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Biomedical subjects

H Refsum

Publications and source records attributed to H Refsum.

At least 73 records · Page 4Linked to original sources
PubMed1986

Determination of in vivo protein binding of homocysteine and its relation to free homocysteine in the liver and other tissues of the rat.

Low concentrations (0.5-6 nmol/g) of homocysteine (Hcy) have recently been demonstrated in acid extracts of various tissues of the mouse and rat (Ueland, P.M., Helland, S., Broch, O.-J., and Schanche, J.-S. (1984) J. Biol. Chem. 259, 2360-2364). This is referred to as free Hcy in tissues. This paper describes a method for the determination of protein-bound Hcy, which involves precipitation and washing of tissue protein with ammonium sulfate, release of Hcy from native proteins in the presence of dithioerythritol, and determination of free Hcy by a sensitive radioenzymic assay. Both free and bound Hcy decreased markedly in rat tissues within a few seconds following death of the animal. The amount of protein-bound Hcy was highest in liver, somewhat lower in kidney, brain, heart, lung, and spleen. The ratio between free and bound Hcy was between 1 and 2 in most tissues, except in cerebellum, containing a large excess of free Hcy (free/bound ratio of 18). Free Hcy was almost exclusively localized to the soluble fraction of rat liver, whereas protein-bound Hcy was about equally distributed between this fraction and the microsomes. Isolated rat hepatocytes contained free and protein-bound Hcy in proportions observed in whole liver, but a large amount of Hcy was exported into the extracellular medium. The half-lives, as determined from pulse-chase experiments with [35S] methionine, were 53 s for S-adenosylmethionine, 2 s for S-adenosylhomocysteine and 3 s for Hcy (free and bound regarded as a single pool). Furthermore, isotope equilibrium between these metabolites and between free and bound Hcy throughout the rapid chase period suggests the turnover rates of S-adenosylhomocysteine and Hcy to be production rate limited, and the dissociation rate of the Hcy-protein complex may greatly exceed the turnover rate of Hcy. Thus, the half-lives of Hcy are such that participation of both free and bound Hcy in metabolic regulation is feasible.

Adenine↗
PubMed1986

Cardiac effects of thoracic epidural morphine caused by increased vagal activity in the dog.

This study was carried out in order to investigate possible side-effects of thoracic epidural morphine on cardiac electrophysiology, haemodynamics and metabolism. In pentobarbital-anaesthetized dogs, intracardiac conduction times were determined by His bundle electrography, and refractoriness by programmed electrical stimulation; monophasic action potential recordings were obtained from the right ventricle by the suction electrode technique. Cardiac output, left ventricular and aortic blood pressures were measured, as well as plasma concentrations of morphine, free fatty acids, glycerol, glucose and lactate. Thoracic epidural morphine (0.12 mg X kg-1) reduced spontaneous heart rate, prolonged atrioventricular nodal conduction time and refractoriness, and reduced left ventricular dP/dt max. Bilateral vagotomy reversed these effects. Intra-atrial, His Purkinje and intraventricular conduction times, atrial and ventricular refractoriness and action potential duration, stroke volume and mean aortic blood pressure, as well as the metabolic variables, were not significantly influenced by thoracic epidural morphine with or without vagotomy. Peak plasma morphine levels of 12-25 ng X ml-1 were measured 10 min after morphine injection. In conclusion, this study demonstrates depressive side-effects of epidural morphine on cardiac function, mediated by an increased vagal activity.

Anesthesia, Epidural↗
PubMed1986

Acute electrophysiologic and blood pressure effects of amiodarone and its solvent in the dog.

Amiodarone has repeatedly been shown to have potent class III antiarrhythmic properties. It has, however, been questioned whether the acute and chronic effects of the drug are due to the same mechanism. In order to investigate the acute electrophysiologic and blood pressure effect of the drug, amiodarone (Cordarone) was given intravenously in cumulative doses of 2.5, 5.0 and 10.0 mg/kg to seven pentobarbital (mebumalum NFN) anaesthetized dogs. Corresponding volumes of the solvent, polysorbatum 80 (Tween 80), were given to two dogs. Cardiac electrophysiologic effects were studied by His bundle electrography and programmed electrical stimulation. Amiodarone decreased heart rate and AV nodal conduction velocity and increased atrial, AV nodal and ventricular refractoriness. A pronounced but transient fall in mean aortic blood pressure (MABP) occurred after the first injection of amiodarone. No fall in MABP occurred, however, after the subsequent two doses. Intravenous injection of the solvent exactly reproduced the effects on MABP, but not the electrophysiologic effects. The present study supports the concept that amiodarone also has acute class III antiarrhythmic effect. After the initial injection, a pronounced fall in blood pressure due to the solvent may be seen, but rapid tachyphylaxis occurs.

