Acute and chronic effects of propionyl-L-carnitine on the hemodynamics, exercise capacity, and hormones in patients with congestive heart failure.

Carnitine is an important cofactor in the intermediary metabolism of the heart, and carnitine deficiency is associated with congestive heart failure. We therefore studied the effects of acute (IV bolus, 30 mg/kg body weight) and chronic administration (1.5 mg/d for 1 month) of propionyl-L-carnitine on hemodynamics, hormone levels, ventricular function, exercise capacity, and peak oxygen consumption in 30 patients with chronic congestive heart failure (NYHA II-III, mean EF 29.5 +/- 7%) in a phase II, parallel, single-blind, randomized, and placebo-controlled study. Acute administration of propionyl-L-carnitine caused a significant reduction in pulmonary artery and pulmonary wedge pressures at both day 1 (P < 0.001) and day 30 (P < 0.05) of the study but no other hemodynamics changes. Hormone levels did not change following acute administration of the drug. Chronic administration of propionyl-L-carnitine increased peak oxygen consumption by 45% (from 16.0 +/- 3 to 23.5 +/- 2 mL/kg/min, P +/- 0.001), exercise time by 21% (from 8.1 +/- 0.5 to 9.8 +/- 0.4 minutes, P < 0.01), and peak exercise heart rate by 12% (P < 0.01). These changes were concomitant with a reduction of pulmonary artery pressure. In the treated group, there was a slight, but significant (P < 0.01), reduction in left ventricular dimensions. Hemodynamics and hormones measured after 1 month of oral therapy remained unchanged, except for a fall in pulmonary artery pressures, with a nonsignificant trend towards a fall in filling pressures and plasma norepinephrine. The chronic changes in the propionyl-L-carnitine group were seen at 15 days of treatment, and no further changes in these parameters were seen at 1 month. We conclude that propionyl-L-carnitine increases exercise capacity and reduces ventricular size in patients with congestive heart failure. The drug has no significant effects on hemodynamics or neurohormone levels. The use of a single-blind design reduces the impact of the positive finding on exercise capacity.

The propionyl-L-carnitine hypothesis: an alternative approach to treating heart failure.

Propionyl-L-carnitine (PLC) is a naturally occurring compound that has been considered for the treatment of congestive heart failure (CHF). The rationale for its use in this pathology is related to its effects on cardiac and skeletal muscle. Chronic treatment with PLC improves the contraction of isolated and aerobic perfused rabbit hearts. The compound improves energy metabolism and myocardial contractility in different experimental models of heart failure, such as pressure-overloaded rats, infarct model of heart failure, and rabbit with streptozotocin-induced diabetes. In general, the effect of PLC is apparent in situations of high energy demand such as those induced by increased workload. It therefore seems likely that PLC is able to correct some metabolic steps of the process that leads to heart failure. In addition, PLC may be helpful in heart failure because of its specific action on peripheral skeletal muscle. Administration of PLC in patients with CHF improves skeletal muscle metabolism by increasing pyruvate flux into the Krebs cycle and by decreasing lactate production. These effects occur in the absence of major hemodynamic and neuroendocrinologic changes and may underlie the ability of PLC to increase exercise performance in patients with heart failure. In a randomized study of 50 patients with mild CHF, PLC increased the maximal exercise time, reduced lactate production, and improved left ventricular ejection fraction. There have been two large-scale trials on the effects of PLC on both cardiac and peripheral muscle function in CHF. One is ongoing; the other one, which just ended, failed to show an improvement in exercise capacity in the population studied. A benefit was evident only in a subgroup of patients with preserved ejection fraction and impaired baseline exercise duration.

Neuroendocrine response to standing and mild exercise in patients with untreated severe congestive heart failure and chronic constrictive pericarditis.

