Hans Gesser

Ca2+ protection from the negative inotropic effect of contraction frequency on teleost hearts

SummaryIsometric tension development by ventricular strips of 9 species of teleosts, a frog and a turtle was assessed at varying contraction frequencies and Cao (external calcium concentration). With teleost hearts an increase in contraction frequency at constant Cao was always associated with a decrease in tension development; however, under comparable conditions a positive staircase was exhibited by the frog and turtle heart preparations. The reaction of the teleost heart was thus very different from the well established response of the hearts of higher vertebrates. Elevations in Cao always resulted in an increase in tension development such that the positive inotropic effect of Cao could compensate for the negative effect of a high contraction frequency.Perfused isolated cod hearts exhibited an increase in cardiac output and pressure development as a result of increases in Cao. At 30 contractions min−1 a transition from 1–2 mM Cao led to a 68% increase in performance defined as the product of cardiac output times pressure development. The response was in excess of that of ventricular strips. At low Cao increases in rate from in situ resting levels to the high end of the physiological range resulted in a decrease in performance. Increases in Cao were able to ameliorate the detrimental effect of high imposed contraction frequency.In conclusion, both ventricular strip and perfused heart experiments show that a positive inotropic effect of increased Cao can compensate for or even surpass the negative effect of high contraction frequency when both variables are at physiological levels. This finding could have relevance to the maintenance of cardiac performance during/or following intense swimming when both heart rate and plasma calcium may be elevated.

Sarcoplasmic reticulum and excitation-contraction coupling at 20 and 10 °C in rainbow trout myocardium

SummaryIsometric force and series membrane potential were recorded in isolated ventricular strips from rainbow trout at 20 and 10 °C. Preparations were electrically stimulated to contract at either 0.5 or 0.2 Hz. Single extrastimulations elicited a twitch force which diminished when the preceding diastole was shortened below the regular value. The stimulation following this extra stimulation evoked no potentiation of force. Apart from a marginal effect on the post extrasystolic force at 20 °C, ryanodine did not affect either of these responses or the steady-state force at 0.5 Hz. At 0.2 Hz the steady-state force was somewhat depressed by ryanodine at 20 but not at 10 °C. In contrast, extrastimulations preceded by diastoles of up to 1 h more than doubled extrasystolic force at 20 °C. This effect was removed by ryanodine. Both the potentiations and the effect of ryanodine were strongly reduced at 10 °C. Apparently, temperature acts on the release of Ca2+ from the sarcoplasmic reticulum, since Ca2+ seems to be taken up at both temperatures. Hence, at both 20 and 10 °C, the contractures evoked in a solution inhibiting sarcolemmal Ca2+ transfer and releasing Ca2+ from the sarcoplasmic reticulum were diminished by pretreatment with 15 mM caffeine. Action potential duration at 20 °C was less than half of that at 10 °C. At both temperatures it tended to be prolonged by periods of prolonged rest. No effect of ryanodine on action potential configuration was detected. The results suggest that trout myocardial sarcoplasmic reticulum, although powerful at unphysiologically low stimulation rates, does not partake in the beat-to-beat regulation of force at heart rates encountered in vivo.

Force frequency relation in the myocardium of rainbow trout

SummaryIsolated 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 μM) 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 μM) 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.

Inotropic effects of adrenaline on the anoxic or hypercapnic myocardium of rainbow trout and eel

SummaryThe effects of adrenaline on the development of force under anoxia and hypercapnic acidosis (13% CO2 in 30 mM HCO3−) were examined in isolated, electrically stimulated cardiac ventricle strips of rainbow trout and eel.During anoxia or acidosis applied 15 min in advance, the adrenaline concentration of the bathing solution was increased in 5 steps from 0 to 10−4 M with 5 min at each step. Before levelling off the contractile tension increased by 145±42% (mean±SE) in the anoxic, 80±14% in the acidotic and 152±41% in the control trout cardiac strips. Except for the acidotic strips the corresponding values tended to be lower for the eel strips being 46±9%, 57±17% and 57±9%, respectively. The inotropic affinity for adrenaline was lower in the trout than in the eel myocardium. For the trout myocardium it remained unchanged, while it decreased somewhat for the eel myocardium under anoxia or acidosis.Adding to the muscle bath 10−5 M adrenaline resulted in an increase in force development by about 90% for the trout myocardium and 50% for the eel myocardium. 5 min later anoxia or hypercapnic acidosis was applied for 30 min followed by 30 min at control conditions. Relative to the force values recorded just before anoxia or acidosis was applied, the changes in contractile force during these periods were the same with and without adrenaline. Thus adrenaline appears to have a persistent positive inotropic effect in the fish myocardium during and after oxygen lack or acidosis.