Amiodarone↗
PubMed1986

Alpha-adrenoceptor blockade and class III antiarrhythmic activity combined: hemodynamic and electrophysiological effects of melperone in the dog.

Melperone has been found to possess vasodilating and slight positive inotropic properties in addition to its class III antiarrhythmic action. To determine whether some of these effects might be related to an alpha-adrenoceptor blocking action of melperone, phenoxybenzamine (10 mg/kg) was given as a 2-h infusion to 12 pentobarbital-anesthetized dogs. In addition, six of the dogs were given atenolol 0.5 mg/kg i.v. After a 1-h stabilizing period, melperone (0.5, 2.5, and 12.5 mg/kg) was given i.v. in cumulative doses to both series of dogs. In the presence of alpha-blockade as well as combined alpha- and beta-blockade, atrial, atrioventricular (AV) nodal, and ventricular refractoriness increased and heart rate and AV nodal conduction time decreased, as previously reported after addition of melperone alone. A slight increase in left ventricular (dP/dt)max occurred after the addition of melperone (2.5 mg/kg) in the presence of alpha- and beta-blockade, but only after the highest dose of melperone were small decreases in blood pressure and total peripheral resistance induced. The present study indicates that melperone combines the properties of class III antiarrhythmic action, slight positive inotropy, and alpha-adrenoceptor mediated vasodilation.

Adrenergic alpha-Antagonists↗
PubMed1986

Disposition of endogenous homocysteine by mouse fibroblast C3H/10T1/2 Cl 8 and the chemically transformed C3H/10T1/2 MCA Cl 16 cells following methotrexate exposure.

The tumorigenic cell line termed "MCA Cl 16" was derived from C3H/10T1/2 clone (Cl) 8 cells by chemical transformation in the presence of 3-methylcholanthrene [(MCA) CAS: 56-49-5]. Transformed (Cl 16) cells were more sensitive toward the cytotoxic effect of methotrexate (MTX) than their normal counterpart Cl 8 cells. The disposition of endogenous L-homocysteine (Hcy) was investigated in these two cell lines after MTX exposure. Both nonmalignant and transformed cells exported Hcy into the extracellular medium, and only small amounts were retained within the cells. The Hcy efflux from the malignant cells was markedly increased after MTX exposure (0.5-10 microM), and this effect was almost completely prevented by 5-formyl-tetrahydrofolate (THF), whereas treatment with thymidine plus hypoxanthine did not inhibit the MTX-dependent Hcy efflux. Cytotoxic concentration of MCA reduced rather than increased the Hcy efflux from these cells. High concentrations of MTX (greater than 10 microM) were required to increase the release of Hcy from nonmalignant cells. The enhancement of Hcy export from the malignant cells in the presence of MTX was not associated with cellular build-up of S-adenosyl-L-homocysteine (AdoHcy), indicating that the amount of intracellular Hcy was kept below the level required for inhibition or reversion of the AdoHcy hydrolase reaction. MTX-dependent Hcy efflux probably reflects cellular deficiency of 5-methyl-THF required for the salvage of Hcy to methionine and may therefore be a measure of lack of this reduced folate relative to the metabolic demand.

Animals↗
PubMed1986

Disposition of homocysteine in rat hepatocytes and in nontransformed and malignant mouse embryo fibroblasts following exposure to inhibitors of S-adenosylhomocysteine catabolism.

