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Biomedical subjects

T Akera

Publications and source records attributed to T Akera.

At least 73 records · Page 4Linked to original sources
PubMed1985

Ascorbic acid: an endogenous inhibitor of isolated Na+,K+-ATPase.

During attempts to isolate and identify an endogenous ligand for the glycoside binding sites on Na+,K+-ATPase, bovine adrenal glands were found to contain a potent inhibitor of isolated Na+,K+-ATPase. The inhibitory principle was extracted from adrenal cortex, following homogenization in NaHCO3 solution and separation on a Sephadex G-10 column. The active principle was recovered from a fraction which eluted from the column after the 3H2O peak. The extract inhibited isolated Na+,K+-ATPase and the specific [3H]ouabain binding reaction. Sensitivity of the enzyme to the inhibitory action of the extract was species and tissue dependent; however, the pattern and the magnitude of the sensitivity were different from those of the digitalis glycosides. Moreover, the inhibitory principle failed to inhibit sodium pump activity, estimated from ouabain inhibitable 86Rb+ uptake by guinea pig brain slices. The activity of the extract to inhibit isolated Na+,K+-ATPase was stable under acidic condition but was lost rapidly at neutral pH, and could be eliminated by EDTA. In an acidic medium, the inhibitory principle had an absorption maximum at 244 nm which shifted to 264 nm and decayed rapidly at neutral pH. By using mass spectrometry, the principle was identified to be ascorbic acid, which has been shown previously to inhibit isolated Na+,K+-ATPase under appropriate conditions. Because ascorbic acid was incapable of inhibiting the sodium pump in intact cells, this inhibitor of the isolated enzyme does not appear to be the endogenous ligand which regulates sodium pump activity in vivo.

Adrenal Glands↗
PubMed1985

Effects of K+ on the interaction between cardiac glycosides and Na,K-ATPase.

Inhibition of Na,K-ATPase by cardiac glycosides is at least partially antagonized by K+. The kinetics of the antagonism, however, appear complicated because K+ is capable of reducing both association and dissociation rate constants for the glycoside-enzyme interaction. In order to better understand the effect of K+, inhibition of partially purified Na,K-ATPase obtained from rat brain, guinea-pig heart and rat heart by ouabain, digoxin, digoxigenin, dihydrodigoxin and cassaine were compared in the presence of 1, 3 or 10 mM K+. Higher concentrations of K+ caused a parallel shift to the right in the concentration-inhibition curves for these compounds. For ouabain or digoxin, the extent of the shift was minimal with rat brain enzyme, intermediate with guinea-pig heart enzyme and more substantial with rat heart enzyme. For digoxigenin, dihydrodigoxin or cassaine, the extent of the shift was substantial in all enzyme preparations. These results could not be explained from either the affinity of the enzyme for the compound or its lipid solubility alone. The concentrations of these compounds required to cause a 50 percent inhibition of enzyme activity were markedly different with rat brain enzyme, but relatively similar with rat heart enzyme. The effects of K+, which depend on the source of the enzyme and chemical structures of the compounds, have to be considered in studies on comparative effects of various compounds on Na,K-ATPase, [3H]ouabain binding, sodium pumping and the force of myocardial contraction.

Abietanes↗
PubMed1985

Aging: effects on sodium- and potassium-activated adenosine triphosphatase activity and ouabain binding sites in rat brain.

Sodium- and potassium-activated adenosine triphosphatase (Na,K-ATPase) activity and [3H]-ouabain binding were examined in homogenates of cerebral cortex, striatum, and hypothalamus of 3-, 8-, and 26-month-old rats to determine if aging-related alterations in energy utilization demonstrated in brain slices and homogenates are potentially associated with alterations in Na,K-ATPase. There were no consistent age-related changes seen in Na,K-ATPase activity, the number of [3H]-ouabain binding sites, or their affinity for ouabain. Moreover, enzyme activities of the two molecular forms of Na,K-ATPase and their inhibition by strophanthidin did not appear to be different in partially purified enzyme preparations obtained from whole brain of 3- and 26-month-old rats. In contrast, the concentration of [3H]-ouabain binding sites was lower in cardiac muscle of senescent rats indicating a reduction in the number of active Na,K-ATPase units. It appears unlikely that aging-related central nervous system changes are associated with alterations in the Na,K-ATPase enzyme system.

Aging↗
PubMed1984

Enhanced ouabain sensitivity of the heart and myocardial sodium pump in aged rats.

