Measurement of an individual rate constant in the presence of multiple exchanges: application to myocardial creatine kinase reaction

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.

Phosphocreatine T1 measurements with and without exchange in the heart

The intrinsic phosphocreatine (PCr) T1 values measured by time-dependent magnetization transfer in isolated perfused rat, hamster, and turkey hearts were indistinguishable. The value of 3.5 +/- 0.3 s for the rat heart is similar to values measured by other magnetization transfer methods. Irreversibly inhibiting the phosphoryl exchange between PCr and ATP in the rat heart using iodoacetamide changed the apparent T1 values of the two exchanging species when measured by inversion recovery: The apparent T1 of PCr increased from 1.92 +/- 0.06 s to 3.55 +/- 0.06 s, in excellent agreement with the intrinsic T1 measured by magnetization transfer. The apparent T1 of [gamma-P]ATP decreased from 0.92 +/- 0.07 s to 0.44 +/- 0.03 s. The value for the T1 of [gamma-P]ATP in hearts with inhibited phosphoryl exchange was similar to T1 values for [alpha-P]ATP and [beta-P]ATP, which remained unchanged. This illustrates that apparent T1 values for PCr and [gamma-P]ATP measured by inversion recovery in the presence of exchange are average T1 values in between the intrinsic values. The large differences between the intrinsic T1 measured by magnetization transfer and the T1 measured by inversion recovery makes the use of the appropriate value in different applications quantitatively important.

Transmural saturation transfer analysis of the creatine kinase system in the mammalian heart

31P NMR spatial localization and saturation transfer techniques were combined to enable the transmural measurement of the forward creatine kinase (CK) rate (ATP:creatine N-phosphotransferase, EC 2.7.3.2.) in the in vivo canine myocardium. Five epicardial towards endocardial regions of the left ventricle (LV) were simultaneously examined using spatially localized voxels. Although intraleft ventricular CP/ATP ratios were constant, the pseudo first order rate constant (k') and the forward creatine kinase rate (Rf) displayed a 61% variation across the LV wall. Because CK levels and calculated [ADP], [CP] and pH are transmurally invariant in the normal left ventricle, the observed changes in the Rf could not be explained by changes in the absolute levels of these substrates and of creatine kinase. In addition, because myocardial oxygen consumption rates are known to be higher in the endocardium, these results imply that forward creatine kinase rates are not directly related to oxidative phosphorylation rates.

Spatially localized 31P NMR measurements of longitudinal relaxation rates in the canine myocardium

FLAX-ISIS spatial localization was combined with inversion recovery to enable the measurement of spatially localized T1 values. This approach was applied to the transmural determination of creatine phosphate longitudinal relaxation times in the canine myocardium. By examining five voxels spanning the left myocardial wall, we observed that transmural T1 values for creatine phosphate ranged from 3.61 +/- 0.20 in the endocardium to 4.00 +/- 0.20 in the epicardium at 4.7 Tesla. As such, the canine myocardium exhibits no transmural variation in the T1 values of creatine phosphate. This simple approach can be extended to enable the in vivo measurement of transmural enzymatic rates.

31P-NMR magnetization transfer study of reperfused rat heart

The relationships between pressure rate product (PRP) and flux (PCr-->ATP) or flux(Pi-->ATP) were studied in isolated perfused rat hearts by the method of saturation transfer using 31P-NMR during the preischemic and reperfusion periods. The hearts were made ischemic for 15 min, followed by 60 min of reperfusion. PRP was almost completely depressed, and recovered to 60% of the control level (preischemic period) after reperfusion. The ATP level during reperfusion was significantly decreased, whereas there was no significant change in PCr level. Pi level of reperfused hearts was significantly higher than that in the control. Both flux(PCr-->ATP) and flux(Pi-->ATP) were significantly decreased during the reperfusion period (both p < 0.05). However, the flux(PCr-->ATP)/PRP ratio during reperfusion did not differ from that of the control. This result indicates that the decrease in flux(PCr-->ATP) was matched by a similar decrease in cardiac performance. In contrast, the flux(Pi-->ATP)/PRP ratio during reperfusion was significantly decreased compared to that of control. These results suggest that the stunned heart needs less ATP turnover in proportion to its depressed contractile activity, and flux(Pi-->ATP) may lmit the recovery of postischemic performance.

