Effects of hypoxia and metabolic inhibition on the intracellular sodium activity of mammalian ventricular muscle

Exercise and fatigue: integrating the role of K+, Na+ and Cl- in the regulation of sarcolemmal excitability of skeletal muscle

Perturbations in K+ have long been considered a key factor in skeletal muscle fatigue. However, the exercise-induced changes in K+ intra-to-extracellular gradient is by itself insufficiently large to be a major cause for the force decrease during fatigue unless combined to other ion gradient changes such as for Na+. Whilst several studies described K+-induced force depression at high extracellular [K+] ([K+]e), others reported that small increases in [K+]e induced potentiation during submaximal activation frequencies, a finding that has mostly been ignored. There is evidence for decreased Cl- ClC-1 channel activity at muscle activity onset, which may limit K+-induced force depression, and large increases in ClC-1 channel activity during metabolic stress that may enhance K+ induced force depression. The ATP-sensitive K+ channel (KATP channel) is also activated during metabolic stress to lower sarcolemmal excitability. Taking into account all these findings, we propose a revised concept in which K+ has two physiological roles: (1) K+-induced potentiation and (2) K+-induced force depression. During low-moderate intensity muscle contractions, the K+-induced force depression associated with increased [K+]e is prevented by concomitant decreased ClC-1 channel activity, allowing K+-induced potentiation of sub-maximal tetanic contractions to dominate, thereby optimizing muscle performance. When ATP demand exceeds supply, creating metabolic stress, both KATP and ClC-1 channels are activated. KATP channels contribute to force reductions by lowering sarcolemmal generation of action potentials, whilst ClC-1 channel enhances the force-depressing effects of K+, thereby triggering fatigue. The ultimate function of these changes is to preserve the remaining ATP to prevent damaging ATP depletion.

Cyanide inhibits the Na+/Ca2+ exchanger in isolated cardiac pacemaker cells of the cane toad

The effects of the metabolic inhibition on the activity of the Na+/Ca2+ exchanger (NCX) were studied in single isolated pacemaker cells from the cane toad. Ca2+ influx on NCX (reverse mode) was estimated by measuring the increase in intracellular calcium concentration ([Ca2+]i) in response to extracellular Na+-free solution. After application of 2 mM sodium cyanide for 3-5 min, the peak [Ca2+]i in Na+-free solution was significantly decreased from 377+/-42 nM to 260+/-46 nM, suggesting inhibition of NCX. To study Ca2+ efflux on NCX (forward mode), we recorded the tail currents on repolarization which were abolished by Ni2+ and by Na+-free solution. Cyanide decreased the amplitude of tail currents by 36+/-3%. To investigate the intrinsic properties of NCX during the metabolic inhibition, we used rapid application of caffeine to trigger sarcoplasmic reticulum Ca2+ release, which then stimulates NCX current (I(NCX) ). Both the caffeine-induced peak [Ca2+]i and the peak I(NCX) were reduced by cyanide exposure. When I(NCX) was plotted against [Ca2+], the slope of the decay phase was decreased in the presence of CN- to 44+/-8% of control, indicating that for a given [Ca2+]i there was less I(NCX) produced. These results show that cyanide (CN-) inhibits NCX activity at least partly through changes in the intrinsic properties of NCX. The inhibition of NCX probably contributes to the slower firing rate of pacemaker cells in CN-.

Effects of regional hypoxia and acidosis on Rb(+) uptake and energetics in isolated pig hearts: (87)Rb MRI and (31)P MR spectroscopic study

The study compared the effects of regional hypoxia and acidosis on Rb(+) uptake and energetics in isolated pig hearts perfused by the Langendorff method. The left anterior descending artery (LAD) was cannulated and the LAD bed was perfused with the same specific flow as the whole heart. Following equilibration with normal Krebs-Henseleit buffer (KHB, pO(2) 568 mm Hg, pH 7.42) the perfusate was switched to one that contained Rb(+) (Rb-KHB). Simultaneously, perfusion through the LAD was carried out with hypoxic (pO(2)=31 mm Hg), an acidemic (pH 7.12) or normal (pO(2)=550 mm Hg) Rb-KHB for 120 min. (87)Rb images of the entire heart or localized (31)P spectra from the left ventricular anterior wall were acquired. Hypoxia decreased the maximal (87)Rb image intensity and Rb(+) flux in the anterior wall to 79+/-9% and 85+/-7%, respectively, of that in the posterior wall. Extracellular acidosis did not affect (87)Rb image intensity and reduced Rb(+) flux (83+/-10%). During hypoxia phosphocreatine and ATP decreased to 36+/-10 and 50+/-15% of baseline, respectively and intracellular pH (pHi) decreased to 6.90+/-0.05. Extracellular acidosis did not affect the phosphocreatine or ATP levels but reduced pHi (7.06+/-0.18 vs. 7.26+/-0.06 in control). We suggest that intracellular acidosis plays a role in the inhibition of Rb(+) uptake during hypoxia.