OBJECTIVES: Plasma hormones at rest in patients with untreated severe congestive cardiac failure are similar to those occurring during heavy exercise in healthy people. This study examines the hypothesis that the neuroendocrine effects of exercise are modified in untreated congestive cardiac failure. DESIGN: The effect of lying, standing, upright exercise, and recovery on several plasma hormones was measured in healthy controls and 2 groups of patients with severe untreated heart failure. The level of exercise was the same in all groups and low enough to be within the capacity of patients with severe failure. PATIENTS: There were 12 healthy controls, 9 patients with untreated severe congestive cardiac failure caused by myocardial disease, and 12 patients with untreated constrictive pericarditis. SETTING: A tertiary referral centre in North India. RESULTS: Heart rate, noradrenaline, renin activity, aldosterone, cortisol, growth hormone and atrial natriuretic peptide (ANP) were higher in the 2 groups of patients than in the healthy controls during both rest and exercise (P < 0.01 for both comparisons). In general, the effects of this mild degree of exercise were no greater than those of standing. The increase in heart rate during exercise was greater in the group with constrictive pericarditis than in the controls (P = 0.04) and (non-significantly) in congestive heart failure. Apart from these differences the pattern of responses to standing and exercise was similar in the three groups. CONCLUSIONS: While there was evidence of a broad neuroendocrine activation in patients with congestive cardiac failure, the only abnormal increase during exercise (of marginal significance) was found for renin activity in those with myocardial disease. In patients with untreated congestive failure, a substantially normal endocrine response to exercise was superimposed on abnormal resting concentrations.

Oxidative stress during reperfusion of human hearts: potential sources of oxygen free radicals.

OBJECTIVE: The aim was to examine the role of neutrophil activation in the genesis of oxidative stress during the early phases of reperfusion after ischaemia in patients subjected to aortocoronary bypass grafting. METHODS: Ten selected patients were studied. All had normal ejection fraction and normal left ventricular end diastolic pressures before operation. Each patient required at least three grafts, so that the duration of aortic crossclamping exceeded 30 min, the minimum ischaemic period required to detect oxidative stress upon reperfusion. Oxidative stress was assessed by measuring the formation and release of oxidised glutathione (GSSG) in the coronary sinus 1 min before and 3 min after the start of the cardiopulmonary bypass, and then 1, 5, 10, and 20 min after removal of the aortic clamp, and again 5 and 10 min after the end of the cardiopulmonary bypass. The arterial-coronary sinus difference for neutrophils, elastase-alpha 1 protease complex (elastase), and creatine phosphokinase was also monitored at the same intervals. RESULTS: Before clamping GSSG was undetectable in arterial and coronary sinus blood. There was no significant arterial-coronary sinus difference for neutrophils or elastase [53(SEM 66) cell.ml-1 and 1.10(2.49) micrograms.litre-1, respectively[. Five minutes after re-establishment of coronary blood flow, there was both a release of GSSG into the coronary sinus [arterial-coronary sinus difference: 11(2.6) nmol.dl-1] and an accumulation of neutrophils in the heart [arterial-coronary sinus difference: 262(33), P < 0.01 cell.ml-1], whereas no elastase release from the heart was measured [arterial-coronary sinus difference 7.6(4.46) microgram.litre-1, NS]. The arterial levels of elastase increased progressively during the operation from 48(5) microgram.litre-1 (preclamping) to 405(62) microgram.litre-1, P < 0.01 (end of the cardiopulmonary bypass). CONCLUSIONS: These data indicate that, in man, neutrophils do accumulate in the myocardium during early reperfusion. However, they are not activated when oxidative stress occurs. It is unlikely that the neutrophil localisation in the heart has pathological significance in the production of oxygen free radicals during early reperfusion. Free radical accumulation in the coronary vessels may contribute to disorders of coronary flow associated with reperfusion.

Left ventricular dysfunction due to stunning and hibernation in patients.

Left ventricular dysfunction is in most cases the consequence of myocardial ischemia. It may occur transiently during an attack of angina and usually it is reversible. It may persist over hours or even days in patients after an episode of ischemia followed by reperfusion, leading to the so-called condition of stunning. In patients with persistent limitation of coronary flow, left ventricular dysfunction may be present over months and years, or indefinitely in subjects with fibrosis, scar formation, and remodeling after myocardial infarction. However, chronic left ventricular dysfunction does not mean permanent or irreversible cell damage. Hypoperfused myocytes can remain viable but akinetic. This type of dysfunction has been called hibernating myocardium. The dysfunction due to hibernation can be partially or completely restored to normal by reperfusion. It is, therefore, important to clinically recognize a hibernating myocardium. In the present article we evaluate stunning and hibernation with respect to clinical decision making and, when possible, we refer to our ongoing clinical experience.

The effects of L-arginine mono(2-mercaptoethanesulfonate) on the ischemic and reperfused heart.

