AJP - Regulatory, Integrative and Comparative Physiology, Vol 250, Issue 2 188-R192, Copyright © 1986 by American Physiological Society
Mechanisms of adrenergic control of blood pressure in developing rats
E. Mills and P. G. Smith
The rat is a species in which the sympathetic nervous system (SNS) is
highly immature at birth. Blood pressure was measured directly in
anesthetized preparations starting on the first postnatal day (days 1, 5,
9, 20, 40, 55, and 85), and pharmacological tests were used to evaluate the
functional development of the vasomotor nerves (maximum pressor response to
tyramine), the sensitivity of the vasculature to direct stimulation of
alpha 1-adrenoceptors (maximum and 50% effective dose of methoxamine
pressor response), and relative magnitude of the SNS contribution to
resting blood pressure (hypotensive response to ganglionic blockade divided
by resting blood pressure). The SNS contribution was not significant on
postnatal day 1, but the relative magnitude was comparable to the adult by
the end of the first postnatal week. During week 1 the vasomotor nerves
were functionally immature (tyramine response was 48% of mature value on
day 5), and the vasculature was supersensitive to alpha 1-adrenoceptor
stimulation (154% of mature value). Conversely, in postnatal weeks 2 and 3,
when the developing SNS is known to be hyperactive, the vasculature was
subsensitive to noradrenergic stimulation (60-70% of adult). The net effect
was to attenuate the SNS contribution to resting blood pressure during this
period (55% of adult value). We conclude that there is an inverse relation
between the level of tonic SNS activity and vascular sensitivity to
noradrenergic stimulation in the developing rat. Supersensitivity may be
critical for cardiovascular adjustments to asphyxia perinatally when the
vasomotor nerves are functionally immature; subsensitivity may act
homeostatically to prevent hypertension during the developmental period
when SNS is hyperactive.
