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.
POSTURAL EFFECTS ON HEART-RATE AND BLOOD-PRESSURE