Compartmentation of high-energy phosphates in resting and beating heart cells

Creatine kinase structural changes induced by substrates

Myofibrillar creatine kinase (CK) buffers the cellular ATP concentration during fluctuating ATP turnover in a muscle. In order to detect structural changes of the CK molecule due to bound substrates, the dynamics of free, ATP-bound, and ATP+creatine-bound CK were examined, using steady-state and time-resolved fluorescence spectroscopy. The intrinsic tryptophan fluorescence of non-labelled CK presented the smaller fluorescence lifetime 2.38 ns and rotation correlation time 27 ns for the CK-ATP (in comparison with the times 2.72 ns and 35 ns for the free CK), and their moderate return to the longer times 2.42 ns and 29 ns for the CK-ATP+creatine complex. Three conformations for the non-labelled CK were indicated also by different quenching of fluorescence by acrylamide. Data were confirmed by anisotropy experiments with CK-(FITC labelled), providing the same substrate dependence of the rotation times (34 ns, 27 ns and returning 30 ns). The results indicate the existence of three conformations arranged according to the "energy minimizing principle" by ligated substrates. In this way the data implicate another essential component of physiological control at the subcellular level in the transition of the nonreactive CK-ATP+creatine complex to the reactive enzyme molecule.

Differences in nucleotide compartmentation and energy state in isolated and in situ rat heart: assessment by 31P-NMR spectroscopy

Free cytosolic concentrations of ATP, PCr, ADP and 5'-AMP, and the cytosolic [ATP]/[ADP].[Pi] ratio, were determined in isolated and in situ rat hearts using 31P-NMR spectroscopy. Total tissue metabolite concentrations were determined by HPLC analysis of freeze-clamped, perchloric acid-extracted tissue. In in situ myocardium the PCr/ATP ratio was 2.7 +/- 0.2 determined from 31P-NMR data (using either PCr/beta-NTP or PCr/gamma-NTP), and 1.9 +/- 0.1 (P < 0.01) determined from total tissue concentrations. 31P-NMR-determined and total tissue [PCr] were in excellent agreement (49.6 +/- 8.4 and 49.5 +/- 1.0 mumol.g-1 dry wt, respectively), whereas 31P-NMR-determined [ATP] (18.6 +/- 3.2 mumol.g-1 dry wt) was only 71% of the total tissue concentration (26.1 +/- 1.7 mumol.g-1 dry wt, P < 0.01). Isolation and Langendorff perfusion of rat hearts with glucose as substrate reduced total tissue [ATP] and [PCr] and the 31P-NMR-determined PCr/ATP ratio fell to 1.5 +/- 0.1. This value agreed well with the total tissue ratio of 1.4 +/- 0.1, and there was excellent agreement between 31P-NMR-determined and total tissue [PCr] and [ATP] values in the perfused heart. Addition of pyruvate to perfusate increased the 31P-NMR-determined PCr/ATP ratio to 1.7 +/- 0.1 due to elevated [PCr], and there remained excellent agreement between NMR-determined and total tissue [PCr] and [ATP] values. Free cytosolic [ADP] (from the creatine kinase equilibrium) was 5% of total tissue ADP, and free cytosolic [5'-AMP] (from the adenylate kinase equilibrium) ranged from 0.2-0.3% of total tissue 5'-AMP. Bioenergetic state, indexed by [ATP]/[ADP].[Pi], was much lower in isolated perfused hearts (30 mM-1) vs. in situ myocardium (approximately 150 mM-1). In summary, we observe a substantial disproportionality between total tissue PCr/ATP and 31P-NMR-determined PCr/ATP in highly energised in situ myocardium but not in isolated perfused hearts. This appears due to an NMR invisible ATP compartment approximating 29% of total tissue ATP in situ. Additionally, more than 95% of ADP and more than 99% of 5'-AMP exist in bound forms in perfused and in situ myocardium. The physiological significance of these observations is unclear. However, substantial differences between 31P-NMR visible and total tissue [ATP] introduces significant errors in conventional estimation of free cytosolic [ADP], [5'-AMP] and [ATP]/[ADP].[Pi] from in vivo 31P-NMR data.

