Biomedical subjects

A Deussen

Publications and source records attributed to A Deussen.

At least 55 records · Page 3Linked to original sources

PubMed1991

Cardiac adenosine production is linked to myocardial pO2.

Experiments were performed on isolated perfused guinea-pig hearts (n = 45) to further evaluate the stimulus that triggers cardiac adenosine production. Stimulation of hearts with isoproterenol (4 nM, 20 min) enhanced left ventricular dP/dtmax, heart rate and myocardial oxygen consumption within 1 min to new steady state values, whereas coronary venous adenosine concentration only transiently increased reaching its maximum between 1 and 3 min of stimulation. Rate of accumulation of S-adenosylhomocysteine (SAH), a measure of the free cytosolic adenosine concentration, was steepest immediately following onset of stimulation and then progressively declined. Similar to adenosine, changes in coronary venous pO2 were phasic and adenosine release and pO2 closely correlated. Norepinephrine (20 nM) which increased myocardial oxygen consumption to a comparable extent as isoproterenol (4 nM) further decreased coronary venous pO2 and increased coronary venous adenosine. When myocardial oxygen supply was systematically varied by changing coronary perfusion pressure from 60 to 90 and 35 cmH2O, respectively, the adenosine release during isoproterenol (2 nM) was markedly enhanced at 35 cmH2O but blunted at 90 cmH2O. Similarly SAH accumulation was greatest at 35 cmH2O and smallest at 90 cmH2O. It is concluded that changing myocardial oxygen consumption is not a sufficient cause to enhance adenosine formation. Myocardial oxygenation as reflected by changes in coronary venous pO2 closely correlates with changes in free cardiac adenosine as evidenced by two independent indices: tissues SAH and coronary venous adenosine concentration. The stimulus triggering cardiac adenosine formation is most likely the imbalance of oxygen supply and oxygen demand.

Adenosine↗

PubMed1991

Release of endothelium derived nitric oxide in relation to pressure and flow.

STUDY OBJECTIVE: Endothelium derived nitric oxide (NO) is an important modulator of resting vascular tone. The aim of the study was to investigate the extent by which the rate of NO release is modulated by the two determinants of vascular conductance: pressure and flow. DESIGN AND EXPERIMENTAL MATERIAL: Porcine macrovascular endothelial cells cultured on microcarrier beads were used as a model in which the rate of NO release was determined photometrically. Columns packed with endothelial cell covered beads were perfused at different flow rates (2, 10, 20 ml.min-1) and perfusion pressures (ranging from 6 to 200 mm Hg). MEASUREMENTS AND MAIN RESULTS: Release of endothelial cell derived NO was continuously quantified under basal and ATP stimulated conditions using a specific difference spectrophotometric assay. Increasing flow flow from 2 to 20 ml.min-1 enhanced the basal NO release from endothelial cells fivefold. ATP (10(-4) M) augmented the NO release from endothelial cells more than 10-fold at each flow level studied. The ATP induced NO release rapidly increased by a factor of 5.8 when flow was enhanced from 2 to 20 ml.min-1 (greater than 180 pmol.min-1.mg endothelial cell protein). Raising perfusion pressure from 6 to 200 mm Hg in endothelial cells did not affect the rate of basal NO release. CONCLUSIONS: (1) The rate of NO release from endothelial cells increases when flow is enhanced. (2) Endothelial cells possess a high capacity for NO production, permitting a rapid adjustment of NO release to changes in flow. (3) The rate of NO release is not causally related to changes in perfusion pressure.

Adenosine Triphosphate↗

PubMed1991

Oxygen partial pressure and free intracellular adenosine of isolated cardiomyocytes.

Adenosine formation by the heart is known to critically depend on the ratio of oxygen supply to oxygen demand, but the sensitivity of cardiomyocytes to defined changes in PO2 is not known. Isolated metabolically stable rat cardiomyocytes were incubated up to 45 min at constant PO2 values ranging from 0.1 to 100 mmHg using a feedback-controlled incubation system (oxystat system). Changes of the free intracellular adenosine concentration were measured after trapping of adenosine by cytosolic S-adenosylhomocysteine (SAH) hydrolase in the presence of 200 microM L-homocysteine thiolactone. Rate of SAH formation was constant at a PO2 between 3 and 100 mmHg and gradually increased at PO2 less than 3 mmHg. Cellular ATP decreased only at PO2 less than 1 mmHg, and this was accompanied by a decline of oxygen consumption. Treatment of cells with 5.5 mM deoxyglucose and 4 micrograms/ml oligomycin increased SAH formation 60-fold and was associated with elevated intra- and to a lesser extent extracellular adenosine levels. Inhibition of nucleoside transport with 20 microM S-(p-nitrobenzyl)-6-thioinosine steepened the transmembrane adenosine gradient. Our findings suggest that the cardiomyocyte responds to metabolic poisoning and oxygen deprivation with an enhanced formation of adenosine. This adenosine is mainly formed intracellularly and reaches the extracellular space by diffusion. Threshold for adenosine formation is as low as 3 mmHg.

