Role of adenosine in mediating myocardial blood flow response to isoproterenol: observations in closed chest, sedated, domestic swine

To test the hypothesis that adenosine is required to mediate the sustained increase in myocardial flow evoked by isoproterenol haemodynamic indices, myocardial blood flow (microspheres), and regional myocardial oxygen consumption were measured in eight closed chest, sedated pigs at control, after isoproterenol (6.9(2.8) ng X kg-1 X min-1 (mean (SD)) infused into the left anterior descending coronary artery, repeat control, and after a simultaneous infusion of the same dose of isoproterenol and adenosine deaminase (10 U X kg-1 X min-1). Data were acquired at one and 10 minutes after each infusion and compared with control measurements. Heart rate was held constant by atrial pacing. Peak left ventricular dP/dt (mm Hg X s-1) increased significantly (control 2190(32) mean (SD)) at both one (2900(302)) and 10 minutes (3010(391)) of isoproterenol infusion alone. At one minute of simultaneous infusion there was no change (control 1970(447)) in left ventricular dP/dt (2290(521)), although dP/dt was significantly increased at 10 minutes of simultaneous infusion (2790(483)). Transmural flow (ml X min-1 X g-1) increased significantly (control 1.49(0.46)) in the distal left anterior descending zone at one (1.94(0.48)) and 10 minutes (2.07(0.27)) of isoproterenol infusion alone. In contrast, flow failed to increase (control 1.65(0.27)) during the first minute of simultaneous infusion (1.73(0.38)), although it did increase significantly by 10 minutes (1.91(0.21). Finally, although myocardial oxygen consumption (ml X min-1 X 100 g-1) increased significantly (control 16.4(4.7)) at both one (20.1(4.7)) and 10 minutes (19.4(3.6)) of isoproterenol infusion alone, it failed to increase (control 18.2(3.8)) at one (19.3(4.6)) and 10 minutes (19.1(3.8)) of simultaneous infusion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cardiovascular response to acute airway obstruction and hypoxia

We wished to evaluate the role of hypoxia in the production of cardiovascular manifestations of acute airway obstruction. We monitored blood pressure, electrocardiogram, and radionuclide ejection fraction in 14 healthy volunteers on exposure to four experimental conditions: expiratory resistive loading while breathing room air (RAL), expiratory resistive loading while hypoxic (HL), hypoxia alone (H), and expiratory resistive loading while voluntarily hyperventilating in a pattern similar to HL trials (VL). Mean respiratory-related oscillation in systolic blood pressure (pulsus paradoxus) increased significantly under each experimental condition compared with those at baseline (2 +/- 2.3 mm Hg): for RAL, 21 +/- 8.4 mm Hg; for HL, 34 +/- 16.3 mm Hg; for H, 10 +/- 5.4 mm Hg; for VL 26 +/- 13.4 mm Hg. Pulsus paradoxus was significantly greater under conditions of moderate hypoxia (saturation, 80%) than of mild hypoxia (saturation, 90%). Electrocardiographic changes were more marked under HL and H conditions than under RAL and VL conditions. HL induced changes in blood pressure and frontal QRS axis that were qualitatively similar to those seen in naturally occurring asthma. Radionuclide ejection fraction showed no dramatic change with any experimental condition. We conclude that hypoxia magnifies the cardiovascular changes induced by acute expiratory resistive loads and may contribute to the degree of pulsus paradoxus seen in severe asthma.     

