Biomedical subjects

H Kammermeier

Publications and source records attributed to H Kammermeier.

At least 37 records · Page 2Linked to original sources

PubMed1995

Hypoxia-induced activation of KATP channels limits energy depletion in the guinea pig heart.

The functional role of ATP-dependent potassium (KATP) in hypoxic cardiac failure was investigated in isolated guinea pig hearts with glibenclamide and rimalkalim as inhibitor and activator, respectively. Monophasic action potential duration at 90% of repolarization (MAP50), left ventricular function, and cardiac energy status (31P nuclear magnetic resonance spectroscopy) were measured during normotoxic (95% O2) and hypoxic (20% O2) perfusion. In normoxic hearts, 1 microM glibenclamide did not affect MAP50, left ventricular function, and coronary flow (n = 4). In contrast, rimalkalim rapidly shortened MAP50 and left ventricular pressure (LVP) in a dose-dependent fashion (e.g., by 60.2 +/- 3.5 and 80.8 +/- 8.2%, respectively, with 0.6 microM rimalkalim). This latter effect was reversed by 1 microM (glibenclamide (n = 4). With hypoxic perfusion, a reduction in LVP was observed, along with a shortening of the action potential (MAP90; 202 +/- 13 vs. 164 +/- 9 ms) and an increase in coronary flow. Glibenclamide (1 microM) reversed the MAP90 shortening and the increase in coronary flow. In addition, glibenclamide increased LVP transiently (n = 4). When coronary flow of hypoxic hearts was kept constant, however, glibenclamide elicited a sustained positive inotropic effect (n = 7). After glibenclamide, an increase in LVP from 54 +/- 4 to 64 +/- 3 mmHg was observed, along with a reduction in the free energy change of ATP hydrolysis from -54.5 +/- 1.9 to -52.9 +/- 0.2 nJ/mol and a further increase in the coronary venous adenosine from 269 +/- 48 to 1,680 +/- 670 nmol/l.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗

PubMed1995

Action of metformin on glucose transport and glucose transporter GLUT1 and GLUT4 in heart muscle cells from healthy and diabetic rats.

The effects of the antidiabetic drug metformin on glucose transport were investigated in freshly isolated heart muscle cells from healthy and streptozotocin-diabetic rats. In vivo treatment of diabetic rats with metformin failed to affect the basal and insulin-stimulated rate of glucose transport measured in isolated cells. In vitro exposure to therapeutic concentrations (< or = 10(-4) M) of metformin did not influence glucose transport, even upon incubation times up to 5 h or in the presence of high glucose (20 nM). In contrast, higher metformin concentrations produced an 8- to 12-fold increase in glucose uptake (with a lag of 90 min, and a maximum at 180 min and approximately 5 mM). In the presence of submaximal insulin concentrations (< or = 3.10(-10) M), the effects of metformin (5 mM) and of insulin were more than additive, whereas, at saturating insulin concentrations (10(-8) M), partial additivity was observed. Like insulin, metformin caused an approximately 1.6-fold increase in the content of both glucose transporter isoforms GLUT1 and GLUT4 in the plasma membrane of cardiac myocytes, with a corresponding decrease in an intracellular membrane fraction. cAMP-elevating treatments depressed the metformin-, but not the insulin-dependent glucose uptake, by 20-30%. In myocytes from diabetic rats, the rate of metformin-activated glucose transport was similar to that of cells from control animals, whereas basal and insulin-stimulated transport were substantially diminished. Finally, metformin (5 mM) induced a slight depression of oxygen consumption and energy metabolism of myocytes (as determined by measuring their level of energy-rich phosphates) comparable to the effects of hypoxia in rat hearts. In conclusion, these data do not provide evidence in favor of the hypothesis that glucose uptake by muscle tissue represents the site of metformin's therapeutic action in vivo. On the other hand, the large, insulin-independent effect of metformin at high concentrations (approximately mM) in vitro may be related to the action of hypoxia and occurs through a redistribution of glucose carriers from an intracellular locus to the plasma membrane. The mechanism (or signal) involved in metformin's action is likely to differ from that triggered by insulin and is not impaired in the diabetic state.

