Age dependence of myocardial Na+-K+-ATPase activity and digitalis intoxication in the dog and guinea pig

Developmental differences in myocardial transmembrane Na+ transport: Implications for excitability and Na+ handling

KEY POINTS

Previous studies showed that myocardial preparations from immature rats are less sensitive to electrical field stimulation than adult preparations. Freshly-isolated ventricular myocytes from neonatal rats showed lower excitability than adult cells, e.g., less negative threshold membrane potential and greater membrane depolarization required for action potential triggering. In addition to differences in mRNA levels for Na⁺ channels isoforms and greater Na⁺ current (I_Na ) density, Na⁺ channel voltage-dependence was shifted to the right in immature myocytes, which seems to be sufficient to decrease excitability, according to computer simulations. Only in neonatal myocytes did cyclic activity promote marked cytosolic Na⁺ accumulation, which was prevented by abolition of systolic Ca²⁺ transients by blockade of Ca²⁺ currents. Developmental changes in I_Na may account for the difference in action potential initiation parameters, but not for cytosolic Na⁺ accumulation, which seems to be due mainly to Na⁺ /Ca²⁺ exchanger-mediated Na⁺ influx.

ABSTRACT

Little is currently known about possible developmental changes in myocardial Na⁺ handling, which may have impact on cell excitability and Ca²⁺ content. Resting intracellular Na⁺ concentration ([Na⁺ ]_i ), measured in freshly-isolated rat ventricular myocytes with CoroNa-green, was not significantly different in neonates (3-5 days old) and adults, but electrical stimulation caused marked [Na⁺ ]_i rise only in neonates. Inhibition of L-type Ca²⁺ current by CdCl₂ abolished not only systolic Ca²⁺ transients, but also activity-dependent intracellular Na⁺ accumulation in immature cells. This indicates that the main Na⁺ influx pathway during activity is the Na⁺ /Ca²⁺ exchanger, rather than voltage-dependent Na⁺ current (I_Na ), which was not affected by CdCl₂ . In immature myocytes, I_Na density was 2-fold greater, inactivation was faster, and the current peak occurred at less negative transmembrane potential (E_m ) than in adults. Na⁺ channel steady-state activation and inactivation curves in neonates showed a rightward shift, which should increase channel availability at diastolic E_m , but also require greater depolarization for excitation, which was observed experimentally and reproduced in computer simulations. Ventricular mRNA levels of Na_v 1.1, Na_v 1.4 and Na_v 1.5 pore-forming isoforms were greater in neonate ventricles, while decrease was seen for the β1 subunit. Both molecular and biophysical changes in the channel profile may contribute to the differences in I_Na density and voltage-dependence, and also to the less negative threshold E_m in neonates, compared to adults. The apparently lower excitability in immature ventricle may confer protection against the development of spontaneous activity in this tissue. Abstract figure legend Little is currently known about possible developmental changes in myocardial Na⁺ transport, which may have impact on cell excitability and other physiological aspects. At the mRNA level, neonatal rat ventricle expresses a greater variety of Na⁺ channel isoforms than in adults. In immature ventricular cardiomyocytes, Na⁺ current (I_Na ) density was greater, but voltage-dependence is shifted to less negative potentials than in adults. This should increase channel availability at diastolic membrane potential, but also require greater depolarization for excitation, which was observed experimentally and reproduced in computer simulation. We also observed that electrical stimulation caused marked intracellular Na⁺ accumulation only in neonates, which was abolished when Ca²⁺ transients and the Na⁺ /Ca²⁺ exchanger (NCX) were inhibited by Cd²⁺ + Ni²⁺ . Thus, it seems that the main Na⁺ influx pathway during activity in neonates is the NCX, rather than voltage-dependent I_Na , which was not affected by these blockers. This article is protected by copyright. All rights reserved.