Role of adenosine in coronary vasodilation during exercise

This study examined the hypothesis that increases in myocardial blood flow during exercise are mediated by adenosine-induced coronary vasodilation. Active hyperemia associated with graded treadmill exercise and coronary reactive hyperemia were examined in chronically instrumented awake dogs during control conditions, after intracoronary infusion of adenosine deaminase (5 units/kg/min for 10 minutes), and after adenosine receptor blockade with 8-phenyltheophylline. Both adenosine deaminase and 8-phenyltheophylline caused a rightward shift of the dose-response curve to intracoronary adenosine; 8-phenyltheophylline was significantly more potent than adenosine deaminase. Adenosine deaminase caused a 33 +/- 7 to 39 +/- 3% decrease in reactive hyperemia blood flow following coronary occlusions of 5-20 seconds duration, respectively, while 8-phenyltheophylline produced a 40 +/- 6 to 62 +/- 8% decrease in reactive hyperemia. Increasing myocardial oxygen consumption during treadmill exercise was associated with progressive increase of coronary blood flow. Neither adenosine deaminase nor 8-phenyltheophylline attenuated the increase in coronary blood flow or the decrease of coronary vascular resistance during exercise. Neither agent altered the relation between myocardial oxygen consumption and coronary blood flow. Thus, although both adenosine deaminase and 8-phenyltheophylline antagonized coronary vasodilation in response to exogenous adenosine and blunted coronary reactive hyperemia, neither agent impaired coronary vasodilation associated with increased myocardial oxygen requirements produced by exercise. These findings fail to support a substantial role for adenosine in mediating coronary vasodilation during exercise.     

Role of the inward rectifier IK1 in the myocardial response to hypoxia

STUDY OBJECTIVE--The aim was to assess the contribution of the inward rectifier IK1 to the electrical responses of the myocardium to hypoxia: action potential shortening, maintenance of resting potential, and myocardial K loss. DESIGN--Hypoxia induced changes of gK1 were inferred from the effects of 40 microns Ba2+ on action potential duration, resting potential, and cellular K content of normoxic and hypoxic rabbit hearts paced at 2.5 Hz and perfused at 33 degrees C and constant coronary flow of 20 ml.min-1. The components (diffusion generated and Na pump related) of the resting potential (Vr) were separated by exposure to 10(-4) M ouabain. The effects of varying the extracellular K concentration (Ko) were also examined. EXPERIMENTAL MATERIAL--New Zealand rabbits were heparinised and anaesthetised and the hearts rapidly excised and perfused in the Langendorff manner. MEASUREMENTS AND MAIN RESULTS--The membrane potential was measured with standard glass microelectrodes and the cellular content of K and Na estimated from determinations of total electrolyte and water content and volume of the inulin space of the tissue. In normoxia, Ba2+ caused a slight depolarisation at Ko below 10 mM and lengthened the action potential duration at 95% repolarisation (APD95) without altering the plateau duration. In hypoxia, Ba2+ caused further depolarisation and markedly reduced the APD95 shortening at normal Ko. The Vr v Ko relationship was considerably flattened and the diffusion component of Vr became insensitive to changes in Ko between 1.5 and 10 mM. Concomitantly, the Na pump related fraction of Vr increased under Ba2+. Barium also attenuated myocardial K loss induced by hypoxia. CONCLUSION--Our data suggest that an increase in the IK1 current underlies the maintenance of Vr in hypoxia despite the decrease in the transmembrane K gradient and contributes to the action potential shortening, speeding up the late repolarisation. The inwardly rectifying K channels would also be involved in the increased efflux of K+. In contrast, the decrease in plateau duration is probably caused by another K current system, less sensitive to Ba2+. Finally, the contribution of electrogenic Na pumping to Vr becomes critical for the maintenance of the resting potential under conditions of decreased resting permeability.     

Melatonin modulates the fetal cardiovascular defense response to acute hypoxia

Experimental studies in animal models supporting protective effects on the fetus of melatonin in adverse pregnancy have prompted clinical trials in human pregnancy complicated by fetal growth restriction. However, the effects of melatonin on the fetal defense to acute hypoxia, such as that which may occur during labor, remain unknown. This translational study tested the hypothesis, in vivo, that melatonin modulates the fetal cardiometabolic defense responses to acute hypoxia in chronically instrumented late gestation fetal sheep via alterations in fetal nitric oxide (NO) bioavailability. Under anesthesia, 6 fetal sheep at 0.85 gestation were instrumented with vascular catheters and a Transonic flow probe around a femoral artery. Five days later, fetuses were exposed to acute hypoxia with or without melatonin treatment. Fetal blood was taken to determine blood gas and metabolic status and plasma catecholamine concentrations. Hypoxia during melatonin treatment was repeated during in vivo NO blockade with the NO clamp. This technique permits blockade of de novo synthesis of NO while compensating for the tonic production of the gas, thereby maintaining basal cardiovascular function. Melatonin suppressed the redistribution of blood flow away from peripheral circulations and the glycemic and plasma catecholamine responses to acute hypoxia. These are important components of the fetal brain sparing response to acute hypoxia. The effects of melatonin involved NO‐dependent mechanisms as the responses were reverted by fetal treatment with the NO clamp. Melatonin modulates the in vivo fetal cardiometabolic responses to acute hypoxia by increasing NO bioavailability.     