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.

Cardiac force-interval relationship, adrenaline and sarcoplasmic reticulum in rainbow trout

The force-interval relationship was examined at 20 and 10 °C in electrically paced atrial and ventricular tissue of rainbow trout,Oncorhynchus mykiss, regarding dependence on the sarcoplasmic reticulum and influence of adrenaline. In both tissues, adrenaline (10-6 mol·l-1) doubled control force developed at 0.5 Hz. In atrial but not in ventricular tissue it also shortened the diastolic interval needed for recovery of a given fraction of the control force. In atrial tissue and in ventricular tissue at 20 °C, the fraction of force recovered in the presence of adrenaline was diminished by 10 μmol·l-1 of ryanodine, a specific inhibitor of the sarcoplasmic reticulum. In atrial tissue not exposed to adrenaline and in ventricular tissue at 10 °C irrespective of adrenaline, ryanodine did not affect recovery. In atrial but not in ventricular tissue it also diminished control force. In conclusion, the cardiac sarcoplasmic reticulum of trout seems to support force development during adrenaline dependent increases in heart rate, and in atrial tissue also the force at steady state.

Roles of nitric oxide, nitrite and myoglobin on myocardial efficiency in trout (Oncorhynchus mykiss) and goldfish (Carassius auratus): implications for hypoxia tolerance

SUMMARY The roles of nitric oxide synthase activity (NOS), nitrite and myoglobin (Mb) in the regulation of myocardial function during hypoxia were examined in trout and goldfish, a hypoxia-intolerant and hypoxia-tolerant species, respectively. We measured the effect of NOS inhibition, adrenaline and nitrite on the O2 consumption rate and isometric twitch force development in electrically paced ventricular preparations during hypoxia, and measured O2 affinity and nitrite reductase activity of the purified heart Mbs of both species. Upon hypoxia (9% O2), O2 consumption and developed force decreased in both trout and goldfish myocardium, with trout showing a significant increase in the O2 utilization efficiency, i.e. the ratio of twitch force to O2 consumption, suggesting an increased anaerobic metabolism. NOS inhibition enhanced myocardial O2 consumption and decreased efficiency, indicating that mitochondrial respiration is under a tone of NOS-produced NO. When trout myocardial twitch force and O2 consumption are enhanced by adrenaline, this NO tone disappears. Consistent with its conversion to NO, nitrite reduced O2 consumption and increased myocardial efficiency in trout but not in goldfish. Such a difference correlates with the lower O2 affinity measured for trout Mb that would increase the fraction of deoxygenated heme available to catalyze the reduction of nitrite to NO. Whereas low-affinity trout Mb would favor O2 diffusion within cardiomyocytes at high in vivo O2 tensions, goldfish Mb having higher O2 affinity and higher nitrite reductase activity appears better suited to facilitate O2 diffusion and nitrite reduction in the heart during severe hypoxia, a condition particularly well tolerated by this species.

Heart performance, Ca2+ regulation and energy metabolism at high temperatures in Bathygobius soporator, a tropical marine teleost