Validation of confocal laser scanning microscopy for detecting intracellular calcium heterogeneity in liver slices

To investigate changes in the intracellular Ca(2+) ([Ca(2+)]i) in liver lobules under aerobic and hypoxic conditions, we measured [Ca(2+)]i in liver slices using a confocal laser scanning microscope (CLSM). The liver lobule is divided into 3 equal parts between the central vein and portal area, Zones 1, 2, and 3 from the portal side. [Ca(2+)]i in each zone of cultured rat liver lobules was measured by CLSM and a fluorescent Ca(2+) indicator (Rhod 2 AM). After the culture solution was changed to an Na(+)-free solution under aerobic conditions, the percentage of cells showing an increase in [Ca(2+)]i was 66.0+/-9.7% in Zone 1, 70.0+/-10.5% in Zone 2, and 94.0+/-9.7% in Zone 3. The percentage was significantly higher in Zone 3 than in Zones 1 and 2 (P< .01). Under hypoxic conditions, the percentage of cells showing an increase in [Ca(2+)]i was 6.0+/-9.7% in Zone 1, 8.0+/-10.3% in Zone 2, and 10.0+/-10.5% in Zone 3. There were no differences among the 3 zones. In all zones, the percentage was higher under aerobic conditions than under hypoxic conditions (P< .01). These results indicated that the increase in [Ca(2+)]i in liver lobules was heterogeneous. Measurement of [Ca(2+)]i in liver slices by CLSM was considered useful for studying heterogeneity between liver lobules, as well as between liver cells.

Altered cardiac sarcoplasmic reticulum function of intact myocytes of rat ventricle during metabolic inhibition

Changes in the behavior of the sarcoplasmic reticulum (SR) in rat ventricular myocytes were investigated under conditions of metabolic inhibition using laser-scanning confocal microscopy to measure intracellular Ca(2+) and the perforated patch-clamp technique to measure SR Ca(2+) content. Metabolic inhibition had several effects on SR function, including reduced frequency of spontaneous releases of Ca(2+) (sparks and waves of Ca(2+)-induced Ca(2+) release), increased SR Ca(2+) content (79.4+/-5.7 to 115.2+/-6.6 micromol/L cell volume [mean+/-SEM; P:<0.001]), and, after a wave of Ca(2+) release, slower reuptake of Ca(2+) into the SR (rate constant of fall of Ca(2+) reduced from 8.5+/-1.1 s(-)(1) in control to 5.2+/-0.4 s(-)(1) in metabolic inhibition [P:<0.01]). Inhibition of L-type Ca(2+) channels with Cd(2+) (100 micromol/L) did not reproduce the effects of metabolic inhibition on spontaneous Ca(2+) sparks. These results are evidence of inhibition of both Ca(2+) release and reuptake mechanisms. Reduced frequency of release could be attributable to either of these effects, but the increased SR Ca(2+) content at the time of reduced frequency of spontaneous release of Ca(2+) shows that the dominant effect of metabolic inhibition is to inhibit release of Ca(2+) from the SR, allowing the accumulation of greater than normal amounts of Ca(2+). In the context of ischemia, this extra accumulation of Ca(2+) would present a risk of potentially arrhythmogenic, spontaneous release of Ca(2+) on reperfusion of the tissue.