Tolerance to arrhythmogenic actions of digitalis decreases with advanced age. To determine if aged myocardium has reduced glycoside tolerance, ouabain sensitivity and associated biochemical changes were examined in young (3-month), adult (8-month) and aged (26-month old) Fisher 344 rats, because age-related changes in physiology and biochemistry are well characterized in this strain of rats. Although rats are uniquely tolerant to digitalis, basic mechanisms leading to inotropic and toxic actions of digitalis are not different from other species. In aged anesthetized rats, intravenous infusion of ouabain produced arrhythmias and cardiac arrest at lower doses. Aged myocardium had lower Na,K-ATPase activity and fewer high affinity [3H]ouabain binding sites. Affinity of the ouabain binding sites on Na,K-ATPase was not altered. Despite these findings, sodium pump activity, estimated from the ouabain-sensitive 86Rb+ uptake by ventricular muscle slices, was higher in the aged myocardium when the uptake was observed in the absence of ouabain and was more sensitive to inhibitory action of the glycoside. Differences in Na,K-ATPase and sodium pump data may be explained if sodium leak influx is increased in aged myocardium. Fewer sodium pumping sites and enhanced ouabain sensitivity of the sodium pump seem to be responsible for the reduced tolerance of aged heart to digitalis-induced toxicity.

Aging↗
PubMed1984

Effects of myocardial hypoxia on digitalis-induced toxicity in the isolated heart of guinea pigs and cats.

The effects of hypoxia on the tolerance of myocardium to the toxic actions of digitalis were studied in isolated heart muscle preparations. Perfusion of Langendorff preparations of guinea-pig heart with a hypoxic solution failed to alter Na, K-ATPase with respect to its activity, its sensitivity to inhibition by dihydrodigoxin, or the number of [3H]ouabain binding sites and their affinity for ouabain, when these activities were estimated in ventricular muscle homogenates obtained after the hypoxic perfusion. Hypoxia potentiated the development of digoxin-induced contracture, as well as digoxin-induced changes in maximal upstroke velocity and the amplitude of action potentials, and the resting transmembrane potential in atrial and ventricular muscle preparations of guinea-pig heart. Under hypoxic conditions, however, digoxin failed to induce arrhythmias. In Purkinje fibers of cat heart, the time to onset of digoxin-induced arrhythmias, or the development of afterdepolarizations, was not affected by hypoxia. These results indicate that hypoxia alters the nature of toxic effects of digitalis in isolated heart muscle preparations, the development of contracture instead of arrhythmias being the primary toxicity in hypoxic muscle.

Animals↗
PubMed1984

The inotropic effects of a synthetic pyrethroid decamethrin on isolated guinea pig atrial muscle.

Pyrethroid insecticides have been reported to increase transmembrane sodium influx and inhibit ion-dependent ATPases in insect, squid, and toad nerve tissues. Since changes in sarcolemmal ion fluxes and inhibition of membrane-bound ATPases can alter myocardial contractility, the effects of a potent synthetic pyrethroid, decamethrin, on mammalian myocardium were examined using isolated left atrial muscle of the guinea-pig heart electrically stimulated at 1.5 Hz. Decamethrin (0.1-10 microM) increased the force of isometric contraction in a dose-dependent manner without affecting the resting tension. Propranolol (5 microM) reduced the magnitude of the pyrethroid's inotropic effect; however, a significant inotropic effect was still observed with 1 and 10 microM decamethrin in the presence of this concentration of propranolol. Similar results were noted in the presence of 500 microM procainamide and in left atrial muscle obtained from reserpine-treated animals. The results suggest that decamethrin increased the force of myocardial contraction by two mechanisms; indirectly by releasing catecholamines from sympathetic nerve terminals and directly by an action on the myocardium. Inotropic concentrations of the pyrethroid did not inhibit partially purified rat brain or guinea pig heart Na+, K+-ATPase. Moreover, twitch tension recordings of isometric contractions showed that, in the presence of propranolol, decamethrin increased tension development without changing either the time to peak tension or the duration of contraction. Tetrodotoxin almost completely abolished the positive inotropic effect of decamethrin. Thus, the positive inotropic effect of decamethrin is apparently due to an increase in transmembrane sodium influx which causes catecholamine release from the sympathetic nerve terminals and also directly enhances muscle contraction.

Animals↗
PubMed1984

Reduced tolerance to digitalis-induced arrhythmias caused by coronary-flow alterations in isolated perfused heart of guinea pigs.

