Biomedical subjects

A H From

Publications and source records attributed to A H From.

At least 37 records · Page 2Linked to original sources

PubMed1993

Bioenergetic abnormalities associated with severe left ventricular hypertrophy.

Transmurally localized 31P-nuclear magnetic resonance spectroscopy (NMR) was used to study the effect of severe pressure overload left ventricular hypertrophy (LVH) on myocardial high energy phosphate content. Studies were performed on 8 normal dogs and 12 dogs with severe left ventricular hypertrophy produced by banding the ascending aorta at 8 wk of age. Spatially localized 31P-NMR spectroscopy provided measurements of the transmural distribution of myocardial ATP, phosphocreatine (CP), and inorganic phosphate (Pi); spectra were calibrated from measurements of ATP content in myocardial biopsies using HPLC. Blood flow was measured with microspheres. In hypertrophied hearts during basal conditions, ATP was decreased by 42%, CP by 58%, and the CP/ATP ratio by 32% in comparison with normal. Increasing myocardial blood flow with adenosine did not correct these abnormalities, indicating that they were not the result of persistent hypoperfusion. Atrial pacing at 200 and 240 beats per min caused no change in high energy phosphate content in normal hearts but resulted in further CP depletion with Pi accumulation in the inner left ventricular layers of the hypertrophied hearts. These changes were correlated with redistribution of blood flow away from the subendocardium in LVH hearts. These findings demonstrate that high energy phosphate levels and the CP/ATP ratio are significantly decreased in severe LVH. These abnormalities are proportional to the degree of hypertrophy but are not the result of persistent abnormalities of myocardial perfusion. In contrast, depletion of CP and accumulation of Pi during tachycardia in LVH are closely related to the pacing-induced perfusion abnormalities and likely reflect subendocardial ischemia.

Adenosine↗

PubMed1991

Ischemic contracture begins when anaerobic glycolysis stops: a 31P-NMR study of isolated rat hearts.

The relationships among myocardial ATP, intracellular pH, and ischemic contracture in Langendorff-perfused rat hearts were investigated by 31P nuclear magnetic resonance spectroscopy during total global normothermic ischemia while the left ventricular pressure was recorded continuously via an intraventricular balloon. Glucose-perfused hearts (n = 63) were divided into five groups based on the time of onset of contracture (TOC), and three other groups of hearts were treated to vary the ischemic glycogen availability. ATP levels, which showed no evidence of accelerated ATP depletion during contracture, were significant and variable at TOC. Intracellular pH initially declined and then leveled off at TOC, with lower final pH in hearts with later TOC. We conclude that contracture began when anaerobic glycolysis (and thus glycolytic ATP synthesis) stopped. These results, though consistent with the concept that ischemic contracture in normal hearts results from rigor bond formation due to low ATP levels at the myofibrils, suggest that TOC is more closely related to glycolytic ATP production than to total cellular ATP content, thus providing evidence of some degree of subcellular compartmentation or metabolite channeling. In glycolytically inhibited hearts, the quite early contracture may have a Ca2+ component.

Anaerobiosis↗

PubMed1990

Digitalis receptor sugar binding site characteristics: a model based upon studies of Na+, K(+)-ATPase preparations with differing digitalis sensitivities.

The structure-activity relationships of the genin moieties of digitalis glycosides are commonly elucidated by determining the inhibitory potency of a variety of genins toward the plasma membrane Na+, K(+)-ATPase; qualitatively these relationships appear to be fairly independent of the specific Na+, K(+)-ATPase preparation utilized for the analysis. To determine whether this is the case with regard to the sugar moieties of glycosides, the inhibitory effects of 12 monoglycosides of digitoxigenin toward four Na+, K(+)-ATPase preparations of different origin were measured. It was found that while recognition of the major structural determinants of sugar activity appeared to be independent of enzyme source, recognition of the minor structural determinants of activity showed some source dependence. It was also observed that the intrinsic sensitivity to sugar potentiation may be source dependent and unrelated to intrinsic sensitivity to inhibition by digitoxigenin. These observations are compatible with a model of the Na+, K(+)-ATPase sugar binding site(s) in which intrinsic sensitivity to sugar attachment as well as recognition characteristics (for sugar structural features) both determine the extent to which a sugar moiety may contribute to the activity of monoglycosides. Further, in these studies one of the Na+, K(+)-ATPase preparations employed was obtained from rat brain, a tissue known to contain a mixture of ouabain sensitive and insensitive isoforms. We have observed that the rigorous purification techniques employed appear to have selectively removed from or denatured the less ouabain sensitive alpha 1 isoform found in this enzyme preparation.