Regulation of fos and jun immediate-early genes by redox or metabolic stress in cardiac myocytes

We have previously demonstrated coordinate inductions of c-fos, c-jun, jun B, and jun D in cardiac myocytes exposed to hypoxia for 2 to 4 hours. Induction of these transcripts occurred before any significant loss of intracellular ATP. In the present study, the origin of the signal(s) that regulates immediate-early gene induction was investigated by comparing the effects of hypoxia with those of the metabolic inhibitors cyanide, deoxyglucose and cyanide combined, and iodoacetic acid. Cyanide, an inhibitor of oxidative metabolism, closely mimicked the metabolic effects of hypoxia, with elimination of oxygen consumption, increased lactate production, and minimal decline in ATP levels under both conditions. Compared with hypoxia, cyanide mediated small transient inductions of fos and jun transcripts that followed a different time course. The combination of cyanide and deoxyglucose resulted in inhibition of lactate production as well as respiration, and ATP dropped rapidly to 20% of control levels. The loss of intracellular ATP was followed by fourfold inductions of c-fos and c-jun with minor changes in jun B and jun D transcript levels. Similarly, iodoacetic acid caused a major (90%) loss of ATP and irreversible cell damage as measured by leakage of creatine phosphokinase enzyme and loss of membrane arachidonic acid; ATP loss was followed by fivefold to sevenfold inductions of c-fos, c-jun and jun B transcripts.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular sodium in cardiomyocytes using 23Na nuclear magnetic resonance

Intracellular sodium content in superfused isolated rat cardiomyocytes was measured using 23Na nuclear magnetic resonance. The shift reagent dysprosium tripolyphosphate was added to the buffer to distinguish between NMR signals from the intracellular region and the extracellular buffer. The NMR visibility of the intracellular sodium signal was experimentally determined by measuring the changes induced in the sodium NMR signals by application of ischemia as an intervention. Intracellular volume was accounted for by determining the change in the sodium signal upon adding cells (in beads) to the buffer solution at the beginning of each experiment and by killing the cells (in beads) with Triton X-100 at the end of each experiment. The visibility of intracellular sodium (relative to extracellular) was 0.47 +/- 0.12 (mean +/- S.D., n = 12). The average intracellular sodium concentration using this visibility is 29 +/- 4.5 mM (n = 12). This value is much higher than results obtained by some investigators using NMR techniques and by others using different standard methods, with the exception of those methods which evaluate the total intracellular sodium (atomic absorption spectroscopy and X-ray microanalysis). We conclude that total Nai is higher than generally reported, using other accepted techniques such as ion-specific electrodes, and that 23Na-NMR analysis can be used to accurately determine Nai in intact cells.

Sodium/calcium exchange modulates intracellular calcium overload during posthypoxic reoxygenation in mammalian working myocardium. Evidence from aequorin-loaded ferret ventricular muscles

We tested the hypothesis that the intracellular Ca2+ overload of ventricular myocardium during the period of posthypoxic reoxygenation is mediated by transsarcolemmal Ca2+ influx via Na+/Ca2+ exchange. In aequorin-loaded, ferret right ventricular papillary muscles, blockers of the sarcolemmal and the sarcoplasmic reticulum Ca2+ channels, slowed the Cai2+ transient, producing a convex ascent during membrane depolarization, followed by a concave descent during repolarization. The magnitude of the Cai2+ transient was affected by changes in the membrane potential, Nai+, Nao+, and Cao2+, and was blocked by Ni2+, or dichlorbenzamil. The calculated Na+/Ca2+ exchange current was in the reverse mode (Ca2+ influx) during the ascending phase of the Cai2+ transient, and was abruptly switched to the forward mode (Ca2+ efflux) at repolarization, matching the time course of the Cai2+ transient. During hypoxic superfusion, the Cai2+ transient was abbreviated, which was associated with a shorter action potential duration. In contrast, immediately after reoxygenation, the Cai2+ transient increased to a level greater than that of the control, even though the action potential remained abbreviated. This is the first demonstration on a beat-to-beat basis that, during reoxygenation, Ca2+ influx via Na+/Ca2+ exchange is augmented and transports a significant amount of Ca2+ into the ventricular myocardial cell. The activation of the exchanger at the time of reoxygenation appears to be mediated by Nai+ accumulation, which occurs during hypoxia.