Adenine Nucleotides↗

PubMed1991

Transmural gradient of adenosine in canine heart during functional hyperemia.

Effects of beta-adrenergic stimulation and atrial pacing on the transmural gradient of intracellular free adenosine were assessed in dog hearts in vivo by measurement of the accumulation of S-adenosylhomocysteine (SAH) in the presence of homocysteine (1.6 mg.kg-1.min-1 iv). Isoproterenol (0.3-0.5 micrograms.kg-1.min-1 iv for 30 min) consistently enhanced left ventricular dP/dtmax (86%), heart rate (38%), and myocardial oxygen consumption (MVO2; 62%) within 3 min while formation and release of adenosine transiently increased. Diastolic aortic pressure fell from 107 +/- 12 to 58 +/- 5 mmHg, and the transmural gradient of SAH was 1.6-, 2.5-, and 4.4-fold increased in subepi-, mid- myo-, and subendocardial layers, respectively. Adenosine formation was inversely related to diastolic aortic pressure. Maintaining aortic pressure greater than 65 mmHg only slightly enhanced venoarterial difference of adenosine despite a greater augmentation of MVO2. Pacing the heart at 209 +/- 4 beats/min enhanced MVO2 by 42% and increased subepi-, midmyo-, and subendocardial levels of SAH by 1.5-, 3.3- and 1.9-fold, respectively. These results demonstrate that in the in situ heart 1) beta-adrenergic stimulation and pacing cause an inhomogeneous transmural increase in free intracellular adenosine mainly affecting the subendocardial and midmyocardial layers; 2) diastolic aortic pressure as the driving force of coronary perfusion is of critical importance for cardiac adenosine formation; and 3) the kinetics of oxygen consumption and adenosine formation are clearly dissociated during beta-stimulation.

Adenosine↗

PubMed1990

Adenosine is a sensitive oxygen sensor in the heart.

Cardiac adenosine is formed both by an oxygen-sensitive (AMP----adenosine) and by an oxygen-insensitive (S-adenosylhomocysteine----adenosine) pathway. The phasic adenosine release during beta-adrenergic stimulation with isoproterenol is closely linked to coronary venous PO2 (isolated heart) and can be almost fully prevented when diastolic aortic pressure is maintained constant (heart in situ). During pressure autoregulation the transmural gradient of free adenosine is only increased when the autoregulatory reserve is exhausted. The critical PO2 below which adenosine formation is enhanced was found to be 3 mm Hg (isolated cardiomyocytes). Collectively, these data indicate that the formation of adenosine is not primarily coupled to the energy expenditure of the heart but to the supply/demand ratio for oxygen.

Adenosine↗

PubMed1990

Contribution of postsynaptic alpha 2-adrenoceptors to reflex sympathetic constriction of stenotic coronary vessels.

