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

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 (INa ), which was not affected by CdCl2 . In immature myocytes, INa density was two-fold greater, inactivation was faster, and the current peak occurred at less negative transmembrane potential (Em ) than in adults. Na+ channel steady-state activation and inactivation curves in neonates showed a rightward shift, which should increase channel availability at diastolic Em , but also require greater depolarization for excitation, which was observed experimentally and reproduced in computer simulations. Ventricular mRNA levels of Nav 1.1, Nav 1.4 and Nav 1.5 pore-forming isoforms were greater in neonate ventricles, while a decrease was seen for the ß1 subunit. Both molecular and biophysical changes in the channel profile may contribute to the differences in INa density and voltage-dependence, and also to the less negative threshold Em , in neonates compared to adults. The apparently lower excitability in immature ventricle may confer protection against the development of spontaneous activity in this tissue. 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+ channel isoforms and greater Na+ current (INa ) 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 INa 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.

Atrial chronotropic reactivity to catecholamines in neonatal rats: Contribution of ß-adrenoceptor subtypes.

Although increase in heart rate is a crucial determinant for enhancement of cardiac output in the neonate, information on the chronotropic reactivity to catecholamines during postnatal development is scarce. The present study was aimed at investigating the role of ß-adrenoceptor subtypes and catecholamine removal mechanisms in the adrenergic chronotropic response during the early post-natal period. Right atria isolated from immature (0-21 day old) and adult (4-6 month old) rats were used for determination of the responsiveness to agonists and quantitation of the transcripts of proteins involved in ß-adrenergic signaling. The main results were: (a) the maximum response (Rmax) to norepinephrine increased with age, whereas sensitivity decreased; (b) age-dependent differences in sensitivity to norepinephrine were abolished by inhibition of the neuronal norepinephrine transporter; (c) Rmax to isoproterenol was similar in immature and adult atria, and depressed only in the former by ß2-adrenoceptor blockade with ICI118,551; (d) neonatal atria showed greater ß2-adrenoceptor mRNA levels, and more prominent positive chronotropic response to the ß2- and ß3-adrenoceptor agonists zinterol and YM178, respectively (nanomolar range); (e) in atria of immature rats, transcript levels of the extraneuronal monoamine transporter were lower, and its inhibition did not affect sensitivity to isoproterenol; and (f) reactivity to forskolin and 3-isobutyl-1-methylxanthine was not affected by age. The increased ß2- and ß3-adrenoceptor participation in the adrenergic chronotropic response, in addition to weaker catecholamine removal, may compensate for the immature cardiac innervation and the apparently reduced efficiency of ß1-adrenoceptor signaling in the neonate, increasing the responsiveness to endogenous and exogenous ß2-adrenoceptor agonists.