Increased work in cardiac trabeculae causes decreased mitochondrial NADH fluorescence followed by slow recovery

Warm ischemic preconditioning improves mitochondrial redox balance during and after mild hypothermic ischemia in guinea pig isolated hearts

Ischemic preconditioning (IPC) induces distinctive changes in mitochondrial bioenergetics during warm (37 degrees C) ischemia and improves function and tissue viability on reperfusion. We examined whether IPC before 2 h of hypothermic (27 degrees C) ischemia affords additive cardioprotection and improves mitochondrial redox balance assessed by mitochondrial NADH and flavin adenine dinucleotide (FAD) autofluorescence in intact hearts. A mediating role of ATP-sensitive K(+) (K(ATP)) channel opening was investigated. NADH and FAD fluorescence was measured in the left ventricular wall of guinea pig isolated hearts assigned to five groups of eight animals each: hypothermia alone, hypothermia with ischemia, IPC with cold ischemia, 5-hydroxydecanoic acid (5-HD) alone, and 5-HD with IPC and cold ischemia. IPC consisted of two 5-min periods of warm global ischemia spaced 5 min apart and 15 min of reperfusion before 2 h of ischemia at 27 degrees C and 2 h of warm reperfusion. The K(ATP) channel inhibitor 5-HD was perfused from 5 min before until 5 min after IPC. IPC before 2 h of ischemia at 27 degrees C led to better recovery of function and less tissue damage on reperfusion than did 27 degrees C ischemia alone. These improvements were preceded by attenuated increases in NADH and decreases in FAD during cold ischemia and the reverse changes during warm reperfusion. 5-HD blocked each of these changes induced by IPC. This study indicates that IPC induces additive cardioprotection with mild hypothermic ischemia by improving mitochondrial bioenergetics during and after ischemia. Because effects of IPC on subsequent changes in NADH and FAD were inhibited by 5-HD, this suggests that mitochondrial K(ATP) channel opening plays a substantial role in improving mitochondrial bioenergetics throughout mild hypothermic ischemia and reperfusion.

NAD(P)H fluorescence imaging of mitochondrial metabolism in contracting Xenopus skeletal muscle fibers: effect of oxygen availability

The blue autofluorescence (351 nm excitation, 450 nm emission) of single skeletal muscle fibers from Xenopus was characterized to be originating from mitochondrial NAD(P)H on the basis of morphological and functional correlations. This fluorescence signal was used to estimate the oxygen availability to isolated single Xenopus muscle fibers during work level transitions by confocal microscopy. Fibers were stimulated to generate two contractile periods that were only different in the PO2 of the solution perfusing the single fibers (PO2 of 30 or 0-2 Torr; pH = 7.2). During contractions, mean cellular NAD(P)H increased significantly from rest in the low PO2 condition with the core (inner 10%) increasing to a greater extent than the periphery (outer 10%). After the cessation of work, NAD(P)H decreased in a manner consistent with oxygen tensions sufficient to oxidize the surplus NAD(P)H. In contrast, NAD(P)H decreased significantly with work in 30 Torr PO2. However, the rate of NAD(P)H oxidation was slower and significantly increased with the cessation of work in the core of the fiber compared with the peripheral region, consistent with a remaining limitation in oxygen availability. These results suggest that the blue autofluorescence signal in Xenopus skeletal muscle fibers is from mitochondrial NAD(P)H and that the rate of NAD(P)H oxidation within the cell is influenced by extracellular PO2 even at high extracellular PO2 during the contraction cycle. These results also demonstrate that although oxygen availability influences the rate of NAD(P)H oxidation, it does not prevent NAD(P)H from being oxidized through the process of oxidative phosphorylation at the onset of contractions.

Functional coupling as a basic mechanism of feedback regulation of cardiac energy metabolism

In this review we analyze the concepts and the experimental data on the mechanisms of the regulation of energy metabolism in muscle cells. Muscular energetics is based on the force-length relationship, which in the whole heart is expressed as a Frank-Starling law, by which the alterations of left ventricle diastolic volume change linearly both the cardiac work and oxygen consumption. The second basic characteristics of the heart is the metabolic stability--almost constant levels of high energy phosphates, ATP and phosphocreatine, which are practically independent of the workload and the rate of oxygen consumption, in contrast to the fast-twitch skeletal muscle with no metabolic stability and rapid fatigue. Analysis of the literature shows that an increase in the rate of oxygen consumption by order of magnitude, due to Frank-Starling law, is observed without any significant changes in the intracellular calcium transients. Therefore, parallel activation of contraction and mitochondrial respiration by calcium ions may play only a minor role in regulation of respiration in the cells. The effective regulation of the respiration under the effect of Frank-Starling law and metabolic stability of the heart are explained by the mechanisms of functional coupling within supramolecular complexes in mitochondria, and at the subcellular level within the intracellular energetic units. Such a complex structural and functional organisation of heart energy metabolism can be described quantitatively by mathematical models.