Animals↗

PubMed1990

Regulation of the oxidative phosphorylation rate in the intact cell.

The mechanisms that underlie the balance between the consumption and oxidative generation of ATP in the intact cell are not well-defined. Cytosolic inorganic phosphate (Pi) and ADP levels, the cytosolic ATP/ADP ratio, and the cytosolic phosphorylation potential (PP) have all been proposed as major regulatory variables, the latter as a component of a "near-equilibrium" thermodynamic regulatory scheme. Therefore, the potential regulatory roles of these variables in the intact cell were evaluated with 31P NMR and Langendorff perfused rat hearts; in this preparation, the tissue oxygen consumption rate (MVO2) can be varied over a wide range. When the exogenous carbon source was varied, none of the proposed regulatory parameters, i.e., the ATP/ADP ratio, PP, or cytosolic ADP level, were found to be uniquely related to MVO2. Rather, ADP levels at a given MVO2 decreased progressively for the exogenous carbon sources in the following order: glucose, glucose + insulin, palmitate + glucose, lactate, pyruvate + glucose, and octanoate + glucose. In the octanoate and pyruvate groups, MVO2(-1) was linearly dependent upon [ADP]-1 with apparent Km values being in the range previously observed in isolated mitochondria. A similar trend was observed in the MVO2-[Pi] relationship. The present findings suggest that exogenous carbon sources which effectuate deregulation of intramitochondrial NADH generation lower cytosolic ADP and Pi to levels which are limiting to the rate of oxidative phosphorylation. For other carbon sources, the processes controlling the rate of NADH generation also participate in determining the rate of oxidative ATP synthesis. However, this control must be exerted kinetically rather than through a near-equilibrium thermodynamic mechanism as indicated by the present data and prior kinetic studies of the ATP synthetic process in both isolated mitochondria and intact myocardium [La Noue, K. F., et al. (1986) Biochemistry 25, 7667-7675; Kingsley-Hickman, P., et al. (1987) Biochemistry 26, 7501-7510].

Adenosine Triphosphate↗

PubMed1990

31P NMR measurement of mitochondrial uncoupling in isolated rat hearts.

Mitochondrial uncoupling is often invoked as a mechanism underlying cellular dysfunction; however, it has not been possible to study this phenomenon directly in intact cells and tissues. In this paper, we report direct evaluation of mitochondrial uncoupling in the intact myocardium using 31P NMR magnetization transfer techniques. Langendorff perfused rat hearts were exposed to either a known uncoupler, 2,4-dinitrophenol (DNP), or a potential uncoupler, octanoate. Both DNP and octanoate decreased mechanical function as measured by the rate pressure product and caused an increase in the oxygen consumption rate (MVO2); with DNP this increase in MVO2 was dose-dependent. The ATP synthesis rate measured by 31P NMR, however, was not elevated commensurately with MVO2; instead, the P/O ratio declined. In contrast, the linear relationship between the ATP synthesis rate and rate pressure product was not altered by the uncoupling agents. These data demonstrate that 1) 31P NMR magnetization transfer can be utilized to measure uncoupling of oxidative phosphorylation in intact organs, 2) octanoate does not induce excess ATP utilization in the intact heart, and 3) high levels of octanoate induce mitochondrial uncoupling in the intact myocardium; and this may, in part, be the cause of the toxic effects associated with fatty acid exposure.

Adenosine Triphosphatases↗

PubMed1990

Measurement of ATP synthesis rates by 31P-NMR spectroscopy in the intact myocardium in vivo.