S-Adenosylhomocysteine (AdoHcy) is catabolized to adenosine and homocysteine through the action of AdoHcy hydrolase, and this reaction is the only known source of L-homocysteine in vertebrates. The disposition of endogenously formed L-homocysteine was investigated in isolated rat hepatocytes and nontransformed and malignant C3H/10T1/2 mouse embryo fibroblasts exposed to 3-deazaaristeromycin or D-eritadenine, compounds which are potent inhibitors of AdoHcy hydrolase. Cells in suspension release large amounts of L-homocysteine into the extracellular medium whereas small amounts are retained within the intracellular compartment. The L-homocysteine egress is inhibited by 3-deazaaristeromycin or D-eritadenine in a manner which closely parallels the inhibitory effect on AdoHcy catabolism, suggesting that L-homocysteine egress may be coupled to its formation from AdoHcy. In liver cells, the accumulation of AdoHcy exceeded the inhibition of L-homocysteine egress, whereas in the fibroblasts inhibition of egress equalled the accumulation of AdoHcy. Inhibition of AdoHcy catabolism was associated with an increase in both free and protein bound L-homocysteine in liver cells, whereas depletion of intracellular L-homocysteine occurred in the mouse embryo fibroblasts under these conditions. These data suggest that some properties of nucleoside analogues may be related to their effects on L-homocysteine metabolism. Furthermore, L-homocysteine is exported into the extracellular medium in proportion to the formation from AdoHcy, and extracellular L-homocysteine may be a measure of the balance between L-homocysteine formation and utilization.

Adenine↗
PubMed1986

Acute and long-term effects of high-dose methotrexate treatment on homocysteine in plasma and urine.

The amino acid, homocysteine, is not supplied by food but is a product formed by cleavage of S-adenosylhomocysteine; a product of transmethylation. Homocysteine is further salvaged to methionine. Since this reaction is in most tissues dependent on 5-methyltetrahydrofolate, we investigated the effect of the antifolate drug, methotrexate (MTX), on homocysteine in patients treated with this drug against cancer. Free and protein-bound homocysteine in plasma and urinary excretion of this amino acid were monitored in seven patients before, during, and after infusion with MTX (1-13.6 g). Each patient was investigated during one to five consecutive MTX treatments, which were separated by intervals of 1 to 4 weeks. Three components of the homocysteine response could be distinguished. An acute effect appeared after a lag period of about 6 h, lasted for about 24 h, and was characterized by a transient increase in free and protein-bound homocysteine and a concomitant increase in urinary excretion of homocysteine. Some patients showed a marked plasma response, whereas in others, enhancement of urinary excretion predominated. A long-term effect developed within 48-72 h after each infusion and was characterized by a progressive decrease in both plasma homocysteine and urinary excretion of homocysteine to amounts below those observed prior to the infusion. This effect lasted for at least 4 weeks. In this way the amount of homocysteine in plasma and urine decreased as a function of the number of MTX infusions. This long-term effect was associated with a decrease in acute homocysteine response in plasma and/or urine. Notably, MTX induced no acute or long-term effect on plasma methionine, suggesting that the homocysteine response is not caused by an imbalance in methionine metabolism due to malignant disease or chemotherapy. The cause and possible consequences of altered homocysteine metabolism during MTX therapy are discussed.

Adolescent↗
PubMed1985

The pericardial hypothesis: a mechanism of acute shifts of the left ventricular diastolic pressure-volume relation.

Changes in LV diastolic P-V relations may be caused by changes in myocardial distensibility and by changes in extraventricular constraint. Experimental studies suggest that the upward shift of the LV diastolic P-V relation associated with pacing tachycardia, in patients with angina pectoris, is due to decreased myocardial distensibility which possibly represents incomplete relaxation. However, shifts in the LV diastolic P-V relation with vasodilator and vasoconstrictor agents seem to be caused by changes in extraventricular constraint. Experimental and clinical data show that such interventions do not significantly change the LV transmural P-V relation. This supports the hypothesis that these shifts are due to changes in pericardial pressure. Our data suggest that such vasoactive agents act by shifting blood between the (splanchnic) venous compartment and the heart, thereby changing heart size and in turn pericardial pressure. These concepts have significantly improved our understanding of the mechanisms of action of vasoactive agents. It seemed a paradox that vasodilators (e.g., nitroglycerine) could substantially lower filling pressure of the failing left ventricle without reducing cardiac output. Because of the downward-shift in the P-V relation with nitroglycerine, preload is virtually unchanged and therefore stroke volume is maintained. Appreciation of these phenomena has considerable impact on how haemodynamic measurements are interpreted. It is obvious that the use of LV end-diastolic pressure as an index of end-diastolic volume may lead to serious misinterpretations of ventricular function. Our demonstration that right atrial pressure might be used to assess pericardial pressure provides a potentially useful way to estimate LV transmural pressure, and therefore an accurate measure of preload.