Neural activity triggers neuronal oxidative metabolism followed by astrocytic glycolysis

We have found that two-photon fluorescence imaging of nicotinamide adenine dinucleotide (NADH) provides the sensitivity and spatial three-dimensional resolution to resolve metabolic signatures in processes of astrocytes and neurons deep in highly scattering brain tissue slices. This functional imaging reveals spatiotemporal partitioning of glycolytic and oxidative metabolism between astrocytes and neurons during focal neural activity that establishes a unifying hypothesis for neurometabolic coupling in which early oxidative metabolism in neurons is eventually sustained by late activation of the astrocyte-neuron lactate shuttle. Our model integrates existing views of brain energy metabolism and is in accord with known macroscopic physiological changes in vivo.

Myocardial contractile function during postischemic low-flow reperfusion: critical thresholds of NADH and O2 delivery

The degree of myocardial oxygen delivery (Do2) that is necessary to reestablish functional contractile activity after short-term global ischemia in heart is not known. To determine the relationship between Do2 and recovery of contractile and metabolic functions, we used tissue NADH fluorometric changes to characterize adequacy of reperfusion flow. Isolated perfused rat hearts were subjected to global ischemia and were reperfused at variable flow rates that ranged from 1 to 100% of baseline flow. Myocardial function and tissue NADH changes were continuously measured. NADH fluorescence rapidly increased and plateaued during ischemia. A strong inverse logarithmic correlation between NADH fluorescence and reperfusion Do2 was demonstrated (r = -0.952). Left ventricular function (rate-pressure product) was inversely related to NADH fluorescence at reperfusion flows from 25 to 100% of baseline (r = -0.922) but not at lower reperfusion flow levels. An apparent reperfusion threshold of 25% of baseline Do2 was necessary to resume contractile function. At very low reperfusion flows (1% of baseline), another threshold flow was identified at which NADH levels increased beyond that observed during global ischemia (3.4 +/- 3.0%, means +/- SE, n = 9), which suggests further reduction of the cellular redox state. This NADH increase at 1% of baseline reperfusion flow was blocked by removing glucose from the perfusate. NADH fluorescence is a sensitive indicator of myocardial cellular oxygen utilization over a wide range of reperfusion Do2 values. Although oxygen is utilized at very low flow rates, as indicated by changes in NADH, a critical threshold of approximately 25% of baseline Do2 is necessary to restore contractile function after short-term global ischemia.

NADH oxidase activity of rat cardiac sarcoplasmic reticulum regulates calcium-induced calcium release

NADH and Ca2+ have important regulatory functions in cardiomyocytes related to excitation-contraction coupling and ATP production. To elucidate elements of these functions, we examined the effect of NADH on sarcoplasmic reticulum (SR) Ca2+ release and the mechanisms of this regulation. Physiological concentrations of cytosolic NADH inhibited ryanodine receptor type 2 (RyR2)-mediated Ca2+-induced Ca2+ release (CICR) from SR membranes (IC50=120 micromol/L) and significantly lowered single channel open probability. In permeabilized single ventricular cardiomyocytes, NADH significantly inhibited the amplitude and frequency of spontaneous Ca2+ release. Blockers of electron transport prevented the inhibitory effect of NADH on CICR in isolated membranes and permeabilized cells, as well as on the activity of RyR2 channels reconstituted in lipid bilayer. An endogenous NADH oxidase activity from rat heart copurified with SR enriched with RyR2. A significant contribution by mitochondria was excluded as NADH oxidation by SR exhibited >9-fold higher catalytic activity (8.8 micromol/mg protein per minute) in the absence of exogenous mitochondrial complex I (ubiquinone) or complex III (cytochrome c) electron acceptors, but was inhibited by rotenone and pyridaben (IC50=2 to 3 nmol/L), antimycin A (IC50=13 nmol/L), and diphenyleneiodonium (IC50=28 micromol/L). Cardiac junctional SR treated with [3H](trifluoromethyl)diazirinyl-pyridaben specifically labeled a single 23-kDa PSST-like protein. These data indicate that NADH oxidation is tightly linked to, and essential for, negative regulation of the RyR2 complex and is a likely component of an important physiological negative-feedback mechanism coupling SR Ca2+ fluxes and mitochondrial energy production.

How Inotropic Drugs Alter Dynamic and Static Indices of Cyclic Myoplasmic [Ca2+] to Contractility Relationships in Intact Hearts

The authors examined effects of positive (dopamine and digoxin) and negative (nifedipine and lidocaine) inotropic interventions on the instantaneous cyclic relationship between myoplasmic [Ca2+] and simultaneously developed left ventricular pressure (LVP) in intact guinea pig hearts. Novel indices were developed to quantify this relationship based on (1) transient [Ca2+] and LVP signal morphology, ie, maxima and minima, peak derivatives, beat areas, durations, and ratios of indices of LVP to [Ca2+]; (2) temporal delay; and (3) LVP versus [Ca2+] loop morphology, ie, orientation, size, hysteresis, position, shape, and duration. These analyses were used to assess the cost of phasic [Ca2+] for contraction and relaxation over one beat after inotropic intervention. It was found that dopamine and digoxin increased contractile and relaxation responsiveness to phasic [Ca2+], cumulative Ca2+, and net Ca2+ flux. Unlike dopamine, digoxin did not decrease relaxation response time. Nifedipine and lidocaine decreased contractile and relaxation responsiveness to phasic [Ca2+], cumulative Ca2+, and net Ca2+ flux. Unlike lidocaine, nifedipine decreased net available Ca2+ and Ca2+ influx. Positive inotropic agents increased [Ca2+]-LVP loop area and hysteresis and resulted in a more vertically oriented loop. Nifedipine and lidocaine decreased these loop indices and lidocaine exhibited greater loop hysteresis than did nifedipine. These novel indices provide a quantitative assessment of myoplasmic [Ca2+] handling for cardiac contractile function.