The ability to measure ATP synthesis rates using 31P-NMR spectroscopy is demonstrated in the normal, ischemic, and postischemic myocardium in vivo. Cardiopulmonary bypass (CBP) was employed to induce 20 min of global myocardial ischemia, and to conduct magnetization transfer measurements during the ischemic episode and following reperfusion and return to normal circulation. For the first few minutes of ischemia, transfer of magnetization from ATP gamma to Pi was extensive and the resultant fractional reduction (delta M/M0) in the Pi resonance intensity reached approximately 100%. Subsequent to reperfusion and stabilization off CPB and on normal circulation, both the fractional reduction and the spin-lattice relaxation time, T1*, of the Pi resonance were determined when ATP gamma spins were saturated. Under these conditions, the unidirectional ATP synthesis rate was 0.41 +/- 0.09 (SEM, N = 4) mumol/s/g wet wt. The data suggest that in the canine myocardium in vivo, glycolytic enzymes mediate a very rapid exchange between Pi and ATP gamma-phosphates during early phases of ischemia; in the postischemic reperfused myocardium, however, the glycolytic contribution to the unidirectional Pi----ATP rate measured by NMR in vivo is relatively small compared to that observed in glucose-perfused, postischemic rat hearts.

Adenosine Triphosphate↗

PubMed1990

Transmural high energy phosphate distribution and response to alterations in workload in the normal canine myocardium as studied with spatially localized 31P NMR spectroscopy.

Spatially localized phosphorus-31 nuclear magnetic resonance (31P NMR) spectroscopy has been applied to the study of the normal canine myocardium to measure the relative content of high energy phosphates across the left ventricular wall. Transmural NMR data were acquired in five voxels spanning the wall of the left ventricle using the FLAX-ISIS technique. The validity of the FLAX-ISIS approach in acquiring localized spectra for transmural studies and in providing quantitative information from the localized spectra was examined rigorously by studies involving phantoms, intact rats, and the canine myocardium in vivo. The results indicated that (1) this technique yields spatially resolved spectra with partial overlap between adjacent voxels and virtually no overlap between every other voxel; (2) in the canine heart, signals from subepicardium, midwall, and subendocardium can be detected separately without cross contamination; and (3) relative metabolite contents within a voxel and among voxels can be quantitated. Transmural 31P NMR spectra were acquired with cardiac gating on 29 separate animals either at early systole or late diastole, and at three different workloads with the heart rate peak systolic pressure product (RPP) increasing from 6000 mmHg/min to 35,000 mmHg/min. The data revealed that in the normal canine myocardium, the creatine phosphate (CP) content and the CP/ATP ratio was significantly lower in the subendocardium than in the subepicardium. ATP levels were transmurally constant. Both the CP content and the CP/ATP ratio measured for each voxel remained unaltered in relation to either the phase of the cardiac cycle or approximately fourfold increase in workload. Free ADP levels calculated for each voxel showed that ADP was relatively higher in the subendocardium than the subepicardium, and in all transmural layers was higher than its apparent Km for oxidative phosphorylation. In this domain changes in ADP content with workload and MVO2 are not expected and were not observed.

Adenosine Diphosphate↗

PubMed1990

Correlation between transmural high energy phosphate levels and myocardial blood flow in the presence of graded coronary stenosis.

Spatially localized nuclear magnetic resonance spectroscopy was used to investigate with transmural differentiation the response of myocardial high energy phosphate compounds and inorganic orthophosphate (Pi) to graded reductions in coronary blood flow caused by sustained coronary stenosis. In an open-chest model, localized 31P nuclear magnetic resonance spectra from five layers across the left ventricular wall were obtained simultaneously with transmural blood flow measurements during control conditions and during sustained graded reductions in intracoronary pressure. Both the blood flow, and high energy phosphate and Pi contents displayed transmural heterogeneity in response to decreases in intracoronary pressure. The subendocardial creatine phosphate (CP) level remained unchanged as blood flow was reduced to approximately 0.7 ml/min/g wet wt and decreased precipitously beyond this critical flow level. The relation between CP and flow in the midmyocardium and especially in the subepicardium was more complex. Subepicardial CP content did not correlate well with blood flow; however, in cases in which a coronary stenosis resulted in subendocardial hypoperfusion but subepicardial flow was near or above normal, a close correlation was present between subepicardial and subendocardial CP levels. ATP levels in all layers remained unaltered until blood flow was severely reduced. These results demonstrate that 1) the myocardial high energy phosphate and Pi levels at any transmural layer are not generally determined by O2 and blood flow limitation under basal conditions; 2) during subtotal coronary occlusion, increased oxygen extraction is able to meet myocardial needs until a critical level of stenosis is reached; 3) below a critical flow level, subendocardial CP and Pi contents are closely correlated with absolute subendocardial blood flow; and 4) in the presence of a coronary stenosis, subepicardial CP and Pi contents may change even in the absence of perfusion deficit secondary to loss of subendocardial function.