Role of nucleus isthmi in the ventilatory response to hypoxia of Bufo paracnemis

Nucleus isthmi (NI) is a mesencephalic structure of the amphibian brain that has recently been reported to participate in CO2-ventilatory response. The present study was designed to test the hypothesis that NI is also involved in hypoxia-induced hyperventilation and in the breathing pattern of the toad Bufo paracnemis. Pulmonary ventilation was directly measured by pneumotachography method in control, sham-operated and NI-lesioned toads exposed to normoxia and hypoxia (7 and 5% inspired O2). Under normoxic conditions, NI lesion caused no significant change in the ventilatory pattern or in the pulmonary ventilation. Hypoxia caused a significant (P < 0.05) increase in ventilation in control and sham-operated animals mainly due to an elevated VT. The hypoxia-induced hyperventilation was greater (P < 0.05) in the NI-lesioned toads, due to increases in both fR and VT. Such increased fR under hypoxia was due to a higher number of breaths per burst. The data indicate that NI plays no role under normoxic conditions but is involved in the ventilatory response to hypoxia, exercising an inhibitory modulation on pulmonary ventilation.     

Lack of NO formation is involved in the vasodilative response to contrast media

Intravascular injection of angiographic contrast media results in peripheral vasodilation and hypotension. The mechanisms underlying these hemodynamic changes are not entirely clear. We hypothesized that increased formation of nitric oxide (NO) could be involved in the vasodilatory response to contrast media. To address this assumption we have investigated whether N^G-monomethyl-L-arginine (L-NMMA, 200 mg/kg) and N^G-nitro-L-arginine methyl ester (L-NAME, 50 mg/kg), two specific NO formation inhibitors, can abolish the hypotensive response to intravascular injection of isopaque amin (1 g/kg), a contrast medium, as well as bradykinin (10 µg/kg), a NO-dependent vasodilator, in anaesthetized normotensive rats. In rats before pretreatment with L-NMMA and L-NAME, the absolute values of the average fall in mean arterial pressure (MAP) induced by intravascular injection of isopaque amin and bradykinin were 21.3 ± 2.1 and 37.2 ± 4.4 mmHg, respectively. Pretreatment with L-NMMA and L-NAME failed to affect the hypotensive response to isopaque amin; by administering isopaque amin in rats pretreated with L-NMMA and L-NAME the absolute values of the average fall in MAP were 25.6±4.9 and 23.4±3.9 mmHg, respectively, similar to the average fall in MAP before treatment with NO formation inhibitors. In contrast, the hypotensive response to bradykinin was significantly inhibited; by administering bradykinin in rats pretreated by L-NMMA and L-NAME, the absolute values of the average fall in MAP were 10.2±2.8 and 7.2±2.2 mmHg, respectively, much less than the average fall in MAP before treatment with NO formation inhibitors. We conclude that intravascular injection of isopaque amin causes reduction in systemic arterial pressure. However, this vasodilative effect seems unrelated majorly to augmented endothelium-derived NO formation.     

Cicletanine blunts the pulmonary pressor response to acute hypoxia in rats.