Cross-bridge kinetics modeled from myoplasmic [Ca2+] and LV pressure at 17 degrees C and after 37 degrees C and 17 degrees C ischemia

We modeled changes in contractile element kinetics derived from the cyclic relationship between myoplasmic [Ca(2+)], measured by indo 1 fluorescence, and left ventricular pressure (LVP). We estimated model rate constants of the Ca(2+) affinity for troponin C (TnC) on actin (A) filament (TnCA) and actin and myosin (M) cross-bridge (A x M) cycling in intact guinea pig hearts during baseline 37 degrees C perfusion and evaluated changes at 1) 20 min 17 degrees C pressure, 2) 30-min reperfusion (RP) after 30-min 37 degrees C global ischemia during 37 degrees C RP, and 3) 30-min RP after 240-min 17 degrees C global ischemia during 37 degrees C RP. At 17 degrees C perfusion versus 37 degrees C perfusion, the model predicted: A x M binding was less sensitive; A x M dissociation was slower; Ca(2+) was less likely to bind to TnCA with A x M present; and Ca(2+) and TnCA binding was less sensitive in the absence of A x M. Model results were consistent with a cold-induced fall in heart rate from 260 beats/min (37 degrees C) to 33 beats/min (17 degrees C), increased diastolic LVP, and increased phasic Ca(2+). On RP after 37 degrees C ischemia vs. 37 degrees C perfusion, the model predicted the following: A x M binding was less sensitive; A x M dissociation was slower; and Ca(2+) was less likely to bind to TnCA in the absence of A. M. Model results were consistent with reduced myofilament responsiveness to [Ca(2+)] and diastolic contracture on 37 degrees C RP. In contrast, after cold ischemia versus 37 degrees C perfusion, A x M association and dissociation rates, and Ca(2+) and TnCA association rates, returned to preischemic values, whereas the dissociation rate of Ca(2+) from A x M was ninefold faster. This cardiac muscle kinetic model predicted a better-restored relationship between Ca(2+) and cross-bridge function on RP after an eightfold longer period of 17 degrees C than 37 degrees C ischemia.

Reduced contraction strength with increased intracellular [Ca2+] in left ventricular trabeculae from failing rat hearts

Intracellular calcium ([Ca2+](i)) and isometric force were measured in left ventricular (LV) trabeculae from spontaneously hypertensive rats (SHR) with failing hearts and normotensive Wistar-Kyoto (WKY) controls. At a physiological stimulation frequency (5 Hz), and at 37 degrees C, the peak stress of SHR trabeculae was significantly (P < or = 0.05) reduced compared to WKY (8 +/- 1 mN mm(-2) (n = 8) vs. 21 +/- 5 mN mm(-2) (n = 8), respectively). No differences between strains in either the time-to-peak stress, or the time from peak to 50 % relaxation were detected. Measurements using fura-2 showed that in the SHR both the peak of the Ca2+ transient and the resting [Ca2+](i) were increased compared to WKY (peak: 0.69 +/- 0.08 vs. 0.51 +/- 0.08 microM(P < or = 0.1) and resting: 0.19 +/- 0.02 vs. 0.09 +/- 0.02 microM(P < or = 0.05), SHR vs. WKY, respectively). The decay of the Ca2+ transient was prolonged in SHR, with time constants of: 0.063 +/- 0.002 vs. 0.052 +/- 0.003 s (SHR vs. WKY, respectively). Similar results were obtained at 1 Hz stimulation, and for [Ca2+ ](o) between 0.5 and 5 mM. The decay of the caffeine-evoked Ca2+ transient was slower in SHR (9.8 +/- 0.7 s (n = 8) vs. 7.7 +/- 0.2 s (n = 8) in WKY), but this difference was removed by use of the SL Ca2+ -ATPase inhibitor carboxyeosin. Histological examination of transverse sections showed that the fractional content of perimysial collagen was increased in SHR compared to WKY (18.0 +/- 4.6 % (n = 10) vs. 2.9 +/- 0.9 % (n = 11) SHR vs. WKY, respectively). Our results show that differences in the amplitude and the time course of the Ca2+ transient between SHR and WKY do not explain the reduced contractile performance of SHR myocardium per se. Rather, we suggest that, in this animal model of heart failure, contractile function is compromised by increased collagen, and its three-dimensional organisation, and not by reduced availability of intracellular Ca2+.