Adenosine Triphosphate↗

PubMed1990

Enhanced chemiluminescence as a measure of oxygen-derived free radical generation during ischemia and reperfusion.

It has been suggested that oxygen-derived free radicals may contribute to the myocardial injury associated with ischemia and reperfusion. As the presence of enhanced free radical generation is a prerequisite for such damage, several techniques have been used to provide evidence of increased oxygen free radical production during reperfusion; however, all such techniques have substantial limitations. In this study, we used enhanced chemiluminescence to evaluate oxygen free radical generation during ischemia and reperfusion in the isolated Langendorff-perfused rat heart. The chemiluminescent technique, which has high sensitivity and can monitor radical generation continuously, avoids some of the limitations of earlier methods. Chemiluminescence (expressed as counts per second) decreased from 219 +/- 11 at baseline to 142 +/- 9 during ischemia and markedly increased to a peak of 476 +/- 36 during the first 3-5 minutes of reperfusion. This was followed by a slow decline over 11-16 minutes to a steady-state level of 253 +/- 14 (each sequential change in chemiluminescence was highly significant; p less than 0.001). Superoxide dismutase (2,000 units/min) significantly decreased peak reperfusion chemiluminescence to 316 +/- 17 (p less than 0.01). Hearts subjected to a second period of ischemia and reperfusion had a higher peak chemiluminescence (626 +/- 62), which also was significantly attenuated by 1,000 units/min superoxide dismutase (398 +/- 16; p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1989

Alterations in oxidative function and respiratory regulation in the post-ischemic myocardium.

In the normal and post-ischemic, isovolumic Langendorff perfused rat hearts, 31P NMR spectra and mechanical performance were evaluated over a wide range of myocardial oxygen consumption rates (MVO2). Hearts were perfused with either glucose and insulin, palmitate and glucose, or pyruvate and glucose as exogenous carbon sources. After ischemia at 38 degrees C until the onset of ischemic contracture and subsequent reperfusion, the "free" ADP levels were significantly reduced as compared to controls. In the control palmitate + glucose and glucose + insulin groups, the ADP levels were virtually independent of approximately 2.5-fold variation in MVO2; in contrast, they changed 4-fold with a approximately 30% variation in MVO2 in the post-ischemic myocardium following ischemia to contracture. In the pyruvate + glucose group, ADP levels varied with MVO2 in controls and post-ischemia; however, MVO2-ADP relationship was significantly altered following ischemia. Analysis of these observations within the concept of kinetic regulation of oxidative phosphorylation yielded the following significant conclusions: 1) the mode of respiratory regulation changed from a non-ADP to an "ADP:Pi limited" domain with non-pyruvate carbon sources; 2) respiratory regulation was in the ADP:Pi limited domain before and after ischemia in the pyruvate + glucose group; however, the Km for the relationship between MVO2 and ADP was reduced following the ischemia/reperfusion insult; 3) the post-ischemic oxidative capacity (Vmax for MVO2) was significantly reduced in all groups and this reduction would limit maximal post-ischemic mechanical performance.

Adenine Nucleotides↗

PubMed1989

Spectroscopic imaging and spatial localization using adiabatic pulses and applications to detect transmural metabolite distribution in the canine heart.