Mechanism of preconditioning. Ionic alterations

The mechanism by which preconditioning (brief intermittent periods of ischemia and reflow) improves recovery of function and reduces enzyme release after a subsequent 30-minute period of ischemia was investigated in perfused rat hearts. Specifically, it was hypothesized that ischemia after preconditioning would result in a decreased production of H+ and therefore a smaller rise in [Na+]i and [Ca2+]i via Na(+)-H+ and Na(+)-Ca2+ exchange. To test this hypothesis we measured pHi, [Na+]i, [Ca2+]i, and cell high-energy phosphates during ischemia and reflow, and we correlated this with recovery of contractile function and release of creatine kinase during reflow. 31P nuclear magnetic resonance (NMR) was used to measure pHi and cell phosphates. [Na+]i was measured by 23Na NMR using the shift reagent thulium 1,4,7,10-tetraazacyclododecane-N,N,'N",N"'-tetramethylenephosph onate to distinguish intracellular from extracellular sodium. [Ca2+]i was measured by 19F NMR using hearts loaded with 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid, termed 5F-BAPTA. Basal time-averaged levels of pHi, [Na+]i, and [Ca2+]i were 7.07 +/- 0.08, 9.4 +/- 0.8 mM, and 715 +/- 31 nM, respectively. After 30 minutes of ischemia, in preconditioned hearts, pHi was 6.5 +/- 0.06, [Na+]i was 2.09 +/- 4.4 mM, [Ca2+]i was 2.1 +/- 0.4 microM, and ATP was negligible. In untreated hearts, after 30 minutes of ischemia, pHi was 6.3 +/- 0.08, [Na+]i was 26.7 +/- 3.8 mM, [Ca2+]i was 3.2 +/- 0.6 microM, and ATP was undetectable. During reperfusion after 30 minutes of ischemia, preconditioned hearts had significantly better recovery of contractile function than untreated hearts (71 +/- 9% versus 36 +/- 8% initial left ventricular developed pressure), and after 60 minutes of ischemia, preconditioned hearts had significantly less release of the intracellular enzyme creatine kinase (102 +/- 12 versus 164 +/- 17 IU/g dry wt). We also found that unpreconditioned hearts arrested with 16 mM MgCl2 (to inhibit calcium entry via calcium channels and Na(+)-Ca2+ exchange) before 30 minutes of ischemia recover function on reflow to the same extent as preconditioned hearts with or without magnesium arrest. Thus, preconditioning has no additional benefit in addition to magnesium arrest. In addition, in hearts that received 16 mM MgCl2 just before the 30-minute period of ischemia, preconditioning had no effect on the rise in [Ca2+]i during the 30-minute period of ischemia. These data support the hypothesis that preconditioning attenuates the increase in [Ca2+]i, [Na+]i, and [H+]i during ischemia, most likely because of reduced stimulation of Na(+)-H+ and Na(+)-Ca2+ exchange.(ABSTRACT TRUNCATED AT 400 WORDS)

Myocardial energetics during ventricular fibrillation investigated by magnetization transfer nuclear magnetic resonance spectroscopy