Cicletanine (CIC) recently has been shown to lower systemic arterial pressure in hypertensive animals and man by a mechanism that may involve potentiation of the vasodilator effect of atrial natriuretic peptide (ANP). We previously have shown that ANP prevents acute hypoxia-induced pulmonary vasoconstriction and modulates the severity of chronic hypoxic pulmonary hypertension. The current study tested the hypothesis that CIC inhibits the pulmonary pressor response to acute hypoxia by a cyclic guanosine monophosphate (cGMP)-dependent mechanism. Catheters were placed in the pulmonary arteries of Sprague-Dawley rats through the right jugular vein using a closed chest technique, and in the aorta through the right femoral artery. After a 24 hour recovery, CIC (600 mg/kg) or vehicle was administered orally by gavage to conscious rats 4 hours prior to exposure to 10% oxygen at ambient pressure or to room air. Mean pulmonary arterial pressure (MPAP) and mean systemic arterial pressure (MSAP) and heart rate (HR) were monitored for 3 hours. CIC attenuated the acute pulmonary pressor response to hypoxia (MPAP = 24.5 +/- 1.0 mm Hg in the "hypoxic+CIC" group vs. 29.9 +/- 1.0 mm Hg in the "hypoxic+vehicle" group; p less than 0.05 at 3 hours of hypoxic exposure), but had no significant effect on MSAP or HR. CIC had no effect on MPAP, MSAP, or HR in air control rats. Acute hypoxia caused significant increases in plasma ANP and cGMP and in kidney cGMP content, but CIC administration did not alter these parameters further. This is the first demonstration that acute administration of CIC attenuates the pulmonary pressor response to acute hypoxia in conscious rats.     

Role of the autonomic nervous system in mediating the response to naloxone in canine hemorrhagic shock

When the opiate antagonist naloxone is administered to anesthetized dogs subjected to hemorrhagic shock, there is a transient decrease in heart rate and sustained increases in mean arterial pressure, maximum left ventricular dp/dt, and cardiac output. Surgical cardiac denervation and pharmacologic blockade of autonomic receptors were employed to investigate the mechanisms of these two responses. The transient bradycardia was prevented by beta-adrenergic receptor blockade or cardiac denervation. The sustained response was unaffected by cardiac denervation, attenuated by either alpha- or beta-adrenergic receptor blockade, and potentiated by cholinergic receptor blockade. Naloxone had no significant effect on plasma catecholamines. The sustained hemodynamic response to naloxone appears to have two components: there is an increase in parasympathetic stimulation which modestly attenuates the adrenergic component of the response. The adrenergic stimulation of the heart after naloxone administration appears to result from potentiation of existing adrenergic stimulation and not from increased sympathoadrenal discharge. These sustained sympathetic and parasympathetic responses appear to result from the action of naloxone at a myocardial site.     

Role of redox signaling in the autonomous proliferative response of endothelial cells to hypoxia

Endothelial cells exhibit an autonomous proliferative response to hypoxia, independent of paracrine effectors. In cultured endothelial cells of porcine aorta, we analyzed the signaling of this response, with a focus on the roles of redox signaling and the MEK/ERK pathway. Transient hypoxia (1 hour) stimulated proliferation by 61+/-4% (n=16; P<0.05 versus control), quantified after 24 hours normoxic postincubation. Hypoxia induced an activation of ERK2 and of NAD(P)H oxidase and a burst of reactive oxygen species (ROS), determined by DCF fluorescence. To inhibit the MEK/ERK pathway, we used PD 98059 (PD, 20 micromol/L); to downregulate NAD(P)H oxidase, we applied p22phox antisense oligonucleotides; and to inhibit mitochondrial ROS generation, we used the ubiquinone derivate mitoQ (MQ, 10 micromol/L). All three inhibitions suppressed the proliferative response: PD inhibited NAD(P)H oxidase activation; p22phox antisense transfection did not inhibit ERK2 activation, but suppressed ROS production; and MQ inhibited ERK2 activation and ROS production. The autonomous proliferative response depends on the MEK/ERK pathway and redox signaling steps upstream and downstream of ERK. Located upstream is ROS generation by mitochondria, downstream is NAD(P)H oxidase.     