Experiments were conducted with Bathygobius soporator (Gobbiidae), a small tropical marine teleost which lives in tide pools along the east coast of South America. In whole animals, VO2 remained constant from 25 to 30°C and then increased until it reached a maximum value at 40°C of about 160 ml·kg−1·h−1. The fH increased progressively and significantly from 25 to 35°C, at which fH reached its maximum value of about 225 beats·min−1. At 40°C, however, the fH decreased to a value similar to that recorded at 25°C. Twitch force and resting tension were determined for isolated ventricle strips. At an extracellular Ca2+ level of 1.25 mM a transition from 25 to 40°C resulted in a decrease in twitch force which was restored upon a return to 25°C. This restoration of twitch force did not occur at an extracellular Ca2+ concentration of 9.25 mM. At 25°C, increments in extracellular Ca2+ from 1.25 to 7.25 mM resulted in increases in twitch force development However, at 40°C only resting tension increased in concert with elevations in Ca2+. At 25°C, twitch force declined as frequency was increased above 30 contractions·min−1 and became irregular above 120 contractions·min−1. At 40°C, twitch force development remained constant at frequencies up to about 150 contractions·min−1 and declined thereafter. Preparations were able to maintain rhythmic response up to about 240 contractions·min−1. An increase in in vitro assay temperature from 25 to 40°C resulted in an elevation of total ATPase activity. Citrate synthase was present in high activities with hexokinase and 3-hydroxyacyl coenzyme A (CoA) being detectable but at lower activities. Heart performance is fragile at high temperature and under conditions which lead to high intracellular Ca2+. A controlled decrease in heart rate at high temperature may have a protective effect in maintaining low levels of intracellular Ca2+.

Tribute to P. L. Lutz: cardiac performance and cardiovascular regulation during anoxia/hypoxia in freshwater turtles

SUMMARY Freshwater turtles overwintering in ice-covered ponds in North America may be exposed to prolonged anoxia, and survive this hostile environment by metabolic depression. Here, we review their cardiovascular function and regulation, with particular emphasis on the factors limiting cardiac performance. The pronounced anoxia tolerance of the turtle heart is based on the ability to match energy consumption with the low anaerobic ATP production during anoxia. Together with a well-developed temporal and spatial energy buffering by creatine kinase, this allows for cellular energy charge to remain high during anoxia. Furthermore, the turtle heart is well adapted to handle the adverse effects of free phosphate arising when phosphocreatine stores are used. Anoxia causes tenfold reductions in heart rate and blood flows that match the metabolic depression, and blood pressure is largely maintained through increased systemic vascular resistance. Depression of the heart rate is not driven by the autonomic nervous system and seems to arise from direct effects of oxygen lack and the associated hyperkalaemia and acidosis on the cardiac pacemaker. These intra- and extracellular changes also affect cardiac contractility, and both acidosis and hyperkalaemia severely depress cardiac contractility. However, increased levels of adrenaline and calcium may, at least partially, salvage cardiac function under prolonged periods of anoxia.

The role of intracellular Ca2+ under hypercapnic acidosis of cardiac muscle: Comparative aspects

Summary1.Under hypercapnic acidosis an initial decrease of myocardial contractile force is seen, but in some vertebrate species it is followed by a recovery. The resulting force level is higher in air-breathers, like a turtle (Pseudemys scripta), a snake (Vipera berus), and the laboratory rat, than in the waterbreathing fishes, trout and carp. The hearts of the leopard frog and grass frog are intermediate although the former tolerate hypercapnic acidosis better than the latter.2.Cyanide does not affect the hypercapnic force development of the leopard frog, while it induces a recovery process in that of the grass frog. Since cyanide is known to release intracellularly bound Ca2+ this suggests that the recovery involves a changed Ca2+ distribution.3.Incubation of cardiac strips fromVipera berus and trout in a Na+- and Ca2+-free Ringer causes an increase in resting tension. Under hypercapnic acidosis a decrease of this tension occurs which, however, for the strips from the viper is followed by recovery. The recovery process thus seems to be independent of extracellular Ca2+ or Na+. It also seems independent of the Ca2+ contained in the sarcoplasmatic reticulum, since the recovery of resting tension in the viper strips is unaffected by pretreatment with caffeine.4.In viper myocardial strips, which develop high force under anoxia, the force loss by acidosis is increased and spontaneous recovery substantially depressed when hypercapnia is imposed after a 60 min pretreatment with agents unloading mitochondrial Ca2+-stores (anoxia and anoxia + oligomycin).5.These findings favour the hypothesis that hypercapnic acidosis triggers the release of Ca2+ from intracellular stores, probably from mitochondria.