The ligation of a main branch of coronary vein elevates the sensitivity of isolated, perfused (Langendorff) preparations of guinea-pig heart to arrhythmogenic actions of digoxin. Retrograde perfusion studies indicate that the presence of an area with a high extracellular K+ concentration, adjacent to an area with normal K+ concentration and exposed to the glycoside, predisposes the heart to digitalis-induced toxicity. This model may be useful in studying the mechanism by which acute myocardial infarction decreases the therapeutic index of cardiac glycosides.

Animals↗
PubMed1984

Positive inotropic and toxic effects of brevetoxin-B on rat and guinea pig heart.

Brevetoxin-B (GbTX-B), a cyclic polyether purified from the marine dinoflagellate Gymnodinium breve, produced positive inotropic and arrhythmogenic effects on isolated rat and guinea pig cardiac preparations at concentrations between 1.25 X 10(-8) and 1.87 X 10(-7) M. The toxin (10(-7) M) transiently increased left ventricular +dP/dt, hydraulic work, and oxygen consumption of paced working rat hearts, then reduced these variables during continuous exposure. Brevetoxin-B exerted a much smaller positive inotropic effect on working guinea pig hearts, but produced a marked and sustained inotropic effect on guinea pig left atria. The toxin also produced arrhythmias in rat and guinea pig hearts, characterized by ventricular tachycardia and A-V blockade. Sympatholytic procedures (beta blockade or reserpine pretreatment) partially blocked the positive inotropic effects, and eliminated the ventricular tachycardia, but not the A-V blockade. Tetrodotoxin markedly inhibited the positive inotropic effect of GbTX-B. Brevetoxin-B did not inhibit guinea pig cardiac Na,K-ATPase activities. The results show that GbTX-B is a potent cardiotoxin and suggest that GbTX-B exerts positive inotropic and arrhythmogenic effects by increasing sarcolemmal sodium permeability, and by releasing catecholamines from sympathetic nerve endings.

Animals↗
PubMed1984

The dual effect of sodium ion on the digitalis-sodium pump interaction.

Digitalis glycosides are specific inhibitors of the sodium pump. While their effects on isolated Na,K-ATPase, an enzymatic representation of the sodium pump, can be quantified easily and precisely, estimation of their effects on the sodium pump and evaluation of the physiological significance of sodium pump inhibition are complicated by several factors. In isolated Na,K-ATPase, the specific binding of cardiac glycosides observed in the presence of Mg2+ and ATP is stimulated by Na+. In intact myocardial cells, conditions which enhance Na+ influx, and hence the amount of Na+ to be transported by the sodium pump, such as stimulation at high frequencies, presence of monensin (a sodium ionophore), batrachotoxin or grayanotoxin I, enhance the glycoside binding to the sodium pump. In left atrial muscle preparations isolated from the guinea-pig heart and stimulated at 0.5 Hz, binding of ouabain to glycoside binding sites on Na,K-ATPase was eliminated by lowering the extracellular Na+ concentration from 145 to 27 mM, a condition reported to lower the intracellular Na+ concentration by more than 60%. When ouabain exposure of atrial muscle preparations was restricted to the quiescent period, glycoside binding to the sodium pump was minimal. Monensin, however, caused ouabain to bind to the sodium pump in quiescent preparations. These results indicate that glycoside binding to the sodium pump is enhanced by intracellular Na+. In addition to enhancing the glycoside binding to the sodium pump, an elevation of intracellular Na+ reduces the reserve capacity of the sodium pump and therefore increases the sensitivity of the myocardium to sodium pump inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1984

Role of sympathetic nervous system in ischemia-induced reduction of digoxin tolerance in anesthetized cats.

Acute myocardial ischemia reduces tolerance of the heart to arrhythmogenic actions of digitalis glycosides. Because both ischemia and the glycoside produce profound changes in activity of the autonomic nervous system and because sympathetic discharge or catecholamines enhance toxic actions of the cardiac glycosides, the possibility that alterations in digitalis sensitivity of ischemic heart involve changes in sympathetic nerve activity was examined using alpha-chloralose-anesthetized cats. Left anterior descending coronary artery (LAD) was completely occluded by ligation and, 40 min later, a slow i.v. infusion of digoxin was started at a rate of 1 microgram/kg/min. LAD ligation alone did not produce arrhythmias in that condition, but shortened the time to onset of digoxin-induced arrhythmias and thereby reduced the amount of digoxin required to produce the toxic manifestation. Concomitantly, digoxin concentration in plasma and nonischemic areas of the heart were lower in LAD-ligated cats at the onset of arrhythmias than those in sham-operated cats. Myocardial digoxin content in the ischemic area of the LAD-occluded heart was lower than that in nonischemic areas of the same heart. At the onset of digoxin-induced arrhythmias, Na,K-adenosine triphosphatase activity of ischemic myocardium was significantly higher than that in the nonischemic area, reflecting a lower digoxin occupancy of the glycoside binding sites on the sodium pump. Spinal cord (C1) transection or propranolol treatment prolonged the time to arrhythmias in both control and LAD-ligated cats, but failed to abolish the effect of LAD ligation to augment digoxin toxicity. Bilateral vagotomy also did not alter the enhancement of digoxin toxicity caused by ligation of LAD.(ABSTRACT TRUNCATED AT 250 WORDS)