Increases in the activity of efferent cardiac sympathetic nerves by 35 +/- 9% were induced by 60 s bilateral occlusion of the common carotid arteries (BCO) in anesthetized dogs. Under control conditions the reflex rise in sympathetic nerve activity enhanced left ventricular pressure (115 +/- 4 mm Hg) by 47% and regional myocardial oxygen consumption (9.7 +/- 1.1 ml/min.100 g) by 56%. Simultaneously, end-diastolic circumflex coronary resistance (0.99 +/- 0.11 mm Hg.min.100 g/ml) decreased by 16%. After exhaustion of coronary dilator reserve by production of a severe coronary stenosis, BCO enhanced left ventricular pressure (107 +/- 4 mm Hg) by 49%, oxygen consumption of the poststenotic area (7.6 +/- 0.8 ml/min.100 g) increased by 21%, and circumflex coronary resistance (0.54 +/- 0.05 mm Hg.min.100 g/ml) also increased by 19%. The reflex increase in coronary resistance during BCO was abolished after infusion of the alpha 2-adrenoceptor antagonist rauwolscine (0.2 mg/kg i.v.). Administration of rauwolscine, however, did not prevent the reflex increase of left ventricular pressure and regional myocardial oxygen consumption. Comparable increases in poststenotic coronary resistance during BCO were found in dogs which either received propranolol (2 mg/kg i.v.) or in which the reflex rise in mean aortic pressure was limited to 13 +/- 3 mm Hg. In both experimental groups, rauwolscine also effectively prevented the BCO-induced rise in coronary resistance. In contrast, the reflex increase of total peripheral resistance was not significantly reduced by rauwolscine, but was blunted after additional administration of the selective alpha 1-adrenoceptor antagonist prazosin (1.2 mg/kg i.v.). We conclude that: 1) Poststenotic coronary vasoconstriction occurs during shortlasting increases in efferent cardiac sympathetic discharge within the physiological range. 2) This increase in poststenotic coronary resistance is significantly reduced after administration of the alpha 2-adrenoceptor antagonist rauwolscine. 3) In contrast to poststenotic coronary resistance, functionally innervated alpha 2-adrenoceptors are of minimal importance for the reflex increase in total peripheral resistance.

Animals↗

PubMed1990

Prevention of alpha-adrenergic coronary constriction by calcium-antagonists.

This manuscript reviews the experimental evidence for a functional antagonism of Ca-antagonists against alpha-adrenoceptor-mediated increases in coronary vasomotor tone. In studies on anesthetized dogs, intravenous nifedipine effectively prevented the alpha 1-adrenoceptor-mediated increase in epicardial coronary resistance, as well as the increase in end-diastolic resistance mediated by both alpha 1- and alpha 2-adrenoceptors during cardiac sympathetic nerve stimulation. Both intracoronary and intravenous administration of nifedipine also prevented the alpha 2-adrenoceptor-mediated increase in coronary resistance distal to severe stenoses, as well as the resulting ischemic dysfunction and net lactate production during cardiac sympathetic nerve stimulation. Felodipine was equally effective as nifedipine in preventing an alpha 2-adrenoceptor-mediated increase in coronary resistance and the resulting contractile dysfunction distal to severe coronary stenoses. alpha 1- and alpha 2-Adrenergic coronary constriction also contribute to the severity of myocardial ischemia in conscious dogs during treadmill exercise. Again, nifedipine improved regional myocardial blood flow and attenuated regional contractile dysfunction during exercise-induced ischemia in conscious dogs with a chronic coronary stenosis. This beneficial effect of nifedipine was attributed to a recruitment of coronary dilator reserve and not to a reduction in heart rate or afterload. In conclusion, there is solid experimental evidence for a functional antagonism of Ca-antagonists against alpha-adrenergic coronary constriction and its contribution to myocardial ischemia.

Animals↗

PubMed1990

Pain and myocardial ischemia: the role of sympathetic activation.

