coronary blood flow (radioactive microspheres), aortic and left ventricular (LV) pressure, and wall thickening of the LV free wall were recorded. Hearts were quickly excised and stored in liquid nitrogen. Cyclic guanosine monophosphate (GMP) and cyclic adenosine monophosphate (AMP) were determined by radioimmunoassay. T4 increased heart weight/body weight ratio, blood pressures, and the first derivative of the maximal rate of increase of LV systolic pressure (dP/dt[max]). Isoproterenol increased heart rate in both groups. Atropine had no effects on hemodynamic parameters either alone or after stimulation with isoproterenol. At this dose, atropine completely blocked the depressant effects of acetylcholine (10 microg/kg). Isoproterenol increased the maximal time derivative of wall thickening (dWT/dt[max]) in control (from 11.0 +/- 1.0 to 16.4 +/- 1.5 mm/s) but not in T4 animals. T4 increased subepicardial (EPI) and subendocardial (ENDO) coronary blood flow. Isoproterenol increased coronary flow (control: EPI, from 173 +/- 11 to 346 +/- 28 ml/min/100 g; ENDO, from 197 +/- 15 to 364 +/- 30 ml/min/100 g; T4: EPI, from 314 +/- 45 to 459 +/- 43 ml/min/100 g; ENDO, from 339 +/- 48 to 458 +/- 43 ml/min/100 g). Cyclic AMP levels were higher in T4 animals. Isoproterenol increased cyclic AMP (control: EPI, from 540 +/- 82 pmol/g to 1,096 +/- 110; ENDO, 596 +/- 58 to 1,050 +/- 145 pmol/g; T4: EPI, from 882 +/- 107 pmol/g to 1,319 +/- 222; ENDO, from 954 +/- 134 to 1 ,409 +/- 261 pmol/g). Atropine, alone or after stimulation with isoproterenol, had no effect on coronary flow or cyclic AMP in either group. Cyclic GMP levels were unaffected by T4-induced hypertrophy or by any of the treatments in either group. Thus it appears that basal muscarinic activity does not significantly influence function or signal transduction either at baseline or during beta-adrenergic stimulation in controls or in T4-induced hypertrophy.

Chronic hypoxia increases beta 1-adrenergic receptor mRNA and density but not signaling in neonatal rat cardiac myocytes

BACKGROUND

It is well recognized that the beta-adrenergic receptor-adenylylcyclase system is altered during myocardial ischemia/hypoxia. However, there are no data regarding either regulation of beta-adrenergic receptors, particularly at the mRNA level, or adenylylcyclase activity in isolated cardiac myocytes exposed to chronic hypoxia.

METHODS AND RESULTS

In a chronic hypoxia model in which neonatal rat ventricular myocytes were exposed to a 1% O2 environment for 72 hours, we investigated (1) beta 1-mRNA and receptor expression and adenylylcyclase activity and (2) beta 1-mRNA and receptor downregulation and adenylylcyclase desensitization induced by prolonged norepinephrine incubation. We found that hypoxia for 72 hours increased myocardial membrane beta 1-adrenergic receptor density by 44%. This increase was not associated with a corresponding decrease in cytosolic beta 1-adrenergic receptors. RNase protection assays demonstrated that hypoxia increased the steady-state levels of beta 1-mRNA by 109%. Adenylylcyclase activity stimulated by isoproterenol, sodium fluoride, guanyl-5'-imidodiphosphate, and forskolin in hypoxic membranes was not altered compared with normoxic controls. Hypoxia for 72 hours also did not affect norepinephrine-induced beta 1-mRNA and receptor downregulation and adenylylcyclase desensitization in response to isoproterenol, guanyl-5'-imidodiphosphate, or forskolin.

CONCLUSIONS

In neonatal rat cardiac myocytes, chronic hypoxia (1) increases beta 1-mRNA and receptor expression but does not alter adenylylcyclase activity stimulated at either the receptor or the postreceptor level and (2) does not affect agonist-induced beta 1-mRNA and receptor downregulation and desensitization of the adenylylcyclase response.

Hypoxia- and normoxia-induced reversibility of autonomic control in Andean guinea pig heart

We herein describe the regulation of cardiac receptors in a typical high-altitude native animal. Heart rate response to isoproterenol (HRIso) (beats.min-1.mg Iso.kg-1) and atropine, the density of beta-adrenergic (beta AR) and muscarinic (M2) receptors, and the ventricular content of norepinephrine (NE) and dopamine (DA) were studied in guinea pigs (Cavia porcellus). Animals native to Lima, Peru (150 m) were studied at sea level (SL) and after 5 wk at 4,300-m altitude (SL-HA). Animals native to Rancas [Pasco, Peru (4,300 m)] were studied at high altitude (HA) and after 5 wk at SL (HA-SL). HA animals had a lower HRIso, maximum number of beta AR binding sites (Bmax), beta AR dissociation constant (Kd), NE, and DA (P < 0.05) and a higher M2 Bmax (P < 0.001) when compared with the SL group. HA-SL showed an increase of the HRIso, beta Ar Kd, and NE (P < 0.05) and a decrease of the M2 Bmax and Kd (P < 0.0001) when compared with the HA group. The present study demonstrates the differential regulation and reversibility of the autonomic control in the guinea pig heart.

