Dependence of myosin-ATPase on structure bound creatine kinase in cardiac myofibrils from rainbow trout and freshwater turtle

The influence of myofibrillar creatine kinase on the myosin-ATPase activity was examined in cardiac ventricular myofibrils isolated from rainbow trout (Oncorhynchus mykiss) and freshwater turtle (Trachemys scripta). The ATPase rate was assessed by recording the rephosphorylation of ADP by the pyruvate kinase reaction alone or together with the amount of creatine formed, when myofibrillar bound creatine kinase was activated with phosphocreatine. The steady-state concentration of ADP in the solution was varied through the activity of pyruvate kinase added to the solution. For rainbow trout myofibrils at a high pyruvate kinase activity, creatine kinase competed for ADP but did not influence the total ATPase activity. When the ADP concentration was elevated within the physiological range by lowering the pyruvate kinase activity, creatine kinase competed efficiently and increased the ATPase activity twice or more for both trout and turtle. As examined for trout myofibrils, the ATPase activity was reduced about four times by inhibiting the activity of myofibril-bound creatine kinase with iodoacetamide and this reduction was only partially counteracted, when the creatine kinase activity was restored by adding creatine kinase to the solution. Hence, the results suggest that myofibril-bound creatine kinase is needed to fully activate the myosin-ATPase activity in hearts from ectothermic vertebrates despite their low energy turn-over relative to endothermic species.

The effect of adrenaline and high Ca2+ on the mechanical performance and oxygen consumption of the isolated perfused trout (Oncorhynchus mykiss) heart

In heart muscle from mammals, catecholamines frequently evoke an oxygen waste and reduce efficiency. It was examined if this also applies to fish in which heart muscle activity is often restricted by oxygen availability. In the isolated perfused heart from rainbow trout, adrenaline (0.5 microM) increased power output by approximately 97%, when afterload was adjusted to maximum both before and after adrenaline addition, and by approximately 68%, when afterload remained at the maximum obtained before adrenaline addition. Oxygen consumption was enhanced by a similar amount (approximately 70%) in both situations. Hence, efficiency, i.e. power output/oxygen consumption, increased significantly from 25 to 30% for the heart always exposed to maximal afterload, whereas it stayed unchanged at 24% for the heart exposed to control afterload only. Adrenaline increases the Ca2+ activity participating in activation, but doing this by elevating perfusate Ca2+ from 1.5 to 5 mM instead of applying adrenaline did not change the mechanical efficiency, although afterload was maximized. The results suggest that catecholamines enhance heart pump activity in the living trout without any oxygen waste. On the contrary, the oxygen budget may even improve when the peripheral resistance is increased by catecholamines.

Force development at elevated [Mg2+]o and [K+]o in myocardium from the freshwater turtle (Trachemys scripta) and influence of factors associated with hibernation

The effects of high [Mg(2+)](o) on force development were examined for heart muscle of freshwater turtle. Plasma [Mg(2+)] during hibernation may increase drastically and like plasma [K(+)] approach values as high as 10 mM. Each experiment performed at either 20 or 5 degrees C involved four ventricular preparations of which one pair was exposed to 10, and one to 1 mMMg(2+). One preparation of each pair was furthermore exposed to 10 mM K(+), whereas the other was maintained at 2.5 mM K(+). During oxygenation, high relative to low [Mg(2+)](o) displayed a weak tendency to reduce twitch force; a tendency that was not reduced by elevations of [Ca(2+)](o). Severe hypoxia accentuated the negative effect of high [Mg(2+)](o). This effect disappeared, however, when hypoxia was combined with acidosis obtained by 24 mM lactic acid. In comparison to [Mg(2+)](o), high [K(+)](o) strongly depressed force development under both oxygenation and hypoxia, but no consistent interplay between the two ions was revealed. The negative inotropic effects of both high [Mg(2+)](o) and high [K(+)](o) were reduced or eliminated by 10 muM adrenaline. In conclusion the cardiac effects of elevations in [Mg(2+)](o) appear to be small during hibernation, in particular when considering the concomitant adrenergic stimulation and acidosis.

