Properties and modulation of the if current in newborn versus adult
cardiac sa node.
Accili, E. A., R. B. Robinson, and D. Difrancesco.
Universit[alpha] di Milano, Dipartimento di Fisiologia e Biochimica
Generali, Elettrofisiologia, Milano, Italy and the Department of
Pharmacology, Columbia University, New York, NY
APStracts 3:0547H, 1996.
The hearts of newborn mammals have greater intrinsic beating rates,
rates of diastolic depolarization, and sensitivity to autonomic
stimulation, than adults. The differences could be explained partly
by altered properties of the hyperpolarization-activated current (if
Cell capacitance of newborn myocytes was approximately 42% smaller
than in the adult.). To test this possibility, sino-atrial (SA) node
myocytes from the hearts of newborn (nb, 9-10 days) and adult (ad,
> 30 days) rabbits were isolated and if was examined using the
perforated-patch clamp technique. The fully-activated I/V relation
yielded a larger slope conductance of if in newborn SA node myocytes
(0.244 ( 0.020 versus 0.158 ( 0.012 pS/pF) compatible with the more
rapid diastolic depolarization. Activation curves of if had similar
mid-activation voltages (-66.71 ( 1.94 mV nb, -66.33 ( 2.60 mV ad)
but the slope was significantly greater in nb (inverse slope factor
-9.57 ( 0.35 mV nb, -11.34 ( 0.54 mV ad). No differences in shifts of
the if activation curve in response to maximal concentrations of
acetylcholine (-9.70 ( 1.8 mV nb, -12.60 ( 2.10 mV, ad) and
isoproterenol (6.90 ( 2.5 mV nb, 5.3 ( 1.5 mV ad) were observed, or
in the total shift in response to these agonists (16.60 ( 3.30 mV nb,
18.00 ( 1.00 mV ad). The greater if current density and steeper
voltage-dependence can contribute to both the greater heart rate and
greater sensitivity of the SA node to autonomic modulation in newborn
animals.
Received 1 October 1996; accepted in final form 10 December 1996.
APS Manuscript Number H878-6.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 31 December 1996