Anesthesia↗
PubMed1983

How increased sodium influx enhances digoxin-induced arrhythmias in guinea-pig atrial muscle.

The hypothesis that digitalis-induced arrhythmias occur when Na,K-ATPase inhibition exhausts the sodium pump reserve capacity, producing an accumulation of intracellular Na+, was tested by reducing the reserve capacity in isolated left atrial muscle of guinea pig heart and estimating specific digoxin binding and Na,K-ATPase activity in atrial muscle homogenized at the onset of digoxin-induced arrhythmias. Reductions in reserve capacity were produced by either increasing the stimulation frequency of the atrial muscle or adding a sodium ionophore, monensin, to the media bathing the tissue. As stimulation frequency was increased, both the time required to produce arrhythmias with a given concentration of digoxin and the amount of digoxin bound to sarcolemmal Na,K-ATPase at the onset of arrhythmias were reduced. Similarly, monensin treatment produced reductions in the time to arrhythmia and in digoxin binding and Na,K-ATPase inhibition observed at the onset of arrhythmias. These results support the above proposal suggesting that a decrease in reserve capacity of the sodium pump enhances cardiac sensitivity to digitalis-induced arrhythmias.

Animals↗
PubMed1983

Effects of progesterone derivatives on sodium pump activity and force of myocardial contraction in isolated guinea pig heart.

A progesterone derivative, chlormadinone acetate, has been reported to inhibit isolated Na,K-ATPase, to increase intracellular sodium, to decrease the force of myocardial contraction and to antagonize the positive inotropic action of ouabain. Because several inhibitors of isolated Na,K-ATPase fail to interact with the sodium pump in intact cells, two progesterone derivatives, chlormadinone acetate and megestrol acetate were examined for their effects on Na,K-ATPase, sodium pump activity and the force of contraction in isolated guinea pig atria. Both megestrol acetate and chlormadinone acetate inhibited specific (ATP-dependent) 3H-ouabain binding to Na,K-ATPase isolated from guinea pig brain or heart, with a corresponding inhibition of enzyme activity. The effects on Na,K-ATPase obtained from brain were more pronounced than that obtained from heart. The inhibitory effect of megestrol acetate was partially antagonized by K+. In electrically stimulated left atrial muscle preparations, neither compound affected ouabain-sensitive 42K+ uptake. Additionally, these compounds failed to affect the force of myocardial contraction or to influence the positive inotropic action of ouabain. These data fail to support the contention that inhibition of the sarcolemmal sodium pump is unrelated to the ouabain-induced increase in the force of myocardial contraction.

Animals↗
PubMed1983

Ischemia-induced enhancement of digitalis sensitivity in isolated guinea-pig heart.