Ventricular fibrillation (VF) is known to produce alterations in myocardial energetics, but the mechanism of these changes remains unclear. To investigate energy metabolism during VF, phosphorus nuclear magnetic resonance spectroscopy and magnetization transfer were applied to isolated perfused ferret hearts. VF was induced either by perfusion with digitalis (strophanthidin, 30 microM) or by high-frequency electrical stimulation. We measured the flux in two critical reactions: from inorganic phosphate (Pi) to ATP (ATP synthesis rate) and from phosphocreatine (PCr) to ATP (energy transfer capacity). During digitalis-induced VF, energy-related phosphates showed changes similar to those during hypoxia: myocardial [Pi] increased and [PCr] decreased. Concomitantly, the ATP synthesis rate increased to levels about threefold higher than control, whereas oxygen consumption increased by only 16%. The ATP synthesis rate exhibited a strong negative correlation with left ventricular pressure during VF (r = -0.95, n = 5, p < 0.02), whereas oxygen consumption did not (r = 0.19, p > 0.05). On the other hand, energy transfer capacity catalyzed by creatine kinase was significantly smaller during VF than in the control condition but still higher than the simultaneous ATP synthesis rate. In contrast to the marked energetic deterioration during VF induced by digitalis, electrically induced VF led to only a small increase in [Pi] and a small decrease in [PCr], and there were no significant changes in the ATP synthesis rate, energy transfer capacity, or O2 consumption. These results indicate that the rundown in energy metabolism during VF induced by digitalis was mainly attributable to a limitation of energy production through oxidative phosphorylation as well as to a marked increase in energy consumption. In contrast, myocardial energy generation remained unimpaired during VF induced by electrical stimulation. Intracellular calcium overload is more severe during VF induced by digitalis than during electrically induced VF (Circ Res 1991;68:1378-1389); severe calcium overload would be expected to compromise the capacity for energy generation by mitochondria. Thus, we propose that known differences in cellular calcium loading underlie the discrepant energetic patterns of the two types of VF.

Intracellular compartmentation, structure and function of creatine kinase isoenzymes in tissues with high and fluctuating energy demands: the 'phosphocreatine circuit' for cellular energy homeostasis

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Complementarity of magnetic resonance spectroscopy, positron emission tomography and single photon emission tomography for the in vivo investigation of human cardiac metabolism and neurotransmission

The three techniques allowing the noninvasive study of cardiac metabolism, namely magnetic resonance spectroscopy (MRS), positron emission tomography (PET) and single photon emission computed tomography (SPET), all use external detection with stable or radioactive isotopes. These techniques yield different information. PET is quantitative and very sensitive, and therefore only tracer amounts of molecules need to be injected. It allows neurotransmitters and receptors to be studied and a global view of metabolism (oxygen consumption, glucose and fatty acid utilization) to be obtained. SPET also has good sensitivity, but uses gamma-emitting isotopes of heteroatoms. Their longer half-lives allow follow-up for hours or days. MRS is based on stable elements with high (hydrogen 1, phosphorus 31, fluorine 19...) or low (carbon 13, Deuterium) natural abundance. It has very low sensitivity and only millimolar concentrations of substrates can be detected, but various parts of metabolism can be studied. The in vivo measurement of myocardial concentration of substances has many problems that are common to all three techniques (measurement of the volume, measurement of the quantity of each molecule, resolution, partial volume effect, improvement of the signal-to-noise ratio, movement of the organ). The complementarity of the techniques is illustrated by their applications to the study of cardiac metabolism. For instance, the energy metabolism can be studied by 31P-MRS, which detects the high-energy compounds ATP and phosphocreatine, and 13C-MRS yields information on the tricarboxylic acid cycle activity. PET and SPET allow the utilization of fatty acids, the normal fuels of the heart, to be studied. During ischaemia, PET with 18F-fluorodeoxyglucose (18FDG) can determine the glucose consumption and 1H-MRS shows the increase in lactic acid, reflecting anaerobic glycolysis. Comparison of the use of acetate labelled with 11C for PET or 13C for MRS shows the potentials and limitations of each technique. Myocardial perfusion can be evaluated directly with various PET tracers or indirectly with thallium 201 or various technetium-99m-labelled tracers by SPET. No MRS marker of perfusion is so far clinically available. Mainly SPET and PET are used clinically for the investigation of ischaemic heart disease as well as cardiomyopathies, but some initial results using 31P-MRS are being obtained.