Animals↗
PubMed1985

Radioenzymic determination of homocysteine in plasma and urine.

Using a modification of the radioenzymic assay described previously (J Biol Chem 259: 2360-2364, 1984) we measured homocysteine in freshly prepared plasma and urine from volunteers. The concentration of free homocysteine--i.e., the amount measurable in plasma after deproteinization by strong acid--was 2.27 (SEM 0.11) mumol/L for 18 men and 1.95 (SEM 0.13) mumol/L for 16 women (p greater than 0.05, not significant). About 70% of the total homocysteine in human plasma was associated with plasma proteins, and was precipitated with strong acid. The concentration of protein-bound homocysteine in plasma was 6.51 (SEM 0.32) mumol/L for men and 7.29 (SEM 0.65) mumol/L for women, a significantly (p less than 0.01) different spread. Homocysteine was rapidly released from plasma proteins in the presence of a reducing agent, dithioerythritol. By gel filtration of plasma on a "high-performance" liquid-chromatographic column, albumin was shown to be the sole carrier of homocysteine in plasma. Because the fraction bound to protein as determined by this procedure equaled that obtained by precipitation of plasma proteins with acid, we conclude that homocysteine is bound to albumin in vivo. The concentration of homocysteine in urine ranged from 3.5 to 9.5 mumol/L, about 6 mumol of homocysteine being excreted per 24 h.

Adult↗
PubMed1985

Cardiac electrophysiologic and hemodynamic effects related to plasma levels of bupivacaine in the dog.

To investigate electrophysiologic and hemodynamic responses to various plasma levels of bupivacaine, especially those in the range normally seen during regional anesthesia, bupivacaine was given intravenously as a bolus dose followed by continuous infusion in pentobarbital-anesthetized dogs. Cardiac electrophysiology was studied by His bundle electrography, programmed electrical stimulation, and monophasic action potential recordings. At plasma bupivacaine concentrations below 1000 ng/ml, no significant electrophysiologic or hemodynamic effects were observed. This indicates that systemic responses to absorbed bupivacaine do not contribute to the cardiac electrophysiologic effects recently demonstrated during thoracic epidural analgesia. At a plasma level of about 2000 ng/ml, a level occasionally achieved during regional anesthesia, bupivacaine prolonged impulse conduction time in all parts of the heart, prolonged atrial and AV nodal refractoriness, decreased left ventricular inotropy, but had no effect on ventricular refractoriness or monophasic action potential duration. These electrophysiologic effects may enhance susceptibility to reentrant arrhythmias.

Action Potentials↗
PubMed1979

Calcium, nifedipine and arrhythmias in isolated rat atria.

Arrhythmias were induced in isolated rat atrial muscle preparations by increasing the calcium concentration of the Ringer solution, while the potassium concentration was kept low. A rise in the resting tension occurred simultaneously. The release of aspartate aminotransferase (ASAT) from the fibrillating atria was not higher than the release from non-fibrillating atria pretreated with a calcium-antagonistic drug, nifedipine 100 microgram/l. It is suggested that calcium-induced rat atrial arrhythmias in the present experiments are caused by a direct effect on calcium influx through the excitable membrane and not as a result of myocardial lesion caused by calcium overload.

Animals↗
PubMed1979

The effect of nifedipine on the monophasic action potential and refractoriness of the right ventricle of the dog heart in situ after beta-adrenergic receptor blockade.