In a first series, we tested whether the relative ischemia distal to a severe stenosis on the left circumflex coronary (CX) artery increases the activity of cardiac sympathetic (CS) nerves which, in turn, may result in a poststenotic vasoconstriction and an aggravation of ischemia. In 23 anesthetized, vagotomized dogs, an acute stenosis that reduced CX blood flow to 50% of control was produced and maintained for 20 min. The activity of postganglionic CS nerves increased by 23 +/- 4% within 20 min. In parallel, poststenotic coronary resistance increased from 0.48 +/- 0.03 (SEM) to 0.61 +/- 0.03 mm Hg.min.100 g/ml, resulting in a net lactate production after 15 min. The selective alpha 2-adrenoceptor antagonist rauwolscine (0.2 mg/kg i.v.; n = 6) and the calcium antagonist nifedipine (10 micrograms/kg i.v.; n = 6) prevented the progressive increase in poststenotic resistance and the net lactate production, but still permitted an increase in CS activity. Segmental anesthesia of CS nerves with epidural infiltration of procaine at segments C7-T6 (n = 6) prevented the sympathetic activation, the progressive increase in poststenotic resistance and the net lactate production. In six additional dogs with intact vagus nerves, CS activation and a concomitant increase in poststenotic resistance resulting in myocardial ischemia were also found. These data suggest a vicious cycle between poststenotic coronary vasoconstriction and CS activation, resulting in severe myocardial ischemia. In a second series, stimulation of high-threshold somatic afferents (= nociceptive stimulation: NCS) was used to cause reflex CS activation. The superficial peroneal nerve was electrically stimulated in 14 anesthetized, vagotomized dogs. With intact CX arteries, a 1 min stimulation resulted in a pronounced increase in CX blood flow and perfusion pressure. In contrast, NCS in the presence of a severe stenosis on the CX artery increased end-diastolic poststenotic coronary resistance by 96 +/- 15% due to a reflex activation of CS nerve fibers. This activation was markedly reduced after injection of fentanyl (27 micrograms/kg i.v.; n = 6). Injection of naloxone (60 micrograms/kg) restored the original effect. Systolic wall thickening (WT; sonomicrometry) in the CX artery-perfused myocardium was increased during NCS (10.9 +/- 3.9 (SD) vs. 13.6 +/- 5.0%) in additional five dogs with intact coronary arteries. In the presence of a stenosis on the CX artery, systolic WT was reduced to 7.0 +/- 2.5% and was further decreased to 4.6 +/- 2.3% during NCS. The additional deterioration of systolic regional function during NCS was prevented after i.v. injection of fentanyl, as was the increase in poststenotic coronary resistance.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1990

Nucleotide levels in human and bovine lenses: a study on regional postmortem changes.

Regional lens adenine nucleotide levels were determined in human and bovine lenses using sensitive HPLC techniques. In adult lenses of both species adenine nucleotide levels were highest in the anterior cortex (plus epithelium and capsule) followed by the posterior cortex (plus capsule) and lowest in the nucleus. With increasing lens age ATP level remained steady in the anterior cortex (plus epithelium and capsule) and nucleus, while those in the posterior cortex showed a tendency toward decrease. In both species, human as well as bovine, adenine nucleotide levels of the anterior cortex (plus epithelium and capsule) underwent early postmortem changes. Thus, immediate postmortem sampling as well as regional determinations are a must for detailed studies of lens nucleotide metabolism.

Adenine Nucleotides↗

PubMed1989

Nucleotide levels in human lens: regional distribution in different forms of senile cataract.

Sensitive high-performance liquid chromatography methods were employed to assess regional distribution of adenine, guanosine and uridine nucleotides in clear and cataractous human eye lenses. According to slit-lamp examination, three forms of senile cataract were distinguished: (1) supranuclear or deep cortical cataract (typical senile cataract), (2) primary nuclear cataract (cataracta brunescens) and (3) subcapsular cortical cataract associated either with a supranuclear (3a) or a secondary nuclear cataract (3b). Except for AMP, which was highest in the nuclear fraction, all other nucleotides (ATP, ADP, GTP, and UTP) were predominantly located in the anterior cortex (plus epithelium) of clear as well as cataractous lenses, that is, ATP levels in the nucleus amounted to 20% of those found in the anterior cortex (plus epithelium); ATP levels in the posterior cortex were about 60% of those in the anterior cortex (plus epithelium). Significant differences in the absolute regional nucleotide level existed between the different forms of cataract. Highest ATP levels were found in the anterior cortex (plus epithelium) of clear lenses and deep or supranuclear cortical cataract. The ATP level was slightly diminished in primary nuclear cataract and in supranuclear cortical cataract when associated with an early subcapsular cortical cataract. ATP levels were depressed to less than 30% in the anterior cortex (plus epithelium) of lenses with a subcapsular cortical cataract when associated with either an early secondary nuclear or a mature cataract. Furthermore, the ATP/ADP ratio was decreased in this form of senile cataract. The decrease in lens nucleotide level did not correlate with increased age. These data suggest that decreases in regional ATP level are a secondary event and do not appear to be causally involved in the genesis of the 'cataracta senilis'.

Adenosine Diphosphate↗

PubMed1989

Contribution of S-adenosylhomocysteine to cardiac adenosine formation.