Regulation of mitochondrial energy production in cardiac cells of rainbow trout (Oncorhynchus mykiss)

In skinned rat cardiac fibres, mitochondrial affinity for endogenous ADP generated by creatine kinase and Ca2+-activated ATPases is higher than for exogenous ADP added to the surrounding medium, suggesting that mitochondria are functionally coupled to creatine kinase and ATPases. Such a coupling may be weaker or absent in ectothermic vertebrate cardiac cells, because they typically have less elaborate intracellular membrane structures, higher glycolytic capacity and lower working temperature. Therefore, we examined skinned cardiac fibres from rainbow trout at 10 degrees C. The apparent mitochondrial affinity for endogenous ADP was obtained by stimulation with ATP and recording of the release of ADP into the surrounding medium. The apparent affinity for endogenous ADP was much higher than for exogenous ADP suggesting a functional coupling between mitochondria and ATPases. The apparent affinity for exogenous ADP and ATP was increased by creatine or an increase in Ca2+-activity, which should increase intrafibrillar turnover of ATP to ADP. In conclusion, ADP seems to be channelled from creatine kinase and ATPases to mitochondria without being released to the surrounding medium. Thus, despite difference in structure, temperature and metabolic capacity, trout myocardium resembles that of rat with regard to the regulation of mitochondrial respiration.

Creatine kinase and mitochondrial respiration in hearts of trout, cod and freshwater turtle

The importance of the creatine kinase system in the cardiac muscle of ectothermic vertebrates is unclear. Mammalian cardiac muscle seems to be structurally organized in a manner that compartmentalizes the intracellular environment as evidenced by the substantially higher mitochondrial apparent Km for ADP in skinned fibres compared to isolated mitochondria. A mitochondrial fraction of creatine kinase is functionally coupled to the mitochondrial respiration, and the transport of phosphocreatine and creatine as energy equivalents of ATP and ADP, respectively, increases the mitochondrial apparent ADP affinity, i.e. lowers the Km. This function of creatine kinase seems to be absent in hearts of frog species. To find out whether this applies to hearts of ectothermic vertebrate species in general, we investigated the effect of creatine on the mitochondrial respiration of saponin-skinned fibres from the ventricle of rainbow trout, Atlantic cod and freshwater turtle. For all three species, the apparent Km for ADP appeared to be substantially higher than for isolated mitochondria. Creatine lowered this Km in trout and turtle, thus indicating a functional coupling between mitochondrial creatine kinase and respiration. However, creatine had no effect on Km in cod ventricle. In conclusion, the creatine kinase-system in trout and turtle hearts seems to fulfil the same functions as in the mammalian heart, i.e. facilitating energy transport and communication between cellular compartments. In cod heart, however, this does not seem to be the case.

Mechanical efficiency of the trout heart during volume and pressure-loading: metabolic implications of the stiffness of the ventricular tissue

In the mammalian heart the metabolic costs of pressure loading exceed those of volume loading. As evidence suggests that the opposite may be true in fish, we evaluated the metabolic costs of volume and pressure loading in the isolated trout heart and compared the results with the mammalian heart based on the biomechanical properties of cardiac muscle. The highest power output (2.33+/-0.32 mW g(-1), n=5) appeared at the highest preload pressure tested (0.3 kPa) and at an afterload of 5 kPa. At a higher afterload, power did not increase because stroke volume fell. The highest mechanical efficiency (20.7+/-2.0%, n=5) was obtained at a preload of 0.15 kPa and an afterload of 5 kPa. Further increases in preload or afterload did not increase mechanical efficiency, probably because of increases in ventricular wall stress which increased the oxygen consumed disproportionately more than the stroke work. Under pressure unloading (25% decrease in power output), mechanical efficiency was significantly higher in comparison with volume unloading. Given that stiffness of the ventricular tissue is larger in trout than in rat papillary muscles, it is suggested that the increased strain during volume loading is energetically disadvantageous for stiff muscles like those of trout, but it is advantageous when muscle stiffness is lower as it occurs in the rat papillary muscle.