We evaluated the effectiveness of L-arginine mono(2-mercaptoethanesulfonate) (argimesna) to limit the extent of myocardial damage resulting from 60 minutes of severe ischemia followed by 30 minutes of reperfusion in the Langendorff-perfused rabbit heart. Argimesna is a sulfhydryl group containing molecule which has no effect on glutathione status or on the total thiol pool. The effects of 10(-6) M argimesna were compared with those of 10(-6) M L-arginine and of 10(-6) M sodium salt of 2-mercaptoethanesulfonate (mesna). Pretreatment of the hearts with 10(-6) M argimesna resulted in marked myocardial protection, measured in terms of improved recovery of developed pressure (p less than 0.01), reduced release of creatine kinase (p less than 0.01), maintenance of mitochondrial function and increased stores of ATP on reperfusion (p less than 0.01). On reperfusion less oxidative stress developed, as indicated by less accumulation of oxidized glutathione (p less than 0.01). These effects were specific for argimesna; no significant protection could be found for mesna and L-arginine. The beneficial effects of argimesna could not be explained by hemodynamic differences or effects on anaerobic metabolism. Neither is it likely that argimesna acts as a free radical scavenger at the concentrations employed. The protection may be achieved by maintenance of -SH groups during ischemia and reperfusion.

Role of timing of administration in the cardioprotective effect of fructose-1,6-bisphosphate.

We administered fructose-1,6-bisphosphate (FDP), 1 mM, to isolated and perfused rabbit hearts submitted, after 90 minutes of equilibration, to an ischemic period (60 minutes at a coronary flow of 0.17 ml/min/g), followed by a period of reperfusion (30 minutes at a coronary flow of 3.6 ml/min/g). FDP was delivered at different times following the experimental protocol: 60 minutes before ischemia and for the entire experiment; 60 minutes before and during ischemia, but not at reperfusion; at the onset of ischemia and during reperfusion; and only during reperfusion. The FDP cardioprotective effect was evaluated in terms of recovery of left ventricular pressure developed during reperfusion, creatine phosphokinase (CPK) and noradrenaline release, mitochondrial function (expressed as yield, RCI, QO2, ADP/O), ATP and creatine phosphate (CP) tissue contents, calcium homeostasis, and by measuring oxidative stress in terms of reduced and oxidized glutathione release and tissue contents. Our data show that the cytoprotective action of FDP is closely related to the time of administration. Optimal myocardial preservation was achieved when it was present prior to ischemia and during reperfusion. When given at the time of ischemia or only on reperfusion, FDP does not exert cardioprotection. The data suggest that the FDP cardioprotective effect is related to improvement of energy metabolism.

Occurrence of oxidative stress during myocardial reperfusion.

Reperfusion, without doubt, is the most effective way to treat the ischaemic myocardium. Late reperfusion may however cause further damage. Myocardial production of oxygen free radicals above the neutralizing capacity of the myocytes is an important cause of this reperfusion damage. There is evidence that prolonged ischaemia reduces the naturally occurring defence mechanisms of the heart against oxygen free radicals, particularly mitochondrial manganese superoxide dismutase, and intracellular pool of reduced glutathione. Consequently, reperfusion results in a severe oxidative damage, as evidenced by tissue accumulation and release of oxidized glutathione. An oxygen free radical-mediated impairment of mechanical function also occurs during reperfusion of human heart. In fact we observed during surgical reperfusion of coronary artery disease (CAD) patients, a prolonged and sustained release of oxidized glutathione; the degree of oxidative stress was inversely correlated with recovery of mechanical and haemodynamic function. These findings represent the rationale for therapeutic interventions which increase the cellular antioxidant capacities and improve the efficacy of myocardial reperfusion.

Temporal relations of the endocrine response to hypotension with sodium nitroprusside.