Elevated nocturnal blood pressure assessed by ambulatory automatic monitoring during a stay at high altitude

The aim of this study was to explore, in healthy children, the arterial blood pressure response to a 3-week stay at high altitude (4200 m). An auscultatory automatic ambulatory pressuremeter was used to avoid undue environmental influence on the measurement. The blood pressure was monitored three times in a group of ten boys, aged 10.5 (CI 0.9 years): at sea level (control values), at an altitude of 2100 m after at least 24 h of acclimatization and after at least 24 h at 4200 m altitude. Each period of monitoring extended over 24 h with 10-min intervals between successive measurements. Arterial blood pressure was evaluated separately for the night and day periods. Nocturnal recordings revealed an increase with altitude in systolic as well as in the diastolic blood pressure. Because of the technique used to gather data, this is thought to have represented an independent effect of altitude without interference from the medical environment or diurnal activity.

Sympathetic response during 21 days at high altitude (4,300 m) as determined by urinary and arterial catecholamines

The sympathoadrenal system plays a major role in adjustments to both short- and long-term high-altitude exposure. Thus, this study investigated catecholamine responses in blood, urine, and muscle during 3 weeks' exposure to 4,300 m in control and beta-blocked subjects. Eleven healthy, sea level (SL)-resident men (aged 26 +/- 1 years) were studied under resting conditions at SL and on arrival and during 21 days at 4,300 m (Pikes Peak). Six subjects received 240 mg/d propranolol, and five were administered a placebo. Compared with SL values (38.7 +/- 4.3 v 32.4 +/- 2.8 micrograms/d for control and beta-blocked, respectively), urinary norepinephrine (NE) excretion increased significantly during altitude exposure, reaching peak values on days 6 to 7 (105.5 +/- 16.1 v 88.4 +/- 12.3 micrograms/d, respectively). Furthermore, resting arterial NE levels (increases 87%), as well as net NE release (decreases 219%) across the leg, both increased during acclimatization, indicating elevated sympathetic activity. Systemic vascular resistance and arterial pressures increased with time at altitude and correlated with NE measurements. Resting heart rates increased initially and then declined steadily after day 4 at altitude in both groups of subjects. Urinary epinephrine (EPI) excretion increased with initial exposure as compared with SL values (5.1 +/- 0.8 to 6.6 +/- 0.7 micrograms/d for control, 4.5 +/- 0.5 to 5.2 +/- 1.3 micrograms/d for beta-blocked); however, no consistent pattern was observed for the following 20 days at altitude. Arterial EPI increased upon acute exposure, but declined after 21 days' acclimatization. No changes in dopamine excretion were observed with beta-blockade or altitude exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Echocardiographic assessment of left ventricular function and wall motion at high altitude in normal subjects

To understand the effects of high-altitude hypoxia on cardiac function and the change in cardiac function during high-altitude acclimatization, precise studies were first performed at greater than 5,000 m of altitude in Himalaya by 2-dimensional echocardiography. In addition to examining well-known indexes of cardiac function, the centerline method was used to assess regional wall motion, which has not been examined under conditions of high-altitude hypoxia. The subjects were 11 climbing members (aged 21 to 43 years) of the Kyoto University Medical Research Expedition of Xixabangma (8,027 m) in 1990. Examinations were performed at sea level, at the base camp (5,020 m), and twice at the advanced base camp (5,650 m). Heart rate, left ventricular (LV) end-diastolic volume, cardiac output, mean rate of circumferential fiber shortening, ejection fraction, % fractional shortening, and regional LV wall motion were measured. At high altitude, heart rate increased to 136% of the sea level value, but gradually decreased in the degree of increment at the early and late advanced base camp measurements. LV end-diastolic volume decreased significantly by 70%. At base camp there were significant increases in ejection fraction, mean rate of circumferential fiber shortening, and % fractional shortening, which showed little change during the long-term stay at high altitude. Regional wall motion at high altitude (measured by the center-line method) decreased at the septal wall and increased at the posterolateral wall. These results demonstrate that: (1) LV cardiac performance at high altitude is enhanced significantly in spite of reduced preload. After good acclimatization, cardiac performance remains augmented, but there is a tendency to decrease the degree of augmentation. (2) In regional LV wall motion, septal wall motion is impaired, but LV posterolateral wall motion shows a compensatory increase.