Oxygen consumption and force development in turtle and trout cardiac muscle during acidosis and high extracellular potassium

Relative to species such as rainbow trout, freshwater turtle shows a high tolerance to challenges involving acidosis and increases in extracellular K+. Therefore, the effects of acidosis or high K+ on twitch force and oxygen consumption were examined in ventricular ring preparations from these two species. The oxygen consumption associated with force development was estimated by net oxygen consumption (oxygen consumption during twitch force development minus that during rest). For turtle, elevation of CO2 from 2% (pH 7.7) to 12% (pH 6.9) in the gas equilibrating the muscle bath decreased twitch force by 20% without any effects on oxygen consumption. Decreasing pH from 7.7 to 6.9 with 22 mM lactic acid had similar effects. For trout, CO2-induced acidosis decreased twitch force by approximately 60%. Furthermore, force development became energetically less efficient as it fell disproportionately more than net oxygen consumption. This was not observed for lactic acidosis. For trout but not for turtle, acidosis resulted in an increase in oxygen consumption during rest. An increase in extracellular K+ from 2.5 mM to 10 mM depressed force and oxygen consumption proportionately for both species. Adrenaline (10 microM) increased twitch force for both species and oxygen consumption for trout; it attenuated the effects of high extracellular K+. Neither adrenaline nor high K+ influenced the ratio of force to net oxygen consumption. As opposed to high extracellular K+, acidosis appears to increase the energetic cost of contractility, particularly for the trout heart.

Effects of high extracellular [K+] and adrenaline on force development, relaxation and membrane potential in cardiac muscle from freshwater turtle and rainbow trout

Increases in extracellular K(+) concentrations reduced the twitch force amplitude of heart muscle from the freshwater turtle (Trachemys scripta elegans) and rainbow trout (Oncorhynchus mykiss). Adrenaline augmented twitch force amplitude and reduced the relative influence of [K(+)]. In the absence of adrenaline, high [K(+)] had less effect in reducing twitch force in turtle than in trout, whereas the reverse was true in the presence of adrenaline. Under anoxic conditions, twitch force was lower in 10 mmol l(−1) than in 2.5 mmol l(−1) K(+) in both preparations, but adrenaline removed this difference. A further analysis of turtle myocardium showed that action potential duration was shorter and resting potential more positive in high [K(+)] than in low [K(+)]. Adrenaline restored the duration of the action potential, but did not affect the depolarisation, which may attenuate Na(+)/Ca(2+) exchange, participating in excitation/contraction coupling. The contractile responses in the presence of adrenaline were, however, similar in both high and low K(+) concentrations when increases in extracellular Ca(2+) were applied to increase the demand on excitation/contraction coupling. The possibilities that adrenaline counteracts the effects of high [K(+)] via the sarcoplasmic reticulum or sarcolemmal Na(+)/K(+)-ATPase were examined by inhibiting the sarcoplasmic reticulum with ryanodine (10 micromol l(−1)) or Na(+)/K(+)-ATPase with ouabain (0.25 or 3 mmol l(−)). No evidence to support either of these possibilities was found. Adrenaline did not protect all aspects of excitation/contraction coupling because the maximal frequency giving regular twitches was lower at 10 mmol l(−1) K(+) than at 2.5 mmol l(−1) K(+).

Ca(2+) uptake in the sarcoplasmic reticulum from the systemic heart of octopod cephalopods

We have measured Ca(2+) uptake in crude homogenates of heart tissue, as well as cell shortening and ionic currents in isolated myocytes exposed to caffeine, to characterize Ca(2+) uptake in the sarcoplasmic reticulum (SR) of the systemic heart of octopus. The maximal rate of SR Ca(2+) uptake in crude homogenates of octopus heart was 43+/−4 (mean +/− s.e.m., N=7), compared with 28+/−2 nmol min(−)(1)mg(−)(1) protein (N=4) in homogenates of rat heart. The Ca(2+)-dependency of SR Ca(2+) uptake was similar for the two species, with a Ca(2+) activity at half-maximal uptake rate (pCa(50)) of 6.04+/−0.02 for octopus and 6.02+/−0.05 for rat. Exposure of isolated myocytes to 10 mmol l(−)(1) caffeine resulted in cell shortening to 53+/−2 % of the resting cell length and an inward trans-sarcolemmal ionic current. The charge carried by this current was 3.28+/−0.70 pC pF(−)(1) (mean +/− s.e.m., N=5) corresponding to extrusion of 34.0+/−0.7 amol Ca(2+)pF(−)(1) from the cell by Na(+)/Ca(2+) exchange. This is approximately 50 times more than the Ca(2+) carried by the Ca(2+) current elicited by a 200 ms depolarization from −80 to 0 mV and corresponds to an increase in the total intracellular [Ca(2+)] of 404+/−86 (μ)mol l(−)(1) non-mitochondrial volume due to Ca(2+) release from the SR. Thus, we find that at 20 degrees C in the SR both Ca(2+) content and Ca(2+) uptake rate in the systemic heart of octopus are comparable with or larger than the corresponding values obtained in the rat heart. These results support the argument that the SR may play an important role in the regulation of contraction in the systemic heart of cephalopods.