Simultaneous measurements of mitochondrial NADH and Ca(2+) during increased work in intact rat heart trabeculae

The main goal of this study is to investigate the role of mitochondrial [Ca(2+)], [Ca(2+)](m), in the possible up-regulation of the NADH production rate during increased workload. Such up-regulation is necessary to support increased flux through the electron transport chain and increased ATP synthesis rates. Intact cardiac trabeculae were loaded with Rhod-2(AM), and [Ca(2+)](m) and mitochondrial [NADH] ([NADH](m)) were simultaneously measured during increased pacing frequency. It was found that 53% of Rhod-2 was localized in mitochondria. Increased pacing frequency caused a fast, followed by a slow rise of the Rhod-2 signal, which could be attributed to an abrupt increase in resting cytosolic [Ca(2+)], and a more gradual rise of [Ca(2+)](m), respectively. When the pacing frequency was increased from 0.25 to 2 Hz, the slow Rhod-2 component and the NADH signal increased by 18 and 11%, respectively. Based on a new calibration method, the 18% increase of the Rhod-2 signal was calculated to correspond to a 43% increase of [Ca(2+)](m). There was also a close temporal relationship between the rise (time constant approximately 25 s) and fall (time constant approximately 65 s) of [Ca(2+)](m) and [NADH](m) when the pacing frequency was increased and decreased, respectively, suggesting that increased workload and [Ca(2+)](c) cause increased [Ca(2+)](m) and consequently up-regulation of the NADH production rate.

Altered NADH and improved function by anesthetic and ischemic preconditioning in guinea pig intact hearts

NADH increases during ischemia because O(2) shortage limits NADH oxidation at the electron transport chain. Ischemic (IPC) and anesthetic preconditioning (APC) attenuate cardiac reperfusion injury. We examined whether IPC and APC similarly alter NADH, i.e., mitochondrial metabolism. NADH fluorescence was measured at the left ventricular wall of 40 Langendorff-prepared guinea pig hearts. IPC was achieved by two 5-min periods of ischemia and APC by exposure to 0.5 or 1.3 mM sevoflurane for 15 min, each ending 30 min before 30 min of global ischemia. During ischemia, NADH initially increased in nonpreconditioned control hearts and then gradually declined below baseline levels. This increase in NADH was lower after APC but not after IPC. The subsequent decline was slower after IPC and APC. On reperfusion, NADH was less decreased after IPC or APC, mechanical and metabolic functions were improved, and infarct size was lower compared with controls. Our results indicate that IPC and APC cause distinctive changes in mitochondrial metabolism during ischemia and thus lead to improved function and tissue viability on reperfusion.

Ischemic and anesthetic preconditioning reduces cytosolic [Ca2+] and improves Ca(2+) responses in intact hearts

Ca(+) loading during reperfusion after myocardial ischemia is linked to reduced cardiac function. Like ischemic preconditioning (IPC), a volatile anesthetic given briefly before ischemia can reduce reperfusion injury. We determined whether IPC and sevoflurane preconditioning (SPC) before ischemia equivalently improve mechanical and metabolic function, reduce cytosolic Ca(2+) loading, and improve myocardial Ca(2+) responsiveness. Four groups of guinea pig isolated hearts were perfused: no ischemia, no treatment before 30-min global ischemia and 60-min reperfusion (control), IPC (two 2-min occlusions) before ischemia, and SPC (3.5 vol%, two 2-min exposures) before ischemia. Intracellular Ca(2+) concentration ([Ca(2+)](i)) was measured at the left ventricular (LV) free wall with the fluorescent probe indo 1. Ca(2+) responsiveness was assessed by changing extracellular [Ca(2+)]. In control hearts, initial reperfusion increased diastolic [Ca(2+)] and diastolic LV pressure (LVP), and the maximal and minimal derivatives of LVP (dLVP/dt(max) and dLVP/dt(min), respectively), O(2) consumption, and cardiac efficiency (CE). Throughout reperfusion, IPC and SPC similarly reduced ischemic contracture, ventricular fibrillation, and enzyme release, attenuated rises in systolic and diastolic [Ca(2+)], improved contractile and relaxation indexes, O(2) consumption, and CE, and reduced infarct size. Diastolic [Ca(2+)] at 50% dLVP/dt(min) was right shifted by 32-53 +/- 8 nM after 30-min reperfusion for all groups. Phasic [Ca(2+)] at 50% dLVP/dt(max) was not altered in control but was left shifted by -235 +/- 40 nM [Ca(2+)] after IPC and by -135 +/- 20 nM [Ca(2+)] after SPC. Both SPC and IPC similarly reduce Ca(2+) loading, while augmenting contractile responsiveness to Ca(2+), improving postischemia cardiac function and attenuating permanent damage.

Oscillations and hypoxic changes of mitochondrial variables in neurons of the brainstem respiratory centre of mice

We studied the functions of mitochondria and their hypoxic modulation in the brainstem slices of neonatal mice (postnatal day (P)6-11). The measurements were made in the preBotzinger complex (pBC), a part of the respiratory centre, and in the hypoglossal (XII) nucleus. Using a CCD camera, changes in the redox state were assessed from cell autofluorescence produced by NADH and FAD, while alterations in mitochondrial membrane potential ([Delta][psi]) and free Ca2+ concentration ([Ca2+]m) were obtained from fluorescence signals after loading the cells with Rh123 and Rhod-2, respectively. In the pBC, the cells were functionally identified by correlating the oscillations in [NADH], [FAD], [Delta][psi] and [Ca2+]m with the respiratory motor output recorded simultaneously from XII rootlets. In the inspiratory cells, NADH fluorescence showed a brief decrease followed by a slow and long-lasting increase during one oscillation period. The initial decrease in NADH fluorescence was accompanied by an increase in FAD fluorescence and coincided with [Delta][psi] depolarization. The slow secondary increase in NADH fluorescence had a time course similar to that of the Rhod-2 signal, indicating the role of Ca2+ uptake by mitochondria in NAD and FADH reduction. Brief (2-4 min) hypoxia reversibly abolished rhythmic changes in mitochondrial variables and brought them to new steady levels. In parallel, ATP-sensitive K+ (KATP) channels were activated and the respiratory output was depressed. The hypoglossal neurons showed much bigger increases in [Delta][psi] and [NADH] during hypoxia than the pBC neurons, which may explain their extreme vulnerability to hypoxia. We show here that mitochondrial function can be monitored in vitro in neurons constituting the respiratory neural network in slice preparations. Since mitochondrial variables demostrate specific, stereotypic fluctuations during a respiratory cycle, we suggest that mitochondrial function is modulated by spontaneous activity in the respiratory network. Therefore mitochondrial depolarization and Ca2+ uptake can contribute to the biphasic reaction of the respiratory network during hypoxia.