Na+/H+ exchange and its inhibition in cardiac ischemia and reperfusion

The characterization of various ion transport systems has led to a better understanding of the effects, which seem to take part in the impairment of ischemic and reperfused cardiac tissue. This review discusses the role of the Na+/H+ exchange system in the pathophysiology of ischemia and reperfusion and the beneficial effects of its inhibition. At the onset of ischemia intracellular pH (pHi) decreases due to anaerobic metabolism and ATP hydrolysis, leading to an activation of Na+/H+ exchange. This in turn increases intracellular Na+ (Na+i) and activates Na+/K+ ATPase, with a consecutive increase of energy consumption. Since cellular Na+ and Ca++ transport are coupled by the Na+/Ca++ exchange system, which depends on the Na+ gradient, the high Na+i leads to increased intracellular Ca++ (Ca++i). After a certain period, Na+/H+ exchange is inactivated by a decrease of extracellular pH. In case of reperfusion the acid extracellular fluid is washed out, which reactivates Na+/H+ exchange, leading to an unfavourably fast restoration of pHi and a second time to Na+ and Ca++i overflow. High Ca++i is assumed to be one of the main reasons for ischemic and reperfusion injury, like arrhythmias, myocardial contracture, stunning and necrosis. It seems that the inhibition of Na+/H+ exchange can interrupt this process at an early phase and prevent or delay the consequences of ischemia and reperfusion as demonstrated by numerous investigators.

Na+/H+ exchanger and reperfusion-induced ventricular arrhythmias in isolated perfused heart: possible role of amiloride

The roles of the Na+/H+ exchange system in the development and cessation of reperfusion induced ventricular arrhythmias were studied in the isolated perfused rat heart. The hearts were perfused in the working heart mode with modified Krebs Henseleit bicarbonate (KHB) buffer and whole heart ischemia was induced by a one-way ball valve with 330 beat/min pacing. Ischemia was continued for 15 min followed by 20 min of aerobic reperfusion (control). Amiloride (1.0 mM), an inhibitor of the Na+/H+ exchange system, was added to the KHB buffer only during reperfusion (group B) or only during ischemic periods (group C). Electrocardiographic and hemodynamic parameters were monitored throughout the perfusion. Coronary effluent was collected through pulmonary artery cannulation and PO2, PCO2, HCO3- and pH were measured by blood-gas analyzer. The incidence of reperfusion induced ventricular arrhythmias was 100%, 100% and 0% in control, group B and group C, respectively. The mean onset time of termination of reperfusion arrhythmias was significantly shorter in group B than in control. PCO2 increased from 39.0 +/- 0.9 to 89.3 +/- 6.0 mmHg at the end of ischemia in control and from 40.6 +/- 0.4 to 60.5 +/- 5.8 in group C, the difference between groups was statistically significant. HCO3- level decreased from 21.8 +/- 0.1 to 18.3 +/- 0.5 mmol/l in control, however, this decrease was significantly inhibited in group C (from 22.0 +/- 0.5 to 20.3 +/- 0.2). The increase in PCO2 and the decrease in HCO3- in group B were similar over time to those observed in control.(ABSTRACT TRUNCATED AT 250 WORDS)

Relation of mitochondrial and cytosolic free calcium to cardiac myocyte recovery after exposure to anoxia

Mitochondrial calcium overload has been suggested as a marker for irreversible injury in the ischemic heart. A new technique is used to measure dynamic changes in mitochondrial free calcium concentration ([Ca2+]m) in electrically stimulated (0.2 Hz) adult rat cardiac myocytes during exposure to anoxia and reoxygenation. Cells were incubated with indo-1 AM, which distributes in both the cytosol and mitochondria. After Mn2+ quenching of the cytosolic signal, cells were exposed to anoxia, and the residual fluorescence was monitored. [Ca2+]m averaged 94 +/- 3 nM (n = 16) at baseline, less than the baseline diastolic cytosolic free calcium concentration ([Ca2+]c, 124 +/- 4 nM, n = 12), which was measured in cells loaded with the pentapotassium salt of indo-1. [Ca2+]m and [Ca2+]c rose steadily only after the onset of ATP-depletion rigor contracture. At reoxygenation 35 minutes later, [Ca2+]c fell rapidly to preanoxic levels and then often showed a transient further rise. In contrast, [Ca2+]m showed only a slight transient fall and a secondary rise at reoxygenation. At reoxygenation, cells immediately either recovered, demonstrating partial relengthening and retaining their rectangular shape and response to stimulation, or they hypercontracted to rounded dysfunctional forms. Recovery occurred only in cells in which [Ca2+]m or [Ca2+]c remained below 250 nM before reoxygenation. Early during reoxygenation, [Ca2+]m remained higher in cells that hypercontracted (305 +/- 36 nM) than in cells that recovered (138 +/- 9 nM, p less than 0.05), whereas [Ca2+]c did not differ between the two groups (156 +/- 10 versus 128 +/- 10 nM, respectively; p = NS).(ABSTRACT TRUNCATED AT 250 WORDS)