Cocultures of fetal and adult cardiomyocytes yield rhythmically beating rod shaped heart cells from adult rats

Different models of isolated cardiomyocytes are generally used for biochemical, biophysical, and pharmacological studies. Fetal cardiomyocytes can be easily cultured for several weeks regaining their ability for rhythmical and synchronous contractions. For investigations, differentiated myocytes derived from adult hearts are closer to the in situ situation. Unfortunately, these cells at best exhibit irregular and asynchronous contractions at very low frequencies. Already 1 d after seeding calcium-tolerant rod-shaped adult cardiomyocytes on a suitable substrate, the differentiated cells begin to dedifferentiate forming a confluent monolayer. After 7-10 d their beating activities are like those of fetal cells. Therefore, we tried to combine the advantages of both cell types to achieve fully differentiated cardiomyocytes, rod-shaped and rhythmically beating, isolated from adult hearts. Using contractile fetal cells as a substrate for the adult cardiomyocytes, freshly seeded differentiated adult myocytes are paced by the contraction frequency of the fetal monolayer. As a consequence, the rod-shaped adult cardiomyocytes reach frequencies of more than 140 cycles/min without external electrical stimulation. This model enables us to study cardiomyocytes in a state very similar to the in situ situation with respect to morphology, integrity, and contractile behavior.

Ischaemic ATP degradation studied by HPLC and 31P-NMR spectroscopy: do the two techniques observe the same ATP pools?

31P-NMR spectroscopy has become the major tool for studying myocardial high energy phosphates. Conflicting results concerning NMR visibility of ATP in ischaemic myocardium were reported. A detailed study was undertaken to resolve this controversy. After cardioplegic arrest, canine hearts were excised and preserved for 24 h at 1 degree C (group 1) or for 6h at 23 degrees C (group 2). ATP breakdown was followed by 31P-NMR spectroscopy in a transmural piece of the anterior wall introduced in the NMR magnet, and by HPLC analysis using serial transmural biopsies from the rest of the anterior wall. At both temperatures, identical relative ATP decay curves were obtained, whether measured by NMR or by HPLC. Absolute quantification of ATP was carried out after varying periods of ischaemia at 1 degree C. The NMR-measured ATP concentration was 106 +/- 8% of the ATP concentration determined by HPLC. From our experiments, we conclude that ATP visibility for 31P-NMR spectroscopy is complete and constant during prolonged periods of hypothermic ischaemia in canine hearts.

A model system of coupled activity of co-immobilized creatine kinase and myosin

Myosin and creatine kinase were co-immobilized onto Immunodyne films to mimic the behaviour of creatine kinase bound to the M-line of myofilaments. The Mg-ATPase activity of bound myosin was studied by a coupled enzymatic assay, which detects Mg-ADP in the bulk solution by means of pyruvate kinase and lactate dehydrogenase. The competition for Mg-ADP between pyruvate kinase and creatine kinase either free in solution or co-immobilized with myosin was studied at various creatine phosphate concentrations. Bound creatine kinase competed efficiently when present in very low amounts, corresponding to an activity ratio higher than 1:20,000 between creatine kinase and pyruvate kinase and a molar ratio higher than 1:1000 between creatine kinase and myosin. The Mg-ADP produced by myosin ATPase in the vicinity of the film did not diffuse into the bulk solution but, in the presence of creatine phosphate, was recycled into Mg-ATP by the neighbouring creatine kinase. The existence of an unstirred layer near the surface of the film is sufficient to explain the channeling of ADP (or ATP) between co-immobilized myosin and creatine kinase, without direct interaction or 'intimate coupling' between the enzymes. The problem now is to determine the importance of this kind of facilitated diffusion in the myofilaments in vivo.

NMR-invisible ATP in heart: fact or fiction?

31P-nuclear magnetic resonance (31P-NMR) spectroscopy is widely used to monitor sequential changes in the nucleoside triphosphate (NTP) pool in intact tissues. Recently, the validity of this technique to quantitate incremental changes in ATP in heart has been challenged. Accordingly, we compared NTP measured by 31P-NMR and by chemical techniques in isolated isovolumic rat hearts at 16 and 56 min of oxygenated perfusion and in hearts subjected to 28 min of hypoxia, with or without 28 min of reoxygenation, and 12 or 28 min of ischemia, with or without 28 min of reperfusion. NTP content was calculated from 31P-NMR spectra using an external standard. At the end of each protocol the heart was freeze-clamped, and NTP and ATP contents were determined by chemical assay. After 16 min of normoxic perfusion the values for NTP and ATP contents measured by both methods in the same hearts were indistinguishable. Results from all seven experimental conditions show no significant difference between methods (P = 0.262). Thus both methods detect the same incremental change in NTP and ATP.