Animals↗

PubMed1995

Effect of a hawthorn extract on contraction and energy turnover of isolated rat cardiomyocytes.

The hawthorn extract LI 132 (crataegus), prepared from leaves and flowers, and standardised to 2.2% flavonoids, was investigated with respect to its effect on (1) the contraction, (2) the energy-turnover and (3) the apparent refractory period (t(ref)) of isolated cardiac myocytes from adult rats. (1) The contractile behaviour of attached myocytes was analyzed by an image processing system. (2) The energy turnover was calculated from the decrease in oxygen content in the myocyte suspension, brought about by cellular respiration. It was differentiated between energy turnover related to cell shortening and that required for ionic transport processes by application of the contraction-inhibiting agent 2,3-butanedione monoxime. (3) The apparent refractory period (t(ref)) was evaluated by pacing the myocytes with increasing stimulation rates and determining the frequency at which failure of single contractions occurred. For these purposes, the myocytes were incubated in a stimulation chamber, which is part of a computer-assisted system allowing to simultaneously evaluate the mechanics and energetics of electrically induced contraction. Within a range of 30-180 microg/ml, the hawthorn extract exhibited a positive inotropic effect on the contraction amplitude accompanied by a moderate increase of energy turnover both for mechanical and ionic processes. In comparison with other positive inotropic interventions, such as application of the beta-adrenergic agonist isoprenaline, or of the cardiac glycoside ouabain (g-strophantin), or elevation of the extracellular Ca++-concentration, the effects of the hawthorn extract were significantly more economical with respect to the energetics of the myocytes. Furthermore the extract prolonged the apparent refractory period in the presence and the absence of isoprenaline, which be indicative for an antiarrhythmic potential.

Adrenergic beta-Agonists↗

PubMed1994

Long-chain fatty acid-binding to albumin: re-evaluation with directly measured concentrations.

In studies on uptake of fatty acids (FA) into organs, the unbound (or free) fatty acid fraction is commonly calculated from the concentration bound to albumin and from published binding constants. However, there is some dispute on the methods used for determining those binding constants. We developed a method allowing direct measurement of unbound FA by extending the previous studies of Svenson et al. [1] and Reed et al. [2]. Albumin was coupled to a solid phase (Sepharose 4B), loaded with FA and equilibrated with an aqueous solution. Laurate, palmitate and oleate concentrations in the aqueous phase were determined at different molar ratios of FA to albumin (r) and at different temperatures. FA albumin-binding constants (Ki) increase with chain length and decrease with temperature, in accordance with data obtained by others. However, the unbound concentrations measured are markedly lower than those obtained from binding constants, and the resulting Ki values markedly higher. This difference is presumed to result from (1) our direct measurement of unbound FA and (2) utilizing different more physiological conditions. Recalculating kinetic parameters from published FA uptake data, we found considerably different Km and Vmax values compared to the original data. Thus, the FA-binding characteristics measured in this study may influence the interpretation of FA uptake substantially.

Albumins↗

PubMed1994

Enzyme release into the interstitial space of the isolated rat heart induced by changes in contractile performance.