Changes in [Na(+)](i), compartmental [Ca(2+)], and NADH with dysfunction after global ischemia in intact hearts

We measured the effects of global ischemia and reperfusion on intracellular Na(+), NADH, cytosolic and mitochondrial (subscript mito) Ca(2+), relaxation, metabolism, contractility, and Ca(2+) sensitivity in the intact heart. Langendorff-prepared guinea pig hearts were crystalloid perfused, and the left ventricular (LV) pressure (LVP), first derivative of LVP (LV dP/dt), coronary flow, and O(2) extraction and consumption were measured before, during, and after 30-min global ischemia and 60-min reperfusion. Ca(2+), Na(+), and NADH were measured by luminescence spectrophotometry at the LV free wall using indo 1 and sodium benzofuran isophthalate, respectively, after subtracting changes in tissue autofluorescence (NADH). Mitochondrial Ca(2+) was assessed by quenching cytosolic indo 1 with MnCl(2). Mechanical responses to changes in cytosolic-systolic (subscript sys), diastolic (subscript dia), and mitochondrial Ca(2+) were tested over a range of extracellular [Ca(2+)] before and after ischemia-reperfusion. Both [Ca(2+)](sys) and [Ca(2+)](dia) doubled at 1-min reperfusion but returned to preischemia values within 10 min, whereas [Ca(2+)](mito) was elevated over 60-min reperfusion. Reperfusion dissociated [Ca(2+)](dia) and [Ca(2+)](sys) from contractile function as LVP(sys-dia) and the rise in LV dP/dt (LV dP/dt(max)) were depressed by one-third and the fall in LV dP/dt (LV dP/dt(min)) was depressed by one-half at 30-min reperfusion, whereas LVP(dia) remained markedly elevated. [Ca(2+)](sys-dia) sensitivity at 100% LV dP/dt(max) was not altered after reperfusion, but [Ca(2+)](dia) at 100% LV dP/dt(min) and [Ca(2+)](mito) at 100% LV dP/dt(max) were markedly shifted right on reperfusion (ED(50) +36 and +125 nM [Ca(2+)], respectively) with no change in slope. NADH doubled during ischemia but returned to normal on initial reperfusion. The intracellular [Na(+)] ([Na(+)](i)) increased minimally during ischemia but doubled on reperfusion and remained elevated at 60-min reperfusion. Thus Na(+) and Ca(2+) temporally accumulate during initial reperfusion, and cytosolic Ca(2+) returns toward normal, whereas [Na(+)](i) and [Ca(2+)](mito) remain elevated on later reperfusion. Na(+) loading likely contributes to Ca(2+) overload and contractile dysfunction during reperfusion.

Mitochondrial NAD(P)H, ADP, oxidative phosphorylation, and contraction in isolated heart cells

To examine the relationship between mitochondrial NADH (NADH(m)) and cardiac work output, NADH(m) and the amplitude and frequency of the contractile response of electrically paced rat heart cells were measured at 25 degrees C. With 5.4 mM glucose plus 2 mM beta-hydroxybutyrate, NADH(m) was reversibly decreased by 23%, and the amplitude of contraction was reversibly decreased by 27% during 4-Hz pacing. With glucose plus 2 mM pyruvate or with 10 mM 2-deoxy-D-glucose, NADH(m) was maintained during rapid pacing, and the contractile amplitude remained high. Phosphocreatine levels decreased with 2-deoxy-D-glucose administration but not with rapid pacing. Respiration increased to meet the increased ATP demand at 30 degrees C. The data suggest that 1) when NADH(m) is decreased during rapid pacing with defined substrates, the amplitude of contraction is decreased; 2) the amplitude of contraction during electrical pacing does not change with rate of pacing when both the ATP and NADH(m) levels are continuously replenished; and 3) the replenishment of NADH(m) during pacing with physiological substrates may be rate-limited by substrate supply to mitochondrial dehydrogenases. During activation of mitochondrial dehydrogenases, or a significant increase in free ADP induced by 2-deoxy-D-glucose, this rate limitation is bypassed or overcome.

Mitochondrial calcium in heart cells: beat-to-beat oscillations or slow integration of cytosolic transients?