The S-adenosylhomocysteine (SAH) hydrolase inhibitor adenosine dialdehyde was used in isolated guinea pig hearts to determine the contribution of the transmethylation pathway to cardiac adenosine formation. This inhibitor did not alter cardiac hemodynamics but effectively inhibited SAH-hydrolase activity under in vitro and in vivo conditions. In normoxic perfused hearts adenosine dialdehyde (10 microM) caused tissue levels of SAH to linearly increase at a rate of 160 pmol/g/min over 60 min. At the same time adenosine dialdehyde decreased release of adenosine into the coronary effluent perfusate by 16 pmol/min (34%). Hypoxic perfusion (30% O2) of guinea-pig hearts increased release of adenosine from 43 to 3700 pmol/min. However, rate of SAH formation in the presence of adenosine dialdehyde was only slightly enhanced from 160 to 200 pmol/g/min and adenosine dialdehyde did not significantly alter the hypoxia induced adenosine release. Since all experiments were performed in the presence of the adenosine deaminase inhibitor EHNA (5 microM) the results demonstrate: (1) the transmethylation pathway of the heart contributes one third to global cardiac adenosine production under normoxic conditions and provides a constant source of adenosine independent of tissue oxygenation. (2) The majority of SAH-derived adenosine is salvaged most likely via adenosine kinase. (3) The hypoxia induced adenosine production is predominantly derived from enhanced 5' AMP hydrolysis.

Adenine↗

PubMed1989

Glutamate degradation in the ischemic dog heart: contribution to anaerobic energy production.

The present study investigated the conversion of amino acids to succinate and the contribution of this pathway to anaerobic energy production during regional ischemia in the dog heart in situ. The relation between regional myocardial blood flow, estimated by the tracer microsphere technique, and myocardial contents of metabolites (glutamate, alanine, succinate, lactate) as well as their local arterio-venous differences (A-V) were determined. During 30 min of coronary artery occlusion, myocardial glutamate decreased from 2.3 mumol/g wet wt in control tissue to 1.2 mumol/g wet wt in severely ischemic areas, while aspartate was unaffected. Myocardial alanine increased in a 1: 1 stoichiometry compared to glutamate, and succinate accumulated. During control perfusion (118 mmHg), A-V of lactate, succinate and glutamate were +470, -0.7 and -3.9 nmol/ml, respectively. Stepwise reduction of perfusion pressure led to the release of lactate and succinate from the underperfused area; extraction of glutamate occurred at the lowest perfusion pressure investigated (34 mmHg; A-V: -500, -10.4 and +4.2 nmol/ml, respectively). The data indicate that during regional ischemia in vivo, succinate is synthetized exclusively from glutamate via 2-oxo-glutarate, following transamination with glycolytic pyruvate yielding alanine, while the contribution of aspartate is negligible. Using tissue levels of glutamate and lactate together with the local arterio-venous concentration differences of these compounds, it can be estimated that degradation of glutamate delivers 20% of the ATP generated by substrate level phosphorylation reactions. Thus energy production by the glutamate degradation pathway is significant in vivo under conditions of flow deprivation.

Adenosine Triphosphate↗

PubMed1989

Turnover of adenosine in plasma of human and dog blood.

To determine half-life and turnover of plasma adenosine, heparinized blood from healthy volunteers was incubated with radiolabeled adenosine in the physiological concentration range of 0.1-1 microM. Plasma levels of adenosine in vitro were 82 +/- 14 nM and were similar to those determined immediately after blood collection with a "stopping solution." Dipyridamole (83 microM) and erythro-9(2-hydroxynon-3yl)-adenine (EHNA) (8 microM) did not measurably alter basal adenosine levels but completely blocked the uptake of added adenosine. Inhibition of ecto-5'-nucleotidase with 100 microM alpha, beta-methyleneadenosine 5'-diphosphate (AOPCP) reduced plasma adenosine to 22 +/- 6 nM. For the determination of adenosine turnover, the decrease in specific radioactivity of added [3H]adenosine was measured using a dipyridamole-containing stopping solution. Without altering basal adenosine levels, the half-life was estimated to be 0.6 s. Similar experiments were carried out with washed erythrocytes or in the presence of AOPCP, yielding half-lives of 0.7 and 0.9 s, respectively. When the initial adenosine concentration was 1 microM, its specific activity decreased by only 11% within 5 s, whereas total plasma adenosine exponentially decreased with a half-life of 1.5 s. Venous plasma concentrations were measured after relief of a 3-min forearm ischemia. Changes in plasma adenosine did not correlate well with changes in blood flow but were augmented in the presence of dipyridamole.(ABSTRACT TRUNCATED AT 250 WORDS)

5'-Nucleotidase↗

PubMed1989

Regional water content of clear and cataractous human lenses.