OBJECTIVE: The aim was to investigate changes in interstitial concentration and release of creatine kinase in isolated perfused rat hearts after an experimental inotropic stimulation or after recovery from a negative inotropic intervention (low Ca2+ or high K+ buffer). METHODS: Interstitial transudate emerging at the surface of the heart and venous effluent were analysed for creatine kinase. RESULTS: The interstitial concentration of creatine kinase was always higher, by a factor of 25 to 100 (range from 10 to 580 mU.ml-1), than the concentration in the venous effluent (close to or below the limit of detection: 0.4 mU.ml-1). Continuous stimulation with a submaximal effective concentration (8 nM) of isoprenaline for 30 min resulted in an initial transient increase in the interstitial release of creatine kinase to about 160% of the control (p < 0.05). Similarly, in a second series, three repeated (5 min) periods of inotropic stimulation also caused a significant and transient increase in the interstitial release of creatine kinase to a maximum of 180%. Change to a buffer containing 2.0 mM Ca2+ after a 60 min period of low Ca2+ perfusion (0.25 mM) led to restoration of contractile function and an immediate and transient increase in the interstitial release of creatine kinase to about 900%. After a period of cardiac arrest (16 mM K+ for 60 min), perfusion with 4 mM K+ also immediately restored cardiac function, and led to an increase in creatine kinase release of 2500%. In additional experiments dilatation of the left ventricle by inflating an intraventricular balloon during cardioplegic perfusion induced a significant fivefold increase in the interstitial release of creatine kinase, which was further enhanced 3.5-fold during the subsequent recovery period. CONCLUSIONS: The coincidence of an increase in or a restoration of cardiac contractile function and an increase in the interstitial enzyme release suggests that myocardial enzyme release may occur in response to physiological stimuli during different episodes of metabolic or mechanical stress, as well as under pathophysiological conditions.

Animals↗

PubMed1993

Phenylarsine oxide and hydrogen peroxide stimulate glucose transport via different pathways in isolated cardiac myocytes.

The aim of this study was to investigate the stimulating effects of sulfhydryl reagents on glucose transport in isolated rat heart muscle cells and to compare them with the action of insulin. Low concentrations of the sulfhydryl oxidants hydrogen peroxide (H2O2) and diamide (5-100 microM), but also of phenylarsine oxide (PAO) (0.5-3 microM), that is known to specifically react with vicinal SH-groups, stimulated the rate of 2-deoxy-D-glucose uptake by a factor of 4 to 8 in these cells, while higher concentrations were inhibitory. The stimulating effects of H2O2 or diamide, and, to a significantly lesser extent, those of PAO or insulin, were depressed in cells pretreated with the sulfhydryl-alkylating agent N-ethylmaleimide (56-100 microM). H2O2 raised the Vmax and lowered the Km of 3-O-methyl-D-glucose uptake, while PAO or insulin solely increased Vmax. The increase in glucose transport caused by H2O2 was antagonized by the beta-adrenergic agonist isoprenaline (1 microM) or by a membrane-permeant cyclic AMP analog, whereas the effects of PAO or insulin were not altered. The action of H2O2 was additive with the stimulation induced by the protein phosphatase inhibitors okadaic acid (1 microM) or vanadate (6 mM), whereas the responses to PAO or insulin were reduced in the presence of these agents. Finally, H2O2 and PAO, but not insulin, acted additively with the protein kinase C ligand phorbol myristate acetate (0.8 microM) and with phospholipase C (0.03 units/ml). We conclude that, in cardiac myocytes, H2O2, on the one hand, and PAO (and possibly insulin), on the other hand, stimulate glucose transport via at least two distinct, SH-dependent pathways. These pathways, in turn, differ from a protein kinase C- and from a phospholipase C-mediated mechanism.

3-O-Methylglucose↗

PubMed1993

Fatty acid transfer across the myocardial capillary wall.

Interstitial transudate of isolated isometrically working perfused rat hearts was analyzed to investigate fatty acid (FA) release and transfer across the capillary wall. Unsaturated FA were released under certain conditions. Lowering medium FA (16:0) with constant or varying FA/A-ratio (A: albumin) decreased interstitial FA-concentration down to 17% of arterial FA-concentration. This was accompanied by reduced uptake-rate. Transcapillary diffusion-resistance, unless markedly altering, cannot be responsible for this observation. Calculated diffusion-rates of FA and FA* protein-complexes in the endothelial cytoplasm and interstitium indicate that FA-transfer takes place almost exclusively bound to carrier-proteins. The apparent permeability surface-area-values of FA for transendothelial FA-transfer are three orders of magnitude higher than those for sucrose; diffusion-coefficients for FA across the endothelium are close to those in water, excluding substantial diffusion-barrier of the capillary wall. Dissociation-rate-constants calculated from experimental data are in the range of those reported for FA* albumin-complexes in vitro. Thus the observed transcapillary FA-gradients are apparently due to the limited dissociation-rate of the FA*albumin-complex in the intravascular space.