Mitochondria have been implicated in intracellular Ca2+ signaling in many cell types. The inner mitochondrial membrane contains Ca2+-transporting proteins, which catalyze Ca2+ uptake and extrusion. Intramitochondrial (matrix) Ca2+, in turn, regulates the activity of Krebs cycle dehydrogenases and, ultimately, the rate of ATP synthesis. In the myocardium, controversy remains whether the fast cytosolic Ca2+ transients underlying excitation-contraction coupling in beating cells are rapidly transmitted into the matrix compartment or slowly integrated by the mitochondrial Ca2+ transporters. This mini-review critically summarizes the recent experimental work in this field.

Analysis of the mechanisms of mitochondrial NADH regulation in cardiac trabeculae

We have previously shown that increased cardiac work initially caused a rapid Ca(2+)-independent fall of mitochondrial [NADH] ([NADH](m)) to a minimum level, and this was followed by a slow Ca(2+)-dependent recovery toward control level (Brandes and Bers, Biophys. J. 71:1024-1035, 1996; Brandes and Bers, Circ. Res. 80:82-87, 1997). The purpose of this study is to improve our understanding of the factors that control [NADH](m) during increased work. [NADH](m) was monitored using fluorescence spectroscopy in intact rat trabeculae isolated from the right ventricular wall. Work was increased by increasing sarcomere length, pacing frequency, external [Ca(2+)], or by decreased temperature. The results were: 1) The initial fall of [NADH](m) during increased pacing frequency depends independently on increased myofilament work and on increased Ca(2+)-transport ATPase activity. 2) The [NADH](m) recovery process depends on average cytosolic [Ca(2+)] (Av[Ca(2+)](c)), but not on absolute work level. 3) The initial fall of [NADH](m) and the [NADH](m) recovery are similar whether increased work is associated with low frequency and high Ca(2+)-transient amplitude or vice versa (at the same myofilament work level and Av[Ca(2+)](c)). 4) The mechanisms associated with the smaller fall and recovery of [NADH](m) at 37 degrees C versus 27 degrees C, may be explained by lowered Av[Ca(2+)](c) and myofilament work. The NADH control mechanisms that operate at lower temperature are thus qualitatively similar at more physiological temperatures.

NADH fluorescence in isolated guinea-pig and rat cardiomyocytes exposed to low or high stimulation rates and effect of metabolic inhibition with cyanide

In this study we investigated whether NADH fluorescence levels changed in response to low or high rates of electrical stimulation in single ventricular myocytes isolated from rat and guinea-pig hearts, either during a single contraction or upon sustained electrical stimulation of cells. NADH levels were determined from cell autofluorescence and cell length monitored using an edge-tracking device. NADH/NAD+ was obtained by addition of cyanide, 100% NADH, and carbonylcyanide-p-trifluoromethoxy phenylhydrazone (FCCP), 100% NAD+. Rat myocytes exhibited slightly higher resting fluorescence levels than guinea-pig cells; however, NADH/NAD+ was higher in rat than guinea-pig cells (P < 0.05), 24.3+/-4.3 (N = 17) vs 14.6+/-1.6 (N = 17), respectively. There was no change in NADH fluorescence during a single contraction when cells were stimulated at either low (0.2 Hz) or high (3 Hz) rates in either species. Furthermore, NADH levels did not change upon sustained stimulation at 3 Hz in either species. Metabolic blockade with cyanide induced a dose dependent rise in NADH fluorescence which was similar for both rat and guinea-pig myocytes and reached a maximum at > or = 1 mM of cyanide. Although a full recovery of NADH fluorescence was seen in both types of cells after brief exposure to cyanide, the rate of recovery was significantly slower in rat myocytes; times to 90% recovery were 110+/-29 sec, N = 6, and 264+/-50 sec, N = 6, for guinea-pig and rat cells, respectively. This work demonstrates that although rat and guinea-pig myocytes have different resting NADH/NAD+, their response to electrical stimulation is the same, whereas in response to metabolic inhibition subtle differences are seen.

Regulation of mitochondrial [NADH] by cytosolic [Ca2+] and work in trabeculae from hypertrophic and normal rat hearts

Pressure overload hypertrophy has previously been shown to reduce contractility but paradoxically to increase O2 consumption rates at a given force. Because O2 consumption rates are related to mitochondrial [NADH] ([NADH]m), we tested the hypothesis that with hypertrophy, control of [NADH]m may be altered. Left ventricular trabeculae were isolated from banded and control rat hearts, and fluorescence spectroscopy was used to monitor [NADH]m and cytosolic [Ca2+] ([Ca2+]c). The hearts from banded rats developed hypertrophy (heart-to-body weight ratio increased from 4.1+/-0.1 to 4.9+/-0.1 mg/g) and hypertension (systolic arterial pressure increased from 117+/-4 to 175+/-5 mm Hg). Muscle workload was increased by stepwise increases in pacing frequency (up to 2 Hz). After increased work, [NADH]m fell and then slowly recovered toward control levels. When work was decreased, [NADH]m overshot control values and then slowly returned. The Ca2+-independent initial fall was larger for trabeculae from rats with hypertrophied hearts than from control rats (eg, 17+/-2% versus 11+/-1% when work was increased by increasing the frequency from 0.25 to 1 Hz). At 1 Hz, average [Ca2+]c was approximately 280 nmol/L, and the Ca2+-dependent [NADH]m recovery was larger for trabeculae from rats with hypertrophied hearts (17+/-4% versus 10+/-2%) despite similar average [Ca2+]c. At steady state after Ca2+-dependent recovery, there was no difference in [NADH]m (fall of 1+/-2% versus 1+/-1%). Furthermore, the Ca2+-dependent overshoot was larger for trabeculae from hypertrophied than from control hearts (increase of 14+/-2% versus 9+/-2% when frequency was decreased from 1 to 0.25 Hz). We conclude that (1) there is initially a larger imbalance in NADH production versus consumption rate in hypertrophy (because NADH fell more) and (2) the Ca2+-dependent recovery mechanism is enhanced in hypertrophy (because NADH recovered and overshot more), thus compensating for the larger imbalance.