Amiloride delays the ischemia-induced rise in cytosolic free calcium

An increase in cytosolic free calcium (Cai) has been shown to occur early during ischemia in perfused rat, ferret, and rabbit hearts. It has been proposed that this increase in Cai may occur as a result of exchange of Nai for Cao, which occurs as a result of an increase in Nai arising from exchange of Nao for H+i. The latter exchange is stimulated by the intracellular acidification that occurs during ischemia. To test this hypothesis, we examined Cai, Nai, ATP, and pHi during ischemia in rats in the presence and absence of 1 mM amiloride, a Na-H exchange inhibitor. Cai was measured using 19F nuclear magnetic resonance (NMR) of 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetra-acetic acid (5F-BAPTA)-loaded rat hearts. Nai was measured using 23Na NMR, and the shift reagent 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetramethylenephosph onate (Tm[DOTP]-5) was used to separate Nai and Nao. ATP and pH were determined from 31P NMR measurements. During 20 minutes of ischemia, amiloride did not significantly alter the ATP decline but did significantly attenuate the rise in Nai and Cai. After 20 minutes of ischemia, time-averaged Cai was 1.0 +/- 0.2 microM (mean +/- SEM) in amiloride-treated hearts compared with 2.3 +/- 0.9 microM in nontreated hearts. After 20 minutes of ischemia, Nai in the untreated heart was threefold greater than control, whereas in the amiloride-treated heart, Nai was not significantly different from control. These data are consistent with the involvement of Na-Ca exchange in the rise in Cai during ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Multisite saturation transfer using DANTE and continuous wave

The DANTE pulse sequence was modified to produce selective resonance saturation similar to that produced by the continuous wave (CW) technique. A combined DANTE and CW saturation technique can be used to perform multisite saturation transfer experiments because of the similar saturation produced by the two techniques.

Combination of 31P-NMR magnetization transfer and radioisotope exchange methods for assessment of an enzyme reaction mechanism: rate-determining steps of the creatine kinase reaction

The theoretical analysis of a reversible enzyme reaction performed in this work shows that the 31P-NMR magnetization (saturation) transfer technique combined with a radioisotope exchange method may potentially provide information on the position of rate-determining step(s). It depends on chemical shifts of NMR signals of nuclei of interest in free and enzyme-bound forms of substrate(s) and product(s) of the reaction. The creatine kinase reaction (MgATP + creatine----MgADP + P-creatine) has been used as a model. Chemical shifts of 31P in binary, ternary and transitional state substrate-enzyme complexes have been estimated by the variable frequency saturation transfer (VFST) method. This method is based on selective irradiation of numerous points in the spectrum and observation of changes in the intensity of visible line(s) which occur due to chemical exchange between it and lines which are not visible in the routine spectrum. Also, dissociation rate constants of MgADP-containing complexes were determined. Magnetization exchange rates, P-creatine----[gamma-P]MgATP and [beta-P]MgADP----[beta-P]MgATP, were compared with radioisotope exchange rates, [gamma-32P-MgATP----P-creatine and [3H]MgADP----MgATP at different [P-creatine]/[creatine] ratios and at different temperatures. All these exchange rates were close to each other at 30-37 degrees C and [PCr]/[Cr] ratios lower than 2. It is concluded that phosphoryl group transfer is the rate-determining step of the overall creatine kinase reaction under these conditions. However, at lower temperatures (below 25 degrees C) or at high [PCr]/[Cr] ratios ([ADP] less than 20 microM) the rate-determining step seems to be shifted toward dissociation of nucleotide substrates from enzyme-substrate complexes, since exchange rates became significantly different. This approach is useful for analysis of mechanism of enzymatic reactions and also can be applied to non-enzymatic reactions and evaluation of small rapidly exchangeable metabolite pools.

Observation of uridine triphosphate:glucose-1-phosphate uridylyltransferase activity in maize root tips by saturation transfer 31P-NMR. Estimation of cytoplasmic PP

Saturation transfer 31P nuclear magnetic resonance was used to estimate the unidirectional rate of phosphorus exchange between Glc-1-P and UDPGlc in maize root tips. The rate was determined to be approx. 4 mumol.min-1 per g fresh weight. This estimated rate is much higher than net rates of other reactions in glucose metabolism (e.g., more than 10-times faster than the maximal glycolytic flux in this tissue). Furthermore, exchange between Glc-1-P and UDPGlc was not significantly inhibited by the metabolic poison KCN. We conclude that the unidirectional rate of conversion of Glc-1P to UDPGlc is much faster than the net rate of UDPGlc synthesis--the UTP:Glc-1-P uridylyltransferase reaction is near-equilibrium in vivo. From the equilibrium constant for this transferase reaction and the concentrations of Glc-1-P, UTP and UDPGlc, the level of cytoplasmic PPi was estimated to be approx. 10 nmol.g-1.