Animals↗

PubMed1993

Interstitial lactate and glucose concentrations of the isolated perfused rat heart before, during and after anoxia.

In isolated rat hearts perfused according to the Langendorff technique lactate and glucose concentrations were determined in the interstitial transudate and the venous effluent before, during and after periods of 15 or 30 min anoxia. The interstitial transudate emerged at the surface of the heart as a result of albumin-free perfusion. During normoxic perfusion the interstitial lactate concentration was 0.144 +/- 0.025 mmol/l (n = 6); the venous lactate concentration was 0.033 +/- 0.005 mmol/l. From the interstitial and the mean vascular concentration, together with the lactate release and the glucose uptake, the apparent permeability surface area products (P.S product) were calculated using Fick's law. The apparent P.S products for lactate and glucose were 4.6 and 3.9 ml/(min x g), respectively. During anoxia we measured a four- to sixfold increase of the interstitial lactate concentration. At the end of the anoxic periods the apparent P.S product was two- to threefold higher than during normoxia; the apparent glucose P.S product increased about fourfold. After 15 min anoxia the increases of permeability were completely reversed in the reoxygenation period. However, after a period of 30 min anoxia the apparent P.S products for lactate and glucose remained raised, which means that there was a prolonged or even irreversible increase of capillary permeability. Besides the marked transcapillary concentration difference for lactate, these data show prolonged functional alteration of the capillary wall after 30 min anoxia.

Animals↗

PubMed1993

Efficiency of energy conversion from metabolic substrates to ATP and mechanical and chemiosmotic energy.

The free energy available from substrate oxidation is a largely invariable figure in biological systems. The extent to which this energy is conserved in high-energy intermediates appears to be optimized during evolution to high efficiency. Since energy transformation by enzymatic and transport processes take place largely with fixed stoichiometrics, high efficiency can only be achieved by adapting free energy levels rather than by adapting turnover (number of molecules synthesized/hydrolyzed, etc.) The relatively small steps (equivalent with high efficiency) in free energy of various metabolic steps implicitly mean that changes in free energy leading to abolition of these steps (e.g., reduction of free energy of ATP) abolish the driving force and interrupt the (net) reaction. However, in the myocardium under limited energy supply (hypoxia) various protective mechanisms appear to be involved which keep free energy levels high for surviving of the cell but for the cost of function (contraction). Those mechanisms might play a role in the phenomena of hibernating and stunning.

Adenosine Triphosphate↗

PubMed1992

Alanine and hyperosmolarity are responsible for the stimulation of cardiomyocyte glucose transport by samples containing a glucose tolerance factor.

Low-molecular-weight, cationic samples, that were previously reported to contain a glucose tolerance factor, were obtained by partial purification from yeast extract. These samples increased the rate of glucose transport in isolated cardiomyocytes 2.0- to 2.5-fold. A further purification by gel filtration led to the separation of two active components that were identified as (i) L-alanine and (ii) an elevated osmolarity. Moreover, the effect of partially purified fractions (before gel filtration) (i) was decreased upon alanine depletion with alanine dehydrogenase and (ii) was mimicked by the additive action of alanine and of a hyperosmolar medium. These findings indicate that the effect of this partially purified material is not accounted for by a putative glucose tolerance factor. Interestingly, alanine elicited its effect at concentrations that correspond to physiological plasma values, which suggests that this amino acid might be involved in the regulation of glucose transport in cardiomyocytes. Furthermore, the effect of alanine was prevented by DL-cycloserine (1 mM) or aminooxyacetate (1 mM), but not by cycloheximide (35 microM), indicating that (a) transamination reaction(s), but not protein synthesis, is required.