Adiabatic pulses have been employed in spectroscopic imaging and relaxation rate measurements at 4.7 T to demonstrate the feasibility of obtaining spectroscopic data from the complete sensitive volume of a surface coil using the surface coil as a transmitter and receiver. With conventional B1 sensitive pulses, spectroscopic localization or imaging techniques, such as chemical-shift imaging, yield resonance intensities that are distorted severely as a function of space, and maximal signal is detected from a small region within the complete sensitive volume of the coil. With adiabatic pulses, however, this problem is eliminated completely. In addition, a new method of spatial localization is introduced. This method, referred to as FLAX-ISIS, is a derivative of longitudinally modulated Fourier series window and ISIS approaches and utilizes adiabatic inversion and excitation pulses. The method allows construction of localized spectra for multiple regions along the surface coil axis by postacquisition data manipulation of a single set of free induction decays. These techniques were applied to the study of the myocardium using an implanted surface coil in an instrumented closed-chest canine model and in an open-chest preparation. The results demonstrate that one-dimensional techniques are adequate for transmural detection of metabolites provided signal origin is restricted to a column perpendicular to the left ventricle wall.

Animals↗

PubMed1989

Transmural metabolite distribution in regional myocardial ischemia as studied with 31P NMR.

Phosphorus-31 nuclear magnetic resonance (31P NMR) has been applied to study the canine heart prior to and during regional myocardial ischemia induced by partial flow reduction in the left anterior descending coronary artery (LAD). NMR data were acquired in a transmural fashion by restricting the signal to a column perpendicular to the heart wall using B0 gradients and obtaining spectroscopic spatial resolution along the third dimension using the B1 gradient and adiabatic excitation. With this approach, transmural spectra were accumulated in five separate voxels spanning the wall of the left ventricle from the epicardium to the endocardium. In the normal canine myocardium the levels of high-energy phosphates CP and ATP were relatively constant throughout the left ventricular wall, with only minor evidence of free inorganic phosphate in any of the transmural voxels. However, during sustained partial occlusion of the LAD, significant regional differences between the epi- and the endocardium were noted. The data demonstrate the importance of studying cardiac bioenergetics with transmural differentiation.

Animals↗

PubMed1989

Metabolic consequences of coronary stenosis. Transmurally heterogeneous myocardial ischemia studied by spatially localized 31P NMR spectroscopy.

Coronary stenosis results in transmurally non-uniform blood flow with the inner layers of the left ventricular wall typically suffering a more severe hypoperfusion relative to the outer layers. Coupled with numerous other transmural non-uniformities such as systolic tension development and oxygen needs, the heterogeneous blood flow distribution in the presence of coronary stenosis is expected to result in transmurally heterogeneous ischemia. All previous NMR spectroscopy studies of myocardial metabolism and bioenergetics under normal and ischemic conditions treated the organ as a homogeneous tissue. We have utilized spatially localized 31P NMR spectroscopy together with non-NMR measurements of regional blood flow to study the myocardium with transmural spatial differentiation under normal and flow-restricted conditions. 31P NMR and blood flow data obtained concurrently on each heart revealed that sustained coronary artery stenosis resulted in transmurally non-uniform ischemia which largely paralleled the hypoperfusion pattern. The reduction in creatine phosphate content (with consequent elevation of Pi) and hypoperfusion was tightly correlated in the subendocardium for flow rates less than approximately 0.7 mL/min per g wet myocardium. The high energy phosphate and Pi content of the epicardium, however, was responsive not only to the flow to this region but also to the extent of ischemia in the subendocardial layers. These results document the utility of localized NMR spectroscopy in physiologic research and suggest potential biomedical applications due to the tight correlation noted between alterations in blood flow and changes in the phosphorylated metabolite levels detected by 31P NMR.

Animals↗

PubMed1989

Hypoxic pulmonary vasoconstriction is unaltered by creatine depletion induced by dietary beta-guanidino propionic acid.

It has been suggested that a specific phosphagen pool might serve a sensor function, allowing direct detection of alveolar hypoxia by the pulmonary vascular smooth muscle. The possibility that phosphocreatine (PCr) levels could serve as such a sensor was assessed in isolated rat lungs. Pulmonary vascular reactivity to angiotensin II and alveolar hypoxia was assessed in lungs from control and PCr-depleted rats. PCr depletion was accomplished by feeding rats a diet containing 2% beta-guanidino propionic acid (beta-GPA), an competitive inhibitor of creatine uptake. Total creatine was depleted in beta-GPA lungs, compared to control lungs (p less than 0.05). Lung PCr levels were undetectable by the available 31P NMR spectroscopy system. PCr and creatine were depleted in hearts from beta-GPA rats relative to control hearts (p less than 0.001). Normoxic pulmonary artery pressure and the pressor responses to angiotensin II and hypoxia were not qualitatively or quantitatively altered by the diet indicating either that PCr is not a critical participant in hypoxic pulmonary vasoconstriction or that the degree of PCr depletion achieved was inadequate to expose its role in the hypoxic pressor response.