The present study evaluates the regional water content of clear and cataractous human lenses. In order to determine the lens water, a freeze-drying technique was used which leaves behind only a very small amount of residual humidity. The effectiveness of freeze-drying was demonstrated in experiments with animal eye lenses (rat, pig, bovine). The results obtained revealed that, like in other mammalian lenses, the human lens cortex exhibits a significantly higher overall water content than the nucleus. This pattern was found in clear lenses as well as in lenses with early 'senile' cataracts. Subcapsulary cataracts present a unique feature as in these lenses the cortical water content is enhanced while the fresh lens weight is significantly reduced. Observations of postmortem human eye lenses indicate that regional differences in water content are greatly abolished 24-48 h after death.

Aged↗

PubMed1988

The transmethylation pathway as a source for adenosine in the isolated guinea-pig heart.

In order to quantify adenosine production from the transmethylation pathway [S-adenosylmethionine (AdoMet)----S-adenosylhomocysteine (AdoHcy) in equilibrium adenosine + L-homocysteine] in the isolated guinea-pig heart under basal conditions (normoxic perfusion with 95% O2) and during elevated adenosine production (hypoxic perfusion with 30% O2), two methods were used. (1) Hearts were perfused with normoxic medium containing [2,5,8-3H]adenosine (5 microM) and L-homocysteine thiolactone (0.1 mM), which brings about net AdoHcy synthesis via reversal of the AdoHcy hydrolase reaction and labels the intracellular pool of AdoHcy. From the decrease in AdoHcy pool size and specific radioactivity of AdoHcy in the post-labelling period, the rate of transmethylation, which is equivalent to the rate of adenosine production, was calculated to be 0.98 nmol/min per g. Adenosine release from the hearts was 40-50 pmol/min per g. (2) Hearts were perfused with hypoxic medium containing [35S]homocysteine (50 microM). Owing to the hypoxia-induced increase in adenosine production, this procedure also results in expansion and labelling of the AdoHcy pool. From the dilution of the specific radioactivity of AdoHcy relative to that of [35S]homocysteine, the rate of AdoHcy synthesis from AdoMet (transmethylation) was calculated to be 1.12 nmol/min per g. It is concluded that in the oxygenated heart the transmethylation pathway is quantitatively an important intracellular source of adenosine, which exceeds the rate of adenosine wash-out by the coronary system by about 15-fold. Most of the adenosine formed by this pathway is re-incorporated into the ATP pool, most likely by adenosine kinase. The transmethylation pathway is essentially O2-independent, and the known hypoxia-induced production of adenosine must be derived from an increase in 5'-AMP hydrolysis.

Adenosine↗

PubMed1988

Formation of S-adenosylhomocysteine in the heart. I: An index of free intracellular adenosine.

To assess the concentration of free intracellular adenosine in the heart the kinetic properties of cytosolic S-adenosylhomocysteine (SAH) hydrolase were utilized at elevated levels of L-homocysteine (adenosine + L-homocysteine in equilibrium with SAH + H2O). Global hypoxia was induced in the isolated perfused guinea pig heart by graded reduction of perfusion medium PO2 in the presence of saturating concentrations of homocysteine (0.2-1.0 mM). Reduction of PO2 from 660 to 165 mm Hg increased the steady-state concentration of total tissue adenosine from 2.0 +/- 0.2 to 2.8 +/- 0.2 nmoles/g, while the rate of SAH formation increased linearly from 0.22 +/- 0.03 to 2.50 +/- 0.13 nmoles/min/g. When adenosine was exogenously applied at a concentration of 100 microM together with homocysteine (1 mM), SAH accumulation rates were much greater: 23.34 +/- 3.31 and 42.11 +/- 1.73 nmoles/min/g with normoxic (95% O2) and hypoxic (30% O2) perfusion, respectively. The apparent Km and Vmax values for SAH-hydrolase in vivo were estimated to be 20 microM and 59 nmoles/min/g wet wt, respectively. Since the relation between SAH formation and adenosine in the physiological concentration range is linear, the measured rate of SAH accumulation during normoxia and hypoxia permitted the calculation of the free intracellular adenosine level, which was 0.061 nmoles/g (0.08 microM) in the normoxic heart. With hypoxia (PO2 165 mm Hg), this value increased to 1.57 nmoles/g (2.0 microM). Free intracellular adenosine closely correlated with the hypoxia-induced changes in coronary flow. The data reveal that measurement of the rate of SAH accumulation during homocysteine infusion can be used for sensitive assessment of free intracellular adenosine levels. Assuming that the intracellular adenosine concentration equals that in the interstitial space, the results furthermore indicate that the degree of intracellular adenosine formation during hypoxic perfusion is quantitatively sufficient to account for most of the observed increases in coronary flow.

