AJP - Regulatory, Integrative and Comparative Physiology, Vol 261, Issue 4 920-R927, Copyright © 1991 by American Physiological Society
Ontogeny of canine intrathoracic cardiac nervous system
C. Haddad and J. A. Armour
Department of Physiology and Biophysics, Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia, Canada.
The functional cardiac innervation of 61 puppies from nine different
litters (2-8 littermates), ranging in age from 1 day to 7 wk, was
investigated. The efferent sympathetic nervous system exerted minimal
effects on the heart of 1-day-old puppies, gradually influencing the heart
more thereafter such that by 7 wk of life it was functionally mature. In
contrast, efferent parasympathetic cardiac innervation was well developed
at birth, maturing thereafter such that by 4-7 wk of age its capacity to
modulate the heart was similar to that found in adults. The right- and
left-sided efferent sympathetic and parasympathetic intrathoracic nervous
systems induced similar cardiac modulation throughout this period of
development. Cardiac myocyte beta-adrenergic receptors were partially
functional at birth, as determined by responses elicited by supramaximal
doses of the beta-agonist isoproterenol. Responses elicited by
isoproterenol became greater over the following 7 wk of life, when they
were found to be similar to those elicited in adults. By 1 wk of age,
synaptic mechanisms in intrathoracic sympathetic ganglia involved in
cardiac regulation were relatively well developed, with
cardiopulmonary-cardiac reflexes present but not functionally mature at
that age. It is concluded that maturation of the efferent sympathetic
nervous system modulating the canine heart depends to a large extent on the
ontogeny of cardiac beta-adrenergic receptors rather than the ontogeny of
synapses in intrathoracic ganglia. Furthermore, even though functional
cardiac efferent parasympathetic innervation is present before efferent
sympathetic innervation, both reach maturity at about the same age.