Correlation between cytosolic free calcium, contracture, ATP, and irreversible ischemic injury in perfused rat heart

The relations between ATP depletion, increased cytosolic free calcium concentration [( Cai]), contracture development, and lethal myocardial ischemic injury, as evaluated by enzyme release, were examined using 19F nuclear magnetic resonance to measure [Cai] in 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (5F-BAPTA)-loaded perfused rat hearts. Total ischemia at 37 degrees C was induced in beating hearts, potassium-arrested hearts, magnesium-arrested hearts, and hearts pretreated with 0.9 microM diltiazem to reduce but not abolish contractility. In the beating hearts, time-averaged [Cai], which is intermediate between the systolic and the basal [Cai], was 544 +/- 74 nM. In contrast, in the potassium- and magnesium-arrested hearts, the time-averaged values are lower than in beating hearts (352 +/- 88 nM for potassium arrest, 143 +/- 22 nM for magnesium arrest). During ischemia, ATP depletion, contracture, and a rise in [Cai] are delayed by cardiac arrest, but all occur more rapidly in the potassium-arrested hearts than in the magnesium-arrested hearts. The diltiazem-treated hearts were generally similar to the magnesium-arrested hearts in their response to ischemia. Under all conditions, contracture development was initiated after tissue ATP had fallen to less than 50% of control; invariably, there was a progressive rise in [Cai] during and following contracture development. Reperfusion with oxygenated perfusate shortly after peak contracture development resulted in a return of [Cai] to its preischemic level, resynthesis of creatine phosphate, no significant enzyme release, and no substantial loss of 5F-BAPTA from the heart. The data demonstrate that an increase in [Cai] precedes lethal myocardial ischemic injury. This rise in [Cai] may accelerate the depletion of cellular ATP and may directly contribute to the development of lethal ischemic cell injury.

Palmitic and erucic acid metabolism in isolated perfused hearts from weanling pigs

Hearts from 4 week-old weanling pigs were capable of continuous work output when perfused with Krebs-Henseleit buffer containing 11 mM glucose. Perfused hearts metabolized either glucose or fatty acids, but optimum work output was achieved by a combination of glucose plus physiological concentrations (0.1 mM) of either palmitate or erucate. Higher concentrations of free fatty acids increased their rate of oxidation but also resulted in a large accumulation of neutral lipids in the myocardium, as well as a tendency to increased acetylation and acylation of coenzyme A and carnitine. When hearts were perfused with 1 mM fatty acids, the work output declined below control values. Erucic acid is known to be poorly oxidized by isolated rat heart mitochondria and, to a lesser degree, by perfused rat hearts. In addition, it has been reported that erucic acid acts as an uncoupler of oxidative phosphorylation. In isolated perfused pig hearts used in the present study, erucic acid oxidation rates were as high as palmitate oxidation rates. When energy coupling was measured by 31P-NMR, the steady-state levels of ATP and phosphocreatine during erucic acid perfusion did not change noticeably from those during glucose perfusion. It was concluded that the severe decrease in oxidation rates and ATP production resulting from the exposure of isolated pig and heart mitochondria to erucic acid are not replicated in the intact pig heart.

31P NMR studies of the thermodynamics and kinetics of the creatine kinase reaction

The thermodynamic and kinetic parameters of the reaction catalyzed by creatine kinase (CK) were measured in vitro in the temperature range 13 to 35 degrees C, using 31P NMR spectroscopy, including magnetization transfer methods. The apparent equilibrium constant of the reaction and the associated enthalpy for the formation of ATP at 35 degrees C, pH 8.2, and excess [Mg2+] were 3.5 x 10(9) M-1 and -2.4 +/- 0.5 kcal/mol, respectively. The rates at equilibrium at 35 degrees C catalyzed by 1 unit/ml CK were 12.4 and 10.7 microM/s at pH 7 and 8, respectively. The rate constants per 1 unit CK/1 ml at 35 degrees C, pH 7, were 1.3 x 10(8) s-1 M-2 and 9.9 x 10(-3) s-1 M-1 in the direction of ATP and PCr formation, respectively. The activation energies in both directions were similar and corresponded to 15 +/- 2 kcal/mol at pH 7 and 17.5 +/- 1.5 kcal/mol at pH 8. Comparison of in vivo results with the above in vitro data may provide information regarding the activity and kinetics of the CK reaction.