Influence of inorganic phosphate and energy state on force in skinned cardiac muscle from freshwater turtle and rainbow trout

Inorganic phosphate, which increases in the hypoxic cardiac cell, depresses force development. The cardiac muscle of freshwater turtle maintains a remarkably high contractility during hypoxia; this may involve a low sensitivity to phosphate. Therefore, freshwater turtle and rainbow trout were compared with regard to Ca(2+)-activated force in skinned atrial trabeculae in a bath containing 3 mM ATP buffered by 15 mM creatine phosphate in the presence of creatine kinase. For turtle, an increase in phosphate from 0 mM to either 6 mM or 12 mM reduced maximal force by 50% and 80% respectively, whereas the Ca2+ activity eliciting half maximal force (Ca0.5) was increased by 70% in 6 mM and could not be reliably recorded in 12 mM. For trout, the effects of phosphate were less pronounced. An increase from 0 mM to 12 mM did not affect maximal force significantly, but elevated Ca0.5 by 70%. Hypoxia increases ADP as creatine phosphate is shifted to creatine, therefore, creatine phosphate was changed from 15 mM to 3 mM and creatine from 0 mM to 12 mM. After these changes, the elevation of phosphate from 0 mM to 12 mM had no significant effects for either turtle or trout. In conclusion, the high performance of turtle cardiac muscle during hypoxia does not involve a low sensitivity of the contractile system to phosphate. In addition, the effect of increased phosphate seems to be offset by a concomitant increase in ADP.

Ca2+ activated myosin-ATPase in cardiac myofibrils of rainbow trout, freshwater turtle, and rat

The Ca(2+)-activated myosin-ATPase and its dependence on hypoxia were assessed in freshwater turtle, rainbow trout, and in some cases rat. At 20 degrees C and pH 7.3, the maximal ATPase activity was (mean +/- SEM): turtle 0.040 +/- 0.003, trout 0.090 +/- 0.005, and rat 0.12 +/- 0.004 mmol*min-1*g-1 myofibrillar dry weight. The turnover number was about three times lower for turtle than for trout. Trout is typically active at lower temperatures than turtle, and its myosin-ATPase activity was about three times lower at 10 degrees than at 20 degrees C. Addition of 12 mM phosphocreatine showed that the myosin-ATPase activity covered by myofibrillar creatine kinase was 22 +/- 2% for turtle, 14 +/- 2% for trout, and 69 +/- 5% for rat. At pH 6.8 relative to 7.3, the maximal M-ATPase activity was the same, whereas the Ca(2+)-sensitivity decreased, and more so for trout than for turtle. This difference disappeared, when trout myocardium was examined at 10 degrees C. P(i) (15 mM) affected neither maximal activity nor Ca(2+)-sensitivity. ADP, however, reduced maximal myosin-ATPase activity, and more so in trout than in turtle. In conclusion, the "slow"-type myosin, the low sensitivity of acidification and ADP, and the high creatine kinase/myosin-ATPase ratio in turtle relative to trout accord with the well-known ability of turtle myocardium to work during hypoxia. However, the difference in living temperature between turtle and trout obscures the situation (e.g. inclusion of rat data suggests that the creatine kinase/myosin-ATPase ratio is related to temperature.

Cardiac force and high-energy phosphates under metabolic inhibition in four ectothermic vertebrates

Isometric twitch tension of ventricular preparations stimulated at 0.2 Hz fell over 30 min of anoxia by a fraction decreasing in the order rainbow trout, cod, eel, and freshwater turtle. Drops in the estimated cytoplasmic energy state were related to larger tension losses for trout than for the other species, possibly due to larger changes in free phosphate. Anoxic energy degradation was slower for turtle than for the other species. Anoxia combined with glycolytic inhibition (1 mmol/l iodoacetate) enhanced the decrease in twitch tension for a drop in energy state and enlarged the increase in ADP/ATP relative to that in creatine/phosphocreatine to an extent inversely related to the creatine kinase activity. Furthermore, it increased resting tension to an extent possibly related to myosin-adenosinetriphosphatase (ATPase) activity and lowered the content of phosphorylated adenylates in trout and turtle myocardium. The results indicate that species differences in performance of the metabolically challenged myocardium depend on energy-degrading processes, e.g., myosin-ATPase activity, phosphate release, creatine kinase activity, and efflux/degradation of ADP and AMP, and that glycolysis offers protection due to its cytoplasmic localization.