Digitalis use in children: an uncertain future

The therapeutic indications for digoxin in the treatment of children with large left-to-right shunts continue to be reassessed. New insights into the alterations in cardiac function imposed by this hemodynamic burden have shown preserved systolic performance. Pharmacological interventions that improve cardiac output by afterload reduction or other modalities have proven useful and potentially have low risk for serious toxicity. Early and successful surgical treatment of most conditions causing pulmonary overcirculation has shortened the duration of medical management and prevented many of the untoward complications of this pathology. As new agents are forthcoming to treat various cardiac conditions, use-by-tradition of cardiac glycosides has appropriately diminished. While the understanding of complicated molecular aspects of cation transport have been enhanced by the unique actions of cardiac glycosides, their clinical utility has decreased. This report summarizes studies on the use of cardiac glycosides in the treatment of large left-to-right shunts in children.

Mechanisms underlying the cost of living in animals

The cost of living can be measured as an animal's metabolic rate. Basal metabolic rate (BMR) is factorially related to other metabolic rates. Analysis of BMR variation suggests that metabolism is a series of linked processes varying in unison. Membrane processes, such as maintenance of ion gradients, are important costs and components of BMR. Membrane bilayers in metabolically active systems are more polyunsaturated and less monounsaturated than metabolically less-active systems. Such polyunsaturated membranes have been proposed to result in an increased molecular activity of membrane proteins, and in this manner the amount of membrane and its composition can act as a pacemaker for metabolism. The potential importance of membrane acyl composition in metabolic depression, hormonal control of metabolism, the evolution of endothermy, as well as its implications for lifespan and human health, are briefly discussed.

Serum digoxin and beta-methyldigoxin in elderly patients on hospital admission: correlation with home compliance and clinical variables

Serum digoxin and beta-methyldigoxin (BMD) were measured in 165 elderly patients (age greater than 60 years) admitted to hospital, of whom 109 had been treated at home with digoxin and 56 with BMD. The mean BMD level was significantly lower than that of digoxin (1.1 vs. 1.4 ng/ml). Creatinine clearance and daily dose were the variables most strongly associated with digoxin level, and the prescribed dose and serum albumin were the best predictors of the BMD concentration. Compliance was assessed by a compliance index (CI), namely the ratio of the measured glycoside concentration, corrected for creatinine clearance, over the expected steady-state dose, calculated from a hospitalized reference group. Compliant individuals in both treatment groups, i.e. those with a CI greater than the median value, were characterized by a lower daily dose and dosage frequency. Toxicity, whether clinical or electrocardiographic, was present in 9% of the patients and was associated only with a significantly higher mean serum level of the drug.

Cardiac glycoside toxicity in small laboratory animals

Cardiac glycosides are frequently administered to laboratory animals for research purposes. The effects achieved depend not only upon the particular glycoside and dose administered, but also upon an entire array of variables from the species of animal to the temperature of the animal housing facility. We review a number of these factors and their influence upon the effects achieved by the administration of cardiac glycosides to laboratory animals.

Age-related digoxin effects in an intact canine model

The inotropic and electrophysiologic effects of digoxin were studied in anesthetized neonatal and adult dogs to test the hypothesis that digoxin had comparable effects in these groups. Recordings of the ECG and central arterial pressure were made starting at 5.75 hours after an intravenous injection of 50 micrograms/kg of the drug. Parameters measured were heart rate (HR); PR interval; mean, systolic, and diastolic blood pressure; preejection period (PEP); and ejection time (ET). Two indices of systolic function were calculated, the systolic time interval (STI = PEP/ET) and total electromechanical systole (TMS = PEP + ET), which was indexed for HR. There was no significant difference from control animals in either the adult or neonatal groups in the PR interval or blood pressure. In the neonatal dogs, HR and STI were also not significantly different from control. However, in the neonatal dogs, there was a significant decrease in the indexed TMS, 288 +/- 7 vs 270 +/- 11 msec (p less than 0.01). In the adult animals, HR decreased from 116 +/- 35 to 66 +/- 25 bpm (p less than 0.01), STI decreased from 0.559 +/- 0.059 to 0.447 +/- 0.069 (p less than 0.01), and indexed TMS decreased from 333 +/- 10 to 291 +/- 13 msec (p less than 0.001). Two-way analysis of variance demonstrated that digoxin differed significantly in its effects on HR (p = 0.005), STI (p = 0.018), and TMS indexed for HR (p = 0.003) in neonatal compared to adult dogs. Pharmacokinetic studies showed a rapid distribution phase and equilibrium conditions at the time of physiologic measurements.(ABSTRACT TRUNCATED AT 250 WORDS)