Animals↗

PubMed1988

ATP synthesis kinetics and mitochondrial function in the postischemic myocardium as studied by 31P NMR.

The effects of ischemia on mitochondrial function and the unidirectional rate of ATP synthesis (Pi----ATP rate) were studied using a Langendorff-perfused heart preparation and 31P NMR spectroscopy. There was significant postischemic depression of mechanical function assessed as the heart rate pressure product, and the myocardial oxygen consumption rate at a given rate pressure product was elevated. Experiments performed on glucose- and pyruvate-perfused hearts demonstrated the presence of a large contribution to the unidirectional Pi----ATP rate catalyzed by glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate kinase. This rate was much greater than the maximal glucose utilization rate in the myocardium, demonstrating that the glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase reactions are near equilibrium both before and after ischemia. In the pyruvate-perfused postischemic hearts, the glycolytic contribution was eliminated and the net rate of ATP synthesis by oxidative phosphorylation was measurable. Despite the reduced mechanical function and increased myocardial oxygen consumption rate, the ratio of the net rate of ATP synthesis by oxidative phosphorylation to oxygen consumption rate (the P:O ratio) was not altered subsequent to ischemia (2.34 +/- 0.12 and 2.36 +/- 0.09 in normal and postischemic hearts, respectively). Therefore, mitochondrial uncoupling cannot be the cause of postischemic depression in mechanical function; instead, the data suggest the existence of ischemia-induced inefficiency in ATP utilization.

Adenosine Triphosphate↗

PubMed1988

Substrate effects in the post-ischemic myocardium.

A study was undertaken to examine the effects of glucose versus pyruvate as the sole substrate following severe myocardial ischemia. Glycolysis usually contributes only a small amount to total ATP production and may be rate limiting in providing tricarboxylic acid (TCA) cycle substrates. Consequently, pyruvate may be a more effective substrate by bypassing glycolysis to feed directly to the TCA cycle and oxidative phosphorylation. Isolated rat hearts were studied in a retrograde (Langendorff) perfusion apparatus while in an NMR spectrometer. Rate pressure product (RPP), myocardial oxygen consumption (MVO2), and the unidirectional Pi----ATP rate were measured in control and postischemic hearts with or without the inotrope dobutamine. The undirectional Pi----ATP rate was higher in the glucose than the pyruvate hearts and the difference increased further postischemia. This increase over that of the pyruvate hearts has been attributed to a glycolytic component of ATP metabolism. Oxygen consumption was higher in pyruvate hearts at equivalent levels of performance. It thus appears that the glycolysis rate is significant and may be elevated following severe myocardial ischemia. Perfusion with pyruvate requires increased rates of oxidative phosphorylation to make up for the loss of glycolytically produced ATP. Optimal postischemic substrate delivery may require several compounds, one of which should be glucose.

Adenosine Triphosphate↗

PubMed1988

Dihydropyridine agonist Bay K 8644 inhibits platelet activation by competitive antagonism of thromboxane A2-prostaglandin H2 receptor.