Creatine kinase, energy-rich phosphates and energy metabolism in heart muscle of different vertebrates

Maximal activities of creatine kinase, pyruvate kinase and cytochrome oxidase and total concentrations of creatine and phosphorylated adenylates were measured in cardiac muscle of hagfish, eight teleost species, frog, turtle, pigeon and rat. The ratio of creatine kinase to cytochrome oxidase with cytochrome oxidase as a rough estimate of aerobic capacity and cellular "energy turnover", was increased in myocardia of hagfish, turtle and crucian carp. These myocardia are likely to be frequently exposed to oxygen deficiency. In agreement with this, they possess a high relative glycolytic capacity as indicated by a high pyruvate kinase/cytochrome oxidase ratio. The creatine kinase/cytochrome oxidase ratio for the other myocardia varied within a factor of 2, except the value for cod myocardium which was below the others. Total creatine varied among species and was high in active species such as herring, pigeon and rat but also high in crucian carp. The variation in total concentration of phosphorylated adenylates was considerably less than the variation in total creatine. The high creatine kinase/cytochrome oxidase ratio in myocardia likely to be challenged by hypoxia may represent an enhanced efficiency for both "spatial" and "temporal" buffering of phosphorylated adenylates to attenuate the impact of a depressed energy liberation. As to the differences in total creatine, this factor influences not only the cellular energy distribution but possibly also contractility via an effect on the free phosphate level.

Sarcoplasmic reticulum, potassium, and cardiac force in rainbow trout and plaice

The role played by the sarcoplasmic reticulum in force development and in cellular Ca2+ balance and its dependence on extracellular K+ were examined in heart ventricular tissue of rainbow trout and plaice. Compared with the steady-state twitch at a stimulation rate of 0.2 Hz, a 30-s rest led to a similar increase in twitch force in trout heart, regardless of whether [K+] was 2.5 or 5 mM. At 5 mM (but not at 2.5 mM) post-rest potentiation increased with increasing rest periods (from 30 to 900 s). These post-rest potentiations were removed or transformed into a loss of force by 10 microM ryanodine or 8 mM caffeine. In the plaice heart, where the sarcoplasmic reticulum is claimed to be sparse, the post-rest potentiation and the influence of ryanodine were small. The Ca2+ uptake measured during 5 min with 45Ca in the trout heart was higher in 5 than in 2.5 mM K+, regardless of the concomitant stimulation rate. This effect of K+ was removed by 10 microM ryanodine. The twitch force after 5 min of rest correlated significantly with the Ca2+ uptake, whereas the twitch force developed at a rate of 0.2 or 1.0 Hz did not. In conclusion, an elevation of K+ appears to stimulate the Ca2+ uptake of the sarcoplasmic reticulum. The twitch force after prolonged rest seems to relate to the Ca2+ contained in this organelle, whereas this does not apply to the twitch force developed at more physiological rates (0.2 or 1 Hz).

Force frequency relation in the myocardium of rainbow trout. Effects of K+ and adrenaline

Isolated heart ventricular preparations from rainbow trout were electrically stimulated to contraction. Following a temporary change in stimulation rate from 0.2 Hz to a higher value, the force fell to a minimum after which it increased and levelled off. Upon the return to 0.2 Hz a further transient increase in force appeared. The latter two responses were stimulated by an increased extracellular K+, which is known to inactivate the Na+ channel. The initial negative inotropic effect, in contrast to the two subsequent positive effects, was associated with a parallel decrease in amplitude of the action potential measured in 15 mM K+, used as an index of the Ca2+ influx. One micromolar (1 microM) ryanodine did not affect either the negative or the positive responses due to an increase in stimulation rate, but depressed the force developed after prolonged periods of rest. Ten micromolar (10 microM) adrenaline strongly inhibited the positive effects of an elevation of frequency. An elevation of extracellular Na+ from 141 to 166 mM had a similar effect. In conclusion, the positive effects occurring in 15 mM K+ do not seem to depend on the initial Na+ current. They may nevertheless depend on changes of the cellular Na+ balance as suggested by the effects of adrenaline, K+ and Na+. The functional role of the sarcoplasmic reticulum is unclear.