Dependence of hypoxic cellular calcium loading on Na(+)-Ca2+ exchange

Na(+)-Ca2+ exchange has been shown to contribute to reperfusion- and reoxygenation-induced cellular Ca2+ loading and damage in the heart. Despite the fact that both [Na+]i and [Ca2+]i have been documented to rise during ischemia and hypoxia, it remains unclear whether the rise in [Ca2+]i occurring during hypoxia is linked to the rise in [Na+]i via Na(+)-Ca2+ exchange before reoxygenation and how this relates to cellular injury. Single electrically stimulated (0.2 Hz) adult rat cardiac myocytes loaded with Na(+)-sensitive benzofuran isophthalate (SBFI), the new fluorescent probe, were exposed to glucose-free hypoxia (PO2 less than 0.02 mm Hg), and SBFI fluorescence was monitored to index changes in [Na+]i. Parallel experiments were performed with indo-1-loaded cells to index [Ca2+]i. The SBFI fluorescence ratio (excitation, 350/380 nm) rose significantly during hypoxia after the onset of ATP-depletion contracture, consistent with a rise in [Na+]i. At reoxygenation, the ratio fell rapidly toward baseline levels. The indo-1 fluorescence ratio (emission, 410/490 nm) also rose only after the onset of rigor contracture and then often showed a secondary rise early after reoxygenation at a time when [Na+]i fell. The increase in both [Na+]i and [Ca2+]i, seen during hypoxia, could be markedly reduced by performing experiments in Na(+)-free buffer. These experiments suggested that hypoxic Ca2+ loading is linked to a rise in Na+i via Na(+)-Ca2+ exchange. To show that Na(+)-Ca2+ exchange activity was not fully inhibited by profound intracellular ATP depletion, cells were exposed to cyanide, and then buffer Na+ was abruptly removed after contracture occurred. The sudden removal of buffer Na+ would be expected to stimulate cell Ca2+ entry via Na(+)-Ca2+ exchange. A large rapid rise in the indo-1 fluorescence ratio ensued, which was consistent with abrupt cell Ca2+ loading via the exchanger. The effect of reducing hypoxic buffer [Na+] on cell morphology after reoxygenation was examined. Ninety-five percent of cells studied in a normal Na(+)-containing buffer (144 mM NaCl, n = 38) and reoxygenated 30 minutes after the onset of hypoxic rigor underwent hypercontracture. Only 12% of cells studied in Na(+)-free buffer (144 mM choline chloride, n = 17) hypercontracted at reoxygenation (p less than 0.05). Myocytes were also exposed to hypoxia in the presence of R 56865, a compound that blocks noninactivating components of the Na+ current. R 56865 blunted the rise in [Na+]i typically seen after the onset of rigor, suggesting that Na+ entry may occur, in part, through voltage-gated Na+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Fluorescence measurements of cytoplasmic and mitochondrial sodium concentration in rat ventricular myocytes

1. The fluorescent Na+ indicator SBFI was incorporated into isolated ventricular myocytes using the acetoxymethyl (AM) ester. 2. The excitation spectrum was found to be shifted about 20 nm in the cell compared to in vitro. In the cell, an increase of [Na+] decreased fluorescence at 380 nm (F380) and had no effect at 340 nm (F340). The ratio (R = F340/F380) was used as a measure of [Na+]i. 3. In vivo calibration of SBFI for [Na+]i was obtained by equilibrating [Na+] across the plasma membrane with a divalent-free solution in the presence of gramicidin D. 4. Selective removal of the surface membrane with saponin or digitonin released only about 50% of the indicator. Following saponin treatment, cyanide or carbonylcyanide m-chlorphenylhydrazone (CCCP) increased the apparent [Na+] measured by the remaining (presumably mitochondrial) SBFI. It is suggested that mitochondrial [Na+] is normally less than cytoplasmic. 5. Attempts to examine the effects of metabolic inhibition on [Na+]i were hampered by changes of autofluorescence due to changes of [NADH]. It is shown that this effect can be corrected for using the isosbestic signal (excited at 340 nm). 6. Inhibition of both aerobic metabolism (with CN-) and glycolysis (glucose removal or iodoacetate) produced a gradual increase of [Na+]i. This began before the resting contracture developed and may (via Na(+)-Ca2+ exchange) account for some of the rise of diastolic [Ca2+]i seen in previous work. The rise of [Na+]i began at about the same time as the decrease of systolic contraction and therefore at a time when [ATP]i had begun to fall.