Sodium nitroprusside was infused intravenously for 10 minutes in normal men, reclining at 45 degrees, in a dose sufficient to decrease the arterial pressure by 10 mmHg. The effect on a variety of plasma hormones was measured during the infusion and for 20 minutes afterwards. The heart rate increased to a maximum of 149%. Norepinephrine rose to a maximum of 196% in 5 minutes. Epinephrine reached a peak of 207% after 10 minutes. Plasma renin activity reached a peak of 449% at 10 minutes. Aldosterone did not change during the infusion, but increased to a maximum of 145% 10 minutes later. Vasopressin increased sharply at the end of the infusion to 893% and then rapidly decreased. Corticotropin, prolactin and growth hormone started to increase toward the end of the infusion, but reached their maxima during recovery. Corticotropin (225%) and prolactin (288%) peaked 10 minutes after the infusion, while growth hormone (414%) appeared still to be rising 20 minutes after the end of the infusion. Cortisol also rose progressively during recovery to a level of 138%. No significant changes were seen in the concentrations of insulin, glucagon, atrial natriuretic peptide, bombesin or neurotensin.

Effect of propionyl-L-carnitine on experimental induced cardiomyopathy in rats.

The purpose of this study is to evaluate the ability of propionyl-L-carnitine, a carnitine derivative to prevent cardiac dysfunction induced by erucic acid and streptozotocin treatment in rats. Rats were fed for 10 days with normal or 10% erucic-acid-enriched diet with or without propionyl-L-carnitine injected intraperitoneally (1 mM/kg daily). Another group of rats was injected for 8-10 weeks with streptozotocin (65 mg/kg) with or without propionyl-L-carnitine intraperitoneally injected at the same dosage. Thereafter the animals were sacrificed and the hearts isolated and perfused aerobically. When isovolumic measurements of left ventricular function were applied, there was no difference in mechanical activity between treated and control hearts. On the contrary, when pressure-volume curves were determined in the paced hearts, the pressure developed by hearts from erucic acid-treated or diabetic rats was reduced. Propionyl-L-carnitine always produced positive inotropy. This was true for the control-saline treated rats that received the drug, as well as for the hearts isolated from cardiomyopathic animals. These data suggest that propionyl-L-carnitine, when given chronically, is able to overcome myocardial dysfunction caused either from erucic acid treatment or diabetes.

Diuretics as initial and sole treatment in chronic cardiac failure.

Six patients with chronic congestive cardiac failure, who had never received any drug treatment, were studied before and after one month of therapy with frusemide alone at a dose of 40 mg a day. Measurements were made at rest of plasma epinephrine, norepinephrine, renin activity, aldosterone, atrial natriuretic peptide, cortisol, growth hormone and prolactin, together with central hemodynamics, body fluid volumes and renal function. The initial measurements of hemodynamics, body fluid compartments and renal function confirmed the presence of the physiopathology typical of congestive cardiac failure. Plasma concentrations of norepinephrine, atrial natriuretic peptide, aldosterone and growth hormone were significantly increased. The mean value of plasma renin activity, although high, was not significantly different from normal. After one month of treatment, body weight, body fluid volumes and exchangeable sodium were reduced. Hemodynamics and renal plasma flow and glomerular filtration were not significantly affected. Plasma norepinephrine fell to within normal limits; atrial natriuretic peptide increased significantly; plasma renin activity and cortisol increased to levels which were abnormally high; growth hormone increased to levels similar to those associated with acromegaly. Increased circulating concentrations of atrial natriuretic peptide during treatment by frusemide may have an important influence on the kidney, blood vessels and neuro-endocrine response.

Temporal relations of the endocrine response to exercise.

We have followed the hormonal response to exercise in twelve normal males cycling at a constant moderate load for ten minutes. Plasma concentrations of a variety of hormones were measured at set times before and during exercise and for twenty minutes afterward. The plasma concentration of norepinephrine and epinephrine and plasma activity of renin rose to a maximum at the end of exercise and then declined. The plasma concentrations of neurotensin and atrial natriuretic peptide followed a similar course. Plasma vasopressin rose to a peak at the end of exercise and then fell transiently below the initial value ten minutes after exercise. The plasma concentrations of aldosterone, prolactin and adrenocorticotropin increased during exercise but continued to do so, reaching a peak at ten minutes after exercise. Plasma growth hormone increased during exercise and continued to increase throughout the period of twenty minutes' recovery. Cortisol did not change during exercise but rose progressively during the recovery period. Plasma concentrations of glucagon did not change while that of insulin decreased during exercise. The plasma concentration of bombesin slowly increased during exercise and declined during recovery, reaching a basal value 10 minutes later.

Role of timing of administration in the cardioprotective effect of iloprost, a stable prostacyclin mimetic.