In vivo functioning of creatine phosphokinase in human forearm muscle, studied by 31P NMR saturation transfer

31P nuclear magnetic resonance (NMR) saturation transfer has been used to measure enzymatic flux through the creatine phosphokinase reaction in the direction of ATP synthesis in the human forearm muscle flexor digitorum superficialis. Modification of the ratio method for measurement of spin-lattice relaxation (R. Freeman, H.D.W. Hill, and R. Kaptein, J. Magn. Reson. 7, 82(1972]was tested and used to appreciably shorten the duration of the measurement. Under conditions of steady state work intracellular pH decreased slightly by 0.06 units and the spin-lattice relaxation time of phosphocreatine in muscle was unchanged, while flux from phosphocreatine to ATP was 64 +/- 10% of the resting value. This is contrary to the increase in flux of 155% predicted from previous saturation transfer studies carried out in vitro on rabbit skeletal muscle creatine phosphokinase using metabolite concentrations to mimic those in vivo (E.A. Shoubridge, J.L. Bland, and G.K. Radda, Biochim. Biophys. Acta 805, 72 (1984]. This discrepancy could be accounted for by an underestimation of the ADP concentrations to which the enzyme is exposed due to inaccurate assumptions about the total metabolite concentrations, or possibly by compartmentation of creatine phosphokinase and its reactants.

ATP synthesis and degradation rates in the perfused rat heart. 31P-nuclear magnetic resonance double saturation transfer measurements

A limitation of magnetization transfer techniques for studying enzyme kinetics in vivo has been the difficulty of treating systems with more than two exchanging species. This problem was addressed in the original papers describing saturation transfer. Since then, a number of approaches have been devised to study these complex situations. Here, we present a method based on the transient saturation transfer experiment in which spin-lattice relaxation time constants and reaction rates are obtained from the same magnetization transfer data. This technique is particularly suitable for biological samples. We apply the method to evaluate flux balance in the three-site linear exchange network composed of ATP, creatine phosphate, and inorganic phosphate in the isolated, perfused rat heart and show that the method yields reasonable values for the reaction velocities of ATP synthesis and degradation.

Rapid 31P NMR test of liver function

The 31P NMR spectrum of perfused rat liver was found to be dependent on the exogenous carbon available to the tissue. When pyruvate was supplied to liver initially perfused with lactate, Pi decreased, phosphoenol pyruvate and phosphomonoesters increased, and nucleotide pools remained the same. It is proposed that these changes can be used to evaluate liver function.

Determination of buffering capacity of rat myocardium during ischemia

To determine the buffering capacity of ischemic rat myocardium, lactate production was altered by glycogen depletion prior to total global ischemia. Lactate production was monitored by 1H-NMR spectroscopy in perfused rat hearts and determined by enzymatic assay of freeze-clamped tissue extracts. Intracellular pH was measured by 31P-NMR spectroscopy. The relationship between total lactate produced and pH varied considerably, depending on the final pH reached. At pH greater than 6.4 this relationship is linear with a total buffering capacity (delta lactate/delta pH) of 25 mumol H+/g wet weight per pH unit. At lower pH values (pH less than 6.4), the total buffering capacity increases progressively. Since ischemia is invariably accompanied by ATP and phosphocreatine (PCr) hydrolysis, the proton production/consumption during high-energy phosphate hydrolysis must be considered when evaluating the intrinsic buffering capacity of the myocardium against proton loads produced by lactate production from glucose and glycogen. Schemes are presented which allow an estimation of the contribution of ATP and PCr hydrolysis and the buffering by the CO2/HCO3- system during ischemia. At pH greater than 6.4, the majority (about 60%) of buffering is due to hydrolysis of adenosine triphosphate, phosphocreatine in the heart, and neutralization of sodium bicarbonate in the perfusate. At pH less than 6.4 an increasing proportion of cardiac buffering is from intrinsic cardiac buffers, most likely from intracellular proteins. After correction for these contributions to the observed total cardiac buffering capacity, the intrinsic buffering capacity of the myocardium can be accounted for by a high capacity (170 mumol/g wet weight) but low pKa (5.2) buffering system.