Extracellular Ca2+, force, and energy state in cardiac tissue of rainbow trout

The importance of ATP and phosphocreatine (PCr) concentrations for myocardial force development was examined in electrically paced ventricular strips from rainbow trout. Three metabolic situations were studied involving either an aerobic block (N2, 5 mM NaCN), both an aerobic and a glycolytic block (1 mM iodoacetate), or no metabolic inhibitors. Increasing extracellular Ca2+ from 1.25 to 5 mM enhanced twitch-force development in all of these situations but caused no change in resting force or ATP and PCr content, except for the noninhibited preparations for which PCr increased. Anaerobiosis caused a decrease in the PCr concentration together with a fall in twitch force and an increase in resting force. Notably the changes in the contractile system associated with a given reduction in PCr were smaller in the absence than in the presence of iodoacetate. The results show that an increased Ca2+ availability stimulates the twitch-force development also at markedly lowered levels of high-energy phosphates. A maintained Ca2+ regulation appears to be one important reason for this. Furthermore, glycolysis seems to protect contractility in a way not reflected in the level of high-energy phosphates.

Electrical and mechanical activity in heart tissue of flounder and rainbow trout during acidosis

1. Twitch force and voltage across the sarcolemma were measured in heart tissue of flounder and rainbow trout. 2. For the trout heart, hypercapnia was followed by a loss of force and an action potential prolongation. 3. This was also observed for the flounder heart, but only initially. 4. About 5 min after the onset of hypercapnia, an increase in force and a shortening of the action potential occurred in the flounder heart. 5. After about 30 min of hypercapnia a decrease in force and a prolongation of the action potential slowly appeared. 6. These results can be interpreted in terms of a species-dependent effect of acidosis on the cellular Ca2+ handling and the influence of intracellular Ca2+ on the action potential.

Contractility and 45Ca fluxes in heart muscle of flounder at a lowered extracellular NaCl concentration

The twitch force of isolated electrically paced ventricular strips of flounder, Platichthys flesus L., increased after lowering the extracellular sodium chloride concentration by 50 mmol l-1. This response was markedly reduced by replacing the sodium chloride with either Tris-HCl or sucrose, so that osmolarity was unchanged. The 45Ca efflux decreased and the 45Ca influx increased when the extracellular sodium concentration Nao+ was lowered. In contrast, changing only the osmolarity had no observable effect on these fluxes. An increased resting tension appeared in strips exposed to a Na+-, Ca2+-free solution. This was transient at an unchanged osmolarity but became permanent at an osmolarity lowered by 100 mosmol l-1. These results suggest that both a lowered Nao and a lowered osmolarity have a positive inotropic effect, due respectively to an increased cellular uptake of Ca2+ and a redistribution of cellular Ca2+.

Effects of [Ca2+]o on contractility in the anoxic cardiac muscle of mammal and fish

Electrically paced atrial strips of hearts from rat and rainbow trout were exposed to increasing extracellular Ca2+ concentration, [Ca2+]o. This resulted in increases in the peak force in oxygenated atria from both species. During anoxia this response was suppressed for the rat, but accentuated for trout atrium.

Acidosis and cardiac muscle contractility: comparative aspects

The evolutionary step involving transition from water- to air-breathing exposed the vertebrate cell to an increased risk of becoming acidotic. This is due to the fact that water-breathers generally excrete CO2 more easily than air-breathers. CO2 rapidly diffuses into the cell, where it may result in an excess of hydrogen ions. This is of interest as to the cardiac muscle, since these ions depress contractility, to a large extent probably by inhibiting the inotropic action of calcium ions in a competitive way. The present review, however, concerns the fact that the heart muscle may have an inherent ability to resist the negative inotropic effects of hydrogen ions. This is not a general property of the vertebrate heart, as it shows a clear tendency to be present in most air-breathers, whereas it is absent in most pure water-breathers, i.e. in most fishes. Measurements of the intracellular pH and of the tissue buffer capacity indicate that this ability to maintain force at a normal level in spite of an ongoing CO2-acidosis involves neither neutralization nor excretion of excess hydrogen ions. Instead, studies involving calcium-flux measurements and interventions in the cellular calcium-distribution suggest that the intracellular calcium ion deficit due to acidosis is compensated for by an increase of the calcium pool involved in the beat to beat regulation of cardiac force. How this is accomplished is unclear, although evidence was obtained that mitochondrial calcium stores may be involved.