We administered iloprost, a stable prostacyclin mimetic, 27 nM, to isolated and perfused rabbit hearts submitted, after 60 min of equilibration, to an ischaemic period (60 min at a coronary flow of 1 ml/min) followed by a period of reperfusion (30 min at a coronary flow of 25 ml/min). Iloprost was delivered at different times during the experimental protocol: 60 min before ischaemia, at the onset and after 30 min of ischaemia and only during reperfusion. The iloprost cardioprotective effect was evaluated in terms of recovery of left ventricular pressure developed during reperfusion, creatine phosphokinase (CPK) and noradrenaline release, mitochondrial function (expressed as yield, RCI (respiratory control index), QO2, ADP/O), ATP and creatine phosphate (CP) tissue contents, calcium homeostasis and by measuring several parameters of oxidative stress: reduced and oxidized glutathione release and tissue contents, Mn and Cu-Zn superoxide dismutase activities; glutathione reductase and peroxidase activities. Our data show that the cytoprotective action of iloprost is closely related to the time of administration. Optimal myocardial preservation was achieved when it was given before or at the onset of ischaemia. Iloprost administration 30 min after the onset of ischaemia was still beneficial, although to a lesser extent. Iloprost lost its protective effect when given only on reperfusion. The data suggest that the iloprost cardioprotective effect is related to maintainance of membrane integrity.

Time course of human atrial natriuretic factor release during cardiopulmonary bypass in mitral valve and coronary artery diseased patients.

We determined the time-course of the release of atrial natriuretic factor (ANF) during cardiopulmonary bypass (CPB) in six patients undergoing coronary artery bypass (CAD) and eight patients undergoing valve replacement for mitral stenosis (MS). Before CPB, the arterial ANF was significantly higher in MS patients than in CAD patients (243 +/- 38 and 29 +/- 5.8 pg/ml respectively, P less than 0.01). With the onset of CPB, the acute pressure unloading of the atria induced a significant, rapid decrease of ANF only in MS patients (-64% of pre-CPB value at 5 min) and no major changes in CAD patients. Clamping of the aorta induced a further progressive reduction of ANF release to almost zero in both groups. Readmission of coronary flow to the empty atria with declamping resulted in an increase in the plasma level of ANF in both groups to reach the concentration present in MS patients before CPB. After CPB, the ANF levels decreased in CAD patients while remaining elevated in MS patients. These data suggest that ANF release from human atria depends on atrial filling pressure and other unknown factors.

Role of oxygen free radicals in ischemic and reperfused myocardium.

In recent years there has been considerable interest concerning the role of oxygen radicals in myocardial ischemia and reperfusion injury. The sequential univalent reduction of oxygen gives rise to very reactive intermediate products. Normally, the tissue concentration of these intermediate products of oxygen is limited and the aerobic myocardium survives because of the existence of a delicate balance between the generation of the various oxidants and the maintenance of the antioxidant defense mechanism. Several possible sources have been identified for the production of active oxygen species after ischemia and reperfusion and these sources may be mutually interactive. The ability of scavengers of oxygen free radicals, including vitamin E, to improve mechanical, mitochondrial, and sarcoplasmic reticulum function in animal models of ischemic-reperfusion injury also suggests that oxygen free radicals are partly responsible for myocardial damage in these models, although caution in the interpretation of these data is necessary.

Effect of congestive heart failure on rate of atrial natriuretic factor release in response to stretch and isoprenaline.

STUDY OBJECTIVE: The aim was to investigate the pattern of release of atrial natriuretic factor induced by mechanical and adrenergic stimulation from atria of rats with or without congestive heart failure. DESIGN: Monocrotaline, a pyrrolizidine alkaloid, was given to rats to cause severe pulmonary hypertension, leading to a marked degree of right ventricular hypertrophy and failure. Measurements of noradrenaline and atrial natriuretic factor were performed in each cardiac chamber and in plasma. Right and left atria of control rats and rats with congestive heart failure were isolated and subjected to mechanical or adrenergic stimulation to study the in vitro release of atrial natriuretic factor. MATERIALS: Studies were performed on plasma, ventricles, and isolated right and left atria of 276 male Wistar rats, 80-100 g weight, with or without congestive heart failure. MEASUREMENTS AND MAIN RESULTS: In monocrotaline rats right and left ventricular concentrations of noradrenaline were significantly reduced. In the same rats concentrations of atrial natriuretic factor fell to 15.2% in the right atria and to 65.5% in the left atria. Whole heart content of atrial natriuretic factor was diminished, while plasma concentrations were increased sevenfold. Isolated hypertrophied right atria of failing hearts did not release atrial natriuretic factor in response to stretch or to isoprenaline (10(-9)M) and they were insensitive to the inotropic action of isoprenaline. On the other hand, non-hypertrophied left atria from the same animals released increased amounts of atrial natriuretic factor under basal conditions and after both stimuli, despite reduced tissue stores of the peptide. CONCLUSIONS: Heart failure may deplete cardiac stores of noradrenaline and atrial natriuretic factor, especially in hypertrophied chambers, and can result in a decrease in the release of atrial natriuretic factor from atrial tissue in response to mechanical and adrenergic stimulation.