Saturation and inversion transfer studies of creatine kinase kinetics in rabbit skeletal muscle in vivo

The steady-state kinetics of the creatine kinase reaction in rabbit skeletal muscle in vivo was investigated using inversion and saturation magnetization transfer techniques. Both techniques determined the forward rate of this reaction (creatine phosphate ATP) as approximately 0.3 s-1. This corresponds to a flux of 10 mumol creatine phosphate/s/g muscle. The saturation transfer technique underestimated the reverse reaction by approximately 56%. This result is likely due to the participation of ATP in other interactions in skeletal muscle not involving creatine phosphate.

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.

31P NMR measurements of myocardial pH in vivo

A 31P NMR magnetization transfer method for measuring myocardial pH in vivo is demonstrated in the lamb, dog and cat. The method involves measuring the difference in chemical shift between the resonances of phosphocreatine and inorganic phosphate in magnetization transfer difference spectra in which the gamma-phosphate resonance of ATP has been saturated. The method has been verified by measuring the chemical shift difference between the resonances of 2-deoxyglucose 6-phosphate and phosphocreatine following infusion of the animals with 2-deoxyglucose. The measured pH values are significantly lower than those obtained in previous studies on the heart in vivo.

NMR as a metabolic tool

We have briefly reviewed the broad range of applications of NMR spectroscopy to metabolism in tissues and biological fluids. Most of these studies are in the exploratory stage, though the potential of NMR for non-invasive and non-destructive monitoring of certain important substrates and reaction pathways is considerable. The limitations of the technique lie in its relative insensitivity and the rather restricted range of substances that it can detect, as well as the current expense. So far, the main clinically useful applications have been in the diagnosis and monitoring of treatment of certain inborn errors of metabolism, namely those that result in altered energy of pH states or the abnormal accumulation of significant amounts of metabolites in body fluids. It might be expected that as localization techniques improve, clinically useful information will be obtained in a wide range of ischaemic or hypoxic states, e.g. stroke and myocardial infarction. The possibility of producing a detailed spatial image of metabolite concentrations (e.g. ATP), in the way that NMR imaging techniques currently do using features of the water proton resonance, is attractive and the initial results are very encouraging (Bogusky et al, 1986; Bailes et al, 1987; Blackledge et al, 1987).

31P-NMR studies of phosphate transfer rates in T47D human breast cancer cells

The concentration of phosphates and the kinetics of phosphate transfer reactions were measured in the human breast cancer cell line, T47D, using 31P-NMR spectroscopy. The cells were embedded in agarose filaments and perifused with oxygenated medium during the NMR measurements. The following phosphates were identified in spectra of perifused cells and of cell extracts: phosphorylcholine (PC), phosphorylethanolamine (PE), the glycerol derivatives of PC and PE, inorganic phosphate (Pi), phosphocreatine (PCr), nucleoside triphosphate (primarily ATP) and uridine diphosphate glucose. The rates of the transfers: PC----gamma ATP (0.2 mM/s), Pi----gamma ATP (0.2 mM/s) and the conversion beta ATP----beta ADP (1.3 mM/s) were determined from analysis of data obtained in steady-state saturation transfer and inversion recovery experiments. Data from spectrophotometric assays of the specific activity of creatine kinase (approx. 0.1 mumol/min per mg protein) and adenylate kinase (approx. 0.4 mumol/min per mg protein) suggest that the beta ATP----beta ADP rate is dominated by the latter reaction. The ratio between the rate of ATP synthesis from Pi and the rate of consumption of oxygen atoms (4 X 10(-3) mM/s) was approx. 50. This high value and preliminary measurements of the rate of lactate production from glucose, indicated that aerobic glycolysis is the main pathway of ATP synthesis.