Effect of vanadate and of removal of extracellular Ca2+ and Na+ on tension development and 45Ca efflux in rat and frog myocardium

Vanadate in the range 0-5 mM has positive inotropic effects on myocardial strips of frog and to a lesser extent on those of rat. Inhibiting the sarcolemmal Na+, Ca2+ exchange by a solution free of Ca2+ and Na+ caused a drop in 45Ca efflux and a transient increase in resting tension. These effects were more expressed for the frog than for the rat myocardium, which suggests that the Na+ for Ca2+ exchange across the cell membrane is more important in the frog than in the rat myocardium. A subsequent addition of vanadate at 2 or 5 mM had no effect on 45Ca efflux, while it increased the resting tension. This increase was higher for the frog than for the rat myocardium. These results suggest that the inotropic effects of vanadate may be due to an effect on membrane-bound Ca2+-ATPase.

pHi, contractility and Ca-balance under hypercapnic acidosis in the myocardium of different vertebrate species

The influence of hypercapnic acidosis upon the heart was examined in four vertebrate species. The CO2 in the tissue bath was increased from 2.7 to 15% at 12 degrees C for flounder (Platichthys flesus) and cod (Gadus morhua) and from 3 to 13% at 22 degrees C for turtle (Pseudemys scripta) and rainbow trout (Salmo gairdneri). During hypercapnia, as previously described, there was a decline and recovery of contractility in heart strips of flounder and turtle, and a sustained decrease in cod and rainbow trout. At high CO2 the increase in contractile force following increases in the extracellular Ca-concentration were smaller for the cod myocardium than for the other myocardia. The intracellular pH (pHi), measured with the DMO method, in heart strips of turtle and trout was significantly lower at high than at low CO2. This acidifying effect expressed as the increase in the intracellular concentration of hydrogen ions was larger in the turtle than in the trout myocardium. Intracellular Ca-activity, measured by efflux of 45Ca from preloaded heart strips, was unaffected by high CO2 in trout, but was raised in the other three species. Thus the ability to counteract the negative inotropic effect of hypercapnia is apparently not due to cellular buffering or extrusion of hydrogen ions. More probably it involves (a) a release of intracellular Ca; (b) a positive inotropic effect of an increase in intracellular Ca-activity.

Relation between non-bicarbonate buffer value and tolerance to cellular acidosis: a comparative study of myocardial tissue

There is a large variation in the tolerance of myocardial tissue to cellular acidosis. Assuming the cytoplasmic acid-base status to be mainly a result of intracellular processes, this variation could be produced by variations in the tissue non-bicarbonate buffer value. In the myocardial tissue from nine vertebrate species, the non-bicarbonate buffer value did not correlate either with ability to develop tension under hypercapnic acidiosis or with the indirectly estimated capacity for anaerobic glycolysis. Therefore, differences in myocardial tolerance to acidosis must be explained either by an active pH regulation or by other compensatory mechanisms.

Myocardial inotrophy of CO2 in water- and air-breathing vertebrates

A negative-inotropic effect of CO2 on myocardial contractility presumably occurs because increasing H+ concentration competes with Ca2+ at cellular membranes and proteins. Since air-breathing vertebrates have higher blood and tissue CO2 concentration than water breathers the question was raised whether the cardiac cell has a modified sensitivity to CO2 correlated with the evolutionary transition of vertebrates from water breathers to air breathers. The water-breathing fish, Salmo gairdneri, and the air-breathing turtle, Pseudemys scripta, were selected as experimental animals, since their total CO2 concentration differs markedly (3.0 and 16.0 mmol.kg-1). Electrically paced isometric ventricular strips from both species were subjected to a stepwise increase in PCO2 from 25 to 114 Torr (pH0 7.80 to 7.0; HCO3- 30 mM). Trout were additionally exposed to the same pH0 changes at 5 mM HCO3- by a stepwise increase in PCO2 (4.5-12 Torr). At each increase in PCO2 the turtle heart showed a lesser negative inotropic effect than trout. The present findings offer direct evidence that the negative inotropic effect of CO2 on heart muscle is inversely proportional to the in vivo levels of tissue CO2 concentration. The results obtained are discussed in relation to phylogenetical and ecological aspects of acid-base balance.