Adenosine↗

PubMed1988

Formation of S-adenosylhomocysteine in the heart. II: A sensitive index for regional myocardial underperfusion.

Rate of accumulation of myocardial S-adenosylhomocysteine (SAH) was used in an open-chest dog preparation as an index of free cytosolic adenosine levels. Following 30 minutes of coronary artery ligation and infusion of L-homocysteine thiolactone (10 mumol/kg/min i.v.) SAH levels increased from 1.3 (control) to 3.3 nmoles/g in the nonischemic and to values over 100 nmoles/g in the ischemic region. Compared with regional myocardial blood flow the enhanced rate of SAH accumulation was strictly confined to the ischemic area. As long as blood flow was 0.6-1.2 ml/min/g, SAH levels remained unchanged. However, they steeply increased when regional myocardial blood flow decreased below 60% of control. Tissue levels of adenine nucleotides, adenosine, and lactate were not significantly affected in the flow range of 0.4-0.6 ml/min/g but rate of SAH accumulation was enhanced by 400%. In the nonischemic myocardium, SAH accumulation was 60% higher in the subendocardium than in the subepicardium. Decreasing coronary perfusion pressure from 110 to 60, 45, and 35 mm Hg was associated with an exponential increase in coronary venous adenosine release only when perfusion pressure was below 60 mm Hg. Transmural mapping of SAH revealed that at 110 mm Hg SAH was homogeneously distributed, while at a perfusion pressure of 60 mm Hg SAH accumulation was enhanced only in the subendocardial layers. Decreasing perfusion pressure further to 40 and 30 mm Hg not only enhanced subendocardial SAH levels to 120 and 170 nmoles/g, respectively, but also considerably steepened the transmural gradient of SAH. SAH-hydrolase exhibited a broad pH-optimum and its activity in different parts of ventricular myocardium was identical. Our findings provide evidence that 1) measurement of SAH accumulation is a sensitive metabolic index for the assessment of regional myocardial ischemia, 2) significant formation of SAH occurs only when regional myocardial blood flow is less than 0.6 ml/min/g, and 3) transmural SAH gradient, a measure of free cytosolic adenosine, and coronary venous adenosine release significantly increase only when the autoregulatory reserve is exhausted.

Adenosine↗

PubMed1986

Contribution of coronary endothelial cells to cardiac adenosine production.

Experiments were performed in isolated non-working guinea pig hearts perfused according to the Langendorff technique (95% O2, 5% CO2), to evaluate the relative contribution of the coronary endothelium to the formation of cardiac adenosine during hypoxia, hypercapnia, and acetylcholine infusion. For this purpose the adenine-nucleotides of the coronary endothelium were prelabeled by perfusion of isolated hearts with 3H-adenosine (10(-8) M) for 35 min. Changes in the relative specific radioactivity (RSA) of adenosine released into the coronary effluent perfusate were used to assess changes in the relative contribution of the coronary endothelium and cardiomyocytes to total cardiac adenosine release. Hypoxic perfusion (15% O2) doubled coronary flow and increased total adenosine release by about two orders of magnitude and in addition, substantially increased the release of 3H-adenosine. The RSA of adenosine, however, was consistently depressed. During hypercapnic acidosis (9% CO2) the increase in coronary flow was associated with only a small and transient rise in cardiac adenosine release, and did not influence the formation of 3H-adenosine. In the unpaced heart, acetylcholine (10(-7) and 2 X 10(-6) M) dose-dependently increased coronary flow and the release of both adenosine and 3H-adenosine. Within the first minute, the RSA of adenosine was increased, but thereafter was decreased relative to control. In the paced heart, the effects of acetylcholine (2 X 10(-6) M) were greatly attenuated. Increasing coronary flow by bradykinin and isosorbide dinitrate or decreasing heart rate by (-)N6-phenylisopropyl-adenosine did not significantly affect effluent perfusate concentration of adenosine or its RSA.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine↗