Continuous read-out of cytochrome b, flavin and pyridine nucleotide oxido-reduction processes in the perfused frog heart and contracting skeletal muscle

The time-sharing instrument for continuous read-out of the fluorescence levels of NAD(P)H and oxidized-flavoproteins, which also makes simultaneous dual beam differential absorption measurements of cytochrome b, has been applied to the study of the metabolic control of the frog heart, perfused in vitro with normal and variously modified Ringer solutions, and to the frog skeletal muscle in situ. In the ionically depolarized resting heart, the long term kinetic analysis of the redox processes during oxygen/anoxia transitions has proved to be an adequate method for identifying and following three fundamental intracellular redox compartments: (1) the respiratory system, (2) the microsomal system, and (3) the glycolytic system. The provision of substrates and/or suitable inhibitors has allowed the description of functional interrelationships within the mitochondrial and the cytoplasmic spaces. The ouabain major site of response has been located on the cytosolic pyridine nucleotides, with a very small response in the mitochondrial space. The switching on and off of substantial fractions of the NAD(P)+/NAD(P)H and Fpoxidized/Fpreduced pools has been observed, which helps to clarify the large phase shift between the overall pyridine nucleotide and flavin oxidation-reduction processes, during the cycle of the spontaneously beating heart and during skeletal muscle contraction and recovery, here originally prompted out.

Differential extractability of creatine phosphate and ATP from cardiac muscle with ethanol and perchloric acid solution

To compare the extractability of creatine phosphate with that of ATP by alcohol extraction, both compounds were extracted from normal perfused rat heart tissues by using various stepwise concentrations of ethanol and 0.4 M HClO4. Powdered samples (6-15 mg wet wt) from the freeze-clamped tissues were homogenized in 2 ml of the ethanol solutions. After centrifugation, the supernatant was removed; each centrifuged sediment was rehomogenized with 2 ml of 0.4 M HClO4 and centrifuged. The supernatant was neutralized with 0.4 m KHCO3. The same powdered samples were directly homogenized with 2 ml of 0.4 M HClO4 and treated in the same manner. Only a small amount of ATP in the tissues was extracted by an 85% or higher concentration of ethanol. Further, about 13% of the tissue ATP was not extractable by the subsequent perchloric acid extraction. In contrast to ATP, creatine phosphate in the tissues was partially extracted by 95% ethanol and nearly all of the tissue creatine phosphate was extracted by 70% ethanol. The total creatine phosphate obtained by 70% ethanol and by subsequent perchloric acid extraction was significantly higher than that obtained by direct perchloric acid extraction. From these results, it was concluded that the extractability of creatine phosphate in the tissue by alcohol extraction is clearly different from that of ATP. Additionally, the stepwise extraction is recommended as a useful method for the extraction of energy metabolites in perfused rat heart tissue.

Effect of inotropic stimulation on cytosolic Mg2+ in isolated rat heart: a 31P magnetic resonance study