Digitalis sensitivity of the heart is increased in patients with ischemic heart disease. Whether this elevation of digitalis sensitivity occurs as the result of ischemia-induced changes in the cardiac tissue and whether changes in the sarcolemmal Na,K-adenosine triphosphatase (ATPase) or reserve capacity of the sodium pump are responsible for the increased digitalis sensitivity were examined using isolated heart preparations obtained from guinea pigs. Ligation of the left anterior descending coronary artery (LAD) in Langendorff preparations 40 min before perfusion with a toxic concentration (either 1.8 or 2.5 microM) of digoxin decreased the time to the onset of arrhythmias. LAD-ligation by itself did not cause arrhythmias. The time to the onset of arrhythmias during digoxin perfusion was slightly longer in preparations obtained from reserpine-treated animals; however, the reserpine pretreatment failed to alter the effect of LAD ligation on digitalis sensitivity, indicating that the release of catecholamines is not involved in the sensitization. The effects of ischemia on Na,K-ATPase and sodium pump activities, glycoside binding to the enzyme and reserve capacity of the sodium pump were examined in globally ischemic Langendorff preparations. The preparations were perfused with a Krebs-Henseleit bicarbonate buffer solution (pH 7.4) saturated with a 95% O2-5% CO2 gas mixture at a control flow rate of 2.5 ml/g of tissue per min or at 5 or 0% of the control flow rate. After 6 hr of zero perfusion, Na,K-ATPase activity and the number of specific ouabain binding sites were reduced in ventricular muscle homogenates. However, the remaining Na,K-ATPase was not altered in its sensitivity to dihydrodigoxin-induced inhibition or affinity of binding sites for ouabain, sodium or potassium. Similar results were observed after reperfusion following 2 or 5 hr of zero perfusion. A 5% perfusion for 2 or 6 hr, or zero perfusion for 2 hr failed to affect Na,K-ATPase activity in muscle homogenates. Sodium pump activity in ventricular slices, estimated from the ouabain-sensitive 86Rb+ uptake, was unchanged after 5% perfusion or zero perfusion for 2 hr, but was significantly reduced after a 20-min reperfusion following 2 hr of zero perfusion. Reserve capacity of the sodium pump, as estimated from the differences in 42K+ uptake by right ventricular strips under 1.5 and 7 Hz stimulation, was unaffected by 2 hr of 5% perfusion. These results indicate that coronary artery occlusion enhances the arrhythmogenic action of digitalis in isolated heart preparations. This change in digitalis sensitivity produced by 40 min of occlusion cannot be explained by reductions in Na,K-ATPase activity or sodium pump reserve capacity as 2 hr of zero perfusion does not alter Na,K-ATPase or sodium pump activity in ventricular tissue.

Animals↗
PubMed1983

Cardiac actions of vanadium.

Vanadate produces positive or negative inotropic effects in isolated cardiac muscle preparations, depending on species, type of the muscle, and experimental conditions. In intact animals, vanadate decreases the force of left ventricular contraction, presumably because of a coronary vasoconstriction, and decreases the heart rate. The blood pressure may be elevated or lowered by vanadate. Although vanadate is a potent inhibitor of isolated Na,K-ATPase, the above actions of vanadate are not associated with sodium pump inhibition, because vanadate is incapable of inhibiting the sodium pump in intact myocardial or vascular smooth muscle cells. In addition to inhibiting isolated Na,K-ATPase, vanadate is capable of affecting many, but not all, biochemical reactions involving formation of phosphorylated intermediates. Whether these effects occur in intact cells, however, should be carefully evaluated. In cardiac muscle preparations, vanadate seems to affect a superficial Ca2+ pool; its augmentation and inhibition corresponding to the positive and negative inotropic effects, respectively. The mechanism for the actions of vanadate on such a superficial Ca2+ pool, however, is presently unknown.

Animals↗
PubMed1983

Negative chronotropic and positive inotropic actions of phencyclidine on isolated atrial muscle in guinea pigs and rats.

Chronotropic and inotropic actions of phencyclidine were studied in spontaneously beating right atrial muscle and electrically paced left atrial muscle preparations isolated from guinea-pig or rat hearts. In right atrial muscle preparations, phencyclidine (10-100 microM) decreased the frequency of spontaneous beating. Guinea-pig and rat heart preparations had similar sensitivities to this action of phencyclidine. The negative chronotropic effect was not altered by atropine. A high concentration of naloxone failed to affect the chronotropic effect of phencyclidine in guinea-pig muscle, but significantly reduced the effect in rat heart muscle preparations. Phencyclidine (1-100 microM) caused positive inotropic effects in both guinea-pig and rat heart left atrial muscle electrically stimulated at 1.5 Hz; rat heart preparations had a higher sensitivity to the positive inotropic action of phencyclidine. The positive inotropic effect was reduced by verapamil, nifedipine and relatively high concentrations of diltiazem, but was not affected by propranolol, phentolamine, tripelennamine, atropine or ryanodine, indicating that the effect is not mediated by adrenergic, histaminergic or cholinergic systems or does not involve ryanodine-sensitive calcium pools. Inactivation of the fast sodium channels by partial membrane depolarization, and subsequent restoration of the contraction by raising the extracellular Ca++ concentration, did not abolish the positive inotropic action of phencyclidine. These results suggest that the negative chronotropic effect of phencyclidine is not mediated by a stimulation of the muscarinic receptor. The positive inotropic effects of phencyclidine seem to result from an increase in Ca++ influx through the slow channels of the cardiac cell membrane.

Action Potentials↗