Adipose Tissue↗

PubMed1992

Negative inotropic effects of the new class I antiarrhythmic agents berlafenone and alprafenone on electrically stimulated isolated cardiomyocytes.

Isolated cardiac myocytes from adult rats were used in a stimulation chamber to investigate the negative inotropic effects of propafenone and the new compounds berlafenone (1-2'-biphenyloxy)-3-tert-butylamino-propanol-2-hydrochloride, GK 23 G; CAS 18965-97-4) and alprafenone (1-(4-methylphenyl)-3-[3'-(2-hydroxy-3-tert-pentylaminopropoxy)-4'- methoxyphenyl]-1-propanonhydrochloride, AH 141; CAS 124316-02-5). This chamber is part of a new device that allows the simultaneous evaluation of mechanics and of the energetics of electrically induced contractions of the myocytes. 1. The contractile behaviour of attaches myocytes was analysed by an image processing system using digitized frames of a CCD camera. 2. The metabolic demand for excitation-contraction coupling was calculated from the drop in oxygen tension (registered by a Clark electrode) caused by suspended myocytes when stimulated in the presence of the contraction inhibiting agent 2,3-butanedione monoxime in the stimulation chamber. 3. The apparent refractory period was evaluated by pacing the myocytes with increasing stimulation rates and determining the frequency at which failure of single contractions occurred. All 3 agents produced a reduction in contraction amplitude of the electrically stimulated myocytes with a similar dose-response relationship (IC50 approx. 10 mumol/l). 4 mumol/l berlafenone reduced the contraction amplitude to 62% of control. Under these conditions the energy expenditure of the contracting cells for excitation and excitation-contraction coupling (ion-cycling) was also reduced (77 +/- 17% of control). Since most of this energy is used for Ca2+ (greater than 80%) it may be concluded that a reduced Ca2+ release causes the negative inotropic action of berlafenone.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Uptake kinetics of technetium-99m-methoxyisobutylisonitrile and thallium-201 in adult rat heart endothelial and fibroblast-like cells in comparison to myocytes.

Endothelial and fibroblast-like cells comprise 5%-10% of total cardiac cell volume and are more resistant to hypoxia than cardiomyocytes. However, their role in the uptake of radiotracers used for myocardial perfusion imaging has largely been ignored. Net uptake of 201TI and 99mTc-hexakis-2-methoxyisobutylisonitrile (99mTc-sestamibi) by cultured rat heart endothelial and fibroblast-like cells and quiescent myocytes was examined. Over a 30-min period, endothelial cells continuously accumulated 99mTc-sestamibi. Initial 201TI accumulation paralleled that of 99mTc-sestamibi; however, after 7 min 201TI accumulation plateaued whereas there was continued slow accumulation of 99mTc-sestamibi. Net uptake of the two radiotracers by fibroblast-like cells was similar to that of 201TI by endothelial cells. In the quiescent myocytes, there was an initial accumulation of both tracers with 201TI net uptake quickly reaching a plateau, whereas 99mTc-sestamibi accumulation continued throughout the 30-min period reaching a level 15-fold greater than that for 201TI. Myocyte uptake of 99mTc-sestamibi was approximately eight times greater than by the endothelial cells when expressed either as activity per mg protein or activity per cell volume. Although significant uptake differences exist among the three cell types, the fact that there is uptake by endothelial and fibroblast-like cells should be considered during image interpretation, particularly in situations in which the hypoxia resistant endothelial and fibroblast-like cells might be the remaining healthiest cell types.

Animals↗

PubMed1991

Highly insulin-responsive isolated rat heart muscle cells yielded by a modified isolation method.