Prescribing digoxin in geriatric units: the unexplained variability in dosage requirements

Some physicians regard patients of Geriatric Units as a homogeneous population with respect to digoxin dosage requirements. Others advocate the use of pharmacokinetic models in prescribing digoxin for the elderly. Sixty in-patients of Geriatric Units were studied and the results compared with those previously obtained from 129 patients of other adult Units; all were receiving maintenance digoxin. For each patient the dose required to achieve a mean steady-state serum digoxin concentration of 1.6 nmol X l-1, the standardized dose, was calculated, assuming proportionality between the dose given and the concentration achieved. A mean of four estimates of standardized dose for each individual was used in the analysis. Threefold ranges of standardized dose covered the requirements of approximately 85% of patients both of Geriatric Units (62.5 to 187.5 micrograms per day) and of other adult Units (125 to 375 micrograms per day). The variables, serum creatinine concentration, sex, age, and body weight were of relatively little value in predicting the standardized dose for the patients in Geriatric Units. There was a sub-group of these in-patients for whom the standardized dose was extremely large.

Sarcolemmal ATPase activities of the rat heart ventricle--dependence on age and sodium ion

Na-K-ATPase activity of the rat heart was similar throughout the postnatal growth when measured from crude unpurified fraction. Instead in the cardiac sarcolemmal fraction, isolated by hypotonic shock LiBr-treatment method, the activity was over two times higher in 10-day old neonates than in adult rats. The conflicting results are partly explained by different effects of the isolation procedure on neonatal and adult tissues. Na concentration for half-maximal activity of the Na-K-ATPase was similar in neonates (7.0 mM) and adults (6.4 mM). Ca-ATPase activity was not affected by Na concentration (2-100 mM) in the two age-groups studied.

Use of digoxin in infants and children, with specific emphasis on dosage

The foregoing discussion leads to several general conclusions regarding the use of digoxin in the pediatric patient. First, pharmacokinetic studies indicate that somewhat higher doses are required in the infant to attain the same serum levels as in the adult. Important sources for this difference appear to be more rapid body clearance of digoxin and larger volume of distribution in the infant. Second, higher serum digoxin levels are not indicated on the basis of decreased myocardial uptake of digoxin in the infant. Tissue uptake of digoxin, as indicated by myocardium/serum digoxin ratios, is higher in infants and children than in adults. Third, according to results of animal studies, the inotropic sensitivity to digoxin in the young is probably greater--certainly not less--than in the adult. This is opposite to a commonly held view that the immature heart is less sensitive to cardiac glycosides and therefore requires higher serum levels for a therapeutic effect. Rather, the infant has decreased sensitivity of the conduction system to digitalis toxicity, and healthy myocardium less prone to arrhythmia than the adult. Therefore the infant may tolerate, but does not require, higher serum levels of digoxin. Fourth, high levels of serum digoxin (greater than 2 ng/ml) are not associated with greater inotropic effects in the pediatric patient. The higher dosages of digoxin are, instead, associated with greater frequency of toxic effects, especially in infants receiving concomitant diuretic therapy. Therefore, a digoxin dosage recommendation is presented, that will result in mean serum digoxin levels of 1.1 to 1.7 ng/ml.(ABSTRACT TRUNCATED AT 250 WORDS)

K+-dependent 3-O-methylfluorescein phosphatase activity in crude homogenate of rodent heart ventricle: effect of K+ depletion and changes in thyroid status