Chronic hypoxia alters structure and transmitter dynamics in dog pulmonary artery

Confinement of dogs to 10% oxygen for 14 days caused erythropoiesis and pulmonary hypertension. Histological sections of the lung tissue showed thickening of the smooth muscle component of muscular arteries and arterioles. Segments of pulmonary artery from dogs exposed to hypoxia were superfused under continuation of hypoxic conditions or after return to oxygenated conditions. Parallel segments of pulmonary artery from normal dogs were also studied. Norepinephrine stores were labeled with [3H]norepinephrine and measurements were made of [3H]norepinephrine and its radiolabeled metabolites (separated by column chromatography) in superfusates using liquid scintillation spectrometry. Chronic hypoxia (1) reduced neuronal uptake of NE from synaptic clefts, (2) reduced the content of DOPEG in superfusate from tissues studied during continuation of hypoxic conditions and in tissues studied after return to oxygenated conditions, (3) increased extraneuronal uptake of NE and (4) increased overflow of NE from synaptic clefts. In similar segments of pulmonary artery removed from the same lung, endogenous free and conjugated norepinephrine and dopamine were measured in pulmonary artery by liquid chromatography with electrochemical detection. The tissue content of free norepinephrine after stimulation was reduced, which was compatible with the reduction in neuronal uptake. Conjugated norepinephrine was a minor metabolite and was increased modestly compared to concentrations reported previously in pulmonary artery from normal dogs.

Effects of hypoxia on catecholamine and cardiorespiratory responses in exercising dogs

The sympathoadrenal contribution to cardiorespiratory response elicited by hypoxia and/or exercise was assessed in the dog. The increased plasma norepinephrine (NE) and dopamine (DA) levels which follow hypoxia (fraction of inspired O2 equals 0.12) while epinephrine (E) remained unchanged ruled out the possibility of a primacy of the adrenal medulla in the response to hypoxia. In contrast to the lack of effect of hypoxic exposure, the adrenal medulla was substantially stimulated during exercise. The exercise-induced sympathoadrenal response remained unchanged during hypoxia as compared to normoxia when expressed as function of relative work intensity. Nevertheless at a given oxygen uptake, all plasma catecholamines were increased by hypoxia. These modifications in hormonal milieu failed, however, to alter the cardiac responses to exercise but were associated with a change in breathing pattern.

Left ventricular function at high altitude examined by systolic time intervals and M-mode echocardiography

To better understand the effects of high-altitude hypoxia on cardiac performance, healthy lowland-residing volunteers were studied in 2 groups: 10 subjects after acute ascent to 12,500 ft (3,810 m) (acute group) and 9 subjects after chronic exposure for 6 weeks to 17,600 ft (5,365 m) and 11,000 ft (3,353 m) (chronic group). Systolic time intervals and M-mode echocardiograms were recorded at low and high altitudes. Heart rate was 21% greater at high altitude for all subjects. Preejection period/left ventricular ejection time (PEP/LVET) increased by 16% in the acute group and by 22% in the chronic group. Heart size was smaller at high altitude in both groups, with left atrial and left ventricular (LV) diameters decreasing by 10 to 12%. These changes were statistically significant (p less than or equal to 0.01). Despite the increase in PEP/LVET, echocardiographic measurements of LV function (percent fractional shortening and mean normalized velocity of circumferential fiber shortening) remained normal. LV isovolumic contraction time was shorter at high altitude, suggesting heightened, rather than depressed, contractility. LV function does not appear to deteriorate at high altitude. Alterations in systolic time intervals probably result from decreased preload, as reflected by smaller heart size, rather than from heart failure or depressed LV contractility.

Alterations in CNS amine levels by acclimatization to hypobaric hypoxia

Rabbits were acclimatized to simulated high altitude (SHA) (hypobaric hypoxia) at 6000 M (350 torr) on alternate days for 70 days. The norepinephrine levels of the midbrain were lower in the acclimatized animal compared to the controls (p less than 0.06) and 3,4 dihydroxyphenylacetic acid (DOPAC) was significantly higher (p less than 0.04) in the striatum of control than in the test animals. The mean dopamine (DA) levels in the striatum of the test animals were higher than the controls. The ratio of DOPAC/DA was 2.0 for the controls and 0.4 for the SHA brains which suggests reduced dopamine turnover in the striatum of the SHA rabbits. Rats acclimatized in the same manner did not show any difference in the NE or DA levels between the control and SHA animals, possibly the result of species differences.

Effects of hypoxia on atrial muscarinic cholinergic receptors and cardiac parasympathetic responsiveness

Chronic exposure of rats to hypoxia resulted in a lower resting heart rate and a supranormal increase in heart rate in response to parasympathetic blockade by atropine. The density of muscarinic cholinergic receptors labeled by the antagonist [3H]quinuclidinyl benzilate was elevated significantly in the atria of animals kept hypoxic for 2-4 weeks. Chronic hypoxia did not change the affinity of the receptor for [3H]quinuclidinyl benzilate, the weight of the atria, or the amount of protein per atrial pair. Thus, the decrease in resting heart rate may be explained by the increase in the density of atrial muscarinic cholinergic receptors.