Effects of left ventricular hypertrophy on force and Ca2+ handling in isolated rat myocardium

To study the effect of left ventricular (LV) hypertrophy on force and Ca2+ handling in isolated rat myocardium, LV hypertrophy was induced in rats by banding of the abdominal aorta. After 16 wk, arterial pressure was assessed by catheterization. LV trabeculae were isolated and loaded with indo 1 salt by iontophoretic injection. Isometric force and intracellular free Ca2+ concentration ([Ca2+]i) were measured at stimulation frequencies between 0.25 and 3 Hz and rest intervals between 2 and 240 s. Sarcoplasmic reticulum (SR) Ca2+ content was also investigated using rapid cooling contractures (RCC). Systolic and diastolic pressure as well as heart weight-to-body weight ratios were significantly elevated in banded compared with control animals (167 vs. 117 mmHg, 108 vs. 83 mmHg, and 4.6 vs. 4.0 mg/g, respectively). At high frequencies, twitch relaxation and [Ca2+]i decline rates were significantly slower in banded compared with control rats, and diastolic [Ca2+]i was higher in the banded rat muscles (at 3 Hz, force half-time = 83 vs. 68 ms; time constant of [Ca2+]i decline = 208 vs. 118 ms; and diastolic [Ca2+]i = 505 vs. 353 nM). These differences could not be ascribed to altered Na+/Ca2+ exchange, since twitch relaxation and Ca2+ handling were not different between groups in the presence of caffeine (or cyclopiazonic acid plus ryanodine), where relaxation depends primarily on Na+/Ca2+ exchange. After long rest intervals (> or = 120 s), control rats showed a significant rest potentiation of force and Ca2+ transients, whereas banded rats did not. In addition, RCC amplitudes increased with rest in control but were unaltered in banded rats. In summary, pressure-overload hypertrophy was associated with slower twitch relaxation and [Ca2+]i decline but also with blunted rest potentiation of twitches and SR Ca2+ content of LV trabeculae. The decrease in SR Ca(2+)-ATPase function in banded rats may contribute to the observed diastolic dysfunction associated with pressure-overload hypertrophy.

Fluorescent probes for living cells

The functional characteristics of fluorescent probes used for imaging and measuring dynamic processes in living cells are reviewed. Initial consideration is given to general design requirements for delivery, targeting, detectability and fluorescence readout, and current technologies for attaining them. Discussion then proceeds to the more application-specific properties of intracellular ion indicators, membrane potential sensors, probes for proteins and lipids, and cell viability markers.

Mechanisms of force inhibition by halothane and isoflurane in intact rat cardiac muscle

1. We investigated the mechanisms underlying the negative inotropic effect of the volatile anaesthetics halothane and isoflurane using twenty-two intact, right ventricular trabeculae of rat. [Ca2+]1 was measured qualitatively using either fluo-3 or fura-2, loaded into the cytosol via the acetoxymethyl (AM) ester form. Diastolic sarcomere length was adjusted to 2.1-2.2 micrograms and experiments were performed at 21-23 degrees C. 2. Halothane (0.25-3%) and isoflurane (0.48-4%) produced dose-dependent decreases in the amplitudes of the intracellular Ca2+ transients and twitch force. When the fluorescent Ca2+ indicator signals were corrected for changes in autofluorescence, neither volatile anaesthetic significantly changed diastolic [Ca2+]. 3. The ability of halothane and isoflurane to induce Ca2+ release from the sarcoplasmic reticulum of quiescent trabeculae was examined. When the superfusate was Ca2+ ad Na+ free (thereby preventing Na(+)-Ca2+ exchange and Ca2+ influx), 2% halothane, but not 4% isoflurane, evoked a transient increase in [Ca2+]i. 4. Halothane and isoflurane produced reversible, dose-dependent changes in cellular autofluorescence, the pattern of which was consistent with an increase in concentration of the reduced forms of nicotinamide adenine nucleotides and flavoproteins. This observation supports the putative inhibitory action of volatile anaesthetics at the site of Complex I of the mitochondrial electron transport chain. 5. Addition of the fatty acid hexanoate, a substrate that can be metabolized in the face of Complex I inhibition, did not appreciably attenuate the anaesthetic-induced negative inotropy; however, it greatly diminished autofluorescence changes. 6. To determine whether direct actions of the volatile anaesthetics on the contractile system contributed to the negative inotropy, external [Ca2+] was varied to modulate the amplitude of the Ca2+ transient. In the presence of 2% halothane or 4% isoflurane, restoration of the peak Ca2+ transient to control levels did not restore peak force. Moreover, halothane (1%) and isoflurane (16%) each reduced maximal Ca2(+)-activated force (attained using ryanodine tetani and a high external [Ca2+]) by around 15%. 7. We conclude that the negative inotropic actions of halothane and isoflurane on intact cardiac muscle reflect both reduced availability of Ca2+ and decreased responsiveness of the contractile system to Ca2+. The inhibitory action of the volatile anaesthetics on mitochondrial function does not contribute significantly to the negative inotropy but may lead to changes in cellular autofluorescence and misinterpretation of fluorescent Ca2+ indicator signals.