Mechanism of pHi regulation by locust neurones in isolated ganglia: a microelectrode study

1. We have measured membrane potential (Em) and intracellular pH (pHi), and sodium and chloride activities (aNai and aCli) in exposed dorsal unpaired median neurones in isolated metathoracic ganglia from the desert locust, Schistocerca gregaria using eccentric double-barrelled ion-sensitive microelectrodes. 2. In the absence of added HCO3- the steady-state pHi was 7.21 +/- 0.13 (mean +/- S.D.) at a mean membrane potential of -37 +/- 7.0 mV (S.D.) (n = 44 cells). The pHi was always more alkaline than predicted for passive H+ distribution. 3. The pHi recovery from acid loads, induced by weak acid application or weak base removal, was pHi dependent and associated, in both the presence and absence of added CO2-HCO3-, with a transient increase in aNai. 4. In the absence of added HCO3-, application of the Na(+)-H+ exchange blocker amiloride or external Na+ removal caused intracellular acidification. Also in the absence of added HCO3- the inhibitor SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid) caused an acidification of about 0.2 pH units which was not additive to the effects of the removal of external Na+. 5. We found that the application of a CO2-HCO3(-)-containing solution increased the rate of pHi recovery from acidification. 6. Intracellular chloride was decreased by intracellular acidification in the presence of added CO2-HCO3-. In the presence of amiloride, intracellular Cl- depletion inhibited pHi regulation. 7. Simultaneous application of SITS (160 microM) and removal of CO2-HCO3- revealed a continuous underlying acid load of 0.03-0.05 pH unit min-1. 8. We conclude that locust neurones possess at least two pHi-regulating mechanisms which operate against a continuous acid load. One is a Na(+)-H+ exchanger which can be blocked by amiloride, while the second is a Na(+)-dependent Cl(-)-HCO3- exchanger. The latter mechanism appears to be able to operate in the absence of added HCO3- and can recover pHi to around pH 7.4; it is probably the main pHi regulating mechanism. The Na(+)-H+ exchanger appears to activate at more acid pHi and being less energy efficient may serve a protective role.

Potentiometric measurements of hydrogen and cyanide ions in buffered media

Potentiometric measurements of hydrogen and cyanide levels were studied in different buffered media that are used in biological experiments. The addition of 1-10mM NaCN increased the pH (from 7.2-7.5 up to 9.5) of either bicarbonate/phosphate-buffered (artificial cerebrospinal fluid (ACSF), sera, and whole blood) or Hepes-buffered solutions in a concentration-dependent manner. While in aerated ACSF this increase was transient (half-relaxation time less than 4 min), the increase in pH was sustained in Hepes buffer. A mathematical model that predicts the maximum cyanide-induced increases in pH was derived. When NaCN was added to either ACSF or Hepes-buffered solutions, a Nernst relation was obtained, but deviations from Nernstian responses resulted when NaCN was added to sera or whole blood. These responses, however, differed substantially from the theoretical results that were based on the relative amounts of ionized cyanide present in the various pH environments. In addition, attempts were made to indirectly examine the formation and escape of HCN by calculating the time constants of decay (tau d) of the cyanide-induced potentiometric responses. With progressively higher NaCN concentrations in ACSF, the values of tau d did not decrease, but were constant. As predicted, however, the values of tau d were larger when solutions were covered or had higher initial pH values. These results suggest that in the solutions tested, the cyanide electrode used in the present study measured the total amount rather than the ionized portion of dissolved cyanide and that the values of tau d do not correspond to formation/escape rates of HCN.