Saturation-transfer studies of ATP-Pi exchange in isolated perfused rat liver

The rate of exchange between inorganic phosphate and ATP was measured in isolated perfused rat livers in the direction of ATP synthesis using 31P NMR spectroscopy and the saturation-transfer technique. Measurement of ATP hydrolysis was not observable, even after treatment of rats with 100 micrograms T3/day per 100 g body wt. When the perfused livers were treated with iodoacetate in order to inhibit glycolysis, NMR measurable exchange between ATP and Pi was eliminated. It is concluded that the inorganic phosphate----ATP conversion detected by saturation transfer is catalyzed by enzymes of the glycolytic pathway and that the mitochondrial ATPase rate is too slow to contribute to the observed effect.

A new colorimetric method for determination of creatine phosphokinase

A new colorimetric method has been developed for the determination of creatine phosphokinase (CPK). This method is based on the reaction of creatine, formed enzymatically from creatine phosphate and ADP, with p-nitrophenylglyoxal (PNPG) under alkaline conditions to produce a colored product which absorbs maximally at 480 nm. At 25 degrees C the reaction was complete after 10 min in 0.1 M sodium pyrophosphate, pH 12, containing 0.15 M sodium ascorbate. The colored product was stable for at least 24 h and obeyed Beer's law in the range 0.005-0.05 mM creatine. The color reaction was used to determine the activity of CPK in serum and tissue extracts. The results obtained by this method agreed well with the results obtained by other available methods for CPK determination. However, the PNPG method was more rapid and more sensitive than other colorimetric methods and required a single chromogenic reagent.

31P magnetization transfer studies of creatine kinase kinetics in living rabbit brain

The kinetics of the CK reaction in the living rabbit brain was studied by three 31P magnetization transfer methods: inversion recovery (IR), inversion transfer (IT), and steady-state saturation transfer (SSST). Longitudinal relaxation rates (tau -1) were determined from IR and IT experiments. The values were 0.88 +/- 0.08 s-1 for PCr and 2.45 +/- 0.39 s-1 for gamma-ATP. Analysis of the results of SSST experiments in which gamma-ATP was saturated yielded a forward rate constant kF of 0.53 +/- 0.07 s-1. Upon saturation of PCr no change in the gamma-ATP signal could be detected in SSST. An average reverse rate constant (kR +/- S.D.) of 0.52 +/- 0.04 s-1 was estimated by analyzing IT data from three experiments with selective inversion of gamma-ATP. The standard error of kR was 50%. The average [PCr]/[ATP] of 1.21 +/- 0.16 together with the values of kF and kR yielded a forward-to-reverse flux ratio of 1.23. Within our limits of accuracy this ratio was not significantly different from 1.0, thus suggesting that in the brain the forward and reverse CK fluxes are equal.

NMR studies of kinetics in cells and tissues

An exciting aspect of NMR spectroscopy is its ability to monitor, non-invasively, a variety of small molecules in cells and tissues. This leads to the possibility of investigating details of cellular biochemistry previously obscured by separation and purification procedures.

31P-NMR saturation transfer measurements of exchange between Pi and ATP in the reactions catalysed by glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate kinase in vitro

Recent 31P-NMR saturation transfer measurements of flux between Pi and ATP in the perfused rat heart (Kingsley-Hickman, P., Sako, E.Y., Andreone, P.A., St. Cyr, J.A., Michurski, S., Foker, J.E., From, A.H.L., Petein, M. and Ugurbil, K. (1986) FEBS Lett. 198, 159-163) have given a P/O ratio (mols ATP synthesised/atoms oxygen consumed) which was close to 6. This anomalously high value was attributed to exchange in the reaction catalysed by the mitochondrial F1F0-ATP synthase. We show here that this exchange could also be catalysed by the glycolytic enzymes glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate kinase. 31P-NMR saturation transfer measurements of the exchange catalysed by these enzymes in vitro, under conditions designed to mimic those present in the perfused rat heart, have shown that they could catalyse a quantitatively significant Pi-ATP exchange in vivo. A three-site exchange model is used to investigate the effects of Pi-ATP exchange on saturation transfer measurements of the reverse flux in the creatine kinase reaction. A discrepancy in the measured and forward and reverse fluxes in this reaction has been attributed previously to the participation of the gamma-phosphate of ATP in other exchange reactions.