The effect of nifedipine, a calcium-antagonistic drug, was studied on the electrophysiology of the right ventricle in the dog heart in situ. Monophasic action potential recordings were obtained by the suction electrode technique and refractoriness was measured by means of programmed electrical stimulation. Pentobarbital anaesthesia was used. As the basic cardiac effects of nifedipine can be altered by the release of catecholamines from sympathetic nerves of the heart and vessels, the dogs were pretreated with the beta-adrenergic receptor blocking agent acebutolol which increased the action potential duration and the refractoriness. Intravenous injection of nifedipine 30 microgram/kg body weight decreased the times for 50 and 90 per cent repolarization of the monophasic action potential and to a smaller extent the effective and functional refractory period. It is suggested that nifedipine decreases the action potential duration and the refractoriness of the right ventricle of the dog heart in situ due to a direct effect of the drug on the myocardium.

Acebutolol↗
PubMed1978

Comparison of the electrophysiological effects of two neuroleptics, melperone and thioridazine, on isolated rat atria.

The effects of the two neuroleptics, melperone (a butyrophenone) and thioridazine (a phenothiazine), were compared on the electrical and mechanical activity of isolated rat atria. Both electrically stimulated and spontaneously beating atria were used. Melperone was found to prolong the effective refractory period while the threshold for electrical stimulation i.e. the excitability, was almost unaffected. Thioridazine caused a similar prolongation of the effective refractory period, but also decreased the excitability significantly. In contrast to melperone, thioridazine had a negative inotropic effect. The spontaneous pacemaker activity was depressed and the sinus node recovery time increased to a greater extent after melperone than after thioridazine. The results taken together with other recent data support the hypothesis that melperone may be a type III anti-arrhythmic according to the classification of Vaughan Williams, in contrast to thioridazine which has a quinidine-like action (type I). The results also indicate that melperone in addition to prolonging the effective refractory period, may act as an anti-arrhythmic agent by depressing automaticity.

Animals↗
PubMed1978

Acebutolol-induced changes in refractoriness and monophastic action potential of the right ventricle of the dog heart in situ.

The effect of acebutolol, a beta-adrenergic receptor blocking agent, on the electrophysiology of the right ventricle was studied in the dog heart in situ. Pentobarbital anaesthesia which is known to increase the sympathetic tone was used. Monophasic action potential recordings were obtained by the suction electode technique and refractoriness was measured by means of programmed electrical stimulation. A stepwise increase in the frequency of stimulation from 170 to 200, 230, and 260 per min caused a progressive decrease in the refractoriness as well as the duration of the monophasic action potential. Intravenous injection of acebutolol 2.0 mg.kg-1 increased the times for 50 and 90% repolarisation of the monophasic action potential. This increase is probably due to beta-adrenergic receptor blockade in the presence of alpha-adrenergic receptor stimulation. The effective and functional refractory periods, however, were increased to an even greater extent than the monophasic action potential duration. It is suggested that this is the result of a blockade of a catecholamine-induced increase in the velocity of the depolarisation.

Acebutolol↗
PubMed1978

Qualitative differences between beta-adrenergic and alpha-adrenergic inotropic effects in rat heart muscle.

If beta- and alpha-adrenergic inotropic effects are cyclic AMP dependent and cyclic AMP independent, respectively, they may be qualitatively different. The inotropic effects of beta-receptor stimulation (isoprenaline) and alpha-receptor stimulation (phenylephrine combined with propranolol) were characterized in isolated perfused rat hearts, rat atria and rat papillary muscles. The beta-effect reached its maximum before the alpha-effect. The alpha-effect followed a three-phasic time-course indicating both stimulatory and inhibitory components. The aortic pressure wave (perfused heart) indicated a shorter contraction phase after beta-stimulation than after alpha-stimulation. The time to peak tension (atrium, papillary muscle) was relatively shorter after isoprenaline than after alpha-stimulation, which tended to prolong it. The contraction-relaxation cycles (atrium, papillary muscle) were examined by recording the isometric tension (T), its first (T') and second (T'') deri derivatives. alpha and beta-stimulation both increased Tmax, T'max (maximal rate of tension rise), T'min (maximal rate of tension decline) and T''min (maximal rate of transition from rise to decline of tension). Isoprenaline increased T'min (papillary muscle) and T''min (atrium, papillary muscle) relatively more than did alpha-stimulation, i.e. the relaxing processes were activated relatively more by beta-stimulation. The results indicate different mechanisms for the two adrenergic inotropic effects. The relatively larger activation of relaxation by beta-stimulation is assumed to be caused by clic AMP.

Adrenergic alpha-Agonists↗