The total Na+-K+-ATPase activity in rodent heart ventricle was quantified by determination of K+-dependent, ouabain suppressible 3-O-methylfluorescein phosphatase activity. A K+-dependent phosphatase activity of 0.80 mumol/min per g wet wt was obtained from crude homogenate of heart ventricle from 4-week-old guinea pigs. The anticipated association between K+-dependent phosphatase activity and Na+-K+-activated ATP hydrolysis could be clearly demonstrated in the crude homogenate. Based upon a molecular activity of 550/min the corresponding cardiac glycoside receptor concentration in the crude homogenate was 1450 pmol/g wet wt. In crude homogenate of heart ventricle from 3-month-old rats the K+-dependent phosphatase activity was 1.16 mumol/min per g wet wt. Following 4 weeks of K+ depletion of the rats, a decrease of 13% in total K+ content of the heart ventricle was seen. This was associated with a 14% decrease in K+-dependent phosphatase activity. The induction of hypo- and hyperthyroidism for 3 weeks in rats was followed by a decrease of 27% and an increase of 13% in K+-dependent phosphatase activity, respectively.

Activation of contractility and sarcolemmal Ca2+-ATPase by Ca2+ during postnatal development of the rat heart

The effects of 0.25-16.0 mM Ca2+ on the contractile force of isolated ventricular strips and sarcolemmal Ca2+-ATPase activity during postnatal development of the rat heart were studied. The half maximal concentrations for contractile activation of ventricular strips were 0.76 and 5.59 mM Ca2+ for adult and 3-day-old rats, respectively. The sensitivity towards Ca2+ began to change from newborn type to that of adult rat 2 weeks after birth and was almost completed after 4 weeks. No significant differences were found in half maximal activation of Ca2+-ATPase by Ca2+ between different age groups. Activation of contractility and Ca2+-ATPase by Ca2+ were linearly related in 30-day-old and adult rats but not in 3- and 10-day-old rats. The observed sensitivity change towards extracellular Ca2+ for contractile activation is suggested to be due to the development of transverse tubular system and sarcoplasmic reticulum during the first 4 weeks of postnatal development.

Insulin action on cardiac glucose transport. Studies on the role of the Na+/K+ pump

Isolated muscle cells from adult rat heart have been used to study the relationship between myocardial glucose transport and the activity of the Na+/K+ pump. 86Rb+-uptake by cardiac cells was found to be linear up to 2 min with a steady-state reached by 40-60 min, and was used to monitor the activity of the Na+/K+ pump. Ouabain (10(-3) mol/l) inhibited the steady-state uptake of 86Rb+ by more than 90%. Both, the ouabain-sensitive and ouabain-insensitive 86Rb+-uptake by cardiac cells were found to be unaffected by insulin treatment under conditions where a significant stimulation of 3-O-methylglucose transport occurred. 86Rb+-uptake was markedly reduced by the presence of calcium and/or magnesium, but remained unresponsive towards insulin treatment. Inhibition of the Na+/K+ pump activity by ouabain and a concomitant shift in the intracellular Na+ :K+ ratio did not affect basal or insulin stimulated rates of 3-O-methylglucose transport in cardiac myocytes. The data argue against a functional relationship between the myocardial Na+/K+ pump and the glucose transport system.

The effect of digoxin on 86rubidium uptake by erythrocytes from mothers and babies

1 The uptake of 86rubidium by erythrocytes from mothers and babies has been used as a model system to investigate possible differences in sensitivity to digoxin in the very young. 2 While total 86rubidium uptake was not significantly different between mothers and babies, the digoxin-sensitive proportion was higher in neonatal erythrocytes. Neonatal cells were less sensitive to digoxin, demonstrated by the requirement for a larger amount of digoxin to inhibit 86rubidium uptake and this was accompanied by an increase in numbers of specific erythrocyte binding sites for digoxin. 3 These results provide further evidence in support of the hypothesis of decreased sensitivity to digoxin in the very young.