Freshly isolated adipocytes or cardiac myocytes appear to be subject to unspecific stimulation during isolation and subsequent handling, e.g. with respect to glucose transport. We have developed a modified procedure that yields rat cardiomyocytes with a very low basal, i.e. non stimulated hexose uptake rate (ca. 3 pmol * s-1 * mg protein-1 at 1 mM sugar), as compared to data reported by others. This low value correlates with the reported oxygen consumption of non-beating, isolated rat hearts, when these are perfused with glucose as the only substrate. The basal rate of glucose uptake in our quiescent cardiomyocytes is slightly lower than the value measured by others in beating rat hearts in vivo. Insulin (10 nM) stimulates 2-deoxy-D-glucose uptake 8- to 20-fold and 3-O-methyl-D-glucose uptake 14- to 20-fold, as compared to control. This insulin effect is markedly larger than that usually observed in isolated cardiomyocytes, but it is similar in magnitude to the stimulation of glucose transport reported for isolated, perfused rat hearts. In these cells, new stimulatory effects on the glucose transport, e.g. that of sulfhydryl reagents like phenylarsine oxide, become apparent. We conclude that the cardiomyocytes obtained by this modified method exhibit a basal glucose transport rate that is close to physiological values. These cells represent a new highly responsive model to detect and to investigate the effects of glucose transport stimulators (insulin, contraction etc.).

3-O-Methylglucose↗

PubMed1991

Simultaneous measurement of contraction and oxygen consumption in cardiac myocytes.

A setup has been developed that simultaneously measures the mechanics and the energetics of electrically induced contractions at physiological frequencies of isolated cardiac myocytes. The core of the setup is a self-manufactured stimulation chamber in which most of the myocytes are in suspension while some are attached to a plastic cover slip prepared from culture Petri dishes. The analysis of the contractile behavior of the attached myocytes is based on an image-processing system with digitized frames of a charge-coupled device camera. Thirty-six frames illuminated by a stroboscope are taken at increasing time intervals between stimulus and flash (snap), allowing one to resolve the contraction cycle with a very high time resolution (down to 1 ms). The number of pixels that differ between each of these frames and a "reference" frame of the cells in the relaxed state (slack cell length) are used to quantify the contractions. An oxygen electrode in the chamber registers the drop of oxygen tension resulting from the consumption by the myocytes, which exhibit a strictly aerobic metabolism. The resulting data are also stored and analyzed in an IBM-AT-compatible computer.

Animals↗

PubMed1990

Possible involvement of alanine and pyruvate in the regulation of glucose transport in heart muscle cells.

In isolated rat heart muscle cells, addition of L-alanine (1.5 mmol/l) or of L-valine (3 mmol/l) resulted in either a ca 1.5- or 1.3-fold increase in glucose transport, resp. half-maximal stimulation was observed in the presence of L-alanine, but not of L-valine, within a physiological plasmatic range of concentrations. D-Alanine (1.5 mmol/l) was ineffective and the stimulating effect of L-alanine could be prevented by an excess of L-serine (15-30 mmol/l). L-Alanine produced an increase in 3-O-methyl-D-glucose transport Vmax (from 44.6 to 81.5 pmol.s-1.mg protein-1) without affecting the Km (12.2 in control vs 12.8 mmol/l in alanine-treated cells). Pyruvate (1.5 mmol/l) inhibited glucose transport by 20% and prevented the stimulating action of L-alanine (1.5 mmol/l). These results suggest that the effect of L-alanine in cardiac myocytes occurs through the interaction with an intracellular site and that both alanine and pyruvate may play a role in the regulation of glucose transport in these cells.

Alanine↗

PubMed1990

Sarcolemmal fatty acid transfer in isolated cardiomyocytes governed by albumin/membrane-lipid partition.