Pacing rate, halothane, and BDM affect fura 2 reporting of [Ca2+]i in intact rat trabeculae

Experiments were done on intact trabeculae from rats. Fura 2 in the salt form was microinjected directly into the myoplasm. The experiments were conducted at 30 degrees C, with 2 mM extracellular Ca2+ concentration and pacing at either 0.5 or 5 Hz. The aims were to establish a new method for in vivo calibration of fura 2 and to determine the effect of autofluorescence changes on intracellular Ca2+ concentration ([Ca2+]i) reported by fura 2. Autofluorescence was recorded under optimal conditions for fura 2 fluorescence (emission at 510 nm). By alteration of the oxidation-reduction state, it was shown that NADH is the main component of autofluorescence in heart. An increase in pacing frequency caused a decrease in autofluorescence. Both halothane and 2,3-butanedione monoxime (BDM) at 5-Hz pacing produced a substantial rise in autofluorescence, approaching the levels observed at 0.5-Hz pacing. The values for the dissociation constant (678 nM) and maximum fluorescence ratio of fura 2 for Ca2+ for the in vivo calibration are 3.4 times larger and 2.6 times smaller, respectively, than those found in vitro. Using the parameters obtained in vivo, we found that the diastolic and systolic [Ca2+]i of a twitch at 30 degrees C were 0.2 and 2.4 microM, respectively. Proper correction of the autofluorescence change unmasks the [Ca2+]i elevation caused by 5-Hz pacing. It was concluded that autofluorescence is not constant and that interventions affecting autofluorescence need correction if fura 2 is used to report [Ca2+]i.

Inhibition of mitochondrial calcium efflux by clonazepam in intact single rat cardiomyocytes and effects on NADH production

The aims of this study were to determine: (i) whether clonazepam and CGP37157, which inhibit the Na+/Ca2+ exchanger of isolated mitochondria, could inhibit mitochondrial Ca2+ efflux in intact cells; and (ii) whether any sustained increase in mitochondrial [Ca2+] ([Ca2+]m) could alter mitochondrial NADH levels. [Ca2+]m was measured in Indo-1/AM loaded rat ventricular myocytes where the cytosolic fluorescence signal was quenched by superfusion with Mn2+. NADH levels were determined from cell autofluorescence. Upon exposure of myocytes to 50 nM norepinephrine (NE) and a stimulation rate of 3 Hz, [Ca2+]m increased from 59 +/- 3 nM to a peak of 517 +/- 115 nM (n = 8) which recovered rapidly upon return to low stimulation rate (0.2 Hz) and washout of NE. In the presence of clonazepam, the peak increase in [Ca2+]m was 937 +/- 192 nM (n = 5) which remained elevated at 652 +/- 131 nM upon removal of the stimulus. CGP37157 in some cells did give the same inhibition of mitochondrial Ca2+ efflux as clonazepam, but the effect was inconsistent since not all cells were capable of following the stimulation rate in presence of this compound. NADH levels increased upon exposure to rapid stimulation in the presence of NE alone and recovered upon return to low stimulation rates, whereas in clonazepam treated cells the recovery of NADH was prevented. We conclude that clonazepam is an effective inhibitor of mitochondrial [Ca2+] efflux in intact cells and also maintains the increase in NADH levels which occurs upon rapid stimulation of cells.

Intracellular Ca2+ increases the mitochondrial NADH concentration during elevated work in intact cardiac muscle

It is not clear how mitochondrial energy production is regulated in intact tissue when energy consumption suddenly changes. Whereas mitochondrial [NADH] ([NADH]m) may regulate cellular respiration rate and energetic state, it is not clear how [NADH]m itself is controlled during increased work in vivo. We have varied work and [Ca2+] in intact cardiac muscle while assessing [NADH]m using fluorescence spectroscopy. When increased work was accompanied by increasing average [Ca2+]c (by increasing [Ca2+]c or pacing frequency), [NADH]m initially fell and subsequently recovered to a new steady state level. Upon reduction of work, [NADH]m overshot and then returned to control levels. In contrast, when work was increased without increasing average [Ca2+]o (by increasing sarcomere length), [NADH]m fell similarly, but no recovery or overshoot was observed. This Ca(2+)-dependent recovery and overshoot may be attributed to Ca(2+)-dependent stimulation of mitochondrial dehydrogenases. We conclude that the immediate initial increase in respiration rate upon elevation of work is not activated by increased [NADH]m (since [NADH]m rapidly fell) or by [Ca2+]o (since work could also be increased at constant [Ca2+]c). However, during sustained high work, a Ca(2+)-dependent mechanism causes slow recovery of [NADH]m toward control values. This demonstrates a Ca(2+)-dependent feed-forward control mechanism of cellular energetics in cardiac muscle during increased work.