The effects of hypoxia and reoxygenation on force development in myocardia of carp and rainbow trout: protective effects of CO2/HCO3

Isometrically mounted and electrically paced myocardial ventricular strips of carp have a much higher capacity to develop force during severe hypoxia and to redevelop force after it than those of rainbow trout. When the concentrations of CO2 and HCO3-in the solutions surrounding the strips were increased together, such that pH remained constant, the force developed during hypoxia increased. The concentration of CO2 was raised from 0–4%; that of HCO3-from 0–25 mM. The effect was much more pronounced in the carp strips than in the trout strips. With the carp strips, the force recovery upon reoxygenation was unaffected by the variations in CO2 and HCO3-. The trout strips, however, recovered better when CO2 and HCO3-had been raised during either hypoxia or reoxygenation.

Tissue metabolism and enzyme activities in the rodent Heterocephalus glaber, a poor temperature regulator

1. Tissue oxygen uptake and enzyme activities were investigated in the naked mole rat, Heterocephalus glaber, a mammal notable for its low body temperature and metabolism and poor temperature regulating ability. 2. Q10 for O2 uptake of Heterocephalus crude liver homogenates ranged from 1.91 for the temperature interval 25-30 degrees C to 1.76 within the range 30-38 degrees C, values similar to those reported for typical homoiotherms. 3. Km pyruvate of lactate dehydrogenase in heart muscle had the same temperature dependence in the mole rat and mouse. 4. O2 uptake and cytochrome oxidase activity of skeletal muscle were higher for mole rat than mouse. The reverse was true for heart muscle. Brain and liver O2 uptake showed similar values for both species, while kidney O2 uptake was highest in the mouse. 5. Pyruvate kinase activity in heart and skeletal muscle was higher in mouse than mole rat, suggesting a greater reliance on glycolysis in the former. 6. Na+, K+ -ATPase activity of liver and kidney was 60% higher in mouse than mole rat, while brain was 30% higher in mouse. 7. The results indicate that the effects of temperature on tissue metabolism in the mole rat conform to those in typical homoiotherms. The low body temperature and O2 uptake in the mole rat find no expression in the tissue respiratory capacity.

Significance of the extracellular bicarbonate buffer system to anaerobic glycolysis in hypoxic muscle

1. The influence of the carbon dioxide-bicarbonate buffer system on anaerobic energy production during severe hypoxia was studied in isolated right hemidiaphragms of rats.--2. When the tissue was incubated in a Ringer solution containing 25 mM HCO-3 aerated with 7% CO2 in N2 at pH 7.4, the lactate production and lactate content of the tissue increased.--3. At an extracellular (tissue bath) pH OF 6.9 the lactate production was stimulated when carbon dioxide and bicarbonate were changed to 19% and 25 mM, respectively. This stimulatory effect disappeared when these values were lowered to 7% and 7 mM.--4. At pH 7.4 the stimulatory effect of the carbon dioxide-bicarbonate system persisted when the buffer value was lowered from 60 to 3 mM by changing the system from an open (i.e. continuous gas equilibration) to a closed one (i.e. without any gas phase). Decreasing the glucose in the media from 22 to 0 mM reduced the lactate production and abolished the stimulatory effect of the carbon dioxide--bicarbonate system.--5. There was no direct effect of this system on the glycolytic enzymes (i.e. lactate production and activity of phosphofructokinase of homogenates).

THE REACTION OF HYDROGEN ATOMS WITH KETENE

The gas-phase reaction of atomic hydrogen with ketene has been investigated over a temperature range of −130° to 232 °C using a low-pressure, fast-flow system. In most cases methane, carbon monoxide, and ethane were the major products, but trace amounts of glyoxal were also detected. Above −96 °C. considerable evidence exists for the occurrence of a chain reaction carried by HCO radicals. The surface reaction at −196 °C produced methane and glyoxal predominantly with only a minor amount of carbon monoxide.

A SIMPLE METHOD FOR TRAPPING AND COLLECTING THE GASES NOT CONDENSABLE AT −196° IN A FAST-FLOWING, LOW-PRESSURE SYSTEM

not available