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

The effects of pH and Pi on tension and Ca2+ sensitivity of ventricular myofilaments from the anoxia-tolerant painted turtle

We aimed to determine how increases in intracellular H⁺ and inorganic phosphate (P_i) to levels observed during anoxic submergence affect contractility in ventricular muscle of the anoxia-tolerant Western painted turtle, Chrysemys picta bellii Skinned multicellular preparations were exposed to six treatments with physiologically relevant levels of pH (7.4, 7.0, 6.6) and P_i (3 and 8 mmol l^-1). Each preparation was tested in a range of calcium concentrations (pCa 9.0-4.5) to determine the pCa-tension relationship for each treatment. Acidosis significantly decreased contractility by decreasing Ca²⁺ sensitivity (pCa₅₀) and tension development (P<0.001). Increasing [P_i] also decreased contractility by decreasing tension development at every pH level (P<0.001) but, alone, did not affect Ca²⁺ sensitivity (P=0.689). Simultaneous increases in [H⁺] and [P_i] interacted to attenuate the decreased tension development and Ca²⁺ sensitivity (P<0.001), possibly reflecting a decreased sensitivity to P_i when it is present as the dihydrogen phosphate form, which increases as pH decreases. Compared with that of mammals, the ventricle of turtles exhibits higher Ca²⁺ sensitivity, which is consistent with previous studies of ectothermic vertebrates.

The effects of hypoxic bradycardia and extracellular HCO3−/CO2 on hypoxic performance in the eel heart

During hypoxia fishes exhibit a characteristic ‘hypoxic bradycardia’, the functional significance of which remains debated. Here, we investigated the hypothesis that hypoxic bradycardia primarily safeguards cardiac performance. In preparations from the European eel (Anguilla anguilla), a decrease in stimulation frequency from 40 to 15 beats per minute, which replicates hypoxic bradycardia in vivo, vastly improved cardiac performance during hypoxia in vitro. As eels display dramatic shifts in extracellular HCO3−/CO2, we further investigated the effect this has upon hypoxic cardiac performance. Elevations from 10 mM HCO3−/ 1% to 40 mM HCO3−/ 4% CO2 had few effects on performance, however further, but still physiologically relevant, increases to 70 mM HCO3−/ 7% CO2 compromised hypoxia tolerance. We revealed a four-way interaction between HCO3−/CO2, contraction frequency, hypoxia and performance over time, whereby the benefit of hypoxic bradycardia was most prolonged at 10 mM HCO3−/ 1% CO2. Together, our data suggest that hypoxic bradycardia greatly benefits cardiac performance, but its significance may be context-specific.

Correlation of cardiac performance with cellular energetic components in the oxygen-deprived turtle heart

The relationship between cardiac energy metabolism and the depression of myocardial performance during oxygen deprivation has remained enigmatic. Here, we combine in vivo (31)P-NMR spectroscopy and MRI to provide the first temporal profile of in vivo cardiac energetics and cardiac performance of an anoxia-tolerant vertebrate, the freshwater turtle (Trachemys scripta) during long-term anoxia exposure (approximately 3 h at 21 degrees C and 11 days at 5 degrees C). During anoxia, phosphocreatine (PCr), unbound levels of inorganic phosphate (effective P(i)(2-)), intracellular pH (pH(i)), and free energy of ATP hydrolysis (dG/dxi) exhibited asymptotic patterns of change, indicating that turtle myocardial high-energy phosphate metabolism and energetic state are reset to new, reduced steady states during long-term anoxia exposure. At 21 degrees C, anoxia caused a reduction in pH(i) from 7.40 to 7.01, a 69% decrease in PCr and a doubling of effective P(i)(2-). ATP content remained unchanged, but the free energy of ATP hydrolysis (dG/dxi) decreased from -59.6 to -52.5 kJ/mol. Even so, none of these cellular changes correlated with the anoxic depression of cardiac performance, suggesting that autonomic cardiac regulation may override putative cellular feedback mechanisms. In contrast, during anoxia at 5 degrees C, when autonomic cardiac control is severely blunted, the decrease of pH(i) from 7.66 to 7.12, 1.9-fold increase of effective P(i)(2-), and 6.4 kJ/mol decrease of dG/dxi from -53.8 to -47.4 kJ/mol were significantly correlated to the anoxic depression of cardiac performance. Our results provide the first evidence for a close, long-term coordination of functional cardiac changes with cellular energy status in a vertebrate, with a potential for autonomic control to override these immediate relationships.