The dihydropyridine calcium channel antagonists, such as nifedipine, inhibit platelet aggregation in vitro and ex vivo, but the mechanism by which this occurs is uncertain. Bay K 8644 (BAY) is a substituted dihydropyridine that has effects on voltage-dependent calcium channels in cardiac and smooth muscle that are opposite the effects of nifedipine. To evaluate the mechanism responsible for dihydropyridine-induced inhibition of platelet function, we studied the in vitro effects of BAY on human platelet aggregation and secretion plus several related biochemical parameters, including cytoplasmic ionized calcium ([Ca2+]i). BAY exerted concentration-dependent effects on platelet aggregation and secretion of [14C]serotonin. BAY (1-10 microns) inhibited the second wave of platelet aggregation and secretion stimulated by adenosine diphosphate or epinephrine and blocked shape change, aggregation, and secretion induced by the thromboxane A2 (TXA2) mimic, U46619. BAY also inhibited U46619-induced phosphorylation of the approximately 40,000-dalton cytoplasmic protein substrate of protein kinase C (40K protein), formation of TXA2, and rise in [Ca2+]i, all biochemical consequences of platelet activation. The (+)-(R) enantiomer of BAY [BAY(+)] was predominantly responsible for the inhibitory effects of racemic BAY. Nifedipine had the same inhibitory effects on platelet function and biochemistry, except it was approximately 10 times less potent than BAY. Since these results suggested inhibition of the TXA2-prostaglandin H2 (PGH2) receptor, we measured binding of [3H]U46619 to intact platelets. BAY, BAY(+), and nifedipine all functioned as competitive antagonists of [3H]U46619 binding (BAY Ki = 1.47 microM). They did not inhibit binding of [3H]yohimbine to platelet alpha 2-adrenergic receptors. At 1-10 nM BAY, BAY(+) and the (-)-(S) enantiomer of BAY [BAY(-)] all resulted in slight stimulation of platelet function and biochemical events. No significant increase in [3H]U46619 binding was demonstrable, however. Therefore, dihydropyridines that function as either calcium channel agonists or antagonists in cardiac or smooth muscle exert concentration-dependent effects on platelet function. In nanomolar concentrations, they augment, and in micromolar concentrations, they inhibit platelet activation induced by TXA2 or U46619. These data indicate that dihydropyridines do not inhibit TXA2-induced platelet activation by an effect on voltage-dependent calcium channels; they define the mechanism of inhibition as competitive antagonism of the TXA2-PGH2 receptor. The mechanism responsible for augmentation of platelet activation is uncertain.(ABSTRACT TRUNCATED AT 400 WORDS)

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1987

31P NMR studies of ATP synthesis and hydrolysis kinetics in the intact myocardium.

The origin of the nuclear magnetic resonance (NMR)-measurable ATP in equilibrium Pi exchange and whether it can be used to determine net oxidative ATP synthesis rates in the intact myocardium were examined by detailed measurements of ATP in equilibrium Pi exchange rates in both directions as a function of the myocardial oxygen consumption rate (MVO2) in (1) glucose-perfused, isovolumic rat hearts with normal glycolytic activity and (2) pyruvate-perfused hearts where glycolytic activity was reduced or eliminated either by depletion of their endogenous glycogen or by use of the inhibitor iodoacetate. In glucose-perfused hearts, the Pi----ATP rate measured by the conventional two-site saturation transfer (CST) technique remained constant while MVO2 was increased approximately 2-fold. When the glycolytic activity was reduced, the Pi----ATP rate decreased significantly, demonstrating the existence of a significant glycolytic contribution. Upon elimination of the glycolytic component, the measured Pi----ATP rates displayed a linear dependence on MVO (micromoles of O consumption rate) with a slope of 2.36 +/- 0.15 (N = 8, standard error of the mean). This linear relationship is expected if the rate determined by CST is the net rate of ATP synthesis by the oxidative phosphorylation process, in which case the slope must equal the P:O ratio. The ATP----Pi rates and rate:MVO ratios measured by the multiple-site saturation transfer method at two MVO2 levels were equal to the corresponding Pi----ATP rates and rate:MVO ratios obtained in the absence of a glycolytic contribution. The following conclusions are drawn from these studies: (1) unless the glycolytic contribution to the ATP in equilibrium Pi exchange is inhibited or is specifically shown not to exist, the myocardial Pi in equilibrium ATP exchange due to oxidative phosphorylation cannot be studied by NMR; (2) at moderate MVO2 levels, the reaction catalyzed by the two glycolytic enzymes glyceraldehyde-3-phosphate dehydrogenase and 3-phosphoglycerate kinase is near equilibrium; (3) the ATP synthesis by the mitochondrial H+-ATPase occurs unidirectionally (i.e., the reaction is far out of equilibrium); (4) the "operative" P:O ratio in the intact myocardium under our conditions is significantly less than the canonically accepted value of 3.

Adenosine Triphosphate↗