Possible source of adenosine triphosphate released from rat myocytes in response to hypoxia and acidosis

Ventricular cells from adult rats were isolated enzymatically and used as a model system for determining what factors affect the release of adenosine triphosphate (ATP) from myocardial cells. The enzyme systems used to isolate cells were trypsin:collagenase; hyaluronidase:collagenase and dispase:collagenase. Adenosine triphosphate was released in greater amounts in response to hypoxia from cells freed by each of the enzymatic procedures. This occurred while the intracellular concentration of ATP remained constant. Experiments were then performed to determine whether the conditions that occur during myocardial ischaemia or hypoxia altered the release of ATP. Cells suspended in either oxygenated or anoxic buffer at a pH of 6.8 released a significantly lower amount of ATP than cells suspended in either condition at pH 7.4. To test the possibility that ATP was released from nucleotide-protein-Ca2+ complexes located in the sarcolemma, artificial disruption of these structures was carried out. Incubation of oxygenated cells with the chelating agent, ethyleneglycol-bis (B-aminoethyl ether)-N, N-tetraacetic acid (EGTA), stimulated the release of ATP in a hyperbolic relationship while incubation of anoxic cells with ethylenediamine tetraacetate (EDTA) stimulated the release of ATP in such a way that the pattern of release followed a sigmoid response with maximal amounts of ATP, 995 +/- 55 pmol.mg-1 protein, occurring in the presence of 0.1 to 2.0 mmol.litre-1 EDTA. By incubating cells with radioactive EDTA, there was no indication that EDTA entered the cells. No release of ATP above control levels occurred when EDTA was chelated with Ca2+ before being applied to isolated cells. These data suggest that the source of ATP found extracellularly may have been nucleotide-protein-Ca2+ complexes located in the sarcolemma, and further support the role of ATP as a coronary vasodilator during hypoxic conditions.     

Indomethacin suppresses the coronary flow response to hypoxia in exercise trained and sedentary rats

STUDY OBJECTIVE--The aim was to determine if prostaglandin like activity might be involved in changes due to exercise training in the coronary flow responses to hypoxia. DESIGN--The coronary flow response to hypoxia was measured under constant perfusion pressure in isolated perfused hearts from 12 endurance exercise trained rats and 12 control rats. Eight hearts were perfused with a solution containing indomethacin, a cyclo-oxygenase inhibitor, to determine its effect on any training induced changes in the coronary flow response to hypoxic stress. EXPERIMENTAL MATERIAL--24 male Sprague-Dawley rats, 517 (SD 51) g, were used for this study. The animals were anesthetised and the hearts rapidly excised and perfused with a modified Langendorff perfusion system. MEASUREMENTS AND MAIN RESULTS--Under constant perfusion pressure, the hearts of endurance exercise trained rats had a greater increase in coronary flow during hypoxia relative to normoxia than did hearts of untrained rats, at 13.52(2.15) v 9.56(1.05) ml.min-1.g-1 dry heart weight. Indomethacin treatment abolished this difference and lowered coronary flow: exercise -3.81(3.75) ml.min-1.g-1; control 0.38(2.44) ml.min-1.g-1. CONCLUSIONS--The inhibition by indomethacin of the endurance exercise training induced potentiation of the coronary fluid flow response to hypoxia suggests that prostacyclin or a related compound may be involved in this adaptation to exercise.     

Effect of insulin and beta-adrenergic blockaders on coronary circulation in response to hypoxia

An experimental study in dogs with intact thoracic cavity under morphine-chloralose narcosis featured coronary catheterization, extracorporeal perfusion and resistography as well as catheterization of the heart cavities and the coronary sinus. Hemodynamic and respiratory parameters were recorded simultaneously. PO2, PCO2 and pH were measured in the inflowing and outflowing blood. The effect of insulin on circulatory response in acute hypoxic hypoxia was examined before and after obsidan block of beta-adrenoreactive systems. Insulin administered after the obsidan beta-adrenergic block was found to limit coronary dilatation and reduce myocardial oxygen consumption in a more pronounced measure. It is suggested that the effect of insulin on coronary dilatation is due to reduced myocardial oxygen consumption resulting from its action on the heart's beta-adrenergic systems.