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

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.

Extracellular Determinants of Cardiac Contractility in the Cold Anoxic Turtle

Painted turtles (Chrysemys picta) survive months of anoxic submergence, which is associated with large changes in the extracellular milieu where pH falls by 1, while extracellular K+, Ca++, and adrenaline levels all increase massively. While the effect of each of these changes in the extracellular environment on the heart has been previously characterized in isolation, little is known about their interactions and combined effects. Here we examine the isolated and combined effects of hyperkalemia, acidosis, hypercalcemia, high adrenergic stimulation, and anoxia on twitch force during isometric contractions in isolated ventricular strip preparations from turtles. Experiments were performed on turtles that had been previously acclimated to warm (25 degrees C), cold (5 degrees C), or cold anoxia (submerged in anoxic water at 5 degrees C). The differences between acclimation groups suggest that cold acclimation, but not anoxic acclimation per se, results in a downregulation of processes in the excitation-contraction coupling. Hyperkalemia (10 mmol L(-1) K+) exerted a strong negative inotropic effect and caused irregular contractions; the effect was most pronounced at low temperature (57%-97% reductions in twitch force). Anoxia reduced twitch force at both temperatures (14%-38%), while acidosis reduced force only at 5 degrees C (15%-50%). Adrenergic stimulation (10 micromol L(-1)) increased twitch force by 5%-19%, but increasing extracellular [Ca++] from 2 to 6 mmol L(-1) had only small effects. When all treatments were combined with anoxia, twitch force was higher at 5 degrees C than at 25 degrees C, whereas in normoxia twitch force was higher at 25 degrees C. We propose that hyperkalemia may account for a large part of the depressed cardiac contractility during long-term anoxic submergence.

Force development, energy state and ATP production of cardiac muscle from turtles and trout during normoxia and severe hypoxia

The effects of hypoxia on energy economy of cardiac muscle were compared between the hypoxia-tolerant freshwater turtle at 20 degrees C and the hypoxia-sensitive rainbow trout at 15 degrees C. Isolated ventricular preparations were left either at rest or stimulated at 30 min(-1) to develop isometric twitch force. Under oxygenated conditions, twitch force and oxygen consumption were similar for the two species. Overall metabolism was reduced during severe hypoxia in both resting and stimulated preparations and under these conditions most of the ATP production was derived from anaerobic metabolism. During hypoxia, a metabolic depression of approximately 2/3 occurred for non-contractile processes in both turtle and trout preparations. During hypoxia, lactate production and residual oxygen consumption were similar in turtle and trout. Cellular energy state and phosphorylation potential decreased during severe hypoxia in both species and this reduction was more severe in preparations stimulated to contraction. However, in turtle ventricular preparations the energy state and phosphorylation potential stabilised at higher levels than in trout, and turtle preparations also maintained a higher twitch force throughout the hypoxic period. Moreover, twitch force relative to total ATP hydrolysis was markedly increased during hypoxia in turtle while this ratio was unchanged for trout. The main findings of this study are: (1) cellular energy liberation and the energy demand of non-contractile processes decreased to similar levels in hypoxic turtle and trout myocardium; (2) turtle myocardium maintained a substantially higher cellular energy state and twitch force development than trout myocardium during hypoxia and (3) the ratio of twitch force to ATP hydrolysis increased during hypoxia in turtle but was unchanged in trout. It is possible that this superior economy of the contracting turtle myocardium contributes to the remarkable hypoxia tolerance of freshwater turtles.