Age-dependent alterations in lipids and function of rat heart sarcolemma

The objective of the present study was to examine the effect of age on heart sarcolemma structure and function. Sarcolemmal fractions were prepared from hearts of young (1-1.5 months) and adult (10-12 months) rats and assayed for marker enzyme activities. The membrane fractions were found to be devoid of other cellular organelles upon examination by electron microscopy. They were enriched with 5'-nucleotidase and devoid of succinate dehydrogenase activity. The only age-related lipid compositional changes noted in these membranes were changes in the fatty acid composition of membrane phospholipids with increasing age. Most changes were detected in phosphatidylcholine and phosphatidylethanolamine with very little alteration of sphingomyelin and phosphatidylserine plus phosphatidylinositol fatty acids. Polyunsaturated fatty acids, especially 18:2 and 20:4, were decreased with saturated fatty acids increased in membrane phosphatidylcholine and phosphatidylethanolamine fractions as the animal develops. There was a decrease in the specific activities of (Na+ + K+)-ATPase and 5'-nucleotidase of these membranes with age. On the other hand, membrane (K+)-rho-nitrophenylphosphatase was not affected by age.

Age-dependent changes in the number of [3H]ouabain-binding sites in rat soleus muscle

The influence of age on the number of (Na+K+)-ATPase units in skeletal muscle has been assessed by measurements of [3H]ouabain binding in vitro and in vivo to rat soleus muscle. In vitro measurements showed that from the 2nd to the 28th day of life, the number of [3H]ouabain-binding sites increases from 120 to 580 pmol/g wet wt. This is followed by a decrease, until a plateau between 150 and 200 pmol/g is reached around 150 days after birth. 60 min after intraperitoneal injection of [3H]ouabain (12.5 mumol/kg body weight), the soleus muscles of 28-day-old rats had accumulated 2.4-times more 3H-activity per g wet wt. than muscles of 85-day-old rats and the 3H-activity in plasma was 54% lower. The results may explain the low sensitivity to digitalis glycosides found in infants as compared to premature or adult individuals.

Effect of age, potassium depletion and denervation on specific displaceable [3H]ouabain binding in rat skeletal muscle in vivo

1. Following intraperitoneal injection of [(3)H]ouabain in rats, the isotope is rapidly distributed in blood plasma available for binding to the Na-K-ATPase in the plasma membranes of most tissues. In skeletal muscle tissue excised and washed 4 x 30 min in ice-cold buffer, 95% of the (3)H activity retained was shown to be [(3)H]ouabain using a specific binding assay.2. The [(3)H]ouabain bound to soleus and extensor digitorum longus (e.d.l.) muscles in vivo and retained following wash-out in the cold showed the same saturation characteristics as those determined when binding took place in vitro.3. In soleus and e.d.l. muscles obtained from 28-day-old rats, the number of [(3)H]ouabain binding sites measured in vivo was 583+/-19 and 720+/-22 pmol/g wet wt., respectively, i.e. in good agreement with previous and present results obtained in vitro.4. In vivo measurements showed that 7 days after denervation, the number of [(3)H]ouabain binding sites in soleus and e.d.l. muscles was reduced by 22 and 13%, respectively.5. In the age interval from 28 to 85 days, the number of [(3)H]ouabain binding sites in soleus was found to decrease by 58%. Following I.P. injection of [(3)H]ouabain, the 85-day-old rats showed a more pronounced and sustained rise in plasma (3)H activity, which in part can be due to the reduced capacity for [(3)H]ouabain binding in skeletal muscle.6. K depletion induced by the administration of K-deficient diet for 3 weeks reduced [(3)H]ouabain binding by 63% in soleus muscles. In the K-depleted animals, the plasma (3)H activity measured 15 min after I.P. injection of [(3)H]ouabain was 77% higher than in controls receiving the same dose per kg body weight.7. The present in vivo results provide further support for the idea that increased digitalis toxicity due to increasing age or K depletion is related to reduced binding capacity for digitalis glycosides in skeletal muscle.