Sympathetic responses of the heart and adrenal medulla in developing Dahl hypertensive rats

Functional development of the sympathetic nervous system was examined in inbred Dahl salt-sensitive (S/JR) and salt-resistant (R/JR) rats by assessing cardiac and adrenal medullary responses to insulin-induced hypoglycemia at 2, 4, 8, 12, and 16 days of age. Heart ornithine decarboxylase (ODC) activity and adrenal catecholamine content were measured in pups of the two strains 3 hours after administration of either saline or insulin. The centrally mediated increase in sympathetic outflow caused by insulin-induced hypoglycemia was attended by induction of heart ODC activity and depletion of adrenal epinephrine (EPI). No significant differences were found overall between R/JR and S/JR strains with regard to either heart ODC activity or adrenal epinephrine. This was true for basal values obtained from saline-injected pups as well as for measures from insulin-injected pups. Functional innervation of the heart was present in pups of both strains as early as 2 days of age, while in the adrenal medulla a significant response to stimulation was not detected until 8 days of age. While the susceptibility for hypertension in the salt-sensitive animals may well be linked to increased sympathetic tone, the present findings indicate that S/JR rats do not have an accelerated development or a hyperresponsiveness of sympathetic input to either the heart or the adrenal medulla during the pre-weanling period.