Effects of prolonged infusions of atrial natriuretic peptide and isoproterenol on the mechanical and endocrine function of isolated atria.

We have measured the rate of release of atrial natriuretic peptide in response to stretch and to isoproterenol from superfused atria isolated from rats after chronic in vivo infusion of either atrial natriuretic peptide or isoproterenol. The infusion lasted seven days, using minipumps filled with: 1) saline; 2) synthetic atrial natriuretic peptide to release 3 micrograms/kg/h; 3) l-isoproterenol HCl to release 400 micrograms/kg/h. Infusion of isoproterenol caused hypertrophy of the left chambers of the heart, a decrease of atrial content of atrial peptide with increased plasma levels. Infusion of atrial natriuretic peptide resulted only in elevated plasma levels of the peptide. Atria from animals infused with atrial natriuretic peptide responded to stretch and to isoproterenol (10(-9) M) with a prompt and transient release of peptide. Atria from animals infused with isoproterenol showed a severe reduction in sensitivity to the inotropic and chronotropic action of the drug. Their basal release of atrial peptide was extremely reduced and they were unable to respond to stretch or to isoproterenol. These data indicate that high circulating levels of atrial natriuretic peptide do not influence the release of the peptide from the atria, while high levels of isoproterenol drastically reduce it. Beta adrenoceptor desensitization, atrial hypertrophy and decrease of stores of atrial peptide are likely to account for this phenomenon.

Protective effects of gallopamil against ischemia and reperfusion damage.

To establish if the administration of gallopamil, a derivative of verapamil, protects heart muscle against the deleterious effect of ischemia and subsequent reperfusion, rabbits were injected subcutaneously twice daily with 2 mg/kg of Gallopamil for 5-6 days. The hearts were isolated and perfused with aerobic Krebs-Henseleit buffer solution by the Langendorff method. The hearts were paced (180 b/min) and wall temperature was controlled. Ischemia was induced by reducing coronary flow from 25 ml/min to 1 ml/min for 90 min and then the hearts were reperfused for 30 min. At the end of either the ischemic period or reperfusion, the hearts were assayed for ATP, CP, and calcium. Others were homogenized, their mitochondria harvested and monitored for oxidative phosphorylating and ATP generating activity as well as calcium content and uptake. The mechanical function of the hearts and noradrenaline release was also measured. Hearts that were made ischemic gained calcium, their endogenous stores of ATP and CP were depleted, their mitochondria had reduced RCI and state 3 respiration and increased calcium concentrations. During reperfusion tissue and mitochondrial calcium was significantly increased, the capacity of mitochondria to use oxygen for state 3 respiration was further impaired and their ATP generating capacity reduced. Diastolic pressure increased and there was no recovery of developed pressure and important noradrenaline release. Pretreatment with gallopamil protected the mitochondria against the ischemically induced changes in RCI, state 3 respiration. There was also a less marked rise in tissue and mitochondrial calcium and a reduced increase of diastolic pressure. Gallopamil also diminished the effect of reperfusion on the calcium accumulating activity of mitochondria and on the decline in the ATP generating and oxygen utilizing capacity of the mitochondria. The tissue levels of ATP and CP were better maintained, and noradrenaline release was reduced, the systolic pressure generating capacity was enhanced by the treatment with gallopamil. These results are discussed in accordance with the hypothesis that this drug protects heart muscle against the deleterious effects of ischemia and reperfusion by ensuring that sufficient ATP remains available to maintain homeostasis with respect to calcium.