The mechanism of transfer of long chain fatty acids across the myocardial sarcolemmal membrane was investigated in isolated, calcium-resistant, rat cardiomyocytes. The initial rate of 14C-palmitate uptake was determined at constant and increasing palmitate/albumin ratios. The latter condition led to a saturable dependence of uptake rate on palmitate concentration. At a constant palmitate/albumin ratio however, there was an almost constant rate of uptake even though the absolute concentration of palmitate increased. The enhanced metabolic rate resulting from electrically induced contractions of the myocytes decreased the apparent Km of uptake from 62 to 23 microM. Thirty seconds after administration, there was no further increase in the [14C]palmitate content of the myocytes. Moreover, from experiments using ghost membrane vesicles the concentration of palmitate in membranes increased almost linearly with increasing palmitate/albumin ratios. This concentration remained virtually constant if vesicles were pre-treated with diamide. Our results do not support the concept of an albumin receptor-mediated uptake but rather suggest that fatty acids are incorporated into cardiomyocytes by a simple diffusion process which is not rate-limiting. The rate of uptake is influenced both by the metabolic rate and by the concentration of fatty acids in the membranes. The rate-limiting step of fatty acid uptake is probably either the formation of acyl-CoA catalyzed by the membrane associated acyl-CoA synthetase, or the transfer of fatty acid carnitine esters across the mitochondrial matrix membrane.

Albumins↗

PubMed1990

Regulation of systolic force and control of free energy of ATP-hydrolysis in hypoxic hearts.

In isolated isometrically working rat hearts during graded, constant pressure hypoxic saline perfusion (PO2 140-700 mmHg) heart rate was changed experimentally between 80 and 400/min. The following parameters were recorded or estimated: peak systolic pressure, dP/dtmax, dP/tmin, oxygen consumption (VO2), venous PO2, ATP, ADP, phosphocreatine, creatine and inorganic P. Free energy of ATP hydrolysis (dG/d xi) was calculated from cytosolic concentration using CK-equilibrium equation. Two predominant responses of the hearts to hypoxia were observed, pronounced negative force frequency relationship; maintained energetic state (Free energy, dG/d xi) at the respective level of hypoxia if contractile force and beating rate were varied by a factor of 4-5. The enhanced force frequency relationship with a maintained free energy level is interpreted in terms of downregulation of EC-coupling (including duration of action potential) which comprises a possible cardioprotective effect.

Adenosine Triphosphate↗

PubMed1990

Free energy of ATP-hydrolysis fails to affect ATP-dependent potassium channels in isolated mouse ventricular cells.

The single channel recording technique has been used to study the adenosine-5'-triphosphate (ATP)-dependent K+ channel in isolated mouse ventricular cells. The aim of this work was to determine if the activity of the K+ channel depends on the free energy of ATP-hydrolytic reaction (phosphorylation potential) in addition to the well-studied direct block by ATP. When the phosphorylation potential was changed from 60 to 50, to 40 and back to 60 kJ/mol at a constant ATP-concentration of 10(-4) mol/l, the ATP-channel activity showed a run down that is best described by a linear function. Changing the ATP-concentration of the bath solution from 0.01 to 1 and back to 0.01 mmol/l by a factor of 10 at a constant phosphorylation potential of 50 kJ/mol, resulted in a run down of the mean average current by a reduction of the open state probability in a concentration dependent manner. A dissociation constant of about 0.14 mmol/l could be estimated. The amplitude of the single channel current was not affected by ATP-concentrations in contrast to changes in phosphorylation potential. A significant increase in the single channel current accompanying a decrease in the phosphorylation potential at constant ATP concentrations was observed. This effect might be due to a decrease of free Mg2(+)-concentration by an increase of the ADP concentration in solutions with lower phosphorylation potentials. An allosteric regulation of the ATP-dependent K+ channel dependent on the free ATP concentration together with the Mg2(+)-ADP concentration seems to be a more likely explanation than regulation by phosphorylation/dephosphorylation secondary to a breakdown of ATP.

Action Potentials↗