Alterations in cytosolic metabolites and [Mg2+]i were monitored using 31P magnetic resonance spectroscopy during inotropic stimulation of isolated rat heart [10 nM isoproterenol, 0.6 microM isobutyl-1-methylxanthine (IBMX), 5 microM ouabain]. All drugs significantly elevated contractile function (rate-pressure product) and MVO2 by approximately 100-150% (P less than 0.001), decreased cytosolic [creatine phosphate] ([CrP]) and [ATP] (approximately 65 and 80% of control values, respectively) (P less than 0.001), increased [Pi] to more than 180% of pretreatment values, and decreased [H+] by less than 15% (P less than 0.05). A significant relative shift in the alpha-P and beta-P resonances of ATP (P less than 0.01) occurred with inotropic stimulation. [Mg2+]i calculated on the basis of these shifts was found to be 0.78 +/- 0.1 mM in control hearts, and increased to maxima of 1.9 +/- 0.2, 2.0 +/- 0.2, and 2.9 +/- 0.2 mM during infusion of isoproterenol, IBMX, and ouabain, respectively. Changes in [Mg2+]i correlate with cytosolic [ATP] + [CrP] in all hearts (r = 0.89, 0.91, and 0.88 in isoproterenol-, IBMX-, and ouabain-treated hearts, respectively). The significantly higher [Mg2+]i with ouabain infusion (P less than 0.01) at similar workloads and [ATP] + [CrP] supports the proposal that a ouabain-inhibited Mg2+ pump exists in the plasma membrane. The data support acute changes in [Mg2+]i during alterations in inotropic state that may be important in modulating metabolic and contractile function.

5'-Nucleotidase activity and adenosine formation in stimulated, hypoxic and underperfused rat heart

Changes in 5'-nucleotidase activity were calculated on the basis of alterations in ATP, ADP, phosphocreatine, Pi, Mg2+, IMP and AMP, determined by using 31P n.m.r. spectroscopy and h.p.l.c., during isoprenaline infusion, graded hypoxia and graded underperfusion in isolated rat heart. Calculated activity changes were compared with the total efflux of purines (adenosine + inosine + hypoxanthine) in order to assess the involvement of various 5'-nucleotidases in formation of adenosine. Purine efflux exhibited an exponential relation with cytosolic [AMP] during isoprenaline infusion and hypoxia (r = 0.92 and 0.95 respectively), supporting allosteric activation of 5'-nucleotidase under these conditions. Purine efflux displayed a linear relation with cytosolic [AMP] during graded ischaemia (r = 0.96), supporting substrate regulation in the ischaemic heart. The calculated activities of membrane-bound ecto-5'-nucleotidase were similar to the observed relations between purine efflux and cytosolic [AMP] in all hearts. The calculated activities of the ATP-activated cytosolic and lysosomal enzymes and of the ATP-inhibited cytosolic 5'-nucleotidase could not explain the observed release of purines under the conditions examined. These results indicate that the kinetic characteristics of the membrane-bound ecto-enzyme are consistent with an important role in the formation of extracellular adenosine, whereas the characteristics of the other 5'-nucleotidases are inconsistent with roles in adenosine formation under the conditions of the present study.

An example of substrate channeling between co-immobilized enzymes. Coupled activity of myosin ATPase and creatine kinase bound to frog heart myofilaments

In myofilaments obtained by Triton X-100 lysis of frog heart cells in high ionic strength medium, the activity of bound creatine kinase cannot be detected by a coupled enzymatic assay. ATP is channelized toward myosin ATPase, through the unstirred layer near myofilaments and cannot diffuse into the bulk solution. Model systems based upon the coupled kinetics of enzymes co-immobilized on the same surface may explain this behaviour. This may also account for why myofilament-bound creatine kinase is more efficient than free enzyme in the cytosol for the physiological recycling of ADP into ATP.

High-energy phosphates in quiescent, beating and contracted cardiac cells

Isolated myocytes from ventricles are quiescent in the presence of 0.9 mM calcium. However, it is possible to induce beating by adding 0.5 mM BaCl2 to the media or to induce a contracture by elevating the external concentration of potassium (72.5 mM K). During the viable stage of contracture, which is up to 1 h, the sarcomere spacing is 1.7 +/- 0.1 micron and no leakage of intracellular components is observed. The metabolic properties of the cells in quiescent, beating and contracted states were compared. The O2 consumption (natom per mg cell protein per min) increased from 10-11 in quiescent cells to 60-66 in beating cells and 90-99 in contractured cells. In contrast no significant difference was found in the metabolite levels in the three cellular states: (nmol per mg cell protein +/- S.E.M.) ATP, 20.9 +/- 1.5; CrP, 22.3 +/- 2.2; ADP, 6.03 +/- 0.67; Cr, 10.8 +/- 1. It is proposed that the combined action of myosine ATPase, ATP synthase and cytosolic and mitochondrial creatine kinases serves to buffer the metabolite levels during